THE EFFECT OF THE MARTENSITIC TRANSFORMATION IN SODIUM ON THE THERMOELECTRIC POWER

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Abstract
When sodium is cooled, its crystal structure partially changes by martensitic transformation from body-centered cubic to close-packed hexagonal below about 40° K. Reversion does not begin until the temperature is raised above about 45° K, so there is a small temperature range in which sodium may consist of either pure body-centered cubic or mixed phases. Using results obtained by Dugdale and Gugan, it is possible to estimate the amount of hexagonal phase present in the mixed samples from electrical resistance measurements. We have measured the thermoelectric power at 43° K for specimens of body-centered cubic sodium and for partially transformed sodium whose composition was thus known. From these measurements the thermoelectric power of pure hexagonal sodium is estimated to be 10% smaller than that of cubic sodium at 43° K. The distribution of the phases in the partially transformed material is represented by two different models and this result is found to be insensitive to which is used. A brief description of the experimental apparatus is given and some theoretical implications of the results are considered.