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Cyanide based framework compounds are known to show large negative thermal expansion behaviour. Here we report the phonon and anomalous lattice behavior of two metal cyanide framework compounds i.e...

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.

The structural dynamics of a quasi-one-dimensional (1D) mixed-metal cyanide, Cu1/3Ag1/3Au1/3CN, with intriguing thermal properties is explored. All the current known related compounds with straight-chain structures, such as group 11 cyanides CuCN, AgCN, AuCN and bimetallic cyanides MxM’1-xCN (M, M’ = Cu, Ag, Au), exhibit 1D negative thermal expansion (NTE) along the chains and positive thermal expansion (PTE) perpendicular to them. Cu1/3Ag1/3Au1/3CN exhibits similar PTE perpendicular to the chains, however PTE, rather than NTE, is also observed along the chains. In order to understand the origin of this unexpected behavior, inelastic neutron scattering (INS) measurements were carried out, underpinned by lattice-dynamical density-functional-theory (DFT) calculations. Synchrotron-based pair-distribution-function (PDF) analysis and 13C solid-state nuclear-magnetic-resonance (SSNMR) measurements were also performed to build an input structural model for the lattice dynamical study. The results indicate that transverse motions of the metal ions are responsible for the PTE perpendicular to the chains, as is the case for the related group 11 cyanides. However NTE along the chain due to the tension effect of these transverse motions is not observed. As there are different metal-to-cyanide bond lengths in Cu1/3Ag1/3Au1/3CN, the metals in neighboring chains cannot all be truly co-planar in a straight-chain model. For this system, DFT-based phonon calculations predict small PTE along the chain due to low-energy chain-slipping modes induced by a bond-rotation effect on the weak metallophilic bonds. However the observed PTE is greater than that predicted with the straight-chain model. Small bends in the chain to accommodate truly co-planar metals provide an alternative explanation for thermal behavior. These would mitigate the tension effect induced by the transverse motions of the metals and, as temperature increases and the chains move further apart, a straightening could occur resulting in the observed PTE. This hypothesis is further supported by unusual evolution in the phonon spectra, which suggest small changes in local symmetry with temperature.

This study explores the relationship between phonon dynamics and negative thermal expansion (NTE) in CuxNi2-x¬(CN)4. The partial replacement of nickel (II) by copper (II) in Ni(CN)2 leads to a line phase, CuNi(CN)4 (x = 1), and a solid solution, CuxNi2-x¬(CN)4 (0 ≤ x ≤ 0.5). CuNi(CN)4 adopts a layered structure related to that of Ni(CN)2¬ (x = 0), and interestingly exhibits 2D NTE which is ~ 1.5 times larger. Inelastic neutron scattering (INS) measurements combined with first principles lattice dynamical calculations provide insights into the effect of Cu2+ on the underlying mechanisms behind the anomalous thermal behavior in all the CuxNi2-x¬(CN)4 compounds. The solid solutions are presently reported to also show 2D NTE. The INS results highlight that as the Cu2+ content increases in CuxNi2-x(CN)4, large shifts to lower energies are observed in modes consisting of localized in- and out-of-plane librational motions of the CN ligand, which contribute to the NTE in CuNi(CN)4. Mode Gruneisen parameters calculated for CuNi(CN)4 show that acoustic and low-energy optic modes contribute the most to the NTE, as previously shown in Ni(CN)2. However, mode eigenvectors reveal a large deformation of the [CuN4] units compared to the [NiC4] units, resulting in phonon modes not found in Ni(CN)2, whose NTE-driving phonons consist predominately of rigid-unit modes. The deformations in CuNi(CN)4 arise because the d9 square-planar center is easier to deform than the d8 one, resulting in a greater range of out-of-plane motions for the adjoining ligands.

