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Material research on perovskite-type oxides (ABO3) has been driven by the recognition of their unique properties primarily attributed to the presence of oxygen octahedron (BO6). Since 2003, the discovery of strong coupling in TbMnO3 and BiFeO3 has stimulated new interests in understanding the relationship between magnetic and electric properties in perovskites. In this article, we report our recent work on the magnetic superexchange interaction and charge formation in copper-doped LaFeO3 using high-temperature neutron diffraction and thermoelectric measurements. In situ neutron diffraction measurements show a loss of antiferromagnetic ordering above 450°C. With an increase in Cu content, the (Fe, Cu)-O bond length decreases and the (Fe, Cu)–O–(Fe, Cu) bond angle increases, which leads to an enhancement of the Fe–O–Fe superexchange interaction. Thermoelectric and electrical measurements show that the formation of electron holes in Cu-doped LaFeO3 is a thermally activated process with two distinct regions with a transition temperature near 450°C, in congruence with the magnetic measurements. Our work show that Cu is in 3+ state in La(Fe,Cu)O3 at room temperature, resulting in the maximum superexchange interaction between Fe3+ ions.

Muon spin rotation experiments on superconducting GdSr2Cu2RuO8 reported a magnetic induction of 720–740 G. Neutron diffraction studies have shown that the Ru spins are ordered antiferromagnetically along the c-axis, while the cuprate planes are ordered ferromagnetically in the a–b plane and are stacked antiferromagnetically. Using this spin configuration with refined spin moments, we have calculated the magnetic induction at potential muon stopping sites; muons stopping in the center of the ferromagnetic CuO2 planes will experience an induction of ∼725 G, a result in excellent agreement with experiment. We predict that muon experiments, carried out at temperatures below the Gd antiferromagnetic ordering temperature, will detect an induction exceeding 1 T at one of the muon sites.

Magnetic and structural properties of La0.7Sr0.3Mn1-xCrxO3 (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) have been studied using neutron and x-ray diffraction and magnetic measurements. All samples are found to be of single phase. At 12 K, we find only ferromagnetic ordering for x < 0.30 and coexisting ferromagnetic and antiferromagnetic domains for x ⩾ 0.30. Our magnetic data indicate temperature dependent ferromagnetic to paramagnetic, and likely antiferromagnetic to paramagnetic transitions. We find no evidence of spin glass or cluster glass behavior, only long-range magnetic order with a transition to antiferromagnetic behavior beginning with x = 0.30 and almost completely antiferromagnetic behavior at x = 0.50.

We have searched, without success, for microwave-frequency magnetic resonance of Ru 4+ and Ru 5+ ions in a number of materials for which the Ru are in octahedral coordination with six oxygen. A number of ruthenates including RuO 2 , SrRuO 3 , Sr 3 Ru 2 O 7 , Ba 3 Ru 2 NiO 9 , Ba 2 GdRuO 6 , Sr 2 YRuO 6 , and Ba 2 YRuO 6 , which include paramagnetic, antiferromagnetic, and ferromagnetic spin configurations, have been examined. We present analysis which shows that the last material provides an optimized opportunity to detect antiferromagnetic Ru resonance for temperatures less than T N =39 K; none is detected for frequencies as high as 35 GHz in magnetic fields up to μ o H =2 T. This result indicates that the antiferromagnetic magnon energy gap exceeds the energy associated with the signal frequency. SrRuO 3 is a known ferromagnetic contaminant phase in the rutheno-cuprates. We report neutron diffraction measurements on SrRuO 3 , finding it to have an appreciable local moment at low temperatures, 1.25(0.1) μ B ; this moment vanishes near 165 K. We show that it also fails to exhibit ferromagnetic resonance, at least in the range 10–35 GHz. As a result of the diffraction and resonance studies, it is concluded that the reports of ferromagnetic resonance in superconducting rutheno-cuprates are actually due to antiferromagnetically ordered Cu in these materials, and the presence of even a few percent of SrRuO 3 as a potential contaminant is of little importance.

Ammonia borane(BH3NH3) is a promising hydrogen storage material because of its high gravimetric (19.6 wt% H2) and volumetric hydrogen density with an accompanying moderate decomposition temperature. Previously reported structures determined by using x-ray and neutron diffraction on hydrides show differences in bond lengths and atomic coordination. Here, the crystal structures of fully and half deuterated ammonia borane were investigated as a function of temperature using powder neutron diffraction. The neutron diffraction patterns show a significant difference due to large difference in the scattering length of D and H. It is evident that an order-disorder phase transition occurs around 225 K for all compounds. At low temperature, the compound crystallizes in the orthorhombic structure with space group Pnm21 and gradually transforms to a high temperature disordered tetragonal structure with space group I/4mm at about 225K. The differential scanning calorimetry studies confirm this phase transformation and also indicate that all compounds melt and decompose at above 370 K. The c cell parameter remains unchanged in the orthorhombic phase from 16 K to 200K and increases liaa nearly above 225K. As the temperature is increased, the BH3-NH3 groups start to reorient along the c axis, and the D/H atoms become disordered, leading to the tetragonal phase transition around 225K.

Hydriding studies on mixed valent intermetallic compound, CeNiAl, have been carried out. Absorption as well as desorption routes have been employed to find the composition range of various hydride phases. Stable hydride composition, CeNiAlHx, with a maximum value of x = 3.2, is obtained. No single phase crystalline hydride could be stabilized for x P 6 ¯ 2 m ) to AlB2 type hexagonal (space group P6/mmm) structure. Structural parameters and D atom occupancies are derived from the neutron diffraction measurements on CeNiAlD3.2. Magnetization measurements show that the structural transformation is also accompanied by a transformation from the mixed valent cerium in the parent compound to trivalent Ce in the hydride phases.

