Polymorphisme sous pression de Sm 2Ge 2O 7 et identification de la nouvelle phase H de la serie des digermanates de terres rares

Abstract

The pressure-temperature phase diagram of Sm 2Ge 2O 7 was determined up to 80 kbar and 1400°C. In this pressure-temperature range only two phases were found. The B″ triclinic phase synthetised at atmospheric pressure is stable up to 40 kbar and at higher pressures appears the H phase which is a new type of structure in the rare earth digermanate series. The lattice parameters of the H phase were determined on small single crystals which were grown with water as mineralizer. The symmetry of the H phase is hexagonal with a space group P 6/m m m. The cell parameters of H-Sm 2Ge 2O 7 are a= 11,26 A ̊ and c= 31,80 A ̊ . The measured density is 7,01 g/cm 3 and therefore the cell contains twenty seven Sm 2Ge 2O 7 units. The calculated density is 7,16 g/cm 3. The X-ray diffraction pattern of H-Gd 2Ge 2O 7 previously obtained can be indexed with similar parameters a= 11,19 A ̊ and c = 31,65 A ̊ .