Vibrational Spectroscopy and X-Ray Diffraction of Perovskite Compounds Sr1−xMxTiO3(M= Ca, Mg; 0 ≤x≤ 1)

Abstract

A series of perovskite compounds, Sr1−xMxTiO3(M= Ca, Mg; 0 ≤x≤ 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 widexrange, whereas traces of orthorhombic CaTiO3would appear forx≥ 0.6. The unit-cell volume of cubic SrTiO3decreases 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−1Raman band assigned to the Ti–O3torsional mode appears atx= 0.6 and shifts upward with further increasingx. On the other hand, the formation of solid solutions is restricted tox≤ 0.2 for Sr1−xMgxTiO3; the Raman/infrared-reflectivity spectra and X-ray diffraction profiles of Sr1−xMgxTiO3are characteristic of the coexistence of cubic SrTiO3and hexagonal MgTiO3. The solubility of Ca or Mg in SrTiO3is discussed based on the correlation between the enthalpy and tolerance factors forAIIBIVO3. It is concluded that inefficient mixing of MgTiO3with SrTiO3is predominated by the substantial difference between the Sr2+and Mg2+cation radii; this accounts for the less stable Sr1−xMgxTiO3and the more limited solubility of Mg in SrTiO3as well, through the effective difference in the tolerance factors of SrTiO3and MgTiO3as compared with CaTiO3.