Discrepancies in Elastic Constant Data for MgO Polycrystals and Single Crystals

Elastic properties and their pressure and temperature derivatives are commonly measured on polyerystalline sintered aggregates and assumed to adequately describe the behavior of an ideal aggregate of single crystals without regard to strain of the individual grains. Similarly, it is commonly assumed that by suitably averaging data for single crystals, the properties of an aggregate can be calculated. Ultrasonic results for polyerystalline MgO measured to 8 kbars and 800°K are presented to point out the limitations of these assumptions and to illustrate the anomalous behavior of polyerystalline material. Previous measurements on single‐crystal and polyerystalline MgO are tabulated, and sources of error are discussed.