Negative Thermal Expansion in the Aluminum and Gallium Phosphate Zeotypes with CHA and AEI Structure types

We present a high-resolution powder X-ray diffraction study as a function of temperature of three structurally related, open-framework phosphates: AlPO 4 and GaPO 4 with the CHA-type structure (AlPO-34 and GaPO-34, respectively) and AlPO 4 with the AEI-type framework (AlPO-18). On the basis of refined lattice parameters (in space group R 3̅ for the CHA materials and Cmcm for AEI) all three materials show negative thermal expansion. In the case of AlPO-34, this is found from 110 to 450 K; for GaPO-34, the volume expansion is net negative above 210 K, below which there is a phase transition to a new monoclinic polymorph. For AlPO-18, the behavior of the negative thermal expansion (110−450 K) can be related directly to AlPO-34 by using polar plots in certain specific crystallographic planes that imply that certain common structural features are responsible for the behavior. We compare the results with published data for SiO 2 with the CHA-type structure; this shows that the isoelectronic AlPO 4 has rather similar thermal behavior but that preparation of the GaPO 4 homologue provides a way of tuning the magnitude and anisotropy of thermal expansivity. Rietveld refinement of atomic structure as a function of temperature for each material reveals that structural units common to each (double six-rings and eight-ring windows) behave similarly for each material as a function of temperature.