Lattice constant, thermal expansion, and isothermal compressibility measurements on argon single crystals
The lattice constant of an argon single crystal has been measured as a function of temperature with a precision, back-reflection x-ray diffractometer. From this determination the total thermal expansion and the coefficient of thermal expansion have been obtained. Also', a preliminary measurement has been made on the isothermal compressibility of solid argon at liquid helium temperature. This was done using the same x-ray measurement technique. No previous compressibility measurements have been made at this temperature, and the thermal expansivity measurements were obtained with 30 times the previously attained accuracy. Essential to the successful completion of these measurements was the growth of high quality single crystals in a rigid cryostat possessing fine temperature control over a large range. Such a cryostat was designed and built with a unique tail assembly which provided the required rigidity and w,ith a heat exchanger which permitted temperature regulation of ±. O.OloK throughout the range from 2.3°K to i. the triple point of argon. The essential growth parameters were investigated and optimized so that single crystals of the necess,ary quality could be grown to a size as large as their container. From the measured compressibility and expansivi ty, valu'es of the thermodynamic properties of argon were compiled and compared with theory. The values of the Gruneisen parameter, the Debye temperature and the thermal vacancy content were compared with theory with agreement within the experimental error.
lattice constant, thermal expansion, isothermal compressibility, argon single crystals, back-reflection x-ray diffractometer
University of Illinois - Urbana-Champaign
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