Flexibility of the framework of zeolite Rho. Structural variation from 11 to 573 K. A study using neutron powder diffraction data

The structure of the dehydrated and deuterium-exchanged form of deammoniated zeolite Rho (Cs_1.15(SiAl)₄₈O₉₆D) has been studied at 11, 295, 423, and 573 K. All data sets were refined by using the Rietveld refinement technique in the noncentrosymmetric space group I43m and are characterized by the presence of elliptically distorted double 8-rings. In agreement with a previous study of the effects of hydration upon the framework, increase in temperature causes a monotonic increase in the cubic-unit-cell parameter (a₀) and a corresponding decrease in Δ, the parameter describing the "degree of ellipticity" as measured by the difference between the major and minor axes of the 8-ring ellipses. The variations of both Δ and a₀ with temperature in the range 11 < T < 573 K appear to fall close to smooth curves. Extrapolation of these curves suggests that the structure may be centrosymmetric above 800 K. This prediction is supported by a study of the changes in the X-ray diffraction pattern of deammoniated Rho up to 773 K.