Phase transition and abnormal compressibility of lanthanide silicate with the apatite structure

The lanthanide silicate La sub(9.33)Si sub(6)O sub( 26), which has the hexagonal apatite structure (P6 sub(3)/m), was investigated at high pressure. A subtle phase transition was observed at ~13.3 GPa by in situ synchrotron x-ray diffraction and confirmed by infrared absorption and Raman scattering measurements. The high-pressure phase has a structure similar to that of the initial hexagonal apatite structure, but the symmetry is reduced to P6 sub(3) through the displacement of one oxygen site and tilting of the SiO sub(4) tetrahedra. Interestingly, the high-pressure phase has an abnormally lower compressibility, which is caused by the change in symmetry that allows the tilting of the SiO sub(4) tetrahedra, and the bulk modulus of the high-pressure phase is only half that of the apatite structure.