Perovskite structure type example compounds

The often mentioned relations between the structure types of cryolite and perovskite (page 41) may be explained best with the example of the elpasoHte type. The elpasoUte structure is really a superstructure of the perovskite-lattice, generated by substituting two divalent Me-ions in KMeFs by two others of valency 1 (Na) and 3 (Me) resp. The resulting compound K (Nao.5Meo.5)F3 crystallizes with an ordered distribution of Na+ and Me + because of the differences in size and charge of the ions. Thus to describe the unit cell the lattice constant of the perovskite ( 4 A) has to be doubled to yield that of the elpasohte structure ( 8 A). 

This type of orthorhombic perovskite structure appears, if the tolerance factor of Goldschmidt is smaller than t — 0.88. The example of the compound NaMnFs [t = 0.78), showing doubled lattice constants a and h (287), is likely to mark the lower limit of the field in which orthorhombic fluoro-perovskits of the GdFe03-t3q>e may occur. Fluoroperovskites which have a smaller tolerance factor than t = 0.78 never have been observed so far, nor do fluoride structures of the ilmenite type seem to exist, which might be expected for ya = Me, corresponding to 1=1/1/2=0.71. 

The fact that the structures of compounds ABO3 are dependent not only on size factors but also on the nature of B has been demonstrated in many comparative studies. For example, while AFe03 (A = lanthanide) all have perovskite-type structures this is true for AMn O3 only if A is La or Ce—Yb. The compounds in which A= Ho-Lu adopt a new hexagonal structure with 5- and 7-coordination of Mn and A respectively. In the series Bax - vSrxRu03 the structure changes from the 9-Iayer to the 4-iayerand then to the perovskite structure forx = 0, 5, and but Bax jcSrxIr03 shows a more complex behaviour. ... 

Most MF2 compounds have the rutile structure, other dihalides forming layer (CdCl2 and Cdl2) types. Many ternary oxides and halides also follow this pattern for example, the LaM03 compounds formed by all elements of the series (M=Sc—Cu) have the perovskite structure (see Topic D5Y... 

A majority of the important oxide ceramics fall into a few particular structure types. One omission from this review is the structure of silicates, which can be found in many ceramics [1, 26] or mineralogy [19, 20] texts. Silicate structures are composed of silicon-oxygen tetrahedral that form a variety of chain and network type structures depending on whether the tetrahedra share comers, edges, or faces. For most nonsilicate ceramics, the crystal structures are variations of either the face-centered cubic (FCC) lattice or a hexagonal close-packed (HCP) lattice with different cation and anion occupancies of the available sites [25]. Common structure names, examples of compounds with those structures, site occupancies, and coordination numbers are summarized in Tables 9 and 10 for FCC and HCP-based structures [13,25], The FCC-based structures are rock salt, fluorite, anti-fluorite, perovskite, and spinel. The HCP-based structures are wurtzite, rutile, and corundum. 

Anion vacancy in perovskites is more common than cation vacancy. Unlike the well-known case of W03, anion-deficient nonstoichiometry is not accommodated by the crystallographic shear mechanism, but by assimilation of vacancies into the structure, resulting in supercells of the basic network. The review by Rao et al. (24) contains numerous examples of this kind of behavior. Anion excess has been described in a more limited number of systems. Structural details of this type of compounds can be found in Rao et al. (24) and Smyth (25). 

As an analogous detailed treatment of the P- and F-family would be beyond the scope of this paper, only examples of structure types for these families are discussed. Table 8a contains some structure types which may be described either in the F-family (index 1) as addition compounds or as belonging to main classes or subclasses of die P-famfly (index 8). An hypothetical structure type with symmetry Pm3m is listed too. The first three structure types of Table 8b are closely related to the perovskite structure. The... 

Example 3 Synthesis of multi-component complex compounds A nmnber of materials such as some lead-containing complex oxides are very difficult to prepare in the form of pine phase. Powder of Pb(Znj,.Mgi jt)i/3Nb2/303 was obtained by soft-mechanochemical procedure, that is, by milling of a stoichiometric mixture of PbO, Mg(OH)2, Nb20s and 2Zn(OH)2 H2O in a multi-ring-type mill up to 3 h. Partial formation of desired phase with perovskite structure already took place during milling. Subsequent heat treatment at 1000°C for 1 h yielded pure perovskite phase. 

In the rare, earth perovskite, RBO3 type compounds the materials whose optical properties have been explored are those in which B is Al, Gd, Cr, or Fe. Generally the bulk properties are determined by the B ion. When this is a transition metal ion the material tends to be opaque, otherwise transparent. This is associated with the localized 3d electrons of the transition metal ion (Allen 1975). The orthoaluminates and orthogallates are transparent throughout the visible region except for the areas where absorption lines due to the crystal field transitions of the rare earth ion are present (Merker and Herrington, 1964). For example, a triplet structure associated with the Tb transitions in TbAlOa has been reported by Hi ner et al. (1968). 

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