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Register a new userPhonons and Anomalous Lattice Behaviour in KMnAg3(CN)6 and KNiAu3(CN)6: Inelastic Neutron Scattering and First-Principles Calculations
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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. KMnAg3(CN)6 and KNiAu3(CN)6. We have studied the role of van der Waals dispersion and magnetic interactions on structural stability of these compounds. The behavior of these compounds under isotropic compression shows the presence of negative linear compressibility. The calculated phonon spectra, validated by inelastic neutron scattering measurements and elastic constants are used to study the negative thermal expansion behavior which is found to arise from low energy phonon modes involving the folding of A-NC-B-CN-A linkage about B atoms.
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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.
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 ({alpha}V = -51 x 10-6 K-1) while for Ni(CN)2, a tetragonal material, the thermal expansion coefficient is negative in the two dimensionally connected sheets ({alpha}a=-7 x 10-6 K-1), but the overall thermal expansion coefficient is positive ({alpha}V=48 x 10-6 K-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, {alpha}V, are used to understand the anomalous behavior in these compounds. Our ab-initio calculations indicate that it is the low-energy rotational modes in Zn(CN)2, which are shifted to higher energies in Ni(CN)2, that are responsible for the large negative thermal expansion. 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).
We present temperature dependent inelastic neutron scattering measurments, accompanied byab-initio calculations of phonon spectra and elastic properties as a function of pressure to understand anharmonicity of phonons and to study the mechanism of negative thermal expansion and negative linear compressibility behaviour of ZnAu2(CN)4. The mechanism is identified in terms of specific anharmonic modes that involve bending of the Zn(CN)4-Au- Zn(CN)4 linkage. The high-pressure phase transition at about 2 GPa is also investigated and found to be related to softening of a phonon mode at the L-point at the Brillouin zone boundary and its coupling with a zone-centre phonon and an M-point phonon in the ambient pressure phase. Although the phase transition is primarily driven by a L-point soft phonon mode, which usually leads to a second order transition with a 2 x 2 x 2 supercell, in the present case the structure is close to an elastic instability that leads to a weakly first order transition.
We report an inelastic x-ray scattering investigation of phonons in FeSe superconductor. Comparing the experimental phonon dispersion with density functional theory (DFT) calculations in the non-magnetic state, we found a significant disagreement between them. Improved overall agreement was obtained by allowing for spin-polarization in the DFT calculations, despite the absence of magnetic order in the experiment. This calculation gives a realistic approximation, at DFT level, of the disordered paramagnetic state of FeSe, in which strong spin fluctuations are present.
We report detailed temperature-dependent inelastic neutron scattering and ab-initio lattice dynamics investigation of magnetic perovskites YCrO3 and LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant changes with temperature and dominates at low momentum transfer regime. Ab-inito calculations performed including magnetic interactions show that the effect of magnetic interaction is very signicant on the low- as well as high-energy phonon modes. We have also shown that the inelastic neutron spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic system, which is consistent with previously reported magnetic structure in the compound. The ab-initio lattice dynamics calculations in both the compounds exhibit anisotropic thermal expansion behaviour in the orthorhombic structure and predict negative thermal expansion along the crystallographic a-axis at low temperatures. We identify the anharmonic phonon modes responsible for this anamolous behaviour in LaCrO3 involving low-energy La vibrations and distortions of the CrO6 octahedra.
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