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Cubic energy materials such as thermoelectrics or hybrid perovskite materials are often understood to be highly disordered. In GeTe and related IV-VI compounds, this is thought to provide the low thermal conductivities needed for thermoelectric applications. Since conventional crystallography cannot distinguish between static disorder and atomic motions, we develop the energy-resolved variable-shutter pair distribution function technique. This collects structural snapshots with varying exposure times, on timescales relevant for atomic motions. In disagreement with previous interpretations, we find the time-averaged structure of GeTe to be crystalline at all temperatures, but with anisotropic anharmonic dynamics at higher temperatures that resemble static disorder at fast shutter speeds, with correlated ferroelectric fluctuations along the $$c direction. We show that this anisotropy naturally emerges from a Ginzburg-Landau model that couples polarization fluctuations through long-range elastic interactions. By accessing time-dependent atomic correlations in energy materials, we resolve the long-standing disagreement between local and average structure probes, and show that spontaneous anisotropy is ubiquitous in cubic IV-VI materials., Published version, substantial changes, many references removed due to journal limits :-( Please see the supporting information online. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format (see conditions)

Using the new polarized neutron diffraction (PND) setup at MLZ the spin reorientation transition in the magnetocaloric orthoferrite HoFeO3 was studied at different wavelength. The various experiments provided reproducible results demonstrating high reliability of the used setup. We show that during the phase transition at TSR=53 K in an external magnetic field applied along crystal c-axis, the ordered magnetic moment of the Fe sublattice rotates from the crystallographic direction b to a not just in the ab plane, but through z axis. This means that the applied field breaks the orthorhombic symmetry allowing some magnetization parallel to z within a short temperature region. Interestingly, this is the same temperature region where large magnetocaloric effect for HoFeO3 was previously reported. A general model of the magnetic structure of HoFeO3, unconstrained by the orthorhombic symmetry, would allow the magnitudes and directions of the moments on each of the 8 magnetic sublattices in the unit cell to be independent of one-another, leading to 24 independent magnetic parameters. PND measurements were used to determine the absolute sign of the Dzyaloshinskii-Moriya interaction (DMI) in the ab plane for the Fe magnetic sublattice at 65 K. DMI plays an important role in the energy balance of the system.

The pyrite mineral MnS 2 was recently shown to undergo a giant pressure-induced volume collapse at ∼ 12 GPa, via a disordered intermediate phase. The high pressure arsenopyrite phase is stabilised by metal-metal bonding, a mechanism now shown to be ubiquitous for Mn 2 + chalcogenides. Here we report a spectroscopic investigation of this transition up to pressures of 22 GPa. Using XANES we show that the transition does not involve a change in oxidation state, consistent with the arsenopyrite crystal structure proposed at high pressure. Notably, the XANES spectrum is almost identical in the pressure-induced disordered phase, and after crystallisation induced by laser-heating. The former is therefore a ‘valence bond glass’, and is likely disordered due to kinetic hindrance of the phase transition. We also detect electronic changes in the compressed pyrite phase, and this is confirmed by Raman scattering which shows that the disulphide vibrations in the pyrite phase saturate before the volume collapse. Together with detailed DFT calculations, these results indicate that electronic changes precede valence bond formation between the Mn 2 + cations.

Physical review / B covering condensed matter and materials physics 100(21), 214432 (2019). doi:10.1103/PhysRevB.100.214432, Published by Inst., Woodbury, NY

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS$_2$ has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this $S=5/2$ system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.

We have investigated the ferromagnetic phase transition of elemental Co by high-resolution neutron backscattering spectroscopy. We monitored the splitting of the nuclear levels by the hyperfine field at the Co nucleus. The energy of this hyperfine splitting is identified as the order parameter of the ferromagnetic phase transition. By measuring the temperature dependence of the energy we determined the critical exponent [Formula: see text] and the ferromagnetic Curie temperature of [Formula: see text] K. The present result of the critical exponent agrees better with the predicted value (0.367) of the three-dimensional Heisenberg model than that determined previously by nuclear magnetic resonance.

