Pressure-induced shear-mode elastic softening in orthorhombic BaCe0.85 Y0.15 O2.925 perovskite

Compressional and shear wave velocities of orthorhombic BaCe0.85 Y0.15 O2.925 perovskite were measured at high pressures up to 8.1GPa by ultrasonic interferometry combined with energy-dispersive synchrotron X-ray diffraction. At room temperature, the adiabatic bulk modulus increases with increasing pressure, as expected for most polycrystalline materials. The shear modulus, however, show a negative dependence on pressure over a pressure range of ~2-8.1GPa. This anomalous elastic softening indicates destabilization of the orthorhombic structure in BaCe0.85Y0.15O2.925 perovskite over the experimental pressure range and is presumably the precursor to the first-order phase transition previously observed at 22GPa using Raman spectroscopic study.