High pressure powder X-ray diffraction of sillenites Bi12MO20 (M=Si, Ge, Ti) and Bi4Ti3O12

Abstract

Powder X-ray diffraction experiments of sillenites Bi12M O20 (M=Si, Ge, Ti) were performed with synchrotron radiation at the ESRF, Grenoble, at pressures up to 39GPa (M=Si), 50GPa (M=Ge), and 37GPa (M=Ti), respectively. These three sillenites were investigated for the first time over such a large pressure range and they show no phase transition up to the highest pressures achieved. Birch–Murnaghan equations of state of third order were fitted to the pressure dependence of the lattice parameters. The resulting bulk moduli B0 and their pressure derivatives B′ are B0=63(1)GPa with B′=6.6(3) (M=Si), B0=63.0(5)GPa with B′=5.90(7) (M=Ge), and B0=48(1)GPa with B′=9.4(5) (M=Ti). In the case of Bi12SiO20 and Bi12TiO20 the equation of state fits were restricted to pressures below 15GPa, because the experimental data were affected by non-hydrostatic stress at higher pressures. The equation of state of neon was redetermined (V0=11.7(6)cm3/mol, B0=5(1)GPa, B′=5.5(3)GPa) and its use as an internal pressure standard for diffraction experiments is discussed. A second cubic phase could be identified in our Bi12TiO20 sample. This was attributed to eulytite-type Bi4Ti3O12, which has B0=50.9(8)GPa and B′=6.9(3).