Oxygen crystal structures, lattice parameters

How can we be sure that the U +(Q2-) complex in a mixed metal oxide is present as the UO octahedron This can be done by studying solid solution series between tungstates (tellurates, etc.) and uranates which are isomorphous and whose crystal structure is known. Illustrative examples are solid solution series with ordered perovskite structure A2BWi aUa 06 and A2BTei-a Ua 06 91). Here A and B are alkahne-earth ions. The hexavalent ions occupy octahedral positions as can be shown by infrared and Raman analysis 92, 93). Usually no accurate determinations of the crystallographic anion parameters are available, because this can only be done by neutron diffraction [see however Ref. (P4)]. Vibrational spectroscopy is then a simple tool to determine the site symmetry of the uranate complex in the lattice, if these groups do not have oxygen ions in common. In the perovskite structure this requirement is fulfilled. 

Between the two possible defects which may be responsible for hyperstoichiometry (i.e. uranium interstitials or oxygen vacancies) the latter is well evidenced by measurements of lattice parameter and densityand neutron diffraction Oxygen interstitials order in U4O9 to provide a crystal structure which can be derived from the fluorite structure of U02+x-... 

In general, a non-stoichiometric compound can be defined as one with variable composition. However, the major structural features are maintained. For example, Figure 6.4 shows the variation in the cubic lattice parameter (a) with oxygen content of Fej 0, which crystallizes with the rock-salt structure. A smooth variation is apparent, gradually reducing as the iron content decreases. 

Uranium(IV) oxide, UOj, crystallizes with the fluorite structure as shown in Figure 6.5. On heating in oxygen, additional oxygen can be taken up by the lattice to form the non-stoichiometric material 002+. There is a gradual increase in the lattice parameters as additional oxygen is added to the structure. 

Fig. 14.1. The crystal structure of YBa2Cu307 x. The lattice parameters are a = 0.383 nm, b — 0.388 nm and c — 1.17 nm. The lattice parameters depend on the oxygen content [e.g. 14.11]. In the following figures, the structure is represented by a rectangular box shown to the right.

Orthorhomh-PhO also has a layered structure the layers are huilt of infinite Pb—O chains [2] as shown in Fig. 5.1. The surfaces of the layers are composed of Ph ions and each oxygen ion is surrounded hy four lead ions. The chain layers are stabilised by van der Waals bonds [2]. Therefore, the orthorhombic-PbO crystals are prone to flaking. In the covalent pattern, the electron pairs are localised and their delocalization requires excitation. This is responsible for the very low dark electric conductivity of orthorhomb-PbO. The following are the lattice parameters of the elementary cell a = 5.489 A, b = 4.755 A and c = 5.891 A. The melting point of orthorhombic PbO is 885 °C and the boiling point is 1480 °C. 

The crystal structure of La202C2 determined from three-dimensional X-ray diffraction on a twin crystal is of monoclinic symmetry, space group C/2m (Seiver and Eick 1976). The lattice parameters are a = 7.069(8) A, b = 3.985(4) A, c = 7.310(9) A and p = 95.70(6)° the calculated density is 5.41 gcm". In this structure, the lanthanum atom has four oxygen and four carbon atoms situated in a distorted bicapped trigonal prismatic arrangement. Interatomic La-O distances range from 2.392(8) to 2.823(9) A and La-C distances from 2.86(1) to 3.11(1) A. The carbon atoms are present as C2 units with an interatomic C-C distance of 1.21(3) A. Oxygen atoms are tetrahedrally coordinated, as in the sesquioxide. 

ZnO normally has the hexagonal (wurtzite) crystal structure with lattice parameters a = 3.25 A and c = 5.12A (space group P63mc). The Zn atoms are tetrahedraDy coordinated to four O atoms, where the Zn d-electrons hybridize with the oxygen p-electrons. Layers occupied by zinc atoms alternate with layers occupied by oxygen atoms [94]. Whilst a bond between the Zn and O atoms exhibits covalent characteristic in the c-direction, it is mostly ionic in the o-direchon [95] consequently, ZnO single crystals have highly anisotropic properties. 

Crystal structure studies of many orthophosphates have confirmed the tetrahedral distribution of four oxygen atoms about a central phosphorus atom. A tetrahedral configuration has also been demonstrated by numerous infrared, Raman and NMR spectroscopic studies of solid, liquid and solution states of these compounds. Slight deviations from perfect tetrahedra (Td) symmetry occur in most crystalline orthophosphates, due to effects of lattice environment, and so on, but this distortion is considerably reduced in dilute aqueous solutions. Experimental parameters for some typical crystalline salts are in (5.31). 

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