X-ray studies of the structure and thermal expansion of methane isotopes

X-ray studies of the structure, lattice parameters and thermal expansion coefficients of solid CH3D, CH2D2, and CHD3 have been carried out in the temperature range from 2 K to Ttr. It was established that the crystallographic symmetry of the orientationally ordered phases of the methane isotopes is similar to that observed in the low-temperature phases of methane and deuteromethane. The quantitative features of the isotope effect in molar volume, thermal expansion coefficient, and volume jumps at the phase transitions were determined. Some conclusions are drawn, on the basis of the results obtained, about the structural properties of phase III and about the second phase transition in pure methane.

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