We demonstrate that small but finite ferroelectric polarization ($\sim$0.01 $\mu$C/cm$^2$) emerges in orthorhombic LuFeO$_3$ ($Pnma$) at $T_N$ ($\sim$600 K) because of commensurate (k = 0) and collinear magnetic structure. The synchrotron x-ray and neutron diffraction data suggest that the polarization could originate from enhanced bond covalency together with subtle contribution from lattice. The theoretical calculations indicate enhancement of bond covalency as well as the possibility of structural transition to the polar $Pna2_1$ phase below $T_N$. The $Pna2_1$ phase, in fact, is found to be energetically favorable below $T_N$ in orthorhombic LuFeO$_3$ ($albeit$ with very small energy difference) than in isostructural and nonferroelectric LaFeO$_3$ or NdFeO$_3$. Application of electric field induces finite piezostriction in LuFeO$_3$ via electrostriction resulting in clear domain contrast images in piezoresponse force microscopy., Comment: 12 pages, 8 figures

Polarised neutron diffraction measurements have been made on HoFeO3 single crystals magnetised in both the [0 0 1] and [1 0 0] directions (Pbnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of [Formula: see text] T parallel to [0 0 1] at [Formula: see text] K and with the lower field [Formula: see text] T parallel to [1 0 0] at [Formula: see text] K. A Fourier projection of magnetization induced parallel to [0 0 1], made using the hk0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70 K, 9 T applied parallel to [0 0 1] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the [Formula: see text] plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature [Formula: see text] structure with a lower field, 0.5 T [Formula: see text] [1 0 0] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the [Formula: see text] domain stabilised by the field were determined relative to the distorted perovskite structure. This relationship fixes the sign of the Dzyalshinski-Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.

AIP Advances 7(4), 045106 (2017). doi:10.1063/1.4979710, Published by American Inst. of Physics, New York, NY

In order to understand the origin of the huge quasielastic magnetic scattering observed previously with a back-scattering neutron spectrometer, we have re-investigated the low energy excitations in HoCrO3 by inelastic neutron scattering in a much wider energy range with time-of-flight neutron spectrometers. The inelastic signals are due to the excitations between the ground state doublet of the Ho ion. The quasielastic signal is due to the fluctuation of the disordered Ho moments. At low temperature the intensity of quasielastic scattering is small. It starts increasing as the temperature increases above 30 K. At the same temperature, the elastic intensity due to Ho moment ordering decreases in a similar way. This observation strengthens the hypothesis that the quasielastic scattering is due the fluctuations of the disordered Ho moments. The time scale of fluctuations has been determine from the quasielastic scattering and was found to vary from about 22 ps at [Formula: see text] K to about 2.5 ps at [Formula: see text] K. The stretched exponential line shape indicates a distribution of decay rates at low temperatures.

The magnetic structures and the magnetic phase transitions in the Mn-doped orthoferrite TbFeO$_3$ studied using neutron powder diffraction are reported. Magnetic phase transitions are identified at $T^\mathrm{Fe/Mn}_N \approx$ 295~K where a paramagnetic-to-antiferromagnetic transition occurs in the Fe/Mn sublattice, $T^\mathrm{Fe/Mn}_{SR} \approx$ 26~K where a spin-reorientation transition occurs in the Fe/Mn sublattice and $T^\mathrm{R}_N \approx$ 2~K where Tb-ordering starts to manifest. At 295~K, the magnetic structure of the Fe/Mn sublattice in TbFe$_{0.5}$Mn$_{0.5}$O$_3$ belongs to the irreducible representation $\Gamma_4$ ($G_xA_yF_z$ or $Pb'n'm$). A mixed-domain structure of ($\Gamma_1 + \Gamma_4$) is found at 250~K which remains stable down to the spin re-orientation transition at $T^\mathrm{Fe/Mn}_{SR}\approx$ 26~K. Below 26~K and above 250~K, the majority phase ($> 80\%$) is that of $\Gamma_4$. Below 10~K the high-temperature phase $\Gamma_4$ remains stable till 2~K. At 2~K, Tb develops a magnetic moment value of 0.6(2)~$\mu_\mathrm{B}/$f.u. and orders long-range in $F_z$ compatible with the $\Gamma_4$ representation. Our study confirms the magnetic phase transitions reported already in a single crystal of TbFe$_{0.5}$Mn$_{0.5}$O$_3$ and, in addition, reveals the presence of mixed magnetic domains. The ratio of these magnetic domains as a function of temperature is estimated from Rietveld refinement of neutron diffraction data. Indications of short-range magnetic correlations are present in the low-$Q$ region of the neutron diffraction patterns at $T < T^\mathrm{Fe/Mn}_{SR}$. These results should motivate further experimental work devoted to measure electric polarization and magnetocapacitance of TbFe$_{0.5}$Mn$_{0.5}$O$_3$., Comment: 16 pages, 7 figures, Accepted to J. Appl. Phys., 2016

