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Register a new userPhonons in the beta-tin, Imma, and sh phases of Silicon from ab initio calculations
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Added by
Katalin Ga\\'al-Nagy
Publication date
2005
fields
Physics
and research's language is
English
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We present a new interpretation of measured Raman frequencies of a high-pressure structure of Silicon which was assigned previously to the beta-tin phase. Our results show that the beta-tin->Imma->sh phase transitions have been already indicated in this experiment which was performed before the discovery of the Imma phase. We have calculated phonon-dispersion curves for the beta-tin, Imma, and sh phases of silicon using the plane-wave pseudopotential approach to the density-functional theory and the density-functional perturbation theory within the local density approximation. With the new assignment, the calculated phonon frequencies display an excellent agreement with the experimental data, and can be also used to determine precisely the transition pressure for the Imma->beta-tin phase transition. The sh->Imma transition is accompanied by soft modes.
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We have investigated the structural sequence of the high-pressure phases of silicon and germanium. We have focussed on the cd->beta-tin->Imma->sh phase transitions. We have used the plane-wave pseudopotential approach to the density-functional theory implemented within the Vienna ab-initio simulation package (VASP). We have determined the equilibrium properties of each structure and the values of the critical parameters including a hysteresis effect at the phase transitions. The order of the phase transitions has been obtained alternatively from the pressure dependence of the enthalpy and of the internal structure parameters. The commonly used tangent construction is shown to be very unreliable. Our calculations identify a first-order phase transition from the cd to the beta-tin and from the Imma to the sh phase, and they indicate the possibility of a second-order phase-transition from the beta-tin to the Imma phase. Finally, we have derived the enthalpy barriers between the phases.
We investigate the pressure-induced metal-insulator transition from diamond to beta-tin in bulk Silicon, using quantum Monte Carlo (QMC) and density functional theory (DFT) approaches. We show that it is possible to efficiently describe many-body effects, using a variational wave function with an optimized Jastrow factor and a Slater determinant. Variational results are obtained with a small computational cost and are further improved by performing diffusion Monte Carlo calculations and an explicit optimization of molecular orbitals in the determinant. Finite temperature corrections and zero point motion effects are included by calculating phonon dispersions in both phases at the DFT level. Our results indicate that the theoretical QMC (DFT) transition pressure is significantly larger (smaller) than the accepted experimental value. We discuss the limitation of DFT approaches due to the choice of the exchange and correlation functionals and the difficulty to determine consistent pseudopotentials within the QMC framework, a limitation that may significantly affect the accuracy of the technique.
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).
Crystalline phases formed in stoichiometric Zr$_9$Ni$_{11}$ and Hf$_9$Ni$_{11}$ have been studied by perturbed angular correlation (PAC) spectroscopy, XRD and TEM/SAED measurements. In Zr$_9$Ni$_{11}$, the phases Zr$_9$Ni$_{11}$ ($sim$89%) and Zr$_8$Ni$_{21}$ ($sim$11%) have been found at room temperature from PAC measurements. At 773 K, Zr$_9$Ni$_{11}$ partially decomposes to Zr$_7$Ni$_{10}$ and at 973 K, it is completely decomposed to ZrNi and Zr$_7$Ni$_{10}$. In Hf$_9$Ni$_{11}$, a predominant phase ($sim$81%) due to HfNi is found at room temperature while the phase Hf$_9$Ni$_{11}$ is produced as a minor phase ($sim$19%). No compositional phase change at higher temperature is found in Hf$_9$Ni$_{11}$. Phase components found from XRD and TEM/SAED measurements are similar to those observed from PAC measurements. Electric field gradients in Zr$_9$Ni$_{11}$ and Hf$_9$Ni$_{11}$ have been calculated by density functional theory (DFT) using all electron full potential (linearized) augmented plane wave plus local orbitals [FP-(L)APW+lo] method in order to assign the phase components.
The four A1-TO Gamma phonon frequencies in lithium tantalate are calculated in the frozen-phonon approach from first principles using the full-potential linearized augmented plane wave method. A good agreement with the experimental data available is found for all modes; reliable displacement pattern of different modes becomes available from the calculated eigenvectors. The Raman spectra recorded for A1 modes in LiNbO3 exhibit a counter-intuitive softening of the A1-TO3 mode frequency with respect to that measured in LiTaO3. We explain this behaviour by a comparatively harder oxygen rotation in LiTaO3 and discuss other differences in lattice dynamics of these two materials, namely a notably delocalization of Ta and Li contributions over more that one corresponding mode in LiTaO3, which is different from the situation in lithium niobate. The Li isotope shift is predicted in the calculation.
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