Ultrasound velocity in solid CH4

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Abstract
We have studied the velocity of 4 and 12 MHz longitudinal and transverse sound in polycrystalline solid CH4 from 2 to 77°K, and calculated the adiabatic compressibility, βs, over this range. Near 20.48°K, the temperature of the known upper lambda transition, the velocity curve shows a discontinuity in slope and βs shows a singularity, as would be expected for a lambda‐type transition. However, no such anomalies are found near 8°K, the expected temperature of a lower transition. Addition of 1% O2 to CH4 produced indications of an anomaly in the velocity at 8°K, governed by an equilibration time on the order of 15 min.
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