Ultrasonic and Infrared Studies of the Lambda Transition in NaNo3

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Abstract
The velocity and attenuation of longitudinal and transverse acoustic waves propagating along the three-fold axis of NaNO3 have been measured from room temperature to 300°C. Strong frequency-dependent ultrasonic attenuation is observed at temperatures immediately above and below the λ point. From the attenuation a relaxation time for the equilibration of the long-range order at constant entropy of 4.3 × 10−10 sec is estimated. The totally symmetric stretching mode of the nitrate ion, inactive in the ordered NaNO3 structure, is possibly active in the high-temperature disordered phase. A weak infrared absorption is found at an elevated temperature at the anticipated frequency of ν1 in NaNO3. The temperature variation of the absorption through the transition is consistent with the notion that there is no abrupt change in short-range order at the λ point.