We studied the crystal structure and magnetic properties of the perovskite manganites Pr 0.63 M 0.07 Sr 0.3 MnO 3 with M = Sm and Bi, as a function of the average size of the A cation site. Neutron diffraction investigations show that our synthesized samples crystallize in orthorhombic symmetry with Pnma space group with distortion of the Mn O Mn angles. Magnetic measurements versus temperature in an applied magnetic field of 500 Oe show that all our samples exhibit a paramagnetic to ferromagnetic transition at low temperature. The Curie temperature T c decreases from 265 K for the parent phase to 325 K for the Sm substituted sample.

Neutron diffraction and Mossbauer spectroscopy were used to characterize the perovskite type cathodes, in terms of the crystal structure, oxygen occupancy, magnetic moment, and Fe valence state. It was found that a lower symmetry trigonal space group P-3C1 gave improved fits to the data including the matching of a peak not fitted in the R3¯c symmetry. A significant lattice distortion from rhombohedral to cubic was observed in La 0.60 Sr 0.40 FeO 3− δ (L6SF) quenched from 1200 °C. Average Fe valence states are ∼3.04 and 3.36 for La 0.60 Sr 0.40 FeO 3− δ quenched from 1500 and 700 °C, respectively. Results from neutron diffraction and Mossbauer spectroscopy indicated that [D.A. Berry, W.A. Surdoval, M.C. Williams, 222nd ACS National Meeting, Chicago, IL, United States, (2001)], in the specimen without quenching, there was no significant oxygen vacancy ( δ =0), and ratio of Fe 3+ over Fe 4+ is 6/4, and [S.P. Simner, J.F. Bonnett, N.L. Canfield, K.D. Meinhardt, V.L. Sprenkle, J.W. Stevenson, Electrochem. Solid State Lett., 5, A173 (2002)] in the sample quenched from 1500 °C, δ was ∼0.2, and Fe exhibited a valence state of ∼3+.

Intermetallic compounds of NdMn6?xFexSn6 () were studied by means of x-ray and neutron diffraction techniques and SQUID magnetic measurements in the temperature range of 30?400?K. The substitution of iron for manganese leads to a phase transition whereby NdMn6Sn6 with the HoFe6Sn6 structure (space group Immm) changes to TbFe6Sn6 (space group Cmcm) for NdMn6?xFexSn6 with . The iron atoms prefer to occupy the 8g sites at iron content x

A Tb-123 phase with the composition, TbSr 2 Cu 2.7 Mo 0.3 O 7+ δ , has been synthesized in single-phase form by the solid-state reaction route. Its phase purity has been confirmed from neutron powder diffraction studies. The as-synthesized Tb-123 sample does not show superconductivity down to 5 K. On the other hand, an unusually high antiferromagnetic ordering temperature ( T N ) of around 7 K is seen for the Tb moments. After 120-atm-O 2 post-annealing, bulk superconductivity is achieved in this compound with a superconducting transition temperature ( T c ) of about 30 K, without any significant effect on T N . To achieve higher oxygen content and higher T c , the as-synthesized sample was subjected to high-pressure oxygenation, carried out in a closed cell, at 5 GPa and 400 °C in the presence of AgO as an excess-oxygen source. This sample exhibited superconductivity onset at around 80 K with a Meissner fraction larger than 10% at 5 K. Our observation of superconductivity at 80 K is the highest T c to-date for the Tb-123 type compounds.

Magnetization, AC susceptibility, Mossbauer spectroscopy, neutron diffraction (ND) and magnetoresistance measurements have been carried out on a polycrystalline perovskite, La0.67Ca0.33Mn0.9Fe0.1O3. Considerable changes occur in magnetic, transport and magnetoresistance properties with respect to the classic composition without Fe but no spin-glass like behavior is observed. ND data confirms that the compound crystallizes in the orthorhombic perovskite structure (space group Pbnm). The Mossbauer spectrum reveals that Fe substitutes for Mn as Fe3+ (S=5/2) and is antiferromagnetically coupled to the Mn host lattice. The substitution of Mn3+ by Fe3+ reduces the number of available hopping sites for the Mn eg(↑) electron and suppresses the double exchange (DE), resulting in the reduction of ferromagnetic exchange as evidenced by a decrease of about 125 K in the value of ferromagnetic ordering temperature (TC) and of about 183 K in the value of metal-to-insulator transition temperature (Tp) without Fe. This compound presents 98% of giant magnetoresistance ratio in the temperature range of 50–80 K at 40 kOe applied magnetic field. The competition between the ferromagnetic DE interactions and the coexisting anti-ferromagnetic super exchange interactions with the introduction of Fe3+ for Mn3+ drive the system supposedly into a random canted ferromagnetic state at low temperatures.

The perovskite La1/3Sr2/3FeO3−δ was investigated by neutron diffraction, magnetic and Mossbauer spectroscopy measurements. La1/3Sr2/3FeO3−δ undergoes magnetic ordering at T = 190–200 K accompanied by charge disproportionation. Magnetic peaks due to charge ordering are observed below 200 K. The charge ordering is gradually developed below 200 K along with a charge disproportionation, 2Fe4+ Fe3+ + Fe5+. La1/3Sr2/3FeO3−δ shows an antiferromagnetic structure at low temperature. Magnetic moments of about 3 and 1.3 μB were obtained from the neutron diffraction data refinement for Fe3+ and Fe5+ at 15 K, respectively, which suggest that both Fe ions are in a low spin state. These values are significantly lower than those reported by Battle et al for La1/3Sr2/3FeO2.98. Mossbauer spectra indicate that full charge ordering might be reached below 20 K with no Fe4+.

A systematic study of the structural and magnetic properties of the diluted perovskite manganite oxides (La 1− y Pr y ) 0.9 Na 0.1 MnO 3 (0.2≤ y ≤0.8) had been carried on. In this study, the average ionic radius 〈 r A 〉 of the A cation site was systematically varied while keeping fixed the carrier concentration (Mn 3+ /Mn 4+ ratio). X-Ray diffraction investigations showed a structural transition from the rhombohedral to the orthorhombic system for 0.33≤ y ≤0.5. The unit cell volume decreases with increasing Pr 3+ content. Magnetic measurements showed that our synthesized samples exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. Neutron diffraction studies show that (La 0.2 Pr 0.8 ) 0.9 Na 0.1 MnO 3 powder sample crystallizes in the orthorhombic system with Pnma space group at room temperature and at 10 K. The magnetic moment of Pr and Mn atoms are coupled parallel to the b -axis.

