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The general performance of diffractometers at the first long pulse spallation source ESS, is compared with their counterparts at J-PARC, a short pulse spallation source. The difference in the inherent pulse structure of these neutron sources presents opportunities for new concepts for instrumentation, where performance does not scale simply with source power. The article describes advantages and disadvantages of those diffractometers, adapting to the very different source characteristics. We find that the two sources offer comparable performance in flux and resolution when operating in high-resolution mode. ESS offers significant advantages in tunability and flexibility, notably in the ability to relax resolution in order to increase flux for a given experiment. The slow repetition rate of ESS favors long instruments. On the other hand, J-PARC instruments perform very well in spite of the lower source power and allow better access to epithermal neutrons, of particular interest for PDF analysis of diffraction data.

The AMnO

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO

Neutron powder diffraction from d-alanine, a d-form amino acid present in higher animals including humans, shows subtle structural differences when compared with its enantiomer, l-alanine. These dissimilarities are due to different rearrangements of the NH3 + group as revealed by Raman scattering., Enantiomeric amino acids have specific physiological functions in complex biological systems. Systematic studies focusing on the solid-state properties of d-amino acids are, however, still limited. To shed light on this field, structural and spectroscopic studies of d-alanine using neutron powder diffraction, polarized Raman scattering and ab initio calculations of harmonic vibrational frequencies were carried out. Clear changes in the number of vibrational modes are observed as a function of temperature, which can be directly connected to variations of the N—D bond lengths. These results reveal dissimilarities in the structural properties of d-alanine compared with l-alanine.

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO$_2$, which consists of a distorted lattice of Mn$^{3+}$ spins with single-ion anisotropy. Physical property measurements on CuMnO$_2$ are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to $\sim$25 meV above TN= 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

With the technological development of affordable and impactful laboratory-based tools and their ever-growing scientific scope, large-science facilities need to review their strategies in order to continue to add value for their scientific communities.

The leading role in science played by crystallography is heavily dependent on Big-Science facilities. The need for Europe-wide coordination of operational resources in Big Science is discussed with particular reference to neutron sources.

Our notion of crystallography and how we communicate it needs to adapt to the major emerging frontiers of science in order to ensure that our community continues to thrive.

Charge carriers in low-doped semiconductors may distort the atomic lattice around them and through this interaction form so-called small polarons. High carrier concentrations on the other hand can lead to short-range ordered polarons (large polarons) and even to a long-range charge and orbital order. These ordered systems should be insulating with a large electrical resistivity. However, recently a polaronic pseudogap was found in a metallic phase of La(2-2x)Sr(1+2x)Mn(2)O(7) (ref. 7). This layered manganite is famous for colossal magnetoresistance associated with a phase transition from this low-temperature metallic phase to a high-temperature insulating phase. Broad charge-order peaks due to large polarons in the insulating phase disappear when La(2-2x)Sr(1+2x)Mn(2)O(7) becomes metallic. Investigating how polaronic features survive in the metallic phase, here we report the results of inelastic neutron scattering measurements showing that inside the metallic phase polarons remain as fluctuations that strongly broaden and soften certain phonons near the wavevectors where the charge-order peaks appeared in the insulating phase. Our findings imply that polaronic signatures in metals may generally come from a competing insulating charge-ordered phase. Our findings are highly relevant to cuprate superconductors with both a pseudogap and a similar phonon effect associated with a competing stripe order.

Raman spectra of deuterated L-alanine have been obtained at high-pressure conditions. A phase transition at ∼1.5 GPa associated with the splitting of some internal modes and increase of the wavenumber of the external modes was observed. Similarly to the hydrogenated L-alanine crystal, this first transition was related to a symmetry change. Moreover, further modifications of the Raman spectra were observed at 4.4 GPa, which may be associated to conformational changes of the molecule. To give further support to such a hypothesis, neutron powder diffraction measurements were performed. Information about the cell parameter at atmospheric pressure gave valuable information about the ND distances, shedding light on the behavior of the torsional vibration of ND3+. Copyright © 2009 John Wiley & Sons, Ltd.

Crystal structures of sodium cobalt oxide bilayer-hydrates were analyzed on the basis of superstructure models by assuming that the host and guest layers are commensurate. The superstructure model gave better fitting results compared to the incommensurate composite crystal model, suggesting that the host and guest layers are commensurate and have a 2 × 2 superstructure. Although the Co−O bonds in the superconductive CoO2 layers were reported to be shorter than that was expected from the Co oxidation states, the bond lengths refined in this study were consistent with the oxidation states.

