Pressure-Induced Changes in the Structure and Band Gap of CsGeX(3) (X = Cl, Br) Studied by Electronic Band Structure Calculations.

Tight-binding electronic band structures of cesium trihalometalates CsGeX(3) (X = Cl, Br) were calculated to examine the pressure-dependence of their crystal structures and band gaps as well as their primitive cubic to rhombohedral structural phase transitions. In agreement with experiment, our calculations show that an increase in the applied pressure decreases the band gap and the stability of CsGeX(3), and the band gap is larger for CsGeCl(3) than for CsGeBr(3). CsGeCl(3) has a much stronger second-order Jahn-Teller instability than does CsGeBr(3) and therefore can adopt a disordered cubic phase unlike CsGeBr(3).