Ultrasonic acoustic wave velocities of neighborite (NaMgF₃) across orthorhombic to cubic phase boundary at high P-T

Neighborite perovskite (NaMgF₃) is a close analogue to Mg or Ca silicate perovskite. It experiences a second-order phase transition at high T that may have a significant effect on the acoustic velocities. Here we report the measurement of elastic wave velocities of neighborite perovskite using the multi-anvil high-pressure apparatus located in beamline 6-BM-B at the Advanced Photon Source. We use the newly installed ultrasonic interferometry equipment using pulse-echo-overlap method coupled with the D-DIA device. X- radiographic imaging is used to measure sample length at high P-T. X-ray diffraction spectrum is used to determine the pressure and sample conditions. Precise measurements of P and S wave velocities are at 60 and 35 MHz respectively and are nearly simultaneous. We use a double reflector method to enable measurement of elastic wave velocities of cold-pressed polycrystalline sample that is sintered in situ at high P-T. Experiments were carried out up to 3 GPa and 1000 °C. Our results indicated that MHz ultrasonic acoustic velocities experience a significant minimum at the phase transition, with the bulk modulus being more affected than the shear modulus. The shear modulus of the cubic phase appears unchanged from those of the orthorhombic phase, while the bulk modulus increases in transforming into the cubic phase.