On the crystal chemistry of alkali-, alkaline-earth-, rare earth-oxozirconates and zirconium oxides [Zur kristallchemie der alkali-, erdalkali-, selten erdmetall-oxozirconate und zirconiumoxide]

摘要

The contribution on oxozirconates gives a view on the crystal chemistry of zirconium compounds with large cations, like alkaline, alkaline-earth and rare earth elements. This is completed by a short overview of the structure types of ZrO_2 synthesized by different methods, temperatures and pressures. The most important observation is the missing reflection of the coordination chemistry of zirconiumoxides into the oxozirconates. Usual coordination numbers of zirconium in zirconiumoxide are CN = 8 and occasionally CN = 9. The described oxozirconates instead show mainly octahedral (CN = 6) coordination (such as Li_2ZrO_3, K_2Zr_2O_5, Li _(0.8)K_(0.9)(LiMgZr)Zr_2O_(6.5), Li _6 Zr_2O_9, K_4Zr_5O _(12), K_2Li_(14)Zr_3O_(14), Pb _(0.5)Ca_(0.5)ZrO_3, Nd_2Zr _2O_7, BaCe_xZr_(1-x)O_3). Exceptions with the coordination number six exhibit trigonal ZrO _6-prisms (K_4Zr_5O_(12)). This facts result in the observation, that zirconium is a member of the anionic parts of the crystal structures. Lower coordination spheres are enforced by the cations. The larger a cation and the larger its amount in the chemical composition is, the lower the coordination number of zirconium gets. For example: CNZr = 5 (tetragonal pyramid) in K_2ZrO_3, Cs_2ZrO _3; CN_(Zr) = 4 (tetrahedron) in Cs_4ZrO _4. Furthermore, Zr~(4+) can also be found in statistical distribution with ions of the same size, like Li~+ and Mg ~(2+). Calculations of the Madelung part of lattice energies of zirconium oxides show unexpected uniform data, independent from shape and size of the coordination polyhedra.