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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.

Gd1.5Ce0.5Sr2Cu2RuO10 exhibits antiferromagnetic resonance at 23.9 GHz for applied fields less than 1000 Oe with a spin–spin relaxation time T 2 of approximately 0.45 ns, and with a spin–lattice relaxation time T 1 of at least 320 μs. Since in the homologue, Eu1.5Ce0.5Sr2Cu2RuO10, the Ru atoms evidently fail to exhibit magnetic order, the antiferromagnetic resonance must arise from the cuprate planes. In other homologues, the cuprate planes are known to order ferromagnetically and are stacked in an antiferromagnetic configuration. The large value of T 1 suggests that phonon mediation plays no role in high temperature superconductivity. In addition, the presence of ferromagnetic cuprate planes is inconsistent with spin-fluctuation models of high temperature superconductivity.

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.

It is widely believed that cuprate (CuO 2 ) planes are the essential structural component of all high temperature superconductors. In contrast, Wu et al. and Chen et al. have published evidence for superconductivity in Cu-doped double pervoskites which consist of alternating layers of either SrO or BaO with YRu 1 − u Cu u O 4 . These structures have no cuprate planes, yet exhibit diamagnetism to temperatures as high as 84 K. Since the chemical components of these materials are common to the rutheno-cuprates, it is widely believed that impurity phases are responsible for the observed superconductivity. Impurity phase formation and decomposition in a homolog, Ba 2 GdRu 1 − u Cu u O 6 , is investigated. It is found that this phase is stable against separation into GdBa 2 Cu 3 O 7 and Ba 2 GdRuO 6 . GdBa 2 Cu 3 O 7 does not survive high temperature processing with Ba 2 GdRuO 6 .

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.

Structural and electrical properties of doped LaFeO3 with Mn, Ni or Cu are studied by neutron diffraction, x-ray diffraction, conductivity measurements and thermoelectrical power measurements. Generation of the carriers (electron holes) in Mn and Cu doped LaFeO3 is a thermally activated process. The carrier fraction reaches a plateau determined by the dopant content. Electrical conductivity of Ni doped LaFeO3 is particularly high with a negligible activation energy, indicating Ni doped LaFeO3 is a metallic conductor, whereas conduction in Mn and Cu doped LaFeO3 is through polaron hopping.

Reflective, pinhole-free Fe/Fe3O4 and Fe3−δO4/Fe2O3 nanocomposite films were obtained by reacting iron pentacarbonyl, Fe(CO)5, in an inductively-coupled radio frequency glow discharge reactor. The conductivity of the Fex/(Fe3O4)1−x (x > 7%) composite films exhibits metallic characteristics and the conductivity decreases as the α-Fe content decreases. The metal-to-insulator transition temperature of the Fex/(Fe3O4)1−x (x ~0.07) films shifts to a higher temperature as compared with Fe3−δO4 due to the increased conductivity from α-Fe. The magnetization versus temperature (M–T) curves show the transition temperature ranging from 95 to 136 K, corresponding to the Verwey temperature, which is dependent on the film composition. The Fe3−δO4 film exhibits a negative magnetoresistance of about 4% and 8% at room temperature and 80 K, respectively.

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.

Crystal structures of a homologous series of Co-based layered cuprates, CoSr2(Y,Ce)sCu2O5+2s (s=1–3), have been investigated by high-resolution electron microscopy (HREM) and electron diffraction (ED) techniques. For all the three phases ED patterns showed double periodicity along a direction parallel to the CoO layers, indicating a regular alternation of two types of CoO4-tetrahedra chains within the layers. Also seen was ordering of the chains along the layer-stacking direction for the s=1 phase (Co-1212); ED patterns simulated based on the proposed superstructure model well reproduced the observed patterns. For the s=2 (Co-1222) and s=3 (Co-1232) phases in which an additional fluorite-type layer-block is inserted between two CuO2 planes, HREM and ED analysis revealed complete disorder of the CoO4 chains along the layer-stacking direction. This implies that the interlayer ordering is mainly controlled by the distance between the neighboring CoO layers.

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.