The discovery of a new family of high-T(C) materials, the iron arsenides (FeAs), has led to a resurgence of interest in superconductivity. Several important traits of these materials are now apparent: for example, layers of iron tetrahedrally coordinated by arsenic are crucial structural ingredients. It is also now well established that the parent non-superconducting phases are itinerant magnets, and that superconductivity can be induced by either chemical substitution or application of pressure, in sharp contrast to the cuprate family of materials. The structure and properties of chemically substituted samples are known to be intimately linked; however, remarkably little is known about this relationship when high pressure is used to induce superconductivity in undoped compounds. Here we show that the key structural features in BaFe2As2, namely suppression of the tetragonal-to-orthorhombic phase transition and reduction in the As-Fe-As bond angle and Fe-Fe distance, show the same behaviour under pressure as found in chemically substituted samples. Using experimentally derived structural data, we show that the electronic structure evolves similarly in both cases. These results suggest that modification of the Fermi surface by structural distortions is more important than charge doping for inducing superconductivity in BaFe2As2.

At ambient pressure CaFe2As2 has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low temperature orthorhombic / antiferromagnetic phase upon cooling through T ~ 170 K. With the application of pressure this phase transition is rapidly suppressed and by ~ 0.35 GPa it is replaced by a first order phase transition to a low temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by ~ 1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe2As2 transforms when it is encased by a frozen media and enters into a low temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures., Comment: review for Special Physica C issue

Incoherent inelastic neutron scattering experiments were performed on Na0.7CoO2 and Na0.28CoO2.1.3H2O in order to understand how the dynamics of the hydrogen-bond network of water is modified in the triangular crystalline lattice NaxCoO2.yH2O. Using quasi-elastic neutron scattering (QENS), we were able to differentiate between two types of proton dynamics: a fast process (due to water strongly bound into the sodium cobalt oxyhydrate structure during the hydration process) and a slow process (likely attributable to a collective motion). High-resolution QENS experiments, carried out on Na0.28CoO2.1.3H2O, show that, at temperatures above 310 K, the water dynamics can be well-described by a random jump diffusion model characterized by a diffusion constant equal to 0.9 x 10(-9)m2/s, which is significantly lower than the rate of diffusion for bulk water. Furthermore, our results indicate that, at room temperature, the sodium ions have no influence on the rotational dynamics of the "fast" water molecules.

Crystal structures of two kinds of cobalt oxide superconductors were investigated by neutron powder diffraction. Comparison between the structures of the two phases, one derived from γ-Na0.7CoO2 and the other from α-NaCoO2, indicated that the guest arrangement is common to the two phases in spite of the difference in stacking made of CoO2 host layers. Structure analysis on the basis of composite crystal models with stacking faults revealed that the hydrated sodium ions in the galleries are ordered to form a 2/ a × 2/ a trigonal lattice, where a is a lattice constant of the fundamental unit cell. The guest layers were found to be stacked along the c-direction with a short-range interlayer ordering: when a guest layer is stacked onto another over a CoO2 layer interposed between them, the stacking must be accompanied by a lateral shift with shift vectors, 1/2a2, 1/2b2, and 1/2a2 +1/2 b2, where a2 and b2 are lattice vectors of the 2/ a × 2/ a lattice with the same probability.

A first study of possible changes instigated by deuteration in amino acids was carried out using neutron diffraction, inelastic neutron scattering, and Raman scattering in l-alanine, C2H4(NH2)COOH. Careful analysis of the structural parameters shows that deuteration of l-alanine engenders significant geometric changes as a function of temperature, which can be directly related to the observation of new lattice vibration modes in the Raman spectra. The combination of the experimental data suggests that C2D4(ND2)COOD undergoes a structural phase transition (or a structural rearrangement) at about 170 K. Considering that this particular amino acid is a hydrogen-bonded system with short hydrogen bonds (O...H approximately 1.8 A), we evoke the Ubbelohde effect to conclude that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions. The structural differences suggest distinct relative stabilities for the hydrogenous and deuterated l-alanine.

