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Lithium tantalate (LiTaO3) has large technological importance due to numerous applications, especially for motion sensors, radiation detectors, and optical waveguides. In some manufacturing processes temperature treatments are applied to obtain the desired structure or composition. However, the temperature treatment influences the optical and electrical properties of the material due to the incorporation of defects. Unfortunately, the exact nature and concentration of these defects are only insufficiently known. In the present study, temperature-induced defect incorporation in congruent LiTaO3 crystals under vacuum at 1224 K is investigated. Inelastic neutron spectra are measured in situ to identify the point defects and separate them from further contributions of structural phase transition as well as ferroelectric domain structure. Inelastic neutron scattering data are analysed with respect to small defect concentrations by comparing with density functional theory results. The influence of ferroelectric domains is analysed by Raman and FT-IR spectroscopy. Combining the results of the different techniques, we will elucidate the role of and vacancies during the annealing of congruent LiTaO3.

Fast-ion conductors are critical to the development of solid-state batteries. The effects of mechanochemical synthesis that lead to increased ionic conductivity in an archetypical sodium-ion conductor Na3PS4 are not fully understood. We present here a comprehensive analysis based on diffraction (Bragg, pair distribution function), spectroscopy (impedance, Raman, NMR, INS) and ab-initio simulations aimed at elucidating the synthesis-property relationships in Na3PS4. We consolidate previously reported interpretations about the local structure of ball-milled samples, underlining the sodium disorder and showing that a local tetragonal framework more accurately describes the structure than the originally proposed cubic one. Through variable-pressure impedance spectroscopy measurements, we report for the first time the activation volume for Na+ migration in Na3PS4, which is ~30% higher for the ball-milled samples. Moreover, we show that the effect of ball-milling on increasing the ionic conductivity of Na3PS4 to ~10-4 S/cm can be reproduced by applying external pressure on a sample from conventional high temperature ceramic synthesis. We conclude that the key effects of mechanochemical synthesis on the properties of solid electrolytes can be analyzed and understood in terms of pressure, strain and activation volume.

β-Eucryptite (LiAlSiO4) is known to show super-ionic conductivity above 700 K. We performed inelastic neutron scattering measurements in β-eucryptite over 300-900 K and calculated the phonon spectrum using classical molecular dynamics (MD) simulations. The MD simulations were used to interpret the inelastic neutron spectra at high temperatures. The calculated diffusion coefficient for Li showed superionic conduction above 1200 K in the perfect crystal. The presence of defects was found to enhance diffusion and lower the temperature for Li diffusion. The calculated trajectory of Li atoms at higher temperatures shows that preferential movement of the Li atom is along the hexagonal c-axis, which is further confirmed by the ab initio calculated activation energy profile for cooperative lithium ion displacements. The inter- and intra-channel correlated motion of Li along the hexagonal c-axis gives the minimum energy pathway for Li ion conduction in LiAlSiO4.

We report measurements of the temperature dependence of phonon densities of states in K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ using inelastic neutron scattering technique. While cooling down to 150 K, a phonon peak splitting \ensuremath{\sim}25 meV is observed and a new peak appears at 31 meV. The measurements support the recent Raman and infrared measurements, indicating a lowering of symmetry of K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ upon cooling below 250 K. Ab initio phonon calculations have been carried out for K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ and KFe${}_{2}$Se${}_{2}$. The comparison of the phonon spectra as obtained from the magnetic and nonmagnetic calculations show pronounced differences. We show that in the two calculations, the energy range of the vibrational contribution from both Fe and Se are quite different. We conclude that Fe magnetism is correlated to the phonon dynamics and plays an important role in stabilizing the structure of K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$, as well as that of KFe${}_{2}$Se${}_{2}$. The calculations highlight the presence of low energy optical vibrational modes in K${}_{0.8}$Fe${}_{1.6}$Se${}_{2}$ compared to KFe${}_{2}$Se${}_{2}$.

