Order-disorder transition and thermal behavior of paracelsian-type MGa2Ge2O8 (M = Sr, Ba) compounds

Two Ga-Ge disordered feldspar-related compounds MGa2Ge2O8 (M = Sr, Ba) with paracelsian topology were synthesized for the first time using the melt crystallization methods (1300-1400 degrees C) and rapid cooling. Their stability was studied under high-temperature conditions using in situ single-crystal (up to 1000 degrees C) and powder (up to 1050 degrees C) X-ray diffraction. Both compounds are stable in the studied temperature range. The refinement of the crystal structure of BaGa2Ge2O8 at different temperatures does not demonstrate any ordering processes. The thermal expansion of both compounds has anisotropic character (alpha max/alpha min= 3.7 and 3.9 for SrGa2Ge2O8 and BaGa2Ge2O8, respectively), wherein the maximum and minimum expansion is observed within the bc plane. The anisotropy degree of MGa2Ge2O8 increases as the size of extraframework cation increasing. Simultaneously this process leads to the decreasing of MGa2Ge2O8 stability range. The influence of the crystallization procedure on order-disorder process of MGa2Ge2O8 is discussed.