Abstract:
Density functional theory with a gradient PBE functional and dispersion correction D3(BJ) in the basis of localized orbitals of the CRYSTAL17 package was used to study the effect of pressure on the structural, electronic, and elastic properties of K$_2$Ca(CO$_3$)$_2$, K$_2$Mg(CO$_3$)$_2$, Na$_2$Ca$_2$(CO$_3$)$_3$, Na$_2$Mg(CO$_3$)$_2$. The parameters of the third-order Birch–Murnaghan equation of state are determined, and it is shown that the equilibrium volume and compressibility modulus depend linearly on the average cation radius. Under pressure, the widths of the upper valence bands increase maximally in K$_2$Ca(CO$_3$)$_2$ and minimally in Na$_2$Ca$_2$(CO$_3$)$_3$, while the centers of gravity of the cationic states shift by $\sim$ 0.2 eV. Elastic constants and polycrystalline moduli are calculated, which increase with decreasing average cation radius. The shear modulus for K$_2$Ca(CO$_3$)$_2$ and K$_2$Mg(CO$_3$)$_2$ decreases with increasing pressure, and this leads to anomalous behavior of the longitudinal and transverse acoustic wave velocities.
Keywords:ab initio calculations, buchliite, eithelite, shortite, equation of state, elastic moduli, density of states, pressure.
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