ZnNi(CN)4 is a 3-D framework material consisting of two interpenetrating PtS-type networks in which tetrahedral [ZnN4] units are linked by square-planar [NiC4] units. Both the parent compounds, cubic Zn(CN)2 and layered Ni(CN)2, are known to exhibit 3-D and 2-D negative thermal expansion (NTE), respectively. Temperature-dependent inelastic neutron scattering (INS) measurements were performed on a powdered sample of ZnNi(CN)4 to probe phonon dynamics. The measurements were underpinned by ab-initio lattice dynamical calculations. Good agreement was found between the measured and calculated generalized phonon density-of-states, validating our theoretical model and indicating that it is a good representation of the dynamics of the structural units. The calculated linear thermal expansion coefficients are αa = -21.2 × 10-6 K-1 and αc = +14.6 × 10-6 K-1, leading to an overall volume expansion coefficient, αV of -26.95 × 10-6 K-1, pointing towards pronounced NTE behaviour. Analysis of the derived mode-Gruneisen parameters shows that the optic modes around 12 and 40 meV make a significant contribution to the NTE. These modes involve localised rotational motions of the [NiC4] and/or [ZnN4] rigid units, echoing what has previously been observed in Zn(CN)2 and Ni(CN)2. However, in ZnNi(CN)4, modes below 10 meV have the most negative Gruneisen parameters. Analysis of their eigenvectors reveals that a large transverse motion of the Ni atom in the direction perpendicular to its square-planar environment induces a distortion of the units. This mode is a consequence of the Ni atom being constrained only in two dimensions within a 3-D framework. Hence, although rigid-unit modes account for some of the NTE-driving phonons, the added degree of freedom compared with Zn(CN)2 results in modes with twisting motions, capable of inducing greater NTE.

We have carried out temperature-dependent inelastic neutron scattering measurements of YMnO3 over the temperature range of 50–1303 K, covering both the antiferromagnetic to paramagnetic transition (70 K), as well as the ferroelectric to paraelectric transition (1258 K). Measurements are accompanied by first principles calculations of phonon spectra for the sake of interpretation and analysis of the measured phonon spectra in the room temperature ferroelectric (P63cm) and high temperature paraelectric (P63/mmc) hexagonal phases of YMnO3. The comparison of the experimental and first-principles calculated phonon spectra highlights unambiguously a spin-phonon coupling character in YMnO3. This is further supported by the pronounced differences in the magnetic and non-magnetic phonon calculations. The calculated atomistic partial phonon contributions of the Y and Mn atoms are not affected by the inclusion of magnetic interactions, whereas the dynamical contribution of the O atoms is found to change. This highlights the role of the super-exchange interactions between the magnetic Mn cations, mediated by O bridges. Phonon dispersion relations have also been calculated, in the entire Brillouin zone, for both the hexagonal phases. In the high-temperature phase, unstable phonon mode at the K point is highlighted. The displacement pattern at the K-point indicates that the freezing of this mode along with the stable mode at the Γ-point may lead to a stabilization of the low-temperature (P63cm) phase, inducing ferroelectricity. Furthermore, we have also estimated the mode Gruneisen parameter and volume thermal expansion behavior. The latter is found to agree with the available experimental data.

We present temperature-dependent inelastic-neutron-scattering measurements, accompanied by ab initio calculations of the phonon spectra and elastic properties as a function of pressure to quantitatively explain an unusual combination of negative thermal expansion and negative linear compressibility behavior of ${\mathrm{ZnAu}}_{2}{(\mathrm{CN})}_{4}$. The mechanism of the negative thermal expansion is identified in terms of specific anharmonic phonon modes that involve bending of the -Zn-NC-Au-CN-Zn- linkage. The soft phonon at the $L$ point at the Brillouin zone boundary quantitatively relates to the high-pressure phase transition at about 2 GPa. The ambient pressure structure is also found to be close to an elastic instability that leads to a weakly first-order transition.

We report inelastic neutron scattering measurements of phonon spectra in the parent superconductor iron-tuned chalcogenide $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Te}$ for two different $x$ contents $(x\ensuremath{\le}0.11)$ using neutron time-of-flight technique. Thermal neutron spectroscopy allowed the collection of the low-temperature Stokes spectra over an extended $Q$ range at 2, 40, and 120 K, hence covering both the magnetic monoclinic and the paramagnetic tetragonal phases, whereas cold neutrons allowed the measurement of high-resolution anti-Stokes spectra at 140, 220, and 300 K, thus covering the tetragonal phase. Our results evidence a spin-phonon coupling behavior towards the observed noticeable temperature-dependent change of the Stokes spectra across the transition temperatures. On the other hand, the anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic region of the phonon spectrum upon heating, indicating a strong anharmonicity and a subtle dependence of phonons on structural evolution within the tetragonal phase. Experimental results are accompanied by ab initio calculations of phonon spectra of the tetragonal stoichiometric phase for a comparison with the high-resolution anti-Stokes spectra. Calculations included different density functional methods. Spin polarization and van der Waals interaction were either considered or neglected, individually or concomitantly, in order to study their respective effect on lattice dynamics description. Our results suggest that including van der Waals interaction has only a slight effect on phonon dynamics; however, phonon spectra are better described when spin polarization is included in a cooperative way with van der Waals interactions.

