Anomaly in the thermal expansion of orientationally disordered CD4–Kr solutions and the problem of an octupole glass

X-ray structural studies of orientationally disordered solid solutions (CD4)x Kr1−x were performed in the temperature range 4.5 K < T < 90 K and for concentrations 0.2 ≤ x ≤ 0.6. Anomalous behavior of the lattice parameters, molar volumes, and coefficients of thermal expansion was observed at temperatures T < 20 K. This behavior is linked with the cooperative character of the hindering of the rotational motion of the deuteromethane molecules. Comparison of the temperature-concentration dependences of the coefficients of thermal expansion obtained with the analogous results for quadrupole disordered systems (ortho-para mixtures of hydrogen) made it possible to determine the temperature range of the expected transition into the state of the hypothetical octupole glass for the CD4-Kr solution studied: Tg = 2.3 ±0.7 K. Theoretical values of the critical temperature of the transition into the octupole glass state are presented.