We have used electron diffraction and neutron powder diffraction to elucidate the structural properties of superconducting NaxCoO 2•yD2O over a wide compositional range. Our measurements show that superconducting samples exhibit a number of supercells ranging from to , but the predominant modulation, observed also in the neutron data, is a double hexagonal cell with dimensions 2a × 2a × c. Rietveld analysis reveals that D2O is inserted between CoO 2 sheets to form a layered network of NaO6 triangular prisms. Our model removes the need to invoke a 5 K superconducting point compound and suggests that a solid solution of Na is possible within a relatively constant amount of water y. © IOP Publishing Ltd.

Neutron diffractometry and X-ray diffractometry were used to characterize the cation disorder and vacancy distribution in nonstoichiometric spinel, MgO-xAl 2 O 3 , where x ≥ 1.0. Both synthetic and natural samples were examined. Least-squares refinements of integrated intensities for single crystals and Rietveld profile refinements for powders yielded the average scattering power from tetrahedral and octahedral sites within the almost-perfect cubic close-packed oxygen sublattice. The cation disorder was calculated assuming models in which the vacancies resided on tetrahedral, octahedral, or both types of sites. No degree of cation disorder was consistent with the tetrahedral vacancy model, and vacancies most likely resided on octahedral sites.

The flexibility offered by molecular-based systems allows us to introduce or replace specific ligands in a material with the aim of radically altering desired structural and magnetic properties. Specifically, Mn(dca) 2 (pyz) {dca = dicyanamide, [N(CN) 2 ] − ; pyz = pyrazine} has a unique interpenetrating ReO 3 -like lattice. The Mn 2+ cations are high-spin ( S= 5 2 ) and the material orders antiferromagnetically below T N = 2.53(2) K. Using neutron powder diffraction we observed a collinear spin structure oriented along the short ac -diagonal of the monoclinic unit cell. Inelastic neutron scattering results show a magnetic excitation at 0.23 meV. The strong dispersion character of this excitation demonstrates that it is related to a low-energy spin wave. Upon warming, the magnon gradually softens and disappears at T N , while critical scattering becomes evident by a broad quasielastic response above T N . The energy of the magnon is consistent with the exchange parameter, J , derived from magnetic susceptibility measurements.

Colossal magnetoresistive materials are perovskite-related mixed-valent (Mn3+/Mn4+) manganese oxides that exhibit both spontaneous (at a Curie transition) and magnetic field-induced insulator−metal transitions. In concert with the dramatic changes in electrical conductivity, these oxides exhibit large lattice anomalies, ferro- and antiferromagnetism, charge ordering of the Mn3+/Mn4+ sites, etc. In this article, we discuss how a particular class of these manganite materials, naturally layered manganites La2-2xSr1+2xMn2O7, has allowed us to experimentally probe many of these tightly coupled phenomena. In particular, we examine the structure−property relationships that determine the critical magnetic ground states, we discuss how conductivity and its field dependence can test prevailing models for the magnetoresistance effect, and we explore the interplay and competition between charge- and magnetic-order on both long- and short range length scales. Finally, we present evidence from neutron and X-ray scatter...

Marokite (CaMn 2 O 4 ) has been identified as a low-level impurity in the solid-state synthesis of Ca 1− x La x MnO 3 perovskites in the low- x region. In order to properly isolate the complex magnetic behavior of these important perovskites, the low- temperature magnetic structure and properties of marokite are determined and reported. All Mn ions are in the high-spin 3+ oxidation state. Jahn–Teller-distorted MnO 6 octahedra share both edges and vertices in a three-dimensional network, through which various exchange mechanisms cooperatively give rise to antiferromagnetic Mn 3+ spin order. The magnetic order is evidenced below T N =220 K by a peak in the magnetic susceptibility and the appearance of symmetry-lowering magnetic Bragg reflections in neutron diffraction.