Samples of the composition CoSr 2 YCu 2 O 7− δ (Co-1212) were synthesized in a single-phase form and studied using neutron diffraction (ND), chemical analyses and magnetization measurements. The structure of Co-1212 is best described in the space group Ima2 that yields good fit to the observed ND data. No cross-migration between Cu and Co is seen for Co-1212. The overall oxygen content, as determined from the refined oxygen occupancies, comes out to be around 7.0. This is close to the values determined from wet-chemical and thermogravimetric analyses. Unlike in Cu-1212, the complete reduction of Co-1212 in 5% H 2 /95% Ar atmosphere occurs in a single step, revealing that the amount of reversibly removable oxygen is considerably less than that in Cu-1212. Zero-field-cooled and field-cooled magnetizations, measured in an applied field of 100 Oe, show branching at around 30 K, presumably due to the magnetic ordering of Co moments. The present Co-1212 sample is found not to be superconducting down to 5 K.

Strontium substituted praseodymium manganite Pr 0.7 Sr 0.3 MnO 3 powder was synthesized by a solid state reaction method at 1400 °C. Neutron diffraction analysis at room temperature and at 14.5 K shows that our sample belongs to the Pnma space group at both temperatures. At low temperature, the refinement gave 3.368μ B for the Mn atoms and 0.273μ B for the Pr atom, aligned ferro-magnetically along the crystallographic y-axis. Magnetic measurements show that our sample exhibits a paramagnetic-ferromagnetic transition with decreasing temperature. The Curie temperature, T C , is found to be 250 K. Electrical studies show a semiconductor-metal transition at T q = 260 K. This transition temperature is observed in the vicinity of the Curie temperature T C . Magneto-transport properties show a sharp drop in resistivity as a function of the magnetic applied field. Our sample displays a magnetoresistance effect at 8 T of about 80% in the vicinity of the Curie temperature T C . 55 Mn NMR measurements at 4.2 K confirm the metallic behavior at low temperature. The time average resonance frequency indicates that electron hopping between Mn 3+ and Mn 4+ occurs over the whole crystal.

Structural studies on Nd 3− x Ba 3+ x Cu 6 O 14−2 δ with 0≤ x ≤1 were carried out by Rietveld refinement procedure using neutron diffraction data. The composition x =1 was prepared both by the solid-state method and the nitrate-decomposition method. Quantitative multiphase analysis coupled with variation in the method of synthesis has shown conclusively that Nd did substitute for Ba even in a stoichiometric composition NdBa 2 Cu 3 O 7− δ , irrespective of the method of synthesis. The fractional atomic substitution of Nd at the Ba site has been determined to be about 15%. The composition x =0, synthesised by the nitrate-decomposition method, has been found to be tetragonal with the space group P 4/ mmm . Nd has been found to substitute for Ba according to Nd(Ba 0.75 Nd 0.25 ) 2 Cu 3 O 7− δ and no random distribution of cations has been found along the c -direction. The observed tetragonality has been attributed to the additional oxygen taken into the basal (001) plane due to the substitution of trivalent Nd at the divalent Ba site.

A Pr0.6Sr0.4MnO3 powder was synthesized by the solid state reaction method at 1400°C. Magnetic measurements show that our sample exhibits a paramagnetic–ferromagnetic transition with decreasing temperature. The paramagnetic to ferromagnetic transition is followed by a decrease of the magnetization below 100 K. The Curie temperature TC is found to be 310 K. Electrical studies show a semiconductor–metallic transition at Tρ=260 K. Our sample displays a magnetoresistance effect in 8 T of about 45% below 250 K. Neutron diffraction studies show that our sample crystallizes in the orthorhrombic system with Pnma space group at room temperature and at 200 K. However, at 10 K, we observe the coexistence of two phases: an orthorhombic phase with Pnma space group (27%) and a monoclinic one with 12/a space group (73%). 55Mn NMR measurements at 4.2 confirm the metallic behavior at low temperature. The observed frequency of 380 MHz is close to the calculated frequency of 378 MHz, which confirms the average value of Mn4+ and Mn3+ resonance frequencies.

The magnetic structure of the transition metal sublattice in induction-melted SmMn 6-x Fe x Sn 6 intermetallic alloys ( x = 0, 0.5, 1.0, 1.5, and 2.0) K have been studied by x-ray and neutron diffraction techniques and SQUID measurements. These intermetallics crystallize in the HoFe 6 Sn 6-type structure with the space group I mmm while the SmMn 6-x Fe x Sn 6 intermetallic alloys ( x = 0.5, 1.0, 1.5, and 2.0) have the TbFe 6 Sn 6-type structure with the space group C mcm. Lattice constant, a, b, c, and unit cell volume contract with increasing iron content The net magnetization exhibits a small magnitude variation within the iron content range investigated. The magnetic moment of the iron sublattice couples positively with ferromagnetically coupled manganese and samarium moments. The Curie temperature (Tc) first increases, goes through a maximum at about x = 1.5, and then decrease again with the increasing iron content. The easy direction of magnetization for all of the samples is parallel to the bc plane of the unit cell at 300 K.

We have successfully synthesized RuSr 2 PrCu 2 O 8− δ in an essentially single-phase form. Structural analysis from room-temperature powder neutron diffraction pattern shows that this compound crystallizes in tetragonal Ru-1212 structure (space group P 4/ mmm ). Magnetization measurements show clear branching of zero-field-cooled and field-cooled magnetization around 150 K indicating magnetic ordering below this temperature (presumably associated with Ru moments), similar to that reported for RuSr 2 GdCu 2 O 8− δ . Magnetization versus applied field hysteresis loops indicate presence of a small ferromagnetic component at 5 K. Though magnetic behaviour of RuSr 2 PrCu 2 O 8− δ is similar to that of reported magneto-superconductor RuSr 2 GdCu 2 O 8− δ , the Pr-based compound is non-superconductive down to 5 K. Results of presently studied RuSr 2 PrCu 2 O 8− δ compound are compared with reported data on both superconductive and non-superconductive RuSr 2 GdCu 2 O 8− δ samples.

