Hydrothermal Synthesis of Lithium Zinc Phosphates: Structural Investigation of Twinned ALPHA-Li4Zn(PO4)2 and a High Temperature Polymorph BETA-Li4Zn(PO4)2

Two polymorphs of Li2CuZrO4 have been synthesized and their structures characterized by X-ray, neutron and electron diffraction. Both have novel types of ordered rock salt structure. beta-Li2CuZrO4 is tetragonal, a=4.165(1) ?, c=9.418(1) ?, I-4m2, Z=4, RI=5.9%; the four sets of octahedral sites contain Zr, Li and partially ordered Li/Cu (two sets). gammaLi2CuZrO4 is orthorhombic, a=9.383(1) ?, b=5.894(1) ?, c=5.862(1) ?, Cccm, Z=8, RI= 6.3%: in this case, the four sets of octahedral cation sites are fully ordered. Both structures show Jahn-;Teller cooperative elongation of CuO6 octahedra. The transition from the low-temperature gammapolymorph to the high-temperature beta phase is first order as indicated by the presence of a transition enthalpy at an onset temperature of 1047°C on heating.