The iron chalcogenide Fe1+y(Te1-xSex) is structurally the simplest of the Fe-based superconductors. Although the Fermi surface is similar to iron pnictides, the parent compound Fe1+yTe exhibits antiferromagnetic order with in-plane magnetic wave-vector (pi, 0). This contrasts the pnictide parent compounds where the magnetic order has an in-plane magnetic wave-vector (pi, pi) that connects hole and electron parts of the Fermi surface. Despite these differences, both the pnictide and chalcogenide Fe-superconductors exhibit superconducting spin resonances around (pi, pi), suggesting a common symmetry for their superconducting order parameter. A central question in this burgeoning field is therefore how (pi, pi) superconductivity can emerge from a (pi, 0) magnetic instability. Here, we report that the magnetic soft mode evolving from the (pi, 0)-type magnetic long-range order is associated with weak charge carrier localization. Bulk superconductivity occurs only as the magnetic mode at (pi, pi) becomes dominant upon doping. Our results suggest a common magnetic origin for superconductivity in iron chalcogenide and pnictide superconductors., 17 pages, 4 figures

The crystallographic and magnetic phase diagram of the $n=2$ layered manganite ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ in the region $xg~0.5$ has been studied using temperature-dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of three-dimensional (3D) perovskites with similar electronic doping: A $(0.5l~xl~0.66)\ensuremath{\rightarrow}C$ $(0.75l~xl~0.90)\ensuremath{\rightarrow}G$ $(0.90l~xl~1.0).$ However, the quasi-2D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: (i) a ``frustrated'' region between the A and C regimes where no long-range magnetic order is observed; (ii) magnetic polytypism arising from weak interbilayer magnetic exchange in the type-C regime; and (iii) a tetragonal-to-orthorhombic phase transition whose temperature evolution directly measures ordering of ${d}_{{3y}^{2}\ensuremath{-}{r}^{2}}$ orbitals in the $a\ensuremath{-}b$ plane. This orbital-ordering transition is precursory to type-C magnetic ordering, where ferromagnetic rods lie parallel to the b axis. These observations support the notion that ${e}_{g}$ orbital polarization is the driving force behind magnetic spin ordering. Finally, in the crossover region between type-C and type-G states, we see some evidence for the development of local type-C clusters embedded in a type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly doped ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ perovskites.

The crystal structure of ${\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ and its variation with pressure and temperature have been investigated by in situ neutron powder diffraction. At room temperature, the cubic \ensuremath{\alpha} phase is stable below 0.21 GPa, where a first-order transition to the orthorhombic \ensuremath{\gamma} phase, accompanied by a 4.95% reduction in volume occurs. The transition involves the inversion of one third of the ${\mathrm{W}}_{2}{\mathrm{O}}_{8}$ units, which is made possible by the migration of oxygen atoms that are bonded to only one W atom in the cubic phase. ${\mathrm{WO}}_{4}$ tetrahedra tilt off the threefold axes of the cubic cell and oxygen atoms that are coordinated to only one W atom in the cubic phase become coordinated to two W atoms in the orthorhombic phase. In spite of its smaller volume, the orthorhombic phase has a volume compressibility $[(\ensuremath{\Delta}V/\ensuremath{\Delta}P)/V=\ensuremath{-}1.53(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}]$ that is slightly larger than that of the cubic phase $[\ensuremath{-}1.38(1)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{GPa}}^{\mathrm{\ensuremath{-}}1}].$ This appears to result from a larger contribution of coordinated tilting of the ${\mathrm{ZrO}}_{6}$ octahedra and ${\mathrm{WO}}_{n}$ polyhedra to the compression. The orthorhombic phase is retained upon release of pressure. Below room temperature, the metastable orthorhombic phase exhibits an average negative volume thermal expansion $[(\ensuremath{\Delta}V/\ensuremath{\Delta}T)/V]$ of $\ensuremath{-}3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}{\mathrm{K}}^{\mathrm{\ensuremath{-}}1},$ which is an order of magnitude smaller than that for the cubic phase $(\ensuremath{-}2.6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}{\mathrm{K}}^{\mathrm{\ensuremath{-}}1}),$ apparently because of the reduced framework flexibility of the orthorhombic phase. Above room temperature, the thermal expansion of the orthorhombic phase becomes positive, prior to a first-order transition back to the cubic phase that occurs at about 390 K.

Neutron diffuse scattering in the form of rod-like features has been observed in single crystals of the layered CMR material La2−2xSr1+2xMn2O7 (x=0.4,0.36), consistent with the presence of 2D ferromagnetic spin correlations. These diffuse features are observed over a wide temperature region. However, their coherence length does not appear to diverge at TC, although there is evidence of the development of three-dimensional correlations around ferromagnetic reflections of the 3D-ordered magnetic structure close to TC. Quasi-elastic neutron scattering on a ceramic sample of x=0.3 shows that the lifetime of these ferromagnetic correlations increases at T→TC. They exhibit a spin-diffusion constant above TC of ∼5 meV A2, much lower than that reported for La2/3Ca1/3MnO3. We discuss the relationship of these magnetic correlations to models of the ferromagnetic transition in CMR compounds.

