Temperature Dependence Of Phonon Spectra Research Articles

Experimental and theoretical study of anomalous temperature dependence of phonons in Y2Ti2O7 pyrochlore

We present Raman spectroscopic as well as first-principles theoretical studies of the temperature dependence of phonon spectra in the pyrochlore ${\mathrm{Y}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$. Our experimental results show that the frequencies of several phonons display anomalous softening upon cooling. Our theoretical calculations of the temperature dependence, via phonon self-energy corrections arising from cubic anharmonic interactions, lead to such anomalies but only for a couple of high-frequency phonon modes. Our work strongly supports the suggestion that anharmonic phonon-phonon interactions are strong in the pyrochlore titanates and that they are responsible for the phonon anomalies observed. The strong anharmonic effects are likely due to the presence of the vacant Wyckoff sites and the shifts of the oxygen atoms from their actual positions in the pyrochlore lattice. However, the theoretical calculations fail to explain the anomalous behavior of low and intermediate frequency modes observed in our experiments. It seems likely that a much more sophisticated, perhaps nonperturbative, treatment of the anharmonic interactions will be required for a full explanation of the anomalies.

Quantum Enhancement of Charge Density Wave in NbS2 in the Two-Dimensional Limit.

At ambient pressure, bulk 2H-NbS2 displays no charge density wave instability, which is at odds with the isostructural and isoelectronic compounds 2H-NbSe2, 2H-TaS2, and 2H-TaSe2, and in disagreement with harmonic calculations. Contradictory experimental results have been reported in supported single layers, as 1H-NbS2 on Au(111) does not display a charge density wave, whereas 1H-NbS2 on 6H-SiC(0001) endures a 3 × 3 reconstruction. Here, by carrying out quantum anharmonic calculations from first-principles, we evaluate the temperature dependence of phonon spectra in NbS2 bulk and single layer as a function of pressure/strain. For bulk 2H-NbS2, we find excellent agreement with inelastic X-ray spectra and demonstrate the removal of charge ordering due to anharmonicity. In the two-dimensional limit, we find an enhanced tendency toward charge density wave order. Freestanding 1H-NbS2 undergoes a 3 × 3 reconstruction, in agreement with data on 6H-SiC(0001) supported samples. Moreover, as strains smaller than 0.5% in the lattice parameter are enough to completely remove the 3 × 3 superstructure, deposition of 1H-NbS2 on flexible substrates or a small charge transfer via field-effect could lead to devices with dynamical switching on/off of charge order.

Lattice dynamics and thermal expansion behavior in the metal cyanidesMCN(M=Cu, Ag, Au): Neutron inelastic scattering and first-principles calculations

We report measurement of temperature dependence of phonon spectra in quasi one-dimensional metal cyanides MCN (M=Cu, Ag and Au). Ab initio lattice dynamics calculations have been performed to interpret the phonon-spectra as well as to understand the anamolous thermal expansion behavior in these compounds. We bring out the differences in the phonon mode behavior to explain the differences in the thermal expansion behavior among the three compounds. The chain-sliding modes are found to contribute maximum to the negative thermal expansion along c axis in the Cu and Ag compounds, while the same modes contribute to positive thermal expansion in the Au compound. Several low energy transverse modes lead to positive thermal expansion along a and b axis in all the compounds. The calculated elastic constants and Born effective charges are correlated with the difference in nature of bonding among these metal cyanides.

Temperature dependence of phonon spectra and structural characteristics in multiferroic LuFe2O4 system

AbstractThe phonon spectra and structural characteristics of the LuFe2O4 system were investigated using X‐ray diffraction as well as Raman and infrared (IR) spectroscopic techniques. Two step‐like anomalies of the wavenumbers of the $E_{\rm {g}}^{3}$ peak in the Raman spectra and the $E_{\rm {u}}^{3}$ peak in IR spectra, as well as some weak anomalies of the lattice parameters, were found around the ferrimagnetic and ferroelectric transition temperatures. The substitutions of Lu with Yb and Fe with Mn lead to the corresponding wavenumber shifts in the optical spectra, which can be attributed to the changes in the atomic mass and lattice parameters. Furthermore, the Raman intensity was found to be related to the changes in the local structure and electrical conductivity. These results illustrate that the structural fluctuations could effectively influence the phonon modes, and a possible interplay among the structural, magnetic, and charge‐ordering properties exists in this multiferroic system. Copyright © 2011 John Wiley & Sons, Ltd.

Relationship between phonons and thermal expansion in Zn(CN)2and Ni(CN)2from inelastic neutron scattering andab initiocalculations

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).

Effects of magnetic doping and temperature dependence of phonon dynamics inCaFe1−xCoxAsFcompounds (x=0, 0.06, and 0.12)

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 CaFe1\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12). The composition as well as temperature dependence of phonon spectra for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) compounds have been measured using time of flight IN4C and IN6 spectrometers at ILL, 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 meV to 25 meV are also found to shift towards high energies only in the 12 % Co doped CaFeAsF compound. The experimental results for CaFe\_{1-x}Co\_{x}AsF (x = 0, 0.06, 0.12) are quite different from our previous phonon studies on parent and superconducting MFe2As2 (M=Ba, Ca, Sr) where low-energy acoustic phonon modes do not react with doping, while the phonon spectra in the intermediate range from 15 to 25 K are found to soften in these compounds. We argue that stronger spin phonon interaction play an important role for the emergence of superconductivity in these compounds. The lattice dynamics of CaFe\_{1-x}Co\_{x}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 with our previous studies on CaFe2As2.

The phase diagram and optical properties of La 1− xSr xMnO 3 for x⩽0.2

We report on detailed susceptibility, magnetization and magnetoresistance studies in La 1− x Sr x MnO 3 for x⩽0.2. We arrive at a phase diagram which shows a ferromagnetic and insulating ground state for 0.1⩽ x⩽0.15, followed by a canted antiferromagnetic phase at higher temperatures. The ferromagnetic transition is strongly coupled to a structural transition from a Jahn–Teller distorted phase to a pseudocubic orthorhombic phase, and is accompanied by the appearance of large positive magnetoresistance effects. We interpret the rich phase diagram close to x= 1 8 in terms of orbital ordering, due to an interplay of superexchange interactions and Jahn–Teller distortions. In addition, we report on the temperature dependence of phonon spectra and on optical conductivities. The temperature dependence of bending and stretching modes clearly indicate the structural and magnetic-phase transitions. The optical conductivities are explained in terms of small polaron absorption.

Raman Study of Molecular Motions in Organic Solids

Abstract The paper presents Raman investigation of the nature of phonon motions in various organic solids and discusses chemical, physical as well as some practical applications of phonon studies. In order to characterize the nature of these motions. delocalization and relaxation of phonons are studied using the mixed crystal technique and temperature dependence of phonon spectra. The application of phonon spectra to investigate chemical perturbation of crystal structure in a group of closely related compounds is discussed. Some preliminary studies are presented on application of phonon spectra in solid state reactions. In regard to practical applications, it is shown that phonon spectra can be used to identify truly random solid solutions as well as various polymorphic forms of a given compound.