The electronic and magnetic properties of the ABO/sub 3/ type manganites (A=rare earth, B=Mn) depend sensitively on the ratio of divalent and trivalent atoms on the A sublattice. The substitution of rare-earth by Ca or Sr not only affects the total valence, but also the lattice spacing, which, in turn may affect the structure and magnetic ordering temperature. If these compounds are synthesized with less than stoichiometric amounts of the A site constituents, (relative to the Mn content), three possible outcomes may be considered: (a) single phase materials with vacancies on the A sublattice may be formed, (b) two phase materials with stoichiometric manganites and Mn/sub 3/O/sub 4/ may result, or (c) some combination of A and B may occur. We have carried out neutron diffraction (ND) studies on samples of Pr/sub 0.7/Sr/sub 0.3-x/MnO/sub 3/ with nominal x=0.0, 0.1 and 0.2, at 300 K and 10 K. The ND data clearly show the presence of Mn/sub 3/O/sub 4/ as a second phase in samples with x=0.1 and 0.2. Refinements of the manganite structure, based on the assumption that the ratio of Pr to Sr on the A site is equal to the nominal concentration, reveal that vacancies can be induced on the A site. In the x=0.2 sample, about 5.5% of the A sites are found to be vacant. The refined Mn moments in the z=0.0 and 0.1 samples at 10 K are found to be nearly equal (3.4 /spl mu//sub B/). The Pr moments are parallel to the Mn moments With magnitude of 0.27 /spl mu//sub B/ and 0.33 /spl mu//sub B/, respectively, in these two samples. The magnetic structure of x=0.2 sample is more complex due to the presence of an antiferromagnetic component.

We have investigated the effect of Fe substitution on the structural and superconducting properties of La2.5Y0.5CaBa3(Cu1−x Fe x )7O z system by Rietveld refinement of the neutron diffraction patterns of three samples with x = 0.02 (labelled B1), x = 0.06 (B2), and x = 0.10 (B3) along with X-ray diffraction, resistivity, AC susceptibility, and oxygen-content measurements. Samples B1, B2, and B3 are superconducting with T c R=0 values of 73, 62, and 41 K, respectively. Neutron diffraction studies confirm (i) the formation of a single phase tetragonal structure (space group P4/mmm) for all three samples, (ii) Ca and Y ions substitution at the La site concomitantly displaces La onto Ba sites, and (iii) increasing x from 0.02 to 0.10 increases oxygen content (the amount of oxygen per unit cell), as well as Cu(1)— O(4) and Cu(1)— O(1) bond lengths whereas Cu(2)— O(4) bond length decreases with corresponding decrease in T c to 41 K due to increasing occupancy of Fe ions at Cu(2) site. The change in bond lengths with oxygen content are essentially the same as those of Fe content (x). Present studies establish a correlation between the bond lengths (Cu(1)— O(1), Cu(1)— O(4), and Cu(2)— O(4)) and the measured T c values of three samples.

The atomic structure of the icosahedral Ti–Zr–Ni quasicrystal is studied using a canonical cell tiling approach. An investigation of the previously proposed structure for W-Ti–Zr–Ni, the 1/1 crystal approximant to the Ti–Zr–Ni icosahedral quasicrystal, has revealed improbable occupation of sites in the region between the Bergman clusters and at the cluster centre. The 1/1 structure is refined for a better-ordered sample of the approximant, using X-ray and neutron diffraction measurements and ab initio relaxation studies. Based on this refined structure, an atomic decoration model for large canonical cell tilings is developed. Calculated diffraction patterns from the generated structures are in reasonable agreement with those measured for the icosahedral quasicrystal.

Elastic neutron diffraction data from icosahedral Ti 45 Zr 38 Ni 17 are presented and analyzed using information from the 1/1 approximant Ti 50 Zr 35 Ni 15 . These data indicate that similar clusters exist in the approximant and the i-phase. This is shown to be consistent with simulated diffraction from an icosahedral glass model of the quasicrystal, placing a Bergman cluster on the glass sites. An electrochemical method was used to hydrogenate Ti-based quasicrystals and their crystal approximants. This technique gives a consistently high hydrogen to metal atom ratio of 1.9, without crystal hydride formation in the quasicrystal.

In the system Er 1− x Ca x Ba 2 Cu 3 O 7− δ with x =0.0 to 0.3, the superconducting transition temperature ( T c ), as measured from AC susceptibility, decreases with increasing Ca substitution, x . Detailed analysis of the powder neutron diffraction patterns of these samples shows a decrease in oxygen content with increasing x . The orthorhombic distortion decreases slightly and the c -parameter increases slightly with increasing x . Both, the buckling angle [(Cu(2)O(2)Cu(2) angle)] and the planer Cu(2)O(2) distance in CuO 2 planes, increase with increasing x . Increased Cu(2)O(2) bond distance indicates a slight decrease in p-type hole carriers in CuO 2 planes. On comparison with reported results on T c vs. p-type carriers in high T c superconducting compounds, it is inferred that decrease in p-type carriers alone cannot account for sharp T c depression with increasing x in Er 1− x Ca x Ba 2 Cu 3 O 7− δ system. With increasing x , oxygen vacancies are created in CuO 2 planes, which along with increased buckling angle might be partly responsible for T c suppression in this system.

We did a detailed study on a series of samples of Eu(Ba 1− x R x ) 2 Cu 3 O 7+δ (where R=La, Pr and Eu) by X-ray diffraction, neutron diffraction, magnetic and thermopower measurements. All R 3+ substitutes went to the Ba 2+ site and suppress superconductivity. The suppression of superconductivity does not have a simple relation to the ionic size of the substitutes. On the other hand, the change in crystal structure from orthorhombic to tetragonal has a strong dependence on the ionic size of the substitutes. Pr on Ba site did not cause unexpected structural distortions yet suppress superconductivity at a rate faster than other rare earths. Data of thermopower measurements and neutron diffraction from our rare earth substituted Eu123 samples show that the suppression of superconductivity by Pr on Ba site is electronic, not magnetic in nature.

