Crystal structure of the mixed conductors phases, Li0.5-3xLa0.5+x+yTi1-3yM3yO3 (M=Mn, Cr) with x=0.133 and y=0.20

Abstract

Perovskite-like solid solutions of general formula Li _0.5-3x La _0.5+x+y Ti _1-3y M _3y O ₃ (M=Mn, Cr) show three polymorphs; A, beta; and C. The crystal structure of the C polymorphs in manganese- and chromium-lanthanum systems, determined from powder neutron diffraction using Rietveld refinement, are of the ordered perovskite type. The structures of both phases are similar, containing a three dimensional framework of corner-sharing MO ₆ (M=Ti or Mn/Cr) octahedra in which the structures are partially collapsed as a result of a cooperative tilting and rotation of octahedra. Orthorhombic unit cell, M=Mn: a=5.5411(11) Å, b=7.8120(14) Å, c=5.4924(10) Å; M=Cr: a=5.5014(9) Å, b=7.7735(15) Å, c=5.4729(9) Å; space group Pnma (no. 62). Ionic conductivity takes place by a hopping mechanism between Li ⁺ -occupied and empty A-sites, while electronic conductivity is along octahedra.