First-principles study of [formula omitted]TcO[formula omitted] pertechnetates

Abstract

The present work reports a study of the electronic, elastic, mechanical, and vibrational properties of tetragonal scheelite-type ATcO4 (A = Na, Ag, K, Tl, Rb, Cs) compounds at ambient pressure using first-principles calculations. Calculated elastic constants and phonon dispersion show that all compounds are elastic and dynamically stable. In alkali-metal pertechnetates, an increasing (decreasing) linear trend with ionic radius was observed in the lattice parameters and direct Γ−Γ electronic band-gap energy (elastic constants and moduli). The elastic anisotropy increases with the ionic radius, which is proposed to be the cause of the crystallization of CsTcO4 in an orthorhombic structure at ambient conditions. Results from TlTcO4 slightly deviate from the linear trend observed in the alkali-metal pertechnetates. In contrast, the outcomes from AgTcO4 present a remarkable difference with the other compounds due to the role of d valence electrons from Ag. Finally, the charge density is analyzed using the quantum-theory of atoms in molecules methodology to complement the electronic structure results. The studied pertechnetates present smaller elastic moduli than other ABO4 compounds, such as molybdates (AMoO4), tungstates (AWO4), and germanates (AGeO4). In particular, calculations also show that ATcO4 compounds are among the most compressible ABO4 ternary oxides.