STRUCTURAL VARIATION IN CA(TIXSI1-X)O-3 PEROVSKITES (1-GREATER-THAN-X-GREATER-THAN-0.65) AND THE ORDERED PHASE CA2TISIO6

High pressure perovskites in the system CaTiO3-CaSiO3 in the compositi on range from 0 to 50% CaSiO3 have been re-examined using powder x-ray diffraction, Raman spectroscopy, and transmission electron microscopy . In runs performed at 9 GPa and 1200 degrees C, a solid solution is o btained with compositions ranging from CaTiO3 to near Ca(Ti0.65Si0.35) O-3, and with the orthorhombic distortion in the CaTiO3 end-member dim inishing to pseudocubic for Ca(Ti0.65Si0.35)O-3. Raman spectra show in tensity changes and band broadening which are attributed to the reduct ion of orthorhombic character, but still indicate lower than cubic sym metry for the entire solid solution range. An ordered intermediate, Ca 2TiSiO6, has been recovered from a run at 14 GPa at 1200 degrees C. Th e space group is Fm (3) over bar m, with a=7.410(2) ii, and the compou nd has the double perovskite structure. In this structure, Si-rich oct ahedral sites and Ti-rich octahedral sites alternate along the three p rincipal axes. Transmission electron microscopy confirms the presence of the Fm (3) over bar m ordered structure, and also documents the pre sence of lesser amounts of at least three other ordered perovskite str uctures which are not resolved in the x-ray data. These may account fo r extra features observed in the Raman spectrum which are inconsistent with mode assignments for thr Fm (3) over bar 3m phase. Non face-cent ered ordering schemes for these regions RTP suggested based nn modelin g Of the HRTEM patterns.