Temperature and composition dependent structural investigation of the defect perovskite series Sr 1-x Ti 1-2x Nb 2x O 3 , 0?x?0.2

Abstract

The crystal structure of the defect perovskite series Sr 1-xTi1-2xNb2xO3 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-xTi1-2xNb2xO3 with minor SrTiO3 intergrowth, 0.125<x≤0.2 was a pure Sr 1-xTi1-2xNb2xO3 solid solution adopting the cubic perovskite type structure (Pm3̄m) and for x>0.2 Sr0.8Ti0.6Nb0.4O3 and Sr 3TiNb4O15 formed a two phase region. The cubic structure for Sr0.8Ti0.6Nb0.4O3 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.