We are interested in the phonon response in the frustrated magnets ${\text{SrCr}}_{9x}{\text{Ga}}_{12\ensuremath{-}9x}{\text{O}}_{19}$ (SCGO) and ${\text{Ba}}_{2}{\text{Sn}}_{2}{\text{ZnCr}}_{7x}{\text{Ga}}_{10\ensuremath{-}7x}{\text{O}}_{22}$ (BSZCGO). The motivation of the study is the recently discovered, phonon-driven, magnetic relaxation in the SCGO compound [Mutka et al., Phys. Rev. Lett. 97, 047203 (2006)] pointing out the importance of a low-energy $(\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}7\text{ }\text{meV})$ phonon mode. In neutron-scattering experiments on these compounds, the phonon signal is partly masked by the magnetic signal from the Cr moments and we have therefore examined in detail the nonmagnetic isostructural counterparts ${\text{SrGa}}_{12}{\text{O}}_{19}$ (SGO) and ${\text{Ba}}_{2}{\text{Sn}}_{2}{\text{ZnGa}}_{10}{\text{O}}_{22}$ (BSZGO). Our ab initio lattice-dynamics calculations on SGO reveal a peak in the vibrational density of states matching with the neutron observations on SGO and SCGO. A strong contribution in the vibrational density of states comes from the partial contribution of the Ga atoms on the $2b$ and $12k$ sites, involving modes at the M point of the hexagonal system. These modes comprise dynamics of the Kagom\'e planes of the pyrochlore slab magnetic sublattice, $12k$ sites, and therefore can drive magnetic relaxation via spin-phonon coupling. Both BSZCGO and BSZGO show a similar low-energy Raman peak but no corresponding peak in the neutron-determined density of states of BSZGO is seen. However, a strong non-Debye enhancement of low-energy phonon response is observed. We attribute this particular feature to the Zn/Ga disorder on the $2d$ site, already evoked earlier to affect the magnetic properties of BSZCGO. We propose that this disorder-induced phonon response explains the absence of a characteristic energy scale and the much faster magnetic relaxation observed in BSZCGO.

We report detailed measurements of composition as well as temperature dependence of the phonon density of states in a new series of FeAs compounds with composition ${\text{CaFe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{AsF}$ $(x=0,0.06,0.12)$. The electronic-structure calculations for these compounds show that bands near the Fermi level are mainly formed by $\text{Fe}\text{ }3d$ states, which is quite different from other 122 and 1111 FeAs compounds where both Fe and As are believed to be related to superconductivity. The difference in electronic structure for fluorine-based compounds may cause phonon spectra to behave differently as a function of composition and temperature in comparison with our previous phonon studies on parent and superconducting $M{\text{Fe}}_{2}{\text{As}}_{2}$ $(M=\text{Ba},\text{Ca},\text{Sr})$. The composition as well as temperature dependence of phonon spectra for ${\text{CaFe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{AsF}$ $(x=0,0.06,0.12)$ compounds have been measured using time-of-flight IN4C and IN6 spectrometers at Institut Laue Langevin, France. The comparison of phonon spectra at 300 K in these compounds shows that acoustic phonon modes up to 12 meV harden in the doped compounds in comparison to the parent CaFeAsF. While intermediate-energy phonon modes from 15 to 25 meV are also found to shift toward high energies only in the 12% Co-doped CaFeAsF compound. The experimental results for ${\text{CaFe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{AsF}$ $(x=0,0.06,0.12)$ are quite different from our previous phonon studies on parent and superconducting $M{\text{Fe}}_{2}{\text{As}}_{2}$ $(M=\text{Ba},\text{Ca},\text{Sr})$ where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 meV are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role in the emergence of superconductivity in these compounds. The lattice dynamics of ${\text{CaFe}}_{1\ensuremath{-}x}{\text{Co}}_{x}\text{AsF}$ $(x=0,0.06,0.12)$ compounds is also investigated using the ab initio as well as shell-model phonon calculations. We show that the nature of the interaction between the Ca and the Fe-As layers in CaFeAsF compounds is quite different compared to our previous studies on ${\text{CaFe}}_{2}{\text{As}}_{2}$.

We report the pressure and temperature dependence of the phonon density-of-states in superconducting ${\text{Ca}}_{0.6}{\text{Na}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ $({T}_{c}=21\text{ }\text{K})$ and the parent compound ${\text{CaFe}}_{2}{\text{As}}_{2}$ using inelastic neutron scattering. We observe no significant change in the phonon spectrum for ${\text{Ca}}_{0.6}{\text{Na}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$ at 295 K up to pressures of 5 kbar. The phonon spectrum for ${\text{CaFe}}_{2}{\text{As}}_{2}$ shows softening of the low-energy modes by about 1 meV when decreasing the temperature from 300 to 180 K. There is no appreciable change in the phonon density of states across the structural and antiferromagnetic phase transition at 172 K. These results, combined with our earlier temperature dependent phonon density of states measurements for ${\text{Ca}}_{0.6}{\text{Na}}_{0.4}{\text{Fe}}_{2}{\text{As}}_{2}$, indicate that the softening of low-energy phonon modes in these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. The phonon spectra are analyzed with ab initio and empirical potential calculations giving partial densities of states and dispersion relations.

