Structure and phase transition of CaGe2O5 revisited

The structure of CaGe2O5 between room temperature and 923 K has been determined by X-ray powder diffraction. A continuous phase transition from triclinic C1macr to monoclinic C2/c symmetry at T-c(star)=714+/-3 K is observed. The transition is accompanied by a weak heat capacity anomaly. This anomaly and the strain analysis based on the measured lattice parameters indicate a classical second-order phase transition. The order parameter, as measured by the strain component e(23), is associated with the displacement of the Ca cation. Electronic structure optimization by density functional methods is used to verify the centric space group of the low-temperature structure of CaGe2O5.