Elastic Properties of Lithium and Sodium Amides

Russian Physics Journal - This paper presents the optimization of the structural parameters of tetragonal and orthorhombic crystals α-LiNH2 and α-NaNH2 and computation of elastic...     

Genesis of the phonon spectra of A2B4C 2 5 and A1B3C 2 6 crystals with a chalcopyrite lattice from the vibrational states of their sublattices

The phonon spectra of a series of A²B⁴C ₂ ⁵ and A¹B³C ₂ ⁶ crystals with the chalcopyrite structure are calculated in the Keating model in the basis set of polarization vectors of their sublattices. The dependences of frequency values and partial contributions of sublattices to the polarization vectors on the chemical composition are studied. The effect of the mass ratio and the strengths of covalent bonds between the compound components on the phonon spectrum formation is found.     

Chemical Bond in Metal Azides and Their Reactivity

Results of experimental investigations into the processes of slow and explosive decomposition of metal azides and the corresponding phenomenological models are analyzed. The energy-band structure and the valence electron density for actual crystallographic planes as well as the hypothetical electron density of the first unoccupied band of KN₃ and AgN₃ are calculated by the density functional method with the use of pseudopotentials. It is demonstrated that the anion-cation chemical bond in KN₃ is predominantly ionic in character, whereas a significant covalent component in AgN₃ is caused by Ag d-states. From the character of distribution of the valence and hypothetical electron densities it follows that the exciton and vibron decomposition mechanisms are improbable. The chain process is also improbable for alkali metal azides, because the gap energy for them is E _g 8 eV, which is close to the energy of azide ion decomposition Q 9–12 eV; the chain process is probable for AgN₃, because E _g(AgN₃) 3 eV. The chain process in AgN₃ is also favored by intense delocalization of the valence electron density through the hybridization of the Ag d-state and the N p-state, which causes a relatively high hole mobility.     

Local structure of the chemical bond in metal azides calculated in a basis set of Wannier functions

The local structure of the chemical bond in molecular ion crystals of alkaline and heavy metal azides (MN₃, M = Li, Tl, Ag) is discussed in terms of Wannier functions. Various schemes of localization are considered in relation to the peculiarities of electronic structure, and hybridization effects, leading to anion-cation complexes in compounds with covalent bonds, are analyzed.     

Electronic Structure of Triple Phosphides MgSiP<Subscript>2</Subscript>, ZnSiP<Subscript>2</Subscript>, and CdSiP<Subscript>2</Subscript>

Pseudopotential calculations of the electronic structure of the compounds are performed to an approximation of the density functional theory within the basis of localized orbitals. The band structure and total and projected state density are calculated, and a complete interpretation of the x-ray spectra is given.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 68–72, January 2005.     

The distribution of the valence electron density in predominantly ionic crystals with different Bravais sublattices

Based on the theory of the local-density functional, self-consistent valence electron densities are calculated for CaO with a rock-salt lattice, CaF₂ with a fluorite lattice, K₂O with an antifluorite lattice, and for their constituent sublattices. It is shown that in the crystals with different Bravais sublattices, the anionic sublattice is a framework with covalent bonds containing a metal sublattice inside of them. The coupling between the sublattices is characterized by the density difference, which is deflned as the difference between the total electron density and the densities of the individual sublattices. The density difference is found to be an order of magnitude smaller than the crystal and sublattice densities, which is evidence of weak hybridization of the sublattices and of the predominately ionic character of the bonding between them.     

Crystals with sublattices belonging to the cubic system and special features of their elementary excitation spectra

Genesis of spectra of elementary excitations of crystals comprising sublattices belonging to the cubic system is investigated. Using the methods of group theory, the Brillouin sublattice zones are transformed into the Brillouin crystal zones, and degeneration is established caused by the convolution of branches of the spectrum. The degeneration is removed with allowance for hybridization of sublattice states. The special features of the band spectra of some crystals belonging to the cubic system are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 21–27, May, 2006.