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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 [100] 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.

Fig. 3.7-1. Schematic illustration of the sapphire buffer rod under construction for shear-wave measurements in the diamond-anvil cell.

Fig. 3.7-2. Orientation and transfer of six sapphire rods on the a-axis into the polishing block for sputtering of the incident P-wave transducer.