The REBaSrCu 3 O 7 compounds with RE = Y, Dy, Nd and La have been synthesized by the standard solid-state reaction route. Superconducting transition temperatures (T c ), as measured by ac susceptibility technique, are 81 K, 79 K, 62 K and 45 K for samples with RE = Y, Dy, Nd and La, respectively. The rare earth dependence of T c in the REBaSrCu 3 O 7 series is quite different from that observed in the REBa 2 Cu 3 O 7 (RE:123) series where T c is nearly independent of the choice of the RE (except when RE = Ce, Pr and Tb). Neutron diffraction studies have been carried out on the REBaSrCu 3 O 7 compounds and structural details have been obtained from the Rietveld analysis of the room-temperature neutron diffraction data. The compounds with RE = Y, Dy are found to crystallize in the orthorhombic RE:123 structure (space group Pmmm) with orthorhombicity considerably smaller than that of the RE:123 compounds. In fact, the orthorhombicity reduces so much that the REBaSrCu 3 O 7 compounds with light rare earths, RE = La and Nd, are tetragonal or almost tetragonal.

We investigate the influence of Ca ions on the structural and superconducting properties of (Nd1−yCay)Ba2(Cu0.94Ga0.06)3Oz system by Rietveld refinement of the neutron diffraction patterns of three samples with y = 0.0 (A1), y = 0.18 (A2), and y = 0.36 (A3) along with X-ray diffraction, resistivity, and ac susceptibility measurements. Samples A2 and A3 are superconducting with Tc values of 37 K and 76 K, respectively, whereas sample A1 is semiconducting to 20 K. Neutron diffraction data confirms (a) the occurrence of a single-phase tetragonal structure (space group Pmmm) for all three samples; (b) Ca ions substitute at the Nd site; and (c) increasing y from 0.0 to 0.36 decreases Cu(1)-O(1), Cu(2)-O(2), and Nd-Cu(2) bond lengths, whereas Cu(2)-O(4) bond length increases with corresponding increase in Tc to 76 K due to increasing occupancy of Ca ions at Nd site. A correlation exists between the Cu(1)-O(1), Cu(2)-O(2), Cu(2)-O(4), and Nd-Cu(2) bond lengths and the measured Tc values of three samples.

Inelastic and elastic neutron scattering studies have been carried out on the mixed-valent, gap-forming compound CeRhSb. For comparison neutron diffraction studies have been also carried out on non-magnetic isostructural compound LaRhSb. Neutron diffraction data down to 18 K do not show any change in crystal structure of LaRhSb and CeRhSb. Broad maxima in the anomalous thermal expansion are seen in CeRhSb (relative to LaRhSb) which suggest lattice thermal expansions and contractions, which may be correlated to their magnetic behaviour. No gap-type excitation, as expected for gap-forming Kondo semimetallic compounds, is evident in CeRhSb from the low energy inelastic neutron scattering studies at 2 K. The high energy neutron scattering studies reveal a broad magnetic response centered at 35 meV, consistent with the mixed-valent behaviour of the Ce ions in CeRhSb.

Titanium/zirconium-based quasicrystals and their related crystal approximants have been identified as potential new materials for hydrogen storage applications. To better understand the local chemistry and atomic ordering in these phases, preferential interstitial sites for hydrogen/deuterium were determined for $\ensuremath{\alpha}(\mathrm{TiCrSiO}).$ This is a bcc 1/1 crystal approximant to the icosahedral quasicrystal phase that contains a two-shell, Mackay-icosahedral cluster of atoms at each bcc site. It absorbs hydrogen or deuterium, without formation of other hydride phases, to a maximum hydrogen to metal atom ratio $(H/M)$ of 0.26. For fully deuterated samples, both tetrahedral and octahedral interstitial sites are occupied with fractions of 0.14 and 0.12, respectively. Here, the hydrogen/deuterium sites are determined from a Rietveld analysis of x-ray and neutron powder diffraction data taken from samples of $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ loaded with deuterium. Only the octahedral sites are occupied in the partially deuterated samples $(D/M=0.11).$ A decrease in the oxygen concentration below the stoichiometric value for $\ensuremath{\alpha}(\mathrm{TiCrSiO})$ leads to an increase in the total amount of hydrogen that can be absorbed, suggesting that these interstitial atoms are competing for the same octahedral interstitial sites.

The results on structural aspects and superconductivity of LaBaCaCu 3 O 7+ δ (La:1113) are reported. Both X-ray and neutron diffraction studies have been carried out on this compound which reveal that this compound crystallizes in the tetragonal structure (space group P 4/ mmm ). Detailed analysis of the neutron diffraction data reveals that the Ca, Ba and La atoms in this compound are intermixed at the nominal La and Ba sites. Nearly 46% of Ca occupy the usual La-site, while the remaining 54% occupies the Ba-site. Consequently, a corresponding amount of La substitutes at the Ba-site. The intermixing of Ca, Ba and La sites randomizes the chain site oxygens leading to a tetragonal structure for this compound despite an oxygen content close to 7. These occupancy refinements explain the tetragonal structure of the compound, despite having an oxygen content close to 7.04. The compound studied presently shows a sharp superconducting transition at about 68 K.

Pr 2-z Ce z Sr2Cu2NbO10 (Pr222Nb10), is widely believed to not superconduct, although its homologues in which Pr is replaced by Nd, Sm, Eu, or Gd do (with Tc ≈ 28 K). On the basis of bulk resistivity, bulk diamagnetism, electron spin resonance, and surface resistance measurements of Pr222Nb10, we conclude that it does exhibit granular superconductivity (as predicted) with a critical temperature in the range 25–28 K, but is not currently a bulk superconductor. Neutron diffraction studies show that our Pr222Nb10 is multi-phase, but does not contain significant amounts of any (known) superconducting phase other than Pr222Nb10. It does contain ~ 23% PrSr defects, but far fewer SrPr impurities (Pr , PrSr ) is probably not responsible for breaking Cooper pairs and destroying superconductivity, but the important half of it, PrSr , almost certainly is.

