Thermoelastic behavior and temperature induced structural evolution of grossite; experimental and ab-initio calculation data

The thermal expansion of grossite (CaAl4O7) has been investigated by single crystal X-ray diffraction at the XRD1 beamline in Elettra, using a Cryostat and a gas blower, in the 100-1080 K temperature range, collecting every 20 K. A 100 K T-range of overlap was collected to scale low-T cryostat data collections with high-T gas blower data collections. This resulted in 55 datasets from which full structure refinement was obtained. No change in symmetry was observed in the whole T-range. Instead, a negative thermal expansion has been observed for the c-lattice parameter up to 400 K. At T > 400 K, this lattice parameter increases with a rate that increases with temperature. Overall, the change is within 0.012 Å. A change of 1.2% in volume is observed in the whole T-range. These results agree with the previous experimental evidence of very limited thermal expansion (αV = 4.1×10-6/K at 1173 K), following Jonas et al. (1998) and Fukuda & Yamauchi (2002), with had much more limited and low precision data. We have also observed negative volume thermal expansion, but only at T < 160K, a temperature value coincident with the start of b-lattice expansion. Therefore, it appears that the c-lattice contraction is responsible for the observed cell-volume contraction below 160 K. A Detailed study of bond geometries and bond-valence variations extracted from the structure refinement sheds light on this particular behaviour. The thermal expansion was also studied by ab-initio calculations using density functional theory and the quasiharmonic approximation (DFT-QHA) and WC1LYP functional. The data obtained for the phonons volume dependence yields αV that is negative < 130K and a very low value (αV,300 = 6.31×10-6/K) in agreement with experimental data.