The low-field magnetization M and susceptibility {chi} are reported for the two-layered Ruddleson-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} for x=0.4 (denoted La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7}) with an in-plane magnetic easy axis. As T approaches the Curie temperature ( T{sub C}=116 K) on cooling, where the metal-insulator transition occurs in zero field, {chi} diverges as H{sup {minus}1/{delta}{sup {prime}}}, with {delta}=21{plus_minus}5. Also, near an extrinsic Curie transition attributed to {approximately}0.1{percent} volume fraction of intergrowths at T{sup {asterisk}}=2.47 T{sub C}, M scales as (1{minus}T/T){sup {beta}}, with {beta}=0.25{plus_minus}0.02. These results can be understood within the context of two-dimensional XY models, and provide some perspective of the layered manganites. {copyright} {ital 1998} {ital The American Physical Society}

In the high temperature superconductor La 1.85 Sr 0.15 Cu 1 - y Zn y O 4 , we directly observed modulated antiferromagnetic correlations of Cu spin orientations via elastic ( y = 0.02 ) and low energy inelastic ( y = 0.004 ) magnetically scattered neutron intensity. A magnetic field was applied perpendicular to the CuO 2 plane to change these correlations. For y = 0.02 we confirmed an earlier observation of elastic scattered intensity at 2 K and 0 T. The component of the magnetic moment perpendicular to the scattering vector is μ ⊥ Q 0.065 μ B , and is enhanced by 32% when a 5.5 T field is applied. For y = 0.004 , we observed a 145% enhancement of 3 meV spin excitations when a 14.5 T magnetic field is applied at 1.6 K. However, we observed no elastic scattered intensity at either 0 or 14.5 T.

The honeycomb lattice material Li2RuO3 undergoes a dimerization of Ru4+ cations on cooling below 270 degrees C, where the magnetic susceptibility vanishes. We use density functional theory calculations to show that this reflects the formation of a valence bond crystal, with a strong structural bond disproportionation. On warming, x-ray diffraction shows that discrete threefold symmetry is regained on average, and the dimerization apparently disappears. In contrast, local structural measurements using high-energy x rays show that disordered dimers survive at the nanoscale up to at least 650 degrees C. The high-temperature phase of Li2RuO3 is thus an example of a valence bond liquid, where thermal fluctuations drive resonance between different dimer coverages, a classic analog of the resonating valence bond state often discussed in connection with high-T-c cuprates. (Less)

Magnetic order and excitations in multiferroic ${\mathrm{DyMnO}}_{3}$ were studied by neutron scattering experiments using a single crystal prepared with an enriched $^{162}\mathrm{Dy}$ isotope. The ordering of Mn moments exhibits pronounced hysteresis arising from the interplay between Mn and Dy magnetism, which has a strong impact on the ferroelectric polarization. The magnon dispersion resembles that reported for ${\mathrm{TbMnO}}_{3}$. We identify the excitations at the magnetic zone center and near the zone boundary in the $b$ direction, which can possess electromagnon character. The lowest frequency of the zone-center magnons is in good agreement with a signal in a recent optical measurement, so this mode can be identified as the electromagnon coupled by the same Dzyaloshinskii-Moriya interaction as the static multiferroic phase.

The evolution of the structural properties of ${A}_{1\ensuremath{-}x}{A}_{x}^{\ensuremath{'}}{\mathrm{MnO}}_{3}$ was determined as a function of temperature, average $A$-site radius $〈{r}_{A}〉,$ and applied pressure for the ``optimal'' doping range $x=0.25,$ 0.30, by using high-resolution neutron powder diffraction. The metal-insulator transition, which can be induced both as a function of temperature and of $〈{r}_{A}〉,$ was found to be accompanied by significant structural changes. Both the paramagnetic charge-localized phase, which exists at high temperatures for all values of $〈{r}_{A}〉,$ and the spin-canted ferromagnetic charge-ordered phase, which is found at low temperatures for low values of $〈{r}_{A}〉,$ are characterized by large metric distortions of the ${\mathrm{MnO}}_{6}$ octahedra. These structural distortions are mainly incoherent with respect to the space-group symmetry, with a significant coherent component only at low $〈{r}_{A}〉.$ These distortions decrease abruptly at the transition into the ferromagnetic metal phase. These observations are consistent with the hypothesis that, in the insulating phases, lattice distortions of the Jahn-Teller type, in addition to spin scattering, provide a charge-localization mechanism. The evolution of the average structural parameters indicates that the variation of the electronic bandwidth is the driving force for the evolution of the insulator-to-metal transition at ${T}_{C}$ as a function of ``chemical'' and applied pressure.