We report the results of inelastic neutron scattering investigation on the model antiferromagnet CoF 2 by time-of-flight neutron spectroscopy. We measured the details of the scattering function S ( Q , ω ) as a function of temperature with two different incident neutron wavelengths. The temperature and Q dependence of the measured scattering function suggests the presence of magnon–phonon coupling in almost all branches. The present results are in agreement with the strong magnetoelastic effects observed previously.

Some selected applications of spin-polarized density functional theory are presented to illustrate the robustness of the applied approaches to help interpretation and to ellucidate some observed, intriguing magnetic behavior, as highlighted by recent neutron scattering experiments. Both structure and dynamics are concerned through four examples covering the elastic and inelastic neutron scattering processes and dealing with the case studies of some complex transition metal containing inorganic materials. In the first two examples, molecular cluster calculations of the magnetic exchange coupling constant and the evaluation of the spin density distribution are shown. Further, periodic simulations of the magnetic bands and the spin-orbit coupling are introduced. Finally, the role of the magnetic interactions in the inelastic case is illustrated through investigating the corresponding effect on phonon dynamics relevant to superconductivity.

β-Eucryptite (LiAlSiO4) shows anisotropic thermal expansion as well as one-dimensional super-ionic conductivity. We have performed the lattice dynamical calculations using ab-initio density functional theory along with inelastic neutron scattering measurements. The anisotropic stress dependence of the phonon spectrum is calculated to obtain the thermal expansion behavior along various axes. The calculations show that the Gruneisen parameters of the low-energy phonon modes around 10 meV have large negative values and govern the negative thermal expansion behavior at low temperatures along both the “a”- and “c”-axes. On the other hand, anisotropic elasticity along with anisotropic positive values of the Gruneisen parameters of the high-energy modes in the range 30–70 meV are responsible for the thermal expansion at high temperatures, which is positive in the a-b plane and negative along the c-axis. The analysis of the polarization vectors of the phonon modes sheds light on the mechanism of the anomalous the...

We have carried out an extensive phonon study on multiferroic ${\mathrm{GaFeO}}_{3}$ to elucidate its dynamical behavior. Inelastic neutron scattering measurements are performed over a wide temperature range, 150 to 1198 K. First principles lattice dynamical calculations are done for the sake of the analysis and interpretation of the observations. The comparison of the phonon spectra from magnetic and nonmagnetic calculations highlights pronounced differences. The energy range of the vibrational atomistic contributions of the Fe and O ions are found to differ significantly in the two calculation types. Therefore, magnetism induced by the active spin degrees of freedom of Fe cations plays a key role in stabilizing the structure and dynamics of ${\mathrm{GaFeO}}_{3}$. Moreover, the computed enthalpy in various phases of ${\mathrm{GaFeO}}_{3}$ is used to gain deeper insights into the high-pressure phase stability of this material. Further, the volume dependence of the phonon spectra is used to determine its thermal expansion behavior.

Sodium niobate (NaNbO3) exhibits most complex sequence of structural phase transitions in perovskite family and therefore provides as excellent model system for understanding the mechanism of structural phase transitions. We report temperature dependence of inelastic neutron scattering measurements of phonon densities of states in sodium niobate. The measurements are carried out in various crystallographic phases of this material at various temperatures from 300 K to 1048 K. The phonon spectra exhibit peaks centered around 19, 37, 51, 70 and 105 meV. Interestingly, the peak around 70 meV shifts significantly towards lower energy with increasing temperature, while the other peaks do not exhibit an appreciable change. The phonon spectra at 783 K show prominent change and become more diffusive as compared to those at 303 K. In order to better analyze these features, we have performed first principles lattice dynamics calculations based on the density functional theory. The computed phonon density of states is found to be in good agreement with the experimental data. Based on our calculation we are able to assign the characteristic Raman modes in the antiferroelectric phase to the A1g symmetry, which are due to the folding of the T (w=95 cm-1) and delta(w=129 cm-1) points of the cubic Brillouin zone., 19 pages, 8 figures, 2 tables

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 have measured phonon spectra of Ag3M(CN)6 (M=Co,Fe) using neutron time-of-flight spectroscopy as a function of temperature and pressure. Ab-initio calculations were carried out for the sake of analysis and interpretation. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes within the energy range 2 to 5 meV are strongly anharmonic and major contributors to thermal expansion in both systems. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes.