The 295 K powder neutron diffraction patterns and the temperature dependence of the Mossbauer spectra of Nd2Fe17−xTix and Nd2Fe17−xTixNy have been measured. A Rietveld refinement of the neutron diffraction patterns yields the Nd2Fe16.32Ti0.68 and Nd2Fe16.32Ti0.68N2.7 stoichiometries for the two compounds and indicates that titanium occupies only the 6c crystallographic site and nitrogen only the 9e site in the rhombohedral Th2Zn17 structure. The insertion of interstitial nitrogen into Nd2Fe16.32Ti0.68 produces a 6.8% increase in the unit cell volume. The relative areas of the Mossbauer spectral components indicate that iron–iron, iron–titanium, and titanium–titanium 6c–6c dumbbell pairs exist in both of these materials. The magnetic hyperfine fields observed for the iron on the 6c site in the iron–titanium dumbbell pair in both compounds is substantially reduced from that found in Nd2Fe17 and Nd2Fe17N2.6 because of the titanium dumbbell near neighbor. For all the remaining hyperfine parameters there is a ...

We report the structural refinement of large-unit-cell bcc crystalline phases found in Ti-Zr-Ni and Ti-Zr-Fe alloys, which are $1/1$ rational approximants of icosahedral quasicrystals in the same alloys. The structure of the stable $1/1$ phase $W(\mathrm{TiZrNi}),$ lattice constant ${a}_{o}=14.317\AA{},$ determined by a Rietveld analysis of x-ray and neutron powder diffraction data, is closely related to that of the $1/1$ phases $R(\mathrm{AlLiCu})$ and Bergman(AlMgZn), containing Bergman-type icosahedral clusters of atoms. Despite the similar chemistry of the $1/1$ phases in Ti-Zr-Ni and Ti-Zr-Fe alloys, the $1/1$ phase $M(\mathrm{TiZrFe})$ contains double-shell Mackay icosahedra, like those found in the $1/1$ phase \ensuremath{\alpha}(TiCrSiO). These results provide starting structures for six-dimensional refinements of the related quasicrystals.

We report on the structural aspects and superconductivity of the La 1− x Pr x CaBaCu 3 O 7− δ system (RE: 1113) with x =0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7 and 1.0. X-ray diffraction studies show that Pr substitutes isostructurally in LaBaCaCu 3 O 7− δ (La: 1113) until complete replacement of La. The superconducting transition temperature, T c , determined from AC susceptibility measurements decreases with x . The relative T c depression due to Pr in the LaBaCaCu 3 O 7− δ system is less in comparison to that found for La site Pr substituted LaBa 2 Cu 3 O 7 system. Normal state DC magnetism of the Pr substituted samples show a Curie–Weiss paramagnetic susceptibility with deviations below about 10 K. The non-superconducting compounds ( x =0.7, 1.0) display Schottky-type anomalies which are attributed to crystalline electric field effects (CEF). The related entropy change for x =1.0 sample amounts to Δ S =7.75 J/mol K. Rietveld analysis on neutron diffraction pattern of x =0.0 sample shows that 46 at% of the La is occupying the Ba site, in normal Re: 123 unit cell. A similar situation is suppose to occur in case of PrBaCaCu 3 O 7 , for which the refinement of Ba and Pr sites is near impossible due to their similar scattering cross-sections for neutrons. Ba site sitting Pr does not contribute to the Pr-4f hybridization with Cu–O conduction band, giving rise to a lower T N and less effect of Pr on superconductivity of La: 1113 system than for La: 123. The results strengthen extended Pr-4f hybridisation with Cu–O conduction band.

Neutron power diffraction studies are performed with different reactors on YFe10.5Mo1.5Cx (x = 0.3 and 0.6) prepared by arc-melting. The refinements of the neutron diffraction data indicate that the carbides retain the ThMn12-type structure with an increase of unit cell volume and most of carbon atoms occupy the interstitial 2b sites. The effects of carbon introduced by arc-melting are similar to that of interstitial nitrogen introduced by the gas-solid phase reaction, which is of significance in the technical applications due to a better stability of these carbides.

The compositional dependence of the Curie temperature of the Tb 2 Fe 17− x Ga x solid solutions shows a primary maximum for x equal to three and a secondary maximum for x equal to eight. The Mossbauer spectra and powder neutron diffraction patterns of Tb 2 Fe 9 Ga 8 indicate that gallium avoids the 9d site, completely fills the 6c site, and occupies preferentially the 18f site over the 18h site. This gallium preferential substitution leads to a nearly ordered two-dimensional antiferromagnetic arrangement of the terbium and iron magnetic moments, an increase in Curie temperature, and a surprisingly sharp Mossbauer spectrum.

The magnetic properties of the rhombohedral Ce 2 Fe 17 − x Ga x solid solutions, with x equal to 0.3, 0.5, 0.7, 1.0, and 2.0, have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. Magnetization studies indicate that, as the unit cell volume expands with Ga addition, the Curie temperature increases from 238 K for Ce 2 Fe 17 to 406 K for Ce 2 Fe 15 Ga 2 . Powder neutron diffraction measurements at 295 and 14 K indicate that the Fe moments are both oriented in the basal plane and increase with increasing Ga content. The Mossbauer spectra, which have been fit with a binomial distribution of the near-neighbor environments, indicate that the maximum hyperfine field, H max , increases with increasing Ga content and is largest for the 6c site. The decremental field, ΔH , per added Ga atom is virtually independent of temperature and Ga content, indicating that the next-nearest-neighbor interactions are very small. The compositional dependence of the isomer shifts for R 2 Fe 17 − x M x , where R is Ce or Tb and M is Al, Si, or Ga, is analyzed in terms of volume and chemical terms, the second of which reveals the importance of the electronic configuration of M in understanding its influence on the magnetic properties of these solid solutions. Finally, the similarities and differences in the changes in crystallographic, magnetic, and Mossbauer spectral properties of R 2 Fe 17 − x M x , at low x values, indicate that electronic rather than structural factors are more important in modifying the magnetic properties and indicate the need for extensive band structure calculations on these solid solutions.

