Lattice Dynamics

Although three forms of lithium phosphate are known, a complete structural description of the highest temperature polymorph has not yet been given. In the present work, the phase transitions of lithium phosphate were investigated at high temperatures using Raman microscopy and x-ray powder diffraction. Both transitions were observed by following the temperature dependence of the totally symmetrical Raman stretching vibration of PO43−. Currently available structural information on the α form, resulting in P—O bond lengths of 1.787–1.899 Å, as determined by valence bond calculations, are disputed here. A correlation between Raman wavenumber and bond length in inorganic phosphates estimates the P—O bond length in α-Li3PO4 to be around 1.57(1) Å, which is in closer agreement with values for other orthophosphates of between 1.50 and 1.58 Å. Copyright © 2002 John Wiley & Sons, Ltd.

We investigate the binding surface along the Bain path and phonon dispersion relations for the cubic phase of the ferromagnetic binary alloys Fe3X (X = Ni, Pd, Pt) for L12 and DO22 ordered phases from first principles by means of density functional theory. The phonon dispersion relations exhibit a softening of the transverse acoustic mode at the M-point in the L12-phase in accordance with experiments for ordered Fe3Pt. This instability can be associated with a rotational movement of the Fe-atoms around the Ni-group element in the neighboring layers and is accompanied by an extensive reconstruction of the Fermi surface. In addition, we find an incomplete softening in [111] direction which is strongest for Fe3 Ni. We conclude that besides the valence electron density also the specific Fe-content and the masses of the alloying partners should be considered as parameters for the design of Fe-based functional magnetic materials.

A force field for the modeling of hydroxyapatite; Ca10(PO4)6OH2 (HAP) is established based upon transferable potentials. Ca–O, P–O and O–O potentials were transported from those previously published for fluorapatite and based on single crystal experimental data. The interactions of hydroxyl oxygen with calcium and phosphate were re-scaled by fitting to experimental data for CaO and AlPO4, respectively, to account for the reduction in the oxygen charge from −2.0 to −1.426. Force field accuracy is tested by comparing the calculated and experimental values for the cell constant and atom positions in the unit cell. The elastic constants and bulk modulus calculated for HAP are in close agreement with the experimental results. The potentials were also used to calculate the compressibility data of HAP and fluorapatite, and these results also agree with the published experimental data. Using formal charges for metal cations allows modeling the complete solid solution of Cd–Ca hydroxyapatite with a good accuracy.

Lattice dynamics of body-centered cubic (bcc) Vb-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated

The results of the thermodynamical functions such as lattice specific heat at constant volume, entropy, Debye temperature, internal energy and vibrational free energy with temperature are presented to understand the thermal behaviour and performance of three indium pnic-tide compounds in wurtzite phase. We have used a first-principles method based on density functional theory and quasi-harmonic approximation. Phonon dispersion curves show no imaginary frequency for any phonon modes throughout the Brillouin zone which confirms the dynam-ical stability of these compounds in wurtzite phase. Our calculated vibrational frequency matches well with experimental Raman spectra of corresponding wurtzite nano-wires. Internal energy and vibrational contribution of Helmholtz free energy, respectively, increases and decreases with temperature. An increase with temperature is observed for entropy. The variation of specific heat at constant volume and Debye temperature is also presented and correlated with speed of phonons and phonon mean free path. The characteristic features of temperature variation of considered thermodynamic functions are in line with those of other crystalline semiconductors.