Subtle high-pressure behaviors of apatite-type La9.33Ge6O26

Abstract

Apatite-type La9.33Ge6O26 powders were synthesized by a facile molten-salt synthesis method, and its high-pressure behaviors were probed in a diamond anvil cell up to 31.8 GPa using in situ angle-dispersive synchrotron radiation technique. A reversible phase transition was observed at ∼16.5 GPa, accompanied by ∼1.8% c-axis shrinkage and ∼2.0% volume collapse. This phase transition should be attributed to a reduction of symmetry (from high P63/m to low P63) induced by the tilting of GeO4 tetrahedra. Furthermore, the high-pressure phase exhibited an unusually higher compressibility than the initial phase, which should be related to its low P63 symmetry which is more flexible to allow the tilting of GeO4 tetrahedron. In contrast with apatite-type lanthanum silicates, the phase transition pressure of La9.33Ge6O26 was higher owing to the larger size of GeO4 tetrahedron than SiO4 tetrahedron. The relevant mechanism was discussed in the present paper.