Structural and superconducting behavior of samples prepared with the nominal compositions of Nd{sub 1.05}(Ba{sub 1{minus}z}Pr{sub z}){sub 1.95}Cu{sub 3}O{sub 7+{delta}} has been investigated by neutron-diffraction and dc magnetization measurements. Neutron data refined by the Reitveld method showed that there is mixing of the rare-earth (R) ions with a substantial fraction of the Pr going to the normal R site in the RBa{sub 2}Cu{sub 3}O{sub 7+{delta}}-type structure and a corresponding fraction of the Nd going to the Ba site. Structural changes due to the trivalent R ions going to the Ba site are very similar to those observed in the Nd{sub 1+x}Ba{sub 2{minus}x}Cu{sub 3}O{sub 7+{delta}} (Nd123ss), indicating that Pr behaves like other trivalent R{close_quote}s on the divalent Ba site. Magnetization measurements show that the depression in T{sub c} is more pronounced for the Pr bearing samples than that for Nd123ss for the same number of trivalent ions on the Ba site. However, by correcting the changes in T{sub c} for the depression associated with the fraction of Pr on the R site, the depression in T{sub c} per trivalent ion on the divalent site is found to be identical to that in other light R123ss. Thus the depression in superconductivity in thismore » series of samples is due to two independent effects: (1) Pr on the R site and (2) hole localization due to trivalent R ions on the Ba site. In this respect, Pr on the Ba site appears to behave like all the other trivalent R ions. {copyright} {ital 1997} {ital The American Physical Society}« less

The crystal structure, site occupancy and intrinsic magnetic properties of R 2 Fe 17− x Al x C (R = Y, Pr, Tb and Ho; x = 0–4) compounds have been studied. Dilute Al substitution for Fe increases the T c and decreases the M s for all four compounds. For x = 2, T c of 501, 476, 529 and 501 K and M s (measured at 5 K) of 141, 115.6, 40 and 38 emu/g were obtained for R = Y, Pr, Tb, and Ho, respectively. From neutron diffraction, it was found unexpectedly that C atoms prefer both the 9e and 18g sites for R = Pr and Tb, which is different from the case of 9e site preference for R = Y and Ho. As the rare earth element changes from light rare earth (R = Y and Pr) to heavy rare earth (R = Tb and Ho), the Al site occupation on 6c and 18f increases while that on the 18h site decreases. However, the 18h site still exhibits the highest Al occupancy for all samples. The T c values obtained on Tb 2 Fe 17− x Al x C and Ho 2 Fe 17− x Al x C are higher than those of Y 2 Fe 17− x Al x C and Pr 2 Fe 17− x Al x C suggesting that T c may be more strongly influenced by the RFe exchange interaction rather than the FeFe exchange interaction.

The equation of state (EOS) of polycrystalline Rb3C60 was accurately determined to 1 GPa at 295 K and 14 K using high-pressure neutron diffraction. In a separate experiment on Rb3C60, the variation of the superconducting transition temperature Tc with hydrostatic pressure was also measured. The resulting dependence of Tc on lattice parameter a is almost linear, with initial slope dT c dlna ∼- +520 K , but is less steep than the Tc(a)-dependence obtained at ambient pressure if Rb is substituted with smaller alkali metal cations. Tc(a) for Rb3C60 thus does not follow a universal curve in high pressure and substitution experiments, as previously claimed. Starting with the measured EOS at room temperature, we use a Debye model to estimate the Murnaghan EOS for both NaCl (used here as a pressure calibrant) and Rb3C60 over the entire temperature region below 400 K.

Superconductivity and structural aspects of Y 1−x Ca x Ba 2 Cu 3 O 7−y high-T c cuprate superconductor near y≈0 have been investigated. The superconducting transition temperature, T c , decreases with increasing Ca 2+ substitution at Y 3+ site in this system. A detailed structural analysis of the system has been carried out through neutron diffraction studies. On partial replacement of Y by Ca , the oxygen content of the system decreases slightly. The in-plane Cu (2)– O (2) bond distance increases with an increased Cu (2)– O (2)– Cu (2) angle, while the bond distances in Cu – O chains are found to decrease. These structural refinement results particularly increased Cu (2)– O (2) indicate that the increase of holes in Cu – O planes due to partial replacement of Y 3+ by Ca 2+ is unlikely to happen in the present situation, implying that the decrease in T c on Ca substitution in this system is not due to overdoping effects.

Neutron diffraction studies have been carried out on superconducting YNi 2 B 2 C ( T c = 15.6 K) at various temperatures between 14 and 300 K to determine its structure, variation of cell constants and interatomic distances as a function of temperature. This compound crystallizes in the tetragonal structure at room temperature and there is no structure change in the temperature range investigated. The lattice parameter shows anomalous temperature dependence in the c -direction.

It is known that Fe moments order antiferromagnetically with Neel temperature, TN, above 700 K in RBa2Fe3O7 − δ with R = Y and Eu, and Pr moments order antiferromagnetically in PrBa2Cu3O7 − δ, with TN of 17 K. We have synthesized PrBa2Fe3O8 to study the ordering of Pr and Fe moments. From X-ray and neutron diffraction studies, this compound is found to crystallize in the tetragonal structure (space group P4/mmm). 57Fe Mossbauer studies show a complex hyperfine split pattern at 77 K, but a doublet superimposed on a six-line pattern at 300 K implying that some of the Fe moments order magnetically between 77 and 300 K while other may have ordering temperature of more than 300 K. Magnetization measurements reveal discontinuities in the susceptibility data around 80 and 20 K, probably an indication of magnetic ordering of the Fe and Pr moments. Neutron diffraction studies at 300 K suggest the presence of additional oxygen in the Pr plane and a magnetic structure very different from that of YBa2Fe3O8.

