RUTILE-TYPE DERIVATIVES

Sixteen structure types derive from the aristotypic rutile, TiO2, structure. Seven of these are based on an ordering of cations: the CoReO4, MgUO4, AlWO4, CuUO4, trirutile, CuSb2O6 and FeNb2O6 types; three on an ordering of anions: the INOOH, the manganite and HgFOH types; and six more on various distortions from the aristotype: the CaCl2, CuF2, V1-xAlxO2 (or VO2-T), V0.8Cr0.2O2, beta-NbO2 and alpha-NbO2 types. The manganite and AlWO4 types could also be considered as distortion variants (as respectively the MoO2 and VO2-M2 types). The trirutile type is defined as a derivative of rutile, with a tripled c cell constant and with space group P4(2)/m2(1)/n2/m. Because other 'polyrutile' types cannot easily be defined as unique structure types the use of these terms is not recommended. Instead the example first described should be used for naming the type (e.g. MgUO4 type or CoReO4 type). The derivatives due to distortions of the aristotypic rutile type are distinguished based on their underlying net of bonds. A structure type should be defined on the basis of its symmetry and the topology of the net formed by its strong primary bonds. Therefore, one should count among the rutile type derivatives only those which are based on a 6,3 net with the same bonding topology as the rutile type proper. This includes the CuF2 type and the CuUO4 type, but not the marcasite type (FeS2), the TeO2 type or the KrF2 type. Relationships between crystal structures can be viewed on three levels: 1. crystal structures of the same stoichiometry, having the same type of space group symmetry and based on the same pattern (net) of bonds are isostructural; 2. crystal structures of the same stoichiometry, crystallizing in a subgroup of a more highly symmetric aristotype and based on the same bonding pattern are derivatives of the aristotype; 3. crystal structures of the same stoichiometry, crystallizing in the same type of space group as an aristotype or in a subgroup, but not based on the same bonding pattern are merely formally related to the aristotype.