Twinning by Merohedry and Thermal Expansion of Zeolitic Clathrasil Deca-dodecasil 3R.

Abstract

Rhombohedral crystal particles of zeolitic clathrasil deca-dodecasil 3R (DDR), hydrothermally synthesized from a mixture consisting of fumed silica, water, and 1-adamantanamine, were characterized by single-crystal and powder X-ray diffractometry as a function of temperature and pole figure analysis. The crystallite was bounded by six equivalent {101̅1} faces and exhibited twin-free appearance, whereas the structure was resolved with the binary twin by merohedry, defined by the twin point group 3̅2'/m'1, consisting of two twin domains with nearly equal volume fractions. This twinning modifies the positions of O atoms in the Si-O-Si framework while preserving the positions of Si atoms that define the topology of polyhedral cages. This type of twinning therefore does not disrupt the microporous channels via the 8-membered rings of the 19-hedral cages and little disturbs the adsorption and permeation of gas molecules in DDR. The cell volume of DDR increased monotonically with an increase in temperature up to ∼673 K accompanied by an elongation perpendicular to the [0001] axis and a shrinkage along the [0001] axis. Above ∼673 K, the cell volume decreased with temperature. These positive and negative volume expansion coefficients observed in this study were roughly one-half and one-third of the values currently available.