The crystal structure of La{sub 1.2}Sr{sub 1.8}Mn{sub 2}O{sub 7} has been determined as a function of temperature in magnetic fields of 0 and 0.6 T. There is a significant magnetostriction at the Curie temperature T{sub C}; the a axis contracts ({Delta}a/a={minus}0.018{percent}) while the c axis expands ({Delta}c/c=0.05{percent}). The latter value is larger than that found for perovskite manganites ({Delta}l/l=0.02{percent} at 1 T). No such effect occurs at lower temperatures. The magnetostriction results from changes in individual Mn-O bond lengths. From T{sub C} to {approximately}300 K, bulk magnetization and magnetoresistance measurements indicate the presence of magnetic order not observed in the neutron-diffraction data. This suggests short-range magnetic order in this temperature range. Our structural measurements show that within this temperature interval there is also a weak magnetostriction. {copyright} {ital 1997} {ital The American Physical Society}

The crystal structure of the layered perovskite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ has been studied under hydrostatic pressure up to $\ensuremath{\sim}6\mathrm{kbar}$, in the paramagnetic and ferromagnetic states, with neutron powder diffraction. The compressibility of the Mn-O apical bonds in the double layer of ${\mathrm{MnO}}_{6}$ octahedra changes sign from the paramagnetic insulator (PI) to the ferromagnetic metal (FM) state; in the FM state the Mn-O-Mn linkage between ${\mathrm{MnO}}_{2}$ planes expands under applied pressure, whereas they contract in the PI state. This counterintuitive behavior is interpreted in terms of exchange striction, which reflects the competition between super and double exchange. An increase of the Mn moment with applied pressure in the FM state is consistent with a positive ${\mathrm{dT}}_{C}/dP$, as well as a cant angle ${\ensuremath{\theta}}_{0}$ between the magnetizations of neighboring ${\mathrm{MnO}}_{2}$ sheets that decreases with pressure.

Colossal magnetoresistance (CMR) is associated with the phase transition from a metallic ferromagnetic to insulating paramagnetic phase, which can be controlled by an applied magnetic field. The insulating phase occurs due to trapping of the charge carriers by polaronic lattice distortions, which raise the resistivity. Theories based on local physics predict that the magnitude of the resistivity jump at T-C is determined by how much, on average, the amplitude of these distortions increases at the phase transition. Using neutron scattering, we measured the average distortion amplitude in La0.7Sr0.3MnO3. Surprisingly, its increase from below to above T-C is just as large as in other manganites, which have a much larger resistivity jump. This result suggests that the strength of CMR is determined not by the size of distortions, but by their cooperative nature, specific to each compound. Existing theories need to be extended to include correlations between different unit cells to explain and predict the strength of CMR. (Less)

The structural properties of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}\mathrm{Mn}{\mathrm{O}}_{3+\ensuremath{\delta}}$ have been studied using neutron powder diffraction as a function of both Sr doping ($0l~xl~0.225$) and oxygen partial pressure during synthesis [2.1\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ atm\ensuremath{\le}P(${\mathrm{O}}_{2}$)\ensuremath{\le}1 atm]. A structural phase diagram constructed as a function of these parameters has a rhombohedral phase ($R\overline{3}c$), an orthorhombic phase ($\mathrm{Pbnm}$), and a monoclinic phase ($\frac{P{2}_{1}}{c}$). For a given $x$, decreasing P(${\mathrm{O}}_{2}$) yields smaller cation vacancy concentrations. At low temperature, the $R$ phase is ferromagnetic, while the $M$ phase is antiferromagnetic. The $O$ phase is ferromagnetic for $xg~0.125$, and the ferromagnetism is independent of the $O\ensuremath{-}R$ phase transition that coincides with the transition from nonmetal to metal. Transport measurements made between 20 and 350 K show that $O$ and $M$ samples are nonmetallic ($\frac{d\ensuremath{\rho}}{\mathrm{dT}}l0$), while the $R$ samples exhibit a temperature-dependent nonmetal-to-metal transition at temperatures close to the Curie temperature. Magnetoresistance (MR) is observed in all three phases. The largest value at 9 T, found in the orthorhombic and monoclinic samples, is of similar order ($\ensuremath{\Delta}\ensuremath{\rho}={\ensuremath{\rho}}_{0}\ensuremath{-}{\ensuremath{\rho}}_{9\mathrm{T}}\ensuremath{\sim}{10}^{4}$ \ensuremath{\Omega}cm) to that reported for ${10}^{6}$% colossal MR powder samples. However, our lower sintering temperatures result in large ${\ensuremath{\rho}}_{9\mathrm{T}}$ values that yield $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{{\ensuremath{\rho}}_{9\mathrm{T}}}\ensuremath{\sim}230%$.

