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 figur_e analysis. The crystallite was bounded by six equivalent {10 (1) over bar1} faces and exhibited twin-free appearance, whereas the structure was resolved with tile binary twin by merohedry, defined by the twin point group (3) over bar2'/m'1, consisting of two twin domains with nearly equal volume fractions. This twinning modifies the positions of 0 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 similar to 673 K accompanied by an elongation perpendicular to the [0001] axis and a shrinkage along the [0001] axis. Above similar to 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.