Strong spin-lattice coupling and prominent frustration effects observed in the 50 Fe-doped frustrated hexagonal IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn1.gif] (h)̑ext{LuMnO}_3} are reported. A Néel transition at IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn2.gif] {T_{\mathrm{N}}≈112\ ̑ext{K}} and a possible spin re-orientation transition at IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn3.gif] {T_{\mathrm{SR}}≈55\ ̑ext{K}} are observed in the magnetization data. From neutron powder diffraction data, the nuclear structure at and below 300 K was refined in polar P 6 3 cm space group. While the magnetic structure of LuMnO 3 belongs to the IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn4.gif] {Γ_4\ (P6'_3c'm)} representation, that of LuFe 0.5 Mn 0.5 O 3 belongs to IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn5.gif] {Γ_1\ (P6_3cm)} which is supported by the strong intensity for the (100) reflection and also judging by the presence of spin-lattice coupling. The refined atomic positions for Lu and Mn/Fe indicate significant atomic displacements at IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn6.gif] {T_{\mathrm{N}}} and IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn7.gif] {T_{\mathrm{SR}}} which confirms strong spin-lattice coupling. Our results complement the discovery of room temperature multiferroicity in thin films of IMG [http://ej.iop.org/images/0295-5075/110/3/37007/epl17080ieqn8.gif] {h̑ext{LuFeO}_3 and would give impetus to study LuFe 1− x Mn x O 3 systems as potential multiferroics where electric polarization is linked to giant atomic displacements.

We study the spontaneous crystallization of an assembly of highly monodisperse steel spheres under shaking, as it evolves from localized icosahedral ordering towards a packing reaching crystalline ordering. Towards this end, real space neutron tomography measurements on the granular assembly are carried out, as it is systematically subjected to a variation of frequency and amplitude. As expected, we see a presence of localized icosahedral ordering in the disordered initial state (packing fraction around 0.62). As the frequency is increased for both the shaking amplitudes (0.2 and 0.6 mm) studied here, there is a rise in packing fraction, accompanied by an evolution to crystallinity. The extent of crystallinity is found to depend on both the amplitude and frequency of shaking. We find that the icosahedral ordering remains localized and its extent does not grow significantly, while the crystalline ordering grows rapidly as an ordering transition point is approached. In the ordered state, crystalline clusters of both face centered cubic (FCC) and hexagonal close packed (HCP) types are identified, the latter of which grows from stacking faults. Our study shows that an earlier domination of FCC gives way to HCP ordering at higher shaking frequencies, suggesting that despite their coexistence, there is a subtle dynamical competition at play. This competition depends on both shaking amplitude and frequency, as our results as well as those of earlier theoretical simulations demonstrate. It is likely that this involves the very small free energy difference between the two structures., Comment: 11 pages, 8 figures

The recent surge of interest in phase-change materials GeTe, ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$, and related compounds motivated us to revisit the structural phase transition in GeTe in more detail than was done before. The rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied using high-resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. The results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral-to-cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe, we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP₂:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

Antisite disorder is observed to have significant impact on the magnetic properties of the double perovskite Y2CoMnO6 which has been recently identified as a multiferroic. A paramagnetic-ferromagnetic phase transition occurs in this material at T-c approximate to 75 K. At 2K, it displays a strong ferromagnetic hysteresis with a significant coercive field of H-c approximate to 15 kOe. Sharp steps are observed in the hysteresis curves recorded below 8K. In the temperature range 2K

The possibility of coupling through the hyperfine interaction of nuclear spins with the electronic spin system has given rise to hope for potential novel applications in spintronics and quantum computations. We investigated the dispersion of nuclear spin waves in such a coupled system, Nd2CuO4, by using neutron-spin-echo spectroscopy at millikelvin temperatures. Our results show the existence of dispersion of nuclear spin waves in Nd2CuO4 at T D 40 mK. A fit of the dispersion data with the spin wave dispersion formula gave the Suhl–Nakamura interaction range to be of the order of 10 A° , which is much smaller than that expected theoretically.

