Temperature and composition dependent structural investigation of the defect perovskite series Sr_1-xTi_1-2xNb_2xO ₃, 0≤x≤0.2

The crystal structure of the defect perovskite series Sr _1-xTi_1-2xNb_2xO₃ has been investigated over a range of temperatures using high-resolution synchrotron X-ray diffraction, neutron diffraction and electron diffraction. Three distinct regions were observed: 0<x≤0.125 was a solid solution of Sr _1-xTi_1-2xNb_2xO₃ with minor SrTiO₃ intergrowth, 0.125<x≤0.2 was a pure Sr _1-xTi_1-2xNb_2xO₃ solid solution adopting the cubic perovskite type structure (Pm3̄m) and for x>0.2 Sr_0.8Ti_0.6Nb_0.4O₃ and Sr ₃TiNb₄O₁₅ formed a two phase region. The cubic structure for Sr_0.8Ti_0.6Nb_0.4O₃ was stable over the temperature range 901248 K and the thermal expansion co-efficient was determined to be 8.72(9)×10^-6 K^-1. Electron diffraction studies revealed diffuse scattering due to local scale Ti/Nb displacements and slightly enhanced octahedral rotations that did not lead to long range order. The octahedral rotations were observed to 'lock-in' at temperatures below ∼75 K resulting in a tetragonal structure (I4/mcm) with anti-phase octahedral tilting about the c-axis.