Correction for ‘Spin-phonon coupling and high-temperature phase transition in multiferroic material YMnO3’ by Mayanak Kumar Gupta et al., J. Mater. Chem. C, 2015, 3, 11717–11728.

Recently colossal positive volume thermal expansion has been found in the framework compounds Ag3Co(CN)6 and Ag3Fe(CN)6. Phonon spectra have been measured using the inelastic neutron scattering technique as a function of temperature and pressure. The data has been analyzed using ab-initio calculations. We find that the bonding is very similar in both compounds. At ambient pressure modes in the intermediate frequency part of the vibrational spectra in the Co compound are shifted to slightly higher energies as compared to the Fe compound. The temperature dependence of the phonon spectra gives evidence for large explicit anharmonic contribution to the total anharmonicity for low-energy modes below 5 meV. We found that modes are mainly affected by the change in the size of unit cell, which in turn changes the bond lengths and vibrational frequencies. Thermal expansion has been calculated via the volume dependence of phonon spectra. Our analysis indicates that Ag phonon modes in the energy range from 2 to 5 meV are strongly anharmonic and major contributors to thermal expansion in both compounds. The application of pressure hardens the low-energy part of the phonon spectra involving Ag vibrations and confirms the highly anharmonic nature of these modes., Comment: 19 pages, 14 figures and one table

Zn (CN)${}_{2}$ and Ni(CN)${}_{2}$ are known for exhibiting anomalous thermal expansion over a wide temperature range. The volume thermal expansion coefficient for the cubic, three-dimensionally connected material, Zn(CN)${}_{2}$, is negative (${\ensuremath{\alpha}}_{V}=\ensuremath{-}51\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6} {\mathrm{K}}^{\ensuremath{-}1}$) while for Ni(CN)${}_{2}$, a tetragonal material, the thermal expansion coefficient is negative in the two-dimensionally connected sheets (${\ensuremath{\alpha}}_{a}=\ensuremath{-}7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6} {\mathrm{K}}^{\ensuremath{-}1}$), but the overall thermal expansion coefficient is positive (${\ensuremath{\alpha}}_{V}=48\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6} {\mathrm{K}}^{\ensuremath{-}1}$). We have measured the temperature dependence of phonon spectra in these compounds and analyzed them using ab initio calculations. The spectra of the two compounds show large differences that cannot be explained by simple mass renormalization of the modes involving Zn (65.38 amu) and Ni (58.69 amu) atoms. This reflects the fact that the structure and bonding are quite different in the two compounds. The calculated pressure dependence of the phonon modes and of the thermal expansion coefficient, $\ensuremath{\alpha}$${}_{V}$, are used to understand the anomalous behavior in these compounds. Our ab initio calculations indicate that phonon modes of energy \ensuremath{\sim} meV are major contributors to negative thermal expansion (NTE) in both compounds. The low-energy modes of \ensuremath{\sim}8 and 13 meV also contribute significantly to the NTE in Zn(CN)${}_{2}$ and Ni(CN)${}_{2}$, respectively. The measured temperature dependence of the phonon spectra has been used to estimate the total anharmonicity of both compounds. For Zn(CN)${}_{2}$, the temperature-dependent measurements (total anharmonicity), along with our previously reported pressure dependence of the phonon spectra (quasiharmonic), is used to separate the explicit temperature effect at constant volume (intrinsic anharmonicity).

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 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 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.

International audience; The filled skutterudite CePt4Ge12 is reported to be at the border between an intermediate valence (IV) state and a Kondo lattice behavior. We report here the first inelastic neutron scattering experiments, carried out on the Ce compound and its non-magnetic counterpart LaPt4Ge12. Prior the neutron studies, the structural and magnetic properties of both samples have been determined. Inelastic neutron scattering spectra are dominated by phonon contribution for both compounds. A rattling vibration mode is observed at 6.9 meV and 7.5 meV for the Ce and La compounds, respectively. In CePt4Ge12 no crystal field excitation could be resolved. Only a small and wide extra contribution at high energy (22-28 meV) is observed. This result would be more consistent with an IV behavior.

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.