We have studied the superconductivity and structural aspects of substituting Y by Ca in oxygen-deficient YBa 2 Cu 3 O 7− y (Y:123) high- T c cuprate superconductor near y ≈ 0.3. Unlike for fully oxygenated systems [1,2], the superconducting transition temperature T c , determined from AC susceptibility measurements, increases with increasing Ca 2+ substitution at the Y 3+ site in this system. A detailed structural analysis of the system has been carried out through neutron diffraction. On replacement of Y by Ca the oxygen content of the Y 1− x Ca x Ba 2 Cu 3 O 7− y ( y ≈ 0.3) system remains nearly unchanged, till x = 0.10, and drops sharply for x = 0.15. The in-plane bond distances in the CuO planes decreases with increasing Cu(2)O(2)Cu(2) angle, while the bond distances in the CuO chains are found to be intact again till x = 0.10. These structural-refinement results indicate that there is an increase in the number of holes with the substitution of Y 3+ by Ca 2+ in Y 1− x Ca x Ba 2 Cu 3 O 7− y ( y ≈ 0.3) system for x = 0.05 and 0.10. Nearly intact bond distances in the CuO chains and decreased Cu(2)O(2) bond distances in the CuO planes indicate that Ca 2+ substitution at Y 3+ directly dopes the mobile holes in the CuO planes without affecting the CuO chains for the low level of substitution, i.e., till x = 0.10. The increased number of holes in the system seems responsible for an increase in the T c of the oxygen-deficient YBa 2 Cu 3 O 7− y system with Y 3+ site Ca 2+ substitution. For higher concentration ( x = 0.15) of Ca the increased number of holes due to Y 3+ by Ca 2+ substitution are compensated with a corresponding decrease in oxygen content.

Stoichiometric and cation-deficient magnetite Fe3−δO4 and γ-Fe2O3 particles have been prepared by the chemical method followed by heat treatments. The magnetic and structural properties were studied using neutron diffraction, magnetic measurements, and Mossbauer spectroscopy. Charge ordering of Fe3+ and Fe2+ and lattice distortion are not observed below the Verwey transition temperature in the stoichiometric and cation-deficient magnetite. It is found that the lattice parameter and the Verwey transition temperature decrease as the cation vacancy increases. The Verwey transition almost disappears in the Fe3−δO4 sample with δ=0.066. Mossbauer spectra show that the ratio of Fe3+/Fe2.5+ in stoichiometric magnetite can be modified by heat treatment. The Fe vacancies on the B sites change the nature of the Verwey transition. No cation vacancy ordering is observed for γ-Fe2O3, due to the small amount of cation vacancies in the compound.

Neutron diffraction has been used to determine the location and concentration of C in Nd2Fe17 and in a Si substituted alloy. The results indicate that two carbide phases exist. One contains nearly three C atoms per formula unit, while the other has a much lower concentration. The high C phase contains C only on the 9e sites, but the low C phase appears to contain C on the tetrahedral 18g site. The changes in interatomic distances and cell volume are associated with the different site occupancies.

The magnetic properties of a series of Ce2Fe17−xAlx solid solutions with x equal to 0.00, 0.88, 2.06, 2.81, 3.98, 5.15, 6.08, 7.21, 8.20, 9.08, 9.84, and 10.62 have been studied by magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. The compounds crystallize in the rhombohedral Th2Zn17‐type structure. Magnetization studies indicate that the Curie temperature increases uniformly from 238 K for Ce2Fe17 to 384 K for Ce2Fe14Al3 and then decreases at higher aluminum content. Powder neutron diffraction results, obtained at 295 K, indicate that aluminum avoids the 9d site for all x values and preferentially occupies the 18h site at low aluminum content. Aluminum shows a marked preference for the 6c site for x≳6. The room‐temperature iron magnetic moments increase from x=0 to 2 and then decrease for x≳2. The Mossbauer spectra have been fit with a binomial distribution of the near‐neighbor environments in terms of a maximum hyperfine field, Hmax, for an iron with zero aluminum near neighbors, a...

Two different samples of the Ce-Ru-Ge ternary system of nearly the same composition, ${\mathrm{Ce}}_{3}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{13}$ and ${\mathrm{CeRuGe}}_{3}$, have been studied, as well as the compound ${\mathrm{Y}}_{3}$${\mathrm{Co}}_{4}$${\mathrm{Ge}}_{13}$, which contained a nonmagnetic rare earth as the reference material. Combined x-ray and neutron-diffraction studies of ${\mathrm{CeRuGe}}_{3}$ and an x-ray study of ${\mathrm{Ce}}_{3}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{13}$ show that we are dealing with internally distorted (probably 3D modulated) crystals with variable composition, i.e., ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ and ranging from at least x=0 (1:1:3) to x=1 (3:4:13). We have measured the dc magnetic susceptibility, the ac susceptibility, the zero-magnetic-field heat capacity, the magnetic heat capacity in fields ranging from 0 to 9.85 T, and the electrical resistivity. The paramagnetic susceptibility and x-ray crystallography data indicate that the ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ alloys cotain both trivalent cerium and tetravalent cerium in a 1 to 3 ratio. As far as we are aware, this is the first time such a valence situation has been reported for a cerium compound. In addition, the heat-capacity results show that ${\mathrm{Ce}}_{4\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ru}}_{4}$${\mathrm{Ge}}_{12+\mathit{x}}$ is a heavy fermion with \ensuremath{\gamma} varying from 428 to 592 mJ/mol ${\mathrm{Ce}}^{3+}$${\mathrm{K}}^{2}$ at the 1:1:3 and 3:4:13 compositions, respectively. Zero-field and magnetic-field heat capacity, and ac magnetic susceptibility data suggest that ${\mathrm{CeRuGe}}_{3}$ is a spin-glass system below \ensuremath{\sim}5 K, which is consistent with the specific features of its crystal structure. ${\mathrm{Y}}_{3}$${\mathrm{Co}}_{4}$${\mathrm{Ge}}_{13}$ displays normal metallic behavior.