Using both powder and single crystal neutron diffraction we have determined the room temperature structure of Pr0.5Sr0.5MnO3from melt-grown crystalline material. Contrary to previous measurements on polycrystalline samples we find the room temperature structure of Pr0.5Sr0.5MnO3is tetragonal, space groupF4/mmc(a= 7.64093(5) A,c= 7.79104(8) A). The structure of Pr0.5Sr0.5MnO3is related to the ideal primitive cubic perovskite structure by a single correlated rotation of MnO6octahedra about thecaxis. We find that MnO6octahedra exhibit a small Jahn–Teller distortion (Δ(Mn–O) ≈ 0.02 A).

The structural, magnetic, and transport behavior of orthorhombic, insulating La{sub 0.875}Sr{sub 0.125}MnO{sub 3+{delta}} were studied as a function of temperature from 20 to 300 K. Upon cooling from room temperature, we observe the rapid development of a large breathing-mode distortion coincident with large positive and negative thermal expansions for the {ital c} and {ital b} axes, respectively. The onset of canted ferromagnetic ordering at 220 K is correlated with the relaxation of this distortion, which reaches values similar to those at room temperature when the magnetic ordering saturates. We observe no first-order-type discontinuity in the volume at the ferromagnetic transition. These results demonstrate a strong coupling between magnetic ordering and crystal structure even though there is no metal-insulator transition associated with the magnetic transition. {copyright} {ital 1996 The American Physical Society.}

We have measured the acoustic phonon dispersion curves of three Cubic Stabilised Zirconias (CDZ) (9.5 and 24mol% Y2O3− and 12.5 mol% CaO−ZrO2) at room temperature using a triple axis spectrometer. Phonons become broader and decrease in intensity as the Brillouin zone boundary is approached. The behaviour of the [ζζ0]TA1 branch is anomalous in that it broadens to the extent that it cannot be observed beyond 2π a (0.4, 0.4, 0) . To a lesser extent the same behaviour is observed for the other LA and TA branches. Despite an intensive search, we have not been able to measure any optic phonons. Previous workers have attributed these effects to the static disorder of the oxygen sublattice. In this paper, we present lattice dynamical calculations which model the effect of oxygen vacancies and relaxations which qualitatively explain the experimental phonon observations.

Time-resolved in situ neutron powder diffraction and Rietveld refinement have been used to study the synthesis of Bi-2212 from hydroxide precursors in a 2% O2 atmosphere. Bi-2212 was found to form within the temperature range 770–800 °C. Studies at 800 °C show that Bi-2212 grows rapidly at the expense of Bi-2201. Upon lowering the temperature to 500 °C and changing the atmosphere to Ar, a rapid increase in the lattice parameters was observed. We attribute this change to the loss of oxygen from the Bi-2212 lattice. The final material exhibited a Tc of 94 K.

Single crystal neutron diffraction has been used to study the relaxations of the O-atom from its ideal fluorite position in yttria stabilised cubic zirconia (Y-CSZ). The samples used in this study cover a compositional range of 9.5–24 mol% Y 2 O 3 ZrO 2 . We find that the O-atom is predominantly displaced in the 〈100〉 direction and to a lesser extent in the 〈111〉, while appreciable amounts remain at the ideal fluorite site. Further the average ZrO distances decrease with Y 2 O 3 content to values similar to that of tetragonal zirconia for compositions close to the cubic-tetragonal zirconia phase boundary. The controversy in the literature over the direction of relaxation of the O-atom is resolved with the aid of probability density functions. We demonstrate that there is more than one way to describe the distribution of the O-atom about its ideal site. A probability maximum is always produced at the ideal site, while probability density in the 〈100〉 directions is strong.

