ECEN I compilations1-3 of single crystal elastic constants do A single crystal was obtained R which had been grown from an arc melt and which gave good Laue back reflection patterns despite some variation in color.The velocity of 45 M H sound waves was measured using a pulse echo technique with the ThOz specimen mounted on the end of a vitreous silica buffer rod with stopcock grease bonds for longitudinal waves and epoxy resin bonds for transverse waves. This rod served two purposes: (1) It delayed the echoes from the specimen so that the receiver would not be overloaded when the first echo returned. (2) It simplified holding the specimen, since no electrical connections had to be made (i.e., no metallic conductive coating on the specimen was required). The bonding material could be set on the silica rod and the specimen placed on top with no holder required. A quartz transducer (x-cut for longitudinal waves, ac-cut for shear waves) was bonded to the other end of the buffer rod. The transit times were determined from the echo envelope displayed on an oscilloscope; measurements using either the peak or the leading edge agreed within the accuracy reported below. Successive echoes differed in having traversed the ThOz specimen one more round trip, but had the same number of passes through the bonds. This technique offered the advantage that transit time in the bonding material canceled out so that the difference in arrival time of successive echoes from within the Tho2 specimen could be used without correction to calculate the velocity of sound. Measurements were made along the [110] direction through 1.942 cm of specimen, and along the [liO] direction through 1.222 cm of specimen. The velocities showed no significant differences for the two directions; the standard deviations of the velocities were about 0.2% of the velocity values themselves. The accuracy is probably somewhat lower than this estimate of precision would indicate and it is felt that 0.5% uncertainty in velocity is a more reasonable estimate considering possible systematic errors in the transit time determinations. The three single-crystal elastic constants were computed as described, e.g. by de Launay4 from the velocity of longitudinal waves (a1 = 5624 m per second, of transverse waves polarized along the [OOl] direction (vz = 2824 m per second, and of transverse waves polarized along the [ IT01 direction (z13 = 3612 mper second). The resulting elastic constants in units of 10" N/m2 ( 1OI2 dyne per cmz) are c11 = 3.67 i 0.04, c12 = 1.06 f 0.02, and c44 = 0.797 i 0.008, where the plus-minus values indicate the estimated overall uncertainties. The results can also be expressed in terms of the elastic compliances in units of cm2 per dyne) and are s11 = 3.13, S I Z =-0.703, and s44 = 12.5. The bulk modulus is given by (c11 + 2c12)/3 = 1.93 X 10l1 N per ma (1 93 X loT2 dyne per cmz).The elastic anisotropy factor is given by A = 2C44/(cll -C~Z ) and has the value 0.61 for ThO2. Comparison with other fluorite structure compounds shows that CaFa6 ( A = 0.61...
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