Temperature-induced phase transitions for stuffed tridymites SrGa 2 O 4 and CaGa 2 O 4

Temperature-induced phase transitions for stuffed tridymite A Ga 2 O 4 ( A = Sr, Ca) were investigated by experimental and theoretical calculations. A simple annealing of SrCO 3 and Ga 2 O 3 led to the formation of γ-SrGa 2 O 4 ( P 2 1 /n ) below 1200 °C, and transform to β-SrGa 2 O 4 ( P 2 1 /c ) when heated at 1200 °C. A similar phenomenon was found for CaGa 2 O 4 , and the temperature boundary between α-CaGa 2 O 4 ( Pna 2 1 ) and the high temperature polymorph β-CaGa 2 O 4 ( P 2 1 /c ) was about 1350 °C. Rietveld refinements provided detailed structural information for these polymorphs and suggest that the driving force of these phase transitions is the under-bonded nature of the alkaline earth cations. In other words, the need of larger space for Sr 2+ /Ca 2+ in the high temperature β-phase forces the 6-membered-ring channel expand through increasing the Ga-O-Ga angles. Density functional theory calculations proved the formation energies for γ-SrGa 2 O 4 and α-CaGa 2 O 4 were both lower than their high temperature β-polymorphs, in accordance with the experimental observations.