The concentration dependence of optical phonons in the ${\mathrm{TiO}}_2$-${\mathrm{SnO}}_2$ system whose phase diagram exhibits a miscibility gap, has been studied by Raman scattering and Fourier-transform infrared spectroscopy. X-ray diffraction confirmed that the mixed oxides ${\mathrm{Ti}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${\mathrm{O}}_2$ with intervals x=0.1 are a single uniform phase (tetragonal) except some x’s; no phase separation by spinodal decomposition could be avoided for x=0.3, 0.4, 0.5, and 0.6, characterizing x-ray-diffraction profiles by sidebands. The 449 ${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathit{A}}_{1\mathit{g}}$ and 610 ${\mathrm{cm}}^{\mathrm{-}1}$ ${\mathit{E}}_{\mathit{g}}$ Raman modes for ${\mathrm{TiO}}_2$ never changed linearly in frequency and linewidth with x. It is stressed that the x dependence of optical phonons is predominated by the octahedral distortion in ${\mathrm{Ti}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${\mathrm{O}}_2$, where different/anisotropic compressibilities and thermal expansivities of each end member play a crucial role. Infrared-reflectance spectra consisting of the infrared-active ${\mathit{A}}_{2\mathit{u}}$ and ${\mathit{E}}_{\mathit{u}}$ modes between 50–900 ${\mathrm{cm}}^{\mathrm{-}1}$, also changed systematically with x in ${\mathrm{Ti}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${\mathrm{O}}_2$. The concentration dependence of optical phonons in ${\mathrm{Ti}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Sn}}_{\mathit{x}}$${\mathrm{O}}_2$ is discussed, from the viewpoint of the mode behavior of mixed crystals. © 1996 The American Physical Society.

• Received 17 August 1995

DOI:https://doi.org/10.1103/PhysRevB.53.8442