Pressure-Induced Changes in the Structure and Band Gap of CsGeX3 (X: Cl, Br) Studied by Electronic Band Structure Calculations.

Tight-binding electronic band structures of cesium trihalometalates CsGeX3 (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 CsGeX3, and the band gap is larger for CsGeCl3 than for CsGeBr3. CsGeCl3 has a much stronger second-order Jahn−Teller instability than does CsGeBr3 and therefore can adopt a disordered cubic phase unlike CsGeBr3.