Stability of bismuth orthogermanate at high pressure and high temperature

The stability of cubic bismuth orthogermanate Bi-4(GeO4)(3) is investigated at high pressure (HP) and high temperature (HT) using x-ray diffraction, Raman spectroscopy, and scanning electron microscopy (SEM). At ambient temperature the compound is found to have a bulk modulus of 48 +/- 2 GPa and it undergoes amorphization at 12.5 GPa, whereas in situ laser heating of amorphous sample in a diamond-anvil cell results in re-crystallization. SEM and Raman spectroscopy of the recovered sample suggest an inhomogeneous sample with regions rich and depleted in Ge, Suggesting a decomposition of the parent compound. The decomposition products are identified from the x-ray diffraction. These results show that at HP-HT the compound exhibits complex decomposition paths leading to several mixed oxide phases including alpha-Bi2O3, Bi2GeO5, and Bi2Ge3O9. The observations of pressure-induced amorphization at ambient temperature in this compound and its decomposition into a mixture of daughter phases at HP-HT are discussed in light of a recent model of pressure-induced amorphization and decomposition.