We present the results of a neutron scattering investigation of Ca 1− x La x MnO 3 in the electron-doped ( x ∼0) regime. For light electron-doping, FM clusters ∼10 A in diameter are identified in a liquid-like distribution within the G-AFM matrix. This mirrors behavior in the hole-doped ( x ∼1) regime. These clusters are not long-range correlated for x ⩽0.05, however, for 0.05⩽ x ⩽0.10, a spontaneous long-range FM moment is observed. This seems to be due to the formation of a ‘spin glass’ of overlapping clusters, as opposed to either the long-range delocalization of doped electrons or long-range phase separation into electron-rich FM and electron-poor AFM regions.

Solid-solution compounds where Cr, Re, and other metals are substituted for Hg in Hg-Ca-Ba-Cu-O superconductors have been reported to exhibit enhanced flux-pinning behavior. We have determined the structural modifications resulting from the incorporation of Cr in Hg{sub 1{minus}{ital x}}Cr{sub {ital x}}Sr{sub 2}CuO{sub 4+{delta}} (for {ital x}{approx}0.4) using neutron powder diffraction, electron diffraction, and lattice imaging. Cr substitutes at the Hg site, but is displaced to allow tetrahedral coordination by oxygen atoms. Additional oxygen is incorporated to provide four oxygen atom neighbors for each Cr atom. These CrO{sub 4} units cluster to form a supercell of approximate dimensions 5{ital a}{times}5{ital a}{times}2{ital c} in which Cr-rich and Hg-rich regions alternate in all three crystallographic directions. Because the Cu-O apical bond associated with the CrO{sub 4} unit is lengthened to 3.13 A, the superconducting planes are best viewed as consisting of CuO{sub 5} pyramids, oriented up or down as dictated by the supercell ordering, rather than CuO{sub 6} octahedra. Local structural constraints associated with individual Cr sites require that considerable disorder is present, even in the supercell. Extended defects in this supercell (e.g., columns of CuO{sub 6} octahedra associated with Hg-rich regions) may contribute to the enhanced flux pinning.

The synthesis of Tl-2201 and Tl-2212 superconductors at 550°C and 1200 psi Ar is described. AC susceptibility indicates onset T c 's of ∼ 93 K for 2201 and ∼ 118 K for 2212. For both materials, the single-phase composition is not that of the nominally stoichiometric cation ratio Tl:Ba:Ca:Cu = 2:2:0:1 or 2:2:1:2. The 2201 phase forms as a point compound under these synthesis conditions, while 2212 may show a more complex solid-solution behavior. Rietveld analysis of combined X-ray and neutron-diffraction data reveals random substitution of Cu on the Tl site for the Tl-2201 compound.

Recently superconductivity has been reported in (Ca,Na${)}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ in which the apical anion position was proposed to be occupied by Cl [Hiroi et al., Nature 371, 139 (1994)]. In this paper we report a neutron powder diffraction investigation of nonsuperconducting ${\mathrm{Ca}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ and superconducting (Ca,Na${)}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ (${\mathit{T}}_{\mathit{c}}$=26 K). We confirm that in both compounds the apical anion position is fully occupied by Cl. Unlike the recently reported superconductor ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{F}}_{2+\mathrm{\ensuremath{\delta}}}$, in (Ca,Na${)}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$ we find little evidence for the presence of interstitial atoms. In our structural data the substitution of ${\mathrm{Na}}^{+}$ for ${\mathrm{Ca}}^{2+}$ is evidenced by a shortening of the Cu-O bond and lengthening of the Cu-Cl bond. The bond valence of the Cu atom increases, consistent with holes being created by the Na substitution.

In this paper we investigate the complex transformation of the magnetoelectric properties from the collinear A-type phase to the multiferroic noncollinear spin cycloid in the solid solution Nd${}_{1\ensuremath{-}x}$Y${}_{x}$MnO${}_{3}$ from compositions of $x=0.3$ to 0.55 in single crystal samples. Neutron diffraction experiments at the structural phase transition ($x=0.35$ to 0.45) reveal the coexistence of the A-type order and several incommensurate orders, varying with composition and temperature.