Dramatic volume collapses under pressure are fundamental to geochemistry and of increasing importance to fields as diverse as hydrogen storage and high-temperature superconductivity. In transition metal materials, collapses are usually driven by so-called spin-state transitions, the interplay between the single-ion crystal field and the size of the magnetic moment. Here we show that the classical S=5/2 mineral Hauerite undergoes an unprecedented 22 % collapse driven by a conceptually different magnetic mechanism. Using synchrotron x-ray diffraction we show that cold compression induces the formation of a disordered intermediate. However, using an evolutionary algorithm we predict a new structure with edge-sharing chains. This is confirmed as the thermodynamic groundstate using in situ laser heating. We show that magnetism is globally absent in the new phase, as low-spin quantum S=1/2 moments are quenched by dimerisation. Our results show how the emergence of metal-metal bonding can stabilise giant spin-lattice coupling in Earth's minerals., Published version, minor changes

Using high resolution powder neutron diffraction data, we show that there is a distinct anomaly in the structural noncentrosymmetry around the magnetic transition point TN for bulk and nanoscale BiFeO3. It appears that the structural noncentrosymmetry - which gives rise to the ferroelectric polarization - is suppressed anomalously by ~1% (of the average noncentrosymmetry at above the magnetic transition) in the bulk sample and by ~12% in the nanoscale sample as the magnetic transition is approached from higher temperature. This observation shows that the multiferroic coupling improves in the nanoscale sample which is expected to brighten the application prospects of nanoscale BiFeO3 in nanospintronics-based sensor devices.PACS Nos. 75.70.Cn, 75.75.-c

We have investigated low energy excitations in metallic HoAl2 by high resolution neutron spectroscopy. At T=3 K we found clear inelastic peaks in the energy loss and energy gain sides along with the central elastic peak. We interpret these inelastic peaks to be due to the transitions from hyperfine-split nuclear levels. The energy which is E = 25.03 ± 0.02 μ eV at T=3 K, decreases continuously and becomes zero at T N ≈ 30 K . The intensity of the inelastic peak remains more or less constant as a function temperature before the it merges with the central elastic peak at TN. The energy of nuclear spin excitations in HoAl2 seems to follow the order parameter of the ferromagnetic phase transition.

We have investigated low energy nuclear spin excitations in double perovskite compounds R$_2$CoMnO$_6$ (R = Y,Tb) by inelastic neutron scattering with a high-resolution back-scattering spectrometer. We observed inelastic signals at about 2.1 $\mu$eV for Y$_2$CoMnO$_6$ and also for Tb$_2$CoMnO$_6$ at T = 2 K in both energy loss and energy gain sides. We interpret these inelastic peaks to be due to the transitions between the hyperfine split nuclear levels of $^{59}$Co nucleus. The inelastic peaks move towards the central elastic peak and finally merge with it at the magnetic ordering temperature $T_C$. The energy of the low energy excitations decreases continuously and becomes zero at $T_C \approx 75$ K for Y$_2$CoMnO$_6$ and $T_C \approx 100$ K for Tb$_2$CoMnO$_6$. For Tb$_2$CoMnO$_6$, which contains magnetic rare-earth ions, additional quasielastic scattering due presumably to the fluctuations of large Tb magnetic moments was observed. The present study reveals the magnetic ordering of the Co sublattice. The results of this investigation along with that obtained by us for other compounds indicate the presence of unquenched orbital moments in some of the Co compounds.

We have investigated the low energy nuclear spin excitations in NdMn$_2$Si$_2$ and NdMn$_2$Ge$_2$ by high resolution inelastic neutron scattering. Previous neutron diffraction investigations gave ambiguous results about Nd magnetic ordering at low temperatures. The present element-specific technique gave direct evidence for the magnetic ordering of Nd ions. We found considerable difference in the process of the Nd magnetic ordering at low temperature in NdMn$_2$Si$_2$ and NdMn$_2$Ge$_2$. Our results are consistent with those of magnetization and recent neutron diffraction measurements.

Using the results of x-ray and neutron diffraction experiments, we show that the ferroelectric polarization, in ~22 nm particles of BiFeO3, exhibits a jump by ~30% around the magnetic transition point T_N (~635 K) and a suppression by ~7% under 5T magnetic field at room temperature (<, Comment: 4 pages including 4 figures and supplementary data; accepted for publication in Appl. Phys. Lett