We present a combination of ab initio calculations, magnetic Compton scattering, and polarized neutron experiments, which elucidate the density distribution of unpaired electrons in the kagome staircase system ${\text{Co}}_{3}{\text{V}}_{2}{\text{O}}_{8}$. Ab initio wave functions were used to calculate the spin densities in real and momentum spaces, which show good agreement with the respective experiments. It has been found that the spin polarized orbitals are equally distributed between the ${t}_{2g}$ and the ${e}_{g}$ levels for the spine $(s)$ Co ions while the ${e}_{g}$ orbitals of the cross-tie $(c)$ Co ions only represent 30% of the atomic spin density. Furthermore, the results reveal that the magnetic moments of the cross-tie Co ions, which are significantly smaller than those of the spine Co ions in the zero-field ferromagnetic structure, do not saturate by applying an external magnetic field of 2 T along the easy axis $a$. In turn, the increasing bulk magnetization, which can be observed by field dependent macroscopic measurements, originates from induced magnetic moments on the O and V sites. The refined individual magnetic moments are $\ensuremath{\mu}({\text{Co}}_{c})=1.54(4){\ensuremath{\mu}}_{B}$, $\ensuremath{\mu}({\text{Co}}_{s})=2.87(3){\ensuremath{\mu}}_{B}$, $\ensuremath{\mu}(\text{V})=0.41(4){\ensuremath{\mu}}_{B}$, $\ensuremath{\mu}(\text{O}1)=0.05(5){\ensuremath{\mu}}_{B}$, $\ensuremath{\mu}(\text{O}2)=0.35(5){\ensuremath{\mu}}_{B}$, and $\ensuremath{\mu}(\text{O}3)=0.36(5){\ensuremath{\mu}}_{B}$ combining to the same macroscopic magnetization value, which was previously only attributed to the Co ions.

We have performed extensive ab initio calculations to investigate phonon dynamics and their possible role in superconductivity in ${\text{BaFe}}_{2}{\text{As}}_{2}$ and related systems. The calculations are compared to inelastic neutron scattering data that offer improved resolution over published data [Mittal et al., Phys. Rev. B 78, 104514 (2008)], in particular at low frequencies. Effects of structural phase transition and full and/or partial structural relaxations, with and without magnetic ordering, on the calculated vibrational density of states are reported. Phonons are best reproduced using either the relaxed magnetic structures or the experimental cell. Several phonon branches are affected by the subtle structural changes associated with the transition from the tetragonal to the orthorhombic phase. Effects of phonon-induced distortions on the electronic and spin structure have been investigated. It is found that for some vibrational modes, there is a significant change in the electronic distribution and spin populations around the Fermi level. A peak at 20 meV in the experimental data falls into the pseudogap region of the calculation. This was also the case reported in our recent work combined with an empirical parametric calculation [Mittal et al., Phys. Rev. B 78, 104514 (2008)]. The combined evidence for the coupling of electronic and spin degrees of freedom with phonons is relevant to the current interest in superconductivity in ${\text{BaFe}}_{2}{\text{As}}_{2}$ and related systems.

The Cu4 OCl6 daca4 system composed of isolated Cu2+ S=1/2 tetrahedra with antiferromagnetic exchange should exhibit the properties of a frustrated quantum spin system. Ab initio density functional theory calculations for electronic structure and molecular dynamics computations suggest a complex interplay between magnetic exchange, electron delocalization, and molecular vibrations. Yet, extensive experimental characterization of Cu4 OCl6 daca4 by means of synchrotron x-ray diffraction, magnetization, specific heat, and inelastic neutron scattering reveal that properties of the real material can be only partly explained by proposed theoretical models as the low-temperature properties seem to be governed by a manifold of molecular configurations coexisting at high temperatures. © 2008 The American Physical Society.

We report inelastic neutron scattering measurements of the phonon spectra in a pure powder sample of the multiferroic material BiFeO3. A high-temperature range was covered to unravel the changes in the phonon dynamics across the Neel (T_N ~ 650 K) and Curie (T_C ~ 1100 K) temperatures. Experimental results are accompanied by ab-initio lattice dynamical simulations of phonon density of states to enable microscopic interpretations of the observed data. The calculations reproduce well the observed vibrational features and provide the partial atomic vibrational components. Our results reveal clearly the signature of three different phase transitions both in the diffraction patterns and phonon spectra. The phonon modes are found to be most affected by the transition at the T_C. The spectroscopic evidence for the existence of a different structural modification just below the decomposition limit (T_D ~ 1240 K) is unambiguous indicating strong structural changes that may be related to oxygen vacancies and concomitant Fe3+ to Fe2+ reduction and spin transition.

We present results of ab initio lattice dynamics calculations for the olivine mineral Ni2SiO4 using first-principles improved approaches within the Kohn–Sham formulation of density functional theory. Dispersion relationships, vibrational density of states and Raman shifts have been evaluated using the direct method. The calculations are compared with data from inelastic neutron scattering and Raman spectroscopy experiments. Non-spin-polarized calculations lead to clear structural instabilities and discrepancies with the measurements. Magnetic effects are taken into account by including first spin-polarization and then also a Hubbard term to describe correctly the strongly correlated character of the 3d electrons of Ni2+ ions. Results of these models are in excellent agreement with the observations, indicating that the structural stability and dynamics of Ni2SiO4 depend strongly on the magnetic interactions, spin–lattice coupling and electron correlation. These calculations will be used to help separate phonon and spin contributions in on-going studies of magnetic excitations.