Vibrational spectroscopy and x-ray diffraction of Perovskite compounds Sr(1-x)M(x)TiO(3) (M=Ca,Mg; 0<=x<=1)

A series of perovskite compounds, Sr(1-x)M(x)TiO(3) (M = Ca, Mg; 0 less than or equal to x less than or equal to 1), has been synthesized by solid-state reaction, and characterized by Fourier-transform infrared spectroscopy, Raman scattering, and X-ray diffraction. As for Sr1-xCaxTiO3, it was found that the substitutional solid solutions are formed over a wide x range, whereas traces of orthorhombic CaTiO3 would appear for x greater than or equal to 0.6. The unit-cell volume of cubic SrTiO3 decreases as Sr is replaced by Ca with a smaller ionic radius, and the Sr-O bond length reduction is somewhat matched by the Ti-O bond shortening. The latter is evidenced by the fact that the 460 cm(-1) Raman band assigned to the Ti-O-3 torsional mode appears at x = 0.6 and shifts upward with further increasing x. On the other hand, the formation of solid solutions is restricted to x less than or equal to 0.2 for Sr1-xMgxTiO3; the Raman/infrared-reflectivity spectra and X-ray diffraction profiles of Sr1-xMgxTiO3 are characteristic of the coexistence of cubic SrTiO3 and hexagonal MgTiO3. The solubility of Ca or Mg in SrTiO3 is discussed based on the correlation between the enthalpy and tolerance factors for A(II)B(IV)O(3). It is concluded that inefficient mixing of MgTiO3 with SrTiO3 is predominated by the substantial difference between the Sr2+ and Mg2+ cation radii; this accounts for the less stable Sr1-xMgxTiO3 and the more limited solubility of Mg in SrTiO3 as well, through the effective difference in the tolerance factors of SrTiO3 and MgTiO3 as compared with CaTiO3. (C) 1996 Academic Press, Inc.