The low-temperature phase of the MnBi alloy has a coercivity μ0Hc of 2.0 T at 400 K and exhibits a positive temperature coefficient from 0 to 400 K. In the higher temperature range it shows a much higher coercivity than that of the NdFeB magnets, which suggests that it has considerable potential as a permanent magnet for use at high temperatures. In the temperature range from 30 to 150 K, the Mn atom is found to change its spin direction from a perpendicular to a parallel orientation with respect to the c axis. The anisotropy field increases with increasing temperature which gives rise to a higher coercivity at the higher temperatures. The maximum energy product (BH)max of the magnet is 7.7 and 4.6 MG Oe at room temperature and 400 K, respectively. The electronic structure of MnBi indicates that the Mn atom possesses a magnetic moment of 3.6μB, and that the Bi atom has a magnetic moment of −0.15μB which is due to the s–d and p–d hybridization between Bi and Mn atoms. We have also investigated the volume dependence of the magnetic moments of Mn and Bi. The results indicate that an increase in the intra-atomic exchange splitting due to the cell volume expansion leads to a large magnetic moment for the Mn atom. The Mn magnetic moment attains a value of 4.6μB at a volume expansion rate of ΔV/V ≈ 100%.

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.

The effects of mixing samarium and gadolinium on the crystallographic and magnetic properties of Sm/sub y/Gd/sub 2-y/Fe/sub 17-x/Si/sub x/ (x=0, 0.33, 1.67) intermetallics have been investigated using X-ray diffraction techniques and SQUID and vibrating magnetometer measurements. The Sm/sub y/Gd/sub 2-y/Fe/sub 17-x/Si/sub x/ samples, except for the sample with x=3 and y=1, crystallized in the rhombohedral Th/sub 2/Zn/sub 17/-structure with little or no impurities. For a given silicon content (fixed x), the unit volume and saturation magnetization increases at the approximate average rates of 4 /spl Aring//sup 3/ and 24 emu/g per substituted samarium atom, respectively. The Curie temperature (T/sub c/) for a given silicon content, on the other hand, decreases with increasing samarium content at an average rate of 30-40/spl deg/C per substitution. The affect of substituting silicon for iron on T/sub c/ is more marked, with T/sub c/ increasing at a rate of almost 70/spl deg/C per silicon substitution at low silicon concentrations. For a given samarium content (fixed y), the unit cell volume and saturation magnetization decreases at approximate rates of 5 /spl Aring//sup 3/ and 9 emu/g per substituted silicon atom, respectively. X-ray diffraction studies of magnetically aligned powders indicate that the easy direction of magnetization is in the basal plane of the rhombohedral unit cell. All of the samples were found to have very little magnetic anisotropy with no apparent dependence of the anisotropy on the samarium or silicon content. The 340/spl deg/C T/sub c/ measured for the SmGdFe/sub 14/Si/sub 3/ sample is unusually high for these intermetallics and its XRD pattern is indicative of a phase other than the rhombohedral Th/sub 2/Zn/sub 17/-type.

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.

The use of bent perfect crystals in focusing three-axis neutron spectrometry with position-sensitive detection (PSD) is analyzed on the basis of a phase-space theory. With PSD, the usual sequential scans are replaced by simultaneous scans. The case of high resolution in energy transfer is considered in detail. With commercial thin silicon wafers, the achievable resolutions are in the 10–150 µeV range, depending on neutron energy. Resolutions around 10 µeV are obtained on the peak of cold-source spectra. To take advantage of the possibilities offered by silicon wafers, PSD with spatial resolution well below 1 mm will be needed. With PSD analysis, a new kind of focusing exists that allows the thickness of bent perfect analyzer crystals to be increased, providing an intensity gain at no resolution loss. With multilamella assemblies a gain in count rate by the number of lamellae in the packet is achievable. Results of a demonstration experiment are presented, confirming that under the right conditions, a multilamella analyzer may resemble a single bent wafer, the individual curves of many wafers having been combined to provide the intensity gain.

The optics of neutron powder diffractometers with bent-crystal monochromators is considered with respect to the computation of instrumental line shapes, accounting for second-order aberrations. A procedure of Monte Carlo simulation of instrumental line shapes has been developed. An analytical approximation of these shapes is described. Computational results for the case of the MURR diffractometer with position-sensitive detection and a focusing silicon monochromator are presented.

