Crystal structure and properties of a new mixed-valence compound LiMn2TeO6 and the survey of the LiMM′XO6 family (X = Sb or Te)

Abstract A new compound, LiMn 2+ Mn 3+ TeO 6 , was prepared by solid-state reactions and characterized by powder X-ray diffraction, redox titration, conductivity, magnetic measurements and cycling behaviour in an electrochemical cell with Li counter electrode. It is triclinic, P1, a  = 5.1077(1), b  = 8.5707(1), c  = 5.0589(1), α  = 92.515(1)°, β  = 92.092(2)°, γ  = 89.818(2)°, Z  = 2. The structure is based on strongly distorted hexagonal close packing of oxygen anions with cations occupying octahedral voids and represents the ordered variant of orthorhombic Li 2 TiTeO 6 structure, derived from LiSbO 3 . TeO 6 octahedra are almost regular but all four independent MnO 6 octahedra display severe distortions. Room-temperature conductivity is unexpectedly low for a mixed-valence compound, 2 × 10 −7  S/cm. Together with the observed antiferromagnetic exchange interaction, this is a prerequisite for the colossal magnetoresistance effect. The unit cell volumes of LiMn 2 TeO 6 , LiSbO 3 , Li 2 M 4+ TeO 6 and LiMM′XO 6 series (a total of 21 compounds) correlate well with average cationic radii, but LiMn 2 SbO 6 deviates considerably, and its preparation could not be reproduced.