Exceptional Negative Thermal Expansion in AlPO₄-17

Anhydrous AlPO 4 -17 has been found to experience a very strong negative thermal expansion over the temperature range 18−300 K. Powder synchrotron X-ray diffraction data taken at six temperatures over this range show an essentially linear decrease in cell volume, with a linear coefficient of thermal expansion of −11.7 × 10 - 6 K - 1 which is significantly more negative than previously reported for any material. The contraction of the structure along the a and b cell edges is considerably greater than along the c edge of this hexagonal structure. Rietveld refinements of the framework as a function of temperature suggest that the negative thermal expansion is related to the harmonic transverse vibrations of bridging oxygen atoms, which lead to dynamic rocking of the essentially rigid tetrahedral building blocks of the structure.