Perovskites parameters

Figure 11 Models of the structures of Ba2YCu3Ox for x = 6.85 (Figure a) and x = 6.15 (Figure b). Both structures are based on a unit cell of parameters 2>/2ac 2y/2ac 3ac where a is the basic perovskite parameter. Also in this case some of the Cu(l) atoms are in three-fold coordination.

When 0.4 < x < 0.53, an orthorhombic phase is observed in the AgxNb02+xFi.x system. This phase undergoes a phase transition at 900°C that leads to the formation of a tetragonal phase, which crystallizes in a tetragonal tungsten bronze-type structure with cell parameters a = 12.343 and c = 3.905 A. When 0.82 < x < 1, solid solutions based on AgNb03 were found, which crystallize in a perovskite-type structure. 

Figure 2. (a) Formation and lattice parameters of perovskite borides versus rare-earth radius values at 1300°C... 

In perovskite-type catalysts the formation of the final phase is completed already at 973 K. XRD and skeletal FTIR/FTFIR data for LalCol, LalMnl and LalFel calcined at 973 K evidence that only LalFel-973 is actually monophasic and consists of a perovskite-type phase with orthorombic structure. A perovskite type phase with hexagonal-rombohedral structure represents the main phase of LalCol-973, but traces of C03O4 and La2C05 are also present. In the case of LalMnl-973 two phases have been detected both with perovskite-type structure, one orthorombic and the other rombohedral. The calculated cell parameters of the dominant perovskite-type phase are reported in Table 1 for the three samples. The results compare well with those reported in the literature [JCPDS 37-1493, 32-484, 25-1060] which refer to similar samples prepared via solid state reartion. All the perovskite-type samples are markedly sintered... 

Blinc R (2007) Order and Disorder in Perovskites and Relaxor Ferroelectrics. 124 51-67 Boca R (2005) Magnetic Parameters and Magnetic Functions in Mononuclear Complexes Beyond the Spin-Hamiltonian Formalism 117 1-268 Bohrer D, see Schetinger MRC (2003) 104 99-138 Bonnet S, see Baranoff E (2007) 123 41-78... 

The most abundant of all minerals in the interior of the earth is (Mg,Fe)Si03 perovskite. It constitutes greater than seventy percent of the lower mantle, so it is of great importance to geophysics. At room temperature the hardness of MgSi03 is VHN = 1800kg/mm2 and its Chin-Gilman parameter is 0.01. 

The hardnesses of some perovskites are given in Table 11.1 (based on the data of Yamanaka et al., 2004). The table shows that these perovskites are moderately hard and the third column which lists their Chin-Gilman parameters indicates that they are predominately ionically bound. 

Double Heck reaction, 42 494 Double layer interface, 30 223-225 Double nucleophilic displacement, capped cyclodextrin, 32 437 Double-pulse method, 38 31 Double recognition models, 32 451 52 Doublet mechanisms, 30 43, 45, 47 Drago parameters, 38 212 Drougard-Decrooq equation, 30 345, 356, 371 Dry evaporation, perovskite preparation, 36 246-247... 

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. 

Equation 13 can be solved numerically for Tc as a function of the proton-lattice coupling. The parameters are chosen so as to fit the experimental value of Tc for KDP. For C = 21 732 K/A and g2ygAyf close to those used for perovskite oxides, Tc Ikdp = 115 K. In Fig. 3 Tc is shown as a function of C with all other parameters fixed. Including the deuteration effects (Table 2), Ter = C Idkdp/C Ikdp 1 2. With this estimate TcIdkdp = 168 K. C itself depends only weakly on /, g2y g4 but a strong dependence on/ is observed, which is the coupling between the PO4 shells and the K" " ions. This, on the other hand, should not be dependent on deuteration. 

The structure of the hexagonal oxide perovskite BaRuOa, recently described by Donohue et al. 84), is also adapted by the ternary fluoride CsCoFs 11). The positional parameters (not listed above) are almost the same in both compounds. 

Table 6. Ligand-field parameters of perovskite compounds (in cm )...

The familiar cubic perovskite structure of ABO3 has of course just one structural parameter, the unit cell edge a. This requires the ratio of the A-O to the B-O bond lengths to be equal to y/2. When this condition cannot be met, the structme distorts in (one of) a number of well-documented ways . By far the largest of the families of derivative structures that arise when A is too small [/(A-0)//(B-0) < /2] is that of the orthorhombic perovskites (GdFeOa type) exemplified by the mineral perovskite (CaTi03) itself. 

A simpler (in the sense of fewer free parameters) stracture derived from that of perovskite is that of caldte, CCaOa. This may be obtained from the cubic structure by concerted rotations of CaOj octahedra about axes parallel to [111], so that the coordination of C is reduced from O12 to 03 In terms of anion coordination, this corresponds... 

For our present purposes, simple crystal structures might be segregated into three groups. The first of these is comprised of those few structures with even fewer parameters than independent bond lengths examples already encountered above include the (cubic) perovskite, pyrochlore and Na3Pt04 structures. 

Of the structures with stoichiometry ABO3 that may be derived from that of perovskite (Sect. 2.2), the calcite structure is the simplest in the sense that it requires the fewest parameters to specify it. We earlier described this structure in terms of the parameters obtained with regular BOe octahedra that are rotated (tilted) about their 3 axes from the positions they have in (cubic) perovskite. The coordination of A goes from 12 in the cubic structure to 3 in the rhombohedral calcite structure. 

The compound NCu02 (N = Ca0 86Sr014) is an insulator, but its structure, which is a simple defect perovskite made of layers (CuOa) sandwiched between layers (N), can be considered as the parent structure of a large family of superconductors. The sequence. ..(Cu0 )OiC(N)e o(Cu02)OjC. .. is in fact one of the building blocks of many compounds considered in this review. The refined parameters for NCu02 are given in Table 3. 

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