Last year, we reported on the successful generation of shear waves with near-optical wavelengths for GHz-frequency ultrasonic interferometry. The new acoustic technique for bench-top ultrasonics features a P-to-S wave conversion by reflection on the oriented facet of a single crystal MgO prism. The technology was put to use this year for determination of the complete set of ambient P-T elastic constants (c11, c12, c44) across the ferropericlase-magnesiowüstite (Mg,Fe)O solid solution. Our next objective is to modify the design for construction of a more durable conversion buffer rod made from single-crystal sapphire, shaped to work with the diamond anvil cell for shear wave measurements at high pressure and temperature. Here we report on our progress.
Whereas we could use the  pure-mode directions in MgO for the orthogonal conversion in the bench-top prism, for the diamond cell we require a pointed buffer rod that fits into the access cone for acoustical contact with the diamond-anvil (Fig. 3.7-1). We have chosen to start with c-axis oriented sapphire rods because the cylinder direction will then support pure-shear mode propagation. We require the conversion to occur at 90° (for symmetrical return of the strain wave), so the incident compression wave (from a transducer) must be applied from the side of the cylinder. Only the a-axis or (110) direction of sapphire supports pure-compressional mode propagation, so we are polishing an a-axis flat on the side of buffer rod. We have completed the orientation and transfer of six sapphire crystals to a polishing platform for production of shear buffer rods (Fig. 3.7-2) using an X-ray precession camera. Completion of the first flat and sputtering of the incident P-wave transducer is expected to occur in early 2001.