Elastic Constants of Polycrystalline BeO as a Function of Pressure and Temperature

The sound velocities of polycrystalline BeO and their pressure and temperature derivatives were determined by a pulse superposition method. The derivatives found from the measurements at pressures up to 2 kbars and near 25° are: (∂ul/∂P)T= 6.48×10−3 km/s kbars, ∂S/∂P)T= 0.33×10−3 km/s kbars, ∂L/∂T)P=−2.8×10−4 km/s deg, and ∂S/∂P)T=−2.0×10−4 km/s deg. The very low value of ∂S/∂P)T is anomalous for crystalline solids but may be expected from the crystal structure in view of similar anomalous behavior found in other solids with the wurtzite structure. The Gruneisen parameter calculated from the pressure derivatives of sound velocities was small compared with the Gruneisen parameter obtained from thermal properties. This result seems to indicate that the thermal expansivity of BeO does not change with temperature in proportion to the specific heat at low temperatures.

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