The tl~ird-order elastic constants and the pressure derivatives of the secondorder elastic constants of magnesium stannide are calculated. The Griineisen parameters of the acoustic modes in six symmetry directions are then computed. It is significant that most of the transverse elastic modes have negative GHineisen parameters. The low-temperature Emit of the Griineisen parameter is then evaluated. Its relevance to the low-temperature thermal expansion of magnesium stannide is discussed.
The six independent elastic constants of sodium nitrate are determined using 10 MHz ultrasonic pulse echo superposition technique over the temperature interval 77 to 300 K. The values obtained at 300 K are C11 = 5.71, C12 = 2.16, C33 = 3.3, C13 = 1.66, C44 = 1.24, C14 = 0.82, and at 77 K C11 = 6.63, C12 = 2.07, C33 = 3.99, C13 = 1.67, C44 = 1.69, C14 = 1.16 all expressed in units of 1011 dyn/cm2.
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