Crystal coordination formulae

A more complex notation is needed for non-stoichiometric phases. Selected simple examples are given below, and more detailed information will be reported when discussing crystal coordination formulae ... 

Coordination and dimensionality symbols in the crystal coordination formula... 

In the description of crystal coordination formulae a coordination number was introduced, defined as the NNN of atoms X around the atom Y under consideration. 

Thus, polymorphism with the formation of monoclinic and tetragonal modifications is typical for U- and Th-compounds with general formula BM(X04)2 (X = P, As, V). These modifications differ in the packing of the XO4 anions, the orientation of anions around a cation, and the form of the (B,M)On coordination polyhedra (n = 8 or 9). The crystal chemical formula of the BM(P04)2 phosphates and their arsenate and vanadate analogues can be written as B0.5M0.5XO4, owing to the cation disorder. 

The crystal chemical formulae should give detailed structural information on the structural units, their constitution and way of packing, the interstitieil atoms and the coordination of the atoms, both interstitial and those contained in the structural units. The nomenclature should be as simple and self-explanatory as possible. 

IngS/ The original crystol structure determination (Wadsten, Amberg Berg, 1980) revealed a defect tetrahedral structure which, based on the reported atom coordinations and atom contacts, can be described by the following crystal chemical formula ... 

In a general crystal-chemical formula T stands for any of several tetra-hedrally coordinated cations and X for any three-coordinated cations. 

With potassium silicate tetravalent cerium forms crystals of formula K2CeSigOi5. The structure consists of a (81205) " framework. Cerium is octahedrally coordinated. Large channels in the structure give it a zeoUte character (fig. 47) (Karpov et al, 1977a). 

Boron coordination of boron partners Code Compound and type Pearson s classif. symbol Space group crystal-chemical formula... 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

The known uranium(VI) carbonate soHds have empirical formulas, 1102(003), M2U02(C03)2, and M4U02(C03)3. The soHd of composition 1102(003) is a well-known mineral, mtherfordine, and its stmcture has been determined from crystals of both the natural mineral and synthetic samples. Rutherfordine is a layered soHd in which the local coordination environment of the uranyl ion consists of a hexagonal bipyramidal arrangement of oxygen atoms with the uranyl units perpendicular to the orthorhombic plane. Each uranium atom forms six equatorial bonds with the oxygen atoms of four carbonate ligands, two in a bidentate manner and two in a monodentate manner. 

Sohd uranium—phosphate complexes have been reported for the IV and VI oxidation states, as well as for compounds containing mixed oxidation states of U(IV) and U(VI). Only a few sohd state stmctures of U(IV) phosphates have been reported, including the metaphosphate U(P03)4, the pyrophosphate U(P202), and the orthophosphate, CaU(PO4)2. The crystal stmcture of orthorhombic CaU(POis similar to anhydrite (194). Compounds of the general formula MU2(PO4)3 have been reported for M = Li, Na, and K, but could not be obtained with the larger Rb and Cs ions (195). In the sohd state, uranium(IV) forms the triclinic metaphosphate, U(P03)4. Each uranium atom is eight-coordinate with square antiprismatic UOg units bridged by... 

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