The magnetic structures adopted by the Fe and Sm sublattices in SmFeAsO have been investigated using element specific x-ray resonant and non-resonant magnetic scattering techniques. Between 110 and 5 K, the Sm and Fe moments are aligned along the c and a directions, respectively according to the same magnetic representation $\Gamma_{5}$ and the same propagation vector (1, 0, 0.5). Below 5 K, magnetic order of both sublattices change to a different magnetic structure and the Sm moments reorder in a magnetic unit cell equal to the chemical unit cell. Modeling of the temperature dependence for the Sm sublattice as well as a change in the magnetic structure below 5 K provide a clear evidence of a surprisingly strong coupling between the two sublattices, and indicate the need to include anisotropic exchange interactions in models of SmFeAsO and related compounds., Comment: 8 pages, 7 figures, accepted for publication in Phys. Rev. B

To shed light on the role of magnetism on the superconducting mechanism of the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering on phonon dynamics in the low-temperature orthorhombic parent compounds, which present a spin density wave. The study covers both the 122 (AFe(2)As(2); A = Ca, Sr, Ba) and 1111 (AFeAsF; A = Ca, Sr) phases. We extend our recent work on the Ca (122 and 1111) and Ba (122) cases by treating, computationally and experimentally, the 122 and 1111 Sr compounds. The effect of magnetic ordering is investigated through detailed non-magnetic and magnetic lattice dynamical calculations. The comparison of the experimental and calculated phonon spectra shows that the magnetic interactions/ordering have to be included in order to reproduce well the measured density of states. This highlights a spin-correlated phonon behavior which is more pronounced than the apparently weak electron-phonon coupling estimated in these materials. Furthermore, there is no noticeable difference between phonon spectra of the 122 Ba and Sr, whereas there are substantial differences when comparing these to CaFe(2)As(2) originating from different aspects of structure and bonding.

We report the magnetocaloric effect (MCE) in a Pr0.52Sr0.48MnO3 single crystal. A peak in the temperature dependence of magnetic entropy change (Delta S-M) with a fairly large negative value (approximate to 3.8 J/ kg K) is observed at 275 K close to Curie temperature. MCE is spread over a wide temperature range resulting in a considerable refrigerant capacity (approximate to 293 J/kg). At low temperature the sign of Delta S-M changes, below which anomalous field dependence of Delta S-M is observed indicating the coexistence of ferromagnetic and antiferromagnetic interactions. Interplay between the interactions is strongly influenced by direction and magnitude of applied magnetic field in the ordered state. (C) 2010 American Institute of Physics. [doi:10.1063/1.3340524]

We reported previously [T. Chatterji et al., Phys. Rev. B 78, 134105 (2008)] negative thermal expansion (NTE) in ReO3 in the limited temperature range from 2 to 220 K. Here we discovered NTE in ReO3 at higher-temperature region from 600 to 680 K. We determined the temperature variation in the lattice parameter and the unit cell volume of ReO3 by neutron diffraction. The temperature variation in the lattice parameter and the unit cell volume show two regions of NTE and two minima. We attribute the NTE of ReO3 to be the result of anharmonicity and anomalous softening of M3 phonon mode.

We have investigated the antiferromagnetic (AF) phase transition and spin correlations in NiO by high-temperature neutron diffraction below and above ${T}_{N}$. We show that AF phase transition is a continuous second-order transition within our experimental resolution. The spin correlations manifested by the strong diffuse magnetic scattering persist well above ${T}_{N}\ensuremath{\approx}530\text{ }\text{K}$ and could still be observed at $T=800\text{ }\text{K}$ which is about $1.5{T}_{N}$. We argue that the strong spin correlations above ${T}_{N}$ are due to the topological frustration of the spins on a fcc lattice. The N\'eel temperature is substantially reduced by this process. We determined the critical exponents $\ensuremath{\beta}=0.328\ifmmode\pm\else\textpm\fi{}0.002$ and $\ensuremath{\nu}=0.64\ifmmode\pm\else\textpm\fi{}0.03$ and the N\'eel temperature ${T}_{N}=530\ifmmode\pm\else\textpm\fi{}1$ K. These critical exponents suggest that NiO should be regarded as a $3dXY$ system.

The question of Mn valence and symmetry and population of doped holes in La1.2Sr1.65Ca0.15Mn2O7 and LaSr2Mn2O7 bilayer single crystals has been studied with polarized soft x-ray absorption spectroscopy. The observed changes in the O K and Mn L spectra with polarization provide a strong indication for the existence of a competition between the charge dynamics and the lattice distortion that leads to transfer of some of the holes doped in the out-of-plane (3z(2)-r(2)) states to the in-plane (x(2)-y(2)) states. The changes observed in these with doping are shown not due to a decrease in the electron population in the d(x2-y2) states but caused by an increase in density of holes in the d(z2-x2) and d(z2-y2) states, and the electrons predominantly occupy the corresponding orthogonal states, i.e., d(3x2-r2) and d(3y2-r2). No evidence is found for the existence of a formal Mn4+ valence state in these systems.

