Lattice dynamics and specific heat of thallium monoselenide

Abstract

The specific heat of thallium monoselenide has been investigated in /1 to 4/.The results of this works a r e in agreement with each other within the temperature range from 6 to 300 K. In /3/ an attempt has been made to explain the temperature dependence of the specific heat of TlSe by its structural peculiarities.According to /5/ there a r e two kinds of thallium ions in the crystalline structure of TlSe, monovalent T1+ and trivalent TI3+, the latter forming chains of tetrahedral bonds disposed along the tetragonal axis. The monovalent T1+ ions a r e located between the chains and provide for a weak coupling between them. In /3/ it is supposed that the arising chain structure leads to an anomaly in the temperature dependence of specific heat and reflects the peculiarities of the dynamics of highly anisotropic lattices.Previously in /6/ we have suggested a lattice dynamics model for TlSe and calculated the phonon spectrum along most of symmetrical directions in dspace inside the Brillouin zone. The parameters of this model, nine force constants, were found by fitting to eleven experimental optical phonon frequencies determined from the infrared lattice reflectivity (five frequencies) and Raman spectrum (six frequencies) /7/ a s well a s to the values of six elastic constants, determined from sound velocity measurements in various crystallographic directions /8, 9/. The calculated phonon spectrum has no appreciable anisotropy. This fact is in agreement with the values of elastic constants.Thus the interpretation of the temperature dependence of the specific heat of TlSe on the basis of the dynamics of highly anisotropic lattices is questionable. It is shown below that the observed temperature dependence of the specific heat of TlSe is quite satisfactorily explained taking into account the real phonon spectrum. The peculiaritiy of the latter is the presence of low-frequency optic a1 branches.1 ) U1. Shachbasi 5, Baku 370122, USSR.

Electronic properties and phase transitions in low-dimensional semiconductors

“…Therefore, though the low temperature phase with alternating univalent and trivalent ions is semiconducting, one can expect that the high temperature phase with equivalent ions will be metallic due to electron hopping between the Tl 1+ and Tl 3+ sites, by analogy with the pressure-induced phases in TlSe that exhibit metallic conductivity caused by removal of the structural constraints upon electron transfer from Tl 1+ to Tl 3+ ions. We note, however, that Brekow et al [149], Kurbanov et al [150], Mamedov et al [151][152][153], Aliev et al [154] and Rzaev et al [129] who measured specific heat, thermal expansion and elastic constants of TlSe, did not report on the temperature-induced phase transitions.…”

In-Plane Optical Anisotropy and Linear Dichroism in Low-Symmetry Layered TlSe

“…19 As shown in Figure 1a,b, the tetragonal structure of TlSe is built up from TlSe 2 chains, in which each Tl 3+ ion is coordinated by four Se 2− ions and each Se 2− ion is connected to two Tl 3+ ions, while Tl + ions are localized between negatively charged chains and these chains are hold together by means of weak ionic interchain forces. 20 This leads to easy cleavage of TlSe crystals along the tetragonal axis. We simulate the atomic-resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) image using bulk atomic models (Figure 1c), in which the c-axis and a-axis of the TlSe crystal are marked, supporting the crystal orientation of TlSe.…”

“…The formula of binary chain-like TlSe is therefore more precisely denoted as Tl + [Tl 3+ (Se 2– ) 2 ] . As shown in Figure a,b, the tetragonal structure of TlSe is built up from TlSe 2 chains, in which each Tl 3+ ion is coordinated by four Se 2– ions and each Se 2– ion is connected to two Tl 3+ ions, while Tl + ions are localized between negatively charged chains and these chains are hold together by means of weak ionic interchain forces . This leads to easy cleavage of TlSe crystals along the tetragonal axis.…”

Phonon Spectra of Chain TlSe and TlInSe2: Density Functional Theory Based Study

“…For the other branches the largest discrepancy with experiment is even smaller, as a detailed comparison has shown. Note that phonon dispersion obtained within a first model [24][25][26] of the phonon spectra of TlSe was far from the agreement with INS data. 23) The present calculations are definitely a step forward since the agreement with INS data 23) is obvious.…”