Coordination-type structure

There are two ways to further reduce the X Me ratio to a value lower than 3. The first is to arrange simple similar to coordination type structure that requests steric similarity of all cations so as to be able to occupy octahedral voids. This method will be discussed separately. The second way is to arrange the linking of the octahedrons so as to render additional structural elements, namely pentagonal "blocks". 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

The lowest coordination number of tantalum or niobium permitted by crystal chemistry formalism is 6, which corresponds to an octahedral configuration. X Me ratios that equal 3, 2 or 1 can, therefore, be obtained by corresponding substitutions in the cationic sub-lattice. A condition for such substitution is no doubt steric similarity between the second cation and the tantalum or niobium ion so as to enable its replacement in the octahedral polyhedron. In such cases, the structure of the compound consists of oxyfluoride octahedrons that are linked by their vertexes, sides or faces, according to the compound type, MeX3, MeX2 or MeX respectively. Table 37 lists compounds that have a coordination-type structure [259-261]. 

Typical examples of compounds with a coordination-type structure are Nb02F and Ta02F, which crystallize in a Re03 type structure [233, 243]. Oxygen and fluorine ions are statistically distributed in the anionic sub-lattice. The compounds are characterized by X Me = 3 and can be described as MeX3 type compounds. 

Table 37. Fluoride and oxyfluoride compounds with coordination-type structures...

When X Me < 3 two possibilities exist coordination-type structure (3, 2, 1) or blocks (2.67), when the stoichiometry of the compounds does not correspond to the formation of any coordination- type compound. 

Me02F Coordination-type structures Shared vertexes - MeX3... 

Framework-type structure Coordination-type structure... 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

The molecular structure input requires atom types to be assigned, which are not the same from one force field to the next. The input also includes a list of bonds in the molecule. There is not a module to automatically assign atom types. Most of the modules use a Cartesian coordinate molecular structure, except for a few that work with torsional space. The same keyword file is read by all the executables. A little bit of input is obtained by the program either interactively or from an ASCII file piped to standard input, which makes for a somewhat cryptic input file. This system of common input files and the user choosing which executables to run give TINKER the ability to run very sophisticated simulations while keeping the input required for simple calculations fairly minimal within the limitations mentioned here. 

High-temperature neutron diffraction studies have shown that this latter phase has the cubic ordered Re03-type structure (p, 1047) with octahedral coordination of both types of Sn atoms by F (Sn -F 229 pm, Sn" -F 186 pm). The fi-phase also features octahedral coordination in a structure closely related to that of rhombohedral LiSbF6. 

Compounds with Sc, Y, lanthanoids and actinoids are of three types. Those with composition ME have the (6-coordinated) NaCl structure, whereas M3E4 (and sometimes M4E3) adopt the body-centred thorium phosphide structure (Th3P4) with 8-coordinated M, and ME2 are like ThAsi in which each Th has 9 As neighbours. Most of these compounds are metallic and those of uranium are magnetically ordered. Full details of the structures and properties of the several hundred other transition metal-Group 15 element compounds fall outside the scope of this treatment, but three particularly important structure types should be mentioned because of their widespread occurrence and relation to other structure types, namely C0AS3,... 

Planar 4-coordinate O occurs uniquely in NbO which can be considered as a defect-NaCl-type structure with O and Nb vacancies at (000) and (555) respectively, thereby having only 3... 

Schematic representation of defect clusters in Fei- jO. The normal NaCl-type structure (a) has Fe (small open circles) and O (large dark circles) at alternate comers of the cube. In the 4 1 cluster (h), four octahedral Fe" sites are left vacant and an Fe" ion (grey) occupies the cube centre, thus being tetrahedrally coordinated by the 40. In (c) a more extended 13 4 cluster is shown in which, again, all anion sites are occupied but the 13 octahedral Fe sites are vacant and four Fe occupy a tetrahedral array of cube centres.

All three metals form a wide variety of binary chalcogenides which frequently differ both in stoichiometry and in structure from the oxides. Many have complex structures which are not easily described, and detailed discussion is therefore inappropriate. The various sulfide phases are listed in Table 22.4 phases approximating to the stoichiometry MS have the NiAs-type structure (p. 556) whereas MS2 have layer lattices related to M0S2 (p. 1018), Cdl2, or CdCl2 (p. 1212). Sometimes complex layer-sequences occur in which the 6-coordinate metal atom is alternatively octahedral and trigonal prismatic. Most of the phases exhibit... 

Violet, easily hydrolysed, PdFp is produced when Pd [Pd F(sl s refluxed with SCF4 and is notable as one of the very few paramagnetic compounds of Pd. The paramagnetism arises from the configuration of Pd which is consequent on its octahedral coordination in the rutile-type structure (p. 961). The dichlorides of both Pd and Pt are obtained from the elements and exist in two isomeric forms which form i.s produced depends on the exact experimenial conditions used. The more usual a form of PdCb is a red material with... 

Since the coordination number of tantalum or niobium in fluoride and oxyfluoride compounds cannot be lower than 6 due to steric limitations, further decrease of the X Me ratio (lower than 6) leads to linkage between complex ions in order to achieve coordination saturation by sharing of ligands between different central atoms of the complexes. The resulting compounds have X Me ratios between 6 and 4, and form crystals with a chain-type structure. 

A further decrease in the X Me ratio, to 4, leads to linkage of the octahedral units by sharing more than one ligand so as to achieve coordination saturation. Sharing of two vertexes (two comers of the each octahedron) leads to the formation of compounds with layered-type structures. 

The structure of LiTa02F2, as reported by Vlasse et al. [218], is similar to a ReC>3 type structure and consists of triple layers of octahedrons linked together through their vertexes. The layers are perpendicular to the c axis, and each layer is shifted, relative to the layer below, by half a cell in the direction (110). Lithium atoms are situated in the centers of the tetragonal pyramids (coordination number = 5). The other lithium atoms are statistically distributed along with tantalum atoms (coordination number = 6) at a ratio of 1 3. The sequence of the metal atoms in alternating layers is (Ta-Li) - Ta - (Ta-Li). Positions of oxygen and fluorine atoms were not determined. The main interatomic distances are (in A) Ta-(0, F) - 1.845-2.114 Li-(0, F) - 2.087-2.048 (O, F)-(0,F) - 2.717-2.844. 

When 8 < X Me < 6 the structure remains an island-type structure with a respective increase in the coordination number. 

---