Anti-perovskite structure

RfLl phases The existence of these phases (cubic AuCu3 type) had been reported for R = La, Ce, Pr, Nd and Sm. Subsequently, however, Buschow and van Vucht (1967) found that many of the R3A1 phases do not form, unless some carbon is present. C atoms occupy the body-centred site, which in the AuCu3 type structure is normally vacant, while the Au atoms occupy the corners, and the Cu atoms the face-centred positions. When the face-centred position is completely filled, the structure is known as the anti-perovskite structure. This occurs for R = Nd, Sm, Gd, Tb, Dy Ho and Er. They also noted that neither N nor O would stabilize these compounds. Notice that Ce3Al and Pr3Al are truly binary compounds and C is not required for these two phases to form. Independently, Nowotny (1968) found that the anti-perovskite structure could be formed by C and N additions to R3M alloys, where M = Al, Ga, In, Tl, Sn and Pb. 

Figure 8.5 Structure of Ca3AsN. An example of a calcium containing ternary nitride in the anti-perovskite structure. White spheres = Arsenic, White prisms = Ca6N octahedra.

The reaction of alkali metal nitrites with alkali metal oxides M2O yields salts M3NO3 that do not contain the orthonitrite anion N03 but are mixed crystals of the salts MNO2 and M2O with anti- perovskite structure (N02)0Na3. Some properties of important nitrite salts are given in Table 28. 

Ternary alkali-metal halide oxides are known and have the expected structures. Thus Na3C10 and the yellow K3BrO have the aqti-perovskite structure (p. 963) whereas Na4Br20, Na4l20 and K4Br20 have the tetragonal anti-K2NiF4 structure. 

Alkaline earth-containing ternary nitrides make up the second largest group of ternary phases. Because the alkaline earth metals form stable binary nitrides, most alkaline earth containing ternary nitrides have been synthesized by the reaction of a binary nitride with a metal or by the reaction of two binary nitrides. This synthesis has resulted in a number of new ternary nitrides with a variety of structures. For example, the reaction of calcium nitride with Group 14 or 15 metals or metalloids forms a series of structurally related ternary nitrides with the anti-perovskite type structure. In Ca3MN (M = P, As, Sb, Bi, Ge, Sn, Pb) (Figure 8.5) the... 

Figure 3a Sheared Sr03-substructure of the hexagonal perovskite structure as model for the hypothetical compound Na3N. N3 -ions are surrounded by Nations forming cubic close packed distorted anti-cube-octahedra. Nations are depicted as black spheres (not drawn to scale). Average Na-Na- and Na-N-distance about 2.6 A (space group PmmnZ, no. 59).

PbFg, but RbF and CsF give the perovskite-type compounds MPbFg. Potassium and rubidium also form non-stoichiometric compounds MnPb4 p2-n where n == 0.2 to 0.3. These have an anti-aAgI structure (p. 153) with additional F ions. 

In the tetragonal 74/ mem structure, in CeAlOa below 314 K, the RO12 polyhedra are based on four short, four medium, and four longer bonds, whereas the AlOg octahedra remain practically regular (Table 28). Deformation from the ideal perovskite structure is also reflected by cooperative anti-phase rotations of... 

These materials have the ilMnOs R = Sc or small, rare earth cation) stoichiometry,and have been erroneously referred to as hexagonal perovskites. The compounds do not exhibit the perovskite structure. The Mn cations are not octahedrally coordinated, rather the cation is surrounded by five oxide anions in a trigonal prismatic coordination environment. Also the R cations are not 12-coordinate, as would be the case in a perovskite, but are in seven-fold coordination. The materials are multi-ferroic, with anti-ferromagnetic and ferroelectric properties.The nature of the polarity and therefore the ferroelectric behaviour was only recently described. Careful structural studies indicated that although the dipole moments are attributable to the R-O bonds and not the Mn-O bonds, the R-cations are not directly responsible for the ferroelectric behaviour. The noncentrosymmetry is attributable to the tilting of the MnOs polyhedra, which in conjunction with the dipole moments in the R-O bonds results in ferroelectric behaviour. Thus the ferroelectric behaviour in these materials is termed improper " and occurs by a much different mechanism than BaTiOs or even BiFeOs. 

Ag+ AgaSI 1 X 10- (25"C) Anti-perovskite-type structure. Non-oxide anti-perovskite Averaged structure for Ag... 

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. 

This phase is stable above 23 GPa, which is the typical pressure at the upper/lower mantle boundary. At low pressures it is metastable, the stabler phases being pyroxenes. The orthorhombic phase is obtained from the cubic perovskite phase (Pm3m) by superposing rotations ofnearly-rigid oxygen octahedra to an anti-ferroelectric displacement of Mg ions perpendicular to the c-axis. This brings to twenty and ten the number of atoms per cell and structural parameters respectively. 

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