We have investigated S = 1/2 triangular lattice Li0.98Ni1.02O2 by a zero-field (ZF) and longitudinal-field (LF) μSR technique. The muon signals from Li0.98Ni1.02O2 could be fitted by a stretched exponential function at low temperatures. At about 30 K the power k of the stretched exponential function becomes about k = 2 for the ZF signal, i.e. the relaxation becomes practically a Gaussian. For the LF-μSR signals the exponent k tends to become 1 above T = 30 K, i.e. the signal becomes exponential. The relaxation rate λ becomes very high at temperature below T = 10 K for the ZF data whereas λ corresponding to LF data shows a maximum at T ~ 10 K. The LF relaxation rate remains finite (0.4 µs-1) at the lowest temperature (T ~ 2 K) investigated. This indicates that even at very low temperature muon spin becomes depolarized because muons pick up low energy excitations from the spin dynamics. We get a rough estimate of the magnetic exchange interaction of about 1 meV from the peak in the spin–spin correlation function.

We report results of a new analysis for the low-temperature structure of alpha'-NaV2O5 from synchrotron x-ray diffraction experiments. We confirm the existence of two inequivalent ladder structures in each vanadium layer. Based on our structural data we perform a bond-valence calculation for the vanadium sites in the low temperature state. Due to an asymmetric charge ordering we obtain only two different vanadium valences despite the three inequivalent sites. This explains the 51V-NMR observation of only two resonant peaks in the charge ordered phase. By use of a Slater-Koster method to obtain hopping matrix elements and cluster calculations we obtain effective vanadium-vanadium hoppings which compare well to LDA results. Using these in a cluster calculation we obtain a superexchange of 0.047 eV between electrons on neighbouring rungs of the same ladder for the undistorted phase. For the distorted phase we find a significant alternation in the shifts of the oxygen atoms along the legs of one of the two ladder types which leads to a significant exchange dimerisation \delta_J \approx 0.25., Comment: 12 pages LaTeX, 3 figures, added references

The magnetic ordering and crystal structure of iron pnictide SrFeAsF was investigated by using neutron powder diffraction method. With decreasing temperature, the tetragonal to orthorhombic phase transition is found at 180 K, while the paramagnetic to antiferromagnetic phase transition set in at 133 K. Similar to the parent compound of other iron pnictide system, the striped Fe magnetism is confirmed in antiferromagnetic phase and the Fe moment of 0.58(6) uB aligned along long a axis. The thermal expansion of orthorhombic phase of SrFeAsF is also investigated. Based on the Grueneisen approximation and Debye approximation for internal energy, the volume of SrFeAsF can be well fitted with Debye temperature of 347(5) K. The experimental atomic displacement parameters for different crystallographic sites in SrFeAsF are analyzed with Debye model. The results suggested that the expansion of FeAs layers plays an important role in determining the thermal expansion coefficient., 4 pages, 3 figures

GeTe is a material of intense topical interest due to its potential in the context of phase-change and nanowire memory devices, as a base for thermoelectric materials and as a ferroelectric. The combination of a soft optic mode and a Peierls distortion contributes large strains at the cubic - rhombohedral phase transition near 625 K and the role of these has been investigated through their influence on elastic and anelastic properties by resonant ultrasound spectroscopy. The underlying physics is revealed by softening of the elastic constants by ~30-45%, due to strong coupling of shear and volume strains with the driving order parameter and consistent with an improper ferroelastic transition which is weakly first order. The magnitude of the softening is permissive of the transition mechanism involving a significant order/disorder component. A Debye loss peak in the vicinity of 180 K is attributed to freezing of the motion of ferroelastic twin walls and the activation energy of ~0.07 eV is attributed to control by switching of the configuration of long and short Ge-Te bonds in the first coordination sphere around Ge. Precursor softening as the transition is approached from above can be described with a Vogel-Fulcher expression with a similar activation energy, which is attributed to coupling of acoustic modes with an unseen central mode that arises from dynamical clusters with local ordering of the Peierls distortion. The strain relaxation and ferroelastic behaviour of GeTe depend on both displacive and order/disorder effects but the dynamics of switching will be determined by changes in the configuration of distorted GeTe6 octahedra, with a rather small activation energy barrier.