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 basics of second-order neutron optics for bent-crystal three-axis spectrometers are presented. Energy-transfer resolution for quasielastic scattering is examined in detail. Experimental data illustrating the effect of second-order aberrations are presented. The resolution of energy transfer at 5 meV neutron energy with bent monochromators made from commercial thin silicon wafers was improved to 8 µeV from a previous record of 16 µeV.

The structural, superconducting and magnetic properties of Nd 1− x Pr x BaCaCu 3 O 7 system with x =0.0, 0.10, 0.25, 0.35, 0.50, 0.75 and 1.0 have been investigated. X-ray diffraction results reveal that Pr substitutes isostructurally in NdBaCaCu 3 O 7 (Nd:1113) superconductor with complete solubility. The superconducting transition temperature ( T c ), measured by ac susceptibility technique, decreases with increasing x . However, suppression of T c with increasing Pr substitution is less in Nd:1113 superconductor compared to that reported for Nd 1− x Pr x Ba 2 Cu 3 O 7 system. Interestingly, in the fully Pr substituted compounds of the above series, i.e., in PrBaCaCu 3 O 7 and PrBa 2 Cu 3 O 7 , the Pr moments order antiferromagnetically with T N of 10 and 17 K, respectively. The present results along those reported earlier [V.P.S. Awana, J. Horvat, S.X. Dou, A. Sedky, A.V. Narlikar, J. Magn. Magn. Mater., 182 (1998) L280; V.P.S. Awana, S.X. Dou, S.K. Malik, Rajvir Singh, A.V. Narlikar, D.A. Landinez Tellez, J.M. Ferreira, J. Albino Aguiar, S. Uma, E. Gmelin, W.B. Yelon, J. Magn. Magn. Mater., 187 (1998) 192], clearly suggest that there is a correlation between the T c suppression due to Pr and the magnetic ordering temperature of the fully substituted Pr moments in these systems. The T N may be taken to be a measure of the strength of hybridization between the Pr-4f electrons with Cu–O conduction band, and hence a lower T N may imply a less deleterious effect on superconductivity.

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.

The correlation between the normal-state anomalies observed in the magnetic and transport properties of the $({\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{)}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_{4}$ system with $0l~xl~0.5$ was studied. The x-ray-diffraction patterns revealed a linear increase of the $(a\ensuremath{-}b)$ orthorhombic parameter with the Pr content. The resistivity curves showed an increasing deviation from linearity below \ensuremath{\sim}100 K. This anomaly was properly accounted by a logarithmic term, whose coefficient C linearly increases with x. Superconducting quantum interference device measurements of the normal-state magnetic susceptibility evidenced a deviation from the ${\mathrm{Pr}}^{3+}$ Curie-Weiss behavior in the same temperature range for which the resistivity anomaly occurs. This behavior is explained in terms of an induced magnetic moment at the ${\mathrm{CuO}}_{2}$ layers under strain. A Dzialoshinsky-Moriya interaction, associated to the orthorhombic distortions, is proposed to be the source of a weak canted ferromagnetic component, which develops in conjunction with an enhancement of the antiferromagnetic correlations. A comprehensive picture of the conduction mechanism for the whole system is presented in terms of a Kondo-like scattering of the mobile holes by the spin fluctuations at the conduction planes. ${T}_{c}$ suppression was found to correlate with C, suggesting that the excitation which interacts with the carriers in the normal state is relevant for superconductivity.

Commercial thin silicon wafers are shown to be superb focusing crystals in three-axis spectrometers. With fully open beams, they allow resolutions that in conventional flat-crystal arrangements can only be achieved with Soller collimators of 2 min of arc. At given resolution, intensities are much higher than in conventional arrangements with vertically focusing pyrolytic graphite crystals. Resolution ellipsoids were measured by diffraction from powder samples. The ellipsoid orientation is controlled by the crystal curvatures and can be set to any specified value, in particular turned by 90° to pass from high resolution in energy transfer to high resolution in wavevector transfer. At neutron energies on the peak of cold sources spectra the focusing technique gives energy transfer resolutions around 10μeV. The technique fills the resolution gap between back-scattering instruments and conventional three-axis spectrometers.

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.