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

A new compound, LiMn2+Mn3+TeO6, 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), alpha = 92.515(1) degree , beta = 92.092(2) degree , gamma = 89.818(2) degree , 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 Li2TiTeO6 structure, derived from LiSbO3. TeO6 octahedra are almost regular but all four independent MnO6 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 LiMn2TeO6, LiSbO3, Li2M4+TeO6 and LiMM'XO6 series (a total of 21 compounds) correlate well with average cationic radii, but LiMn2SbO6 deviates considerably, and its preparation could not be reproduced.