STRUCTURAL PHASE-TRANSITIONS AND PSEUDOMEROHEDRALLY TWINNING OF LI2FEBR4

The crystal structure of the quenched high-temperature phase Li2FeBr4, which is stable between 668 and 749 If, was determined by fi-ray diff raction of a powdered sample and a twinned crystal (space group Cmmm, Z = 2, a = 773.50(8), b = 1093.6(1), c = 382.58(4) pm, R(1) = 5.9%, R( F) = 3.2%, 185 unique reflections, respectively). The six twin individ uals are connected by one threefold rotation axis and, in addition, by three twofold rotation axes. Raman spectra and the ionic conductivity revealed Li2FeBr4 to be very similar to the orthorhombic low-temperat ure forms of Li(2)MBr(4) (RI = Mg, Mn). High-temperature X-ray and neu tron diffraction experiments of quenched powder samples show the forma tion of an inverse spinel-type polymorph (space group <Fd(3)over bar m >, Z = 8, a = 1115.2(1) pm, R(1) = 10.1%, R(wp) = 14.5% at 660 K) in t he temperature range 441-668 K. (C) 1996 Academic Press, Inc.