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: AlPO4 and GaPO4 with the CHA-type structure (AlPO-34 and GaPO-34, respectively) and AlPO4 With the AEI-type framework (AlPO-18). On the basis of refined lattice parameters (in space group R (3) over bar 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 SiO2 with the CHA-type structure; this shows that the isoelectronic AlPO4 has rather similar thermal behavior but that preparation of the GaPO4 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 it function of temperature.