Perovskite distortion

Keywords Perovskite distortions. Magnetic configurations. Fluctuation band narrowing. Interatomic exchange... 

The reddish pigments with perovskite structure of type doped chromium lanthanides aluminates LnAli.xCrxOs is when Al is substituted by Cr occupying octahedral sites in the structure forming solid solution, because Cr radium is 12.3% in size bigger than the size of Al, (0.63A) and (0.51 A) respectively. This makes that the Cr-0 distance decreases and the Al-0 distance and the perovskite distortion increase. 

Ca is replaced by a rare-earth element, resulting in a distorted perovskite stmcture, which is essentially orthorhombic. Orthoferrites, studied extensively in the early 1970s as potential data storage materials based on magnetic bubble domains (10), have been largely replaced by the garnet materials (see... 

The ternary Ge halides, MGeX3 (M = Rb, Cs X = Cl, Br, I) are polymorphic with various distorted perovskite-like (p. 963) structures which reflect the influence of the nonbonding pair of electrons on the Ge" centre. Thus, at room temperature, rhombohedral CsGel3 has three Ge-I at 275 pm and three at 327 pm whereas in the high-temperature cubic form (above 277°C) there are six Ge-I distances at 320 pm as a result of position changes of the Ge atoms (reversible order-disorder transition). Again, RbGel3 has a lemon-yellow, orthorhombic form below —92° an intermediate, bordeaux-red orthorhombic perovskite form (—92° to —52°) a black rhombohedral form (—52° to —29°) and... 

Chlorates and bromates feature the expected pyramidal ions X03 with angles close to the tetrahedral (106-107°). With iodates the interatomic angles at iodine are rather less (97-105°) and there are three short I-O distances (177-190 pm) and three somewhat longer distances (251-300 pm) leading to distorted perovskite structures (p. 963) with pseudo-sixfold coordination of iodine and piezoelectric properties (p. 58). In Sr(I03)2.H20 the coordination number of iodine rises to 7 and this increases still further to 8 (square antiprism) in Ce(I03)4 and Zr(I03)4. 

F can he suppressed hy the high site symmetry of the central atom In many perovskite-like structures of the ABO3 type the lone pair of the B-cat-ion leads not to a structural distortion. In CsPbF3 under ambient conditions no lone-pair activity observed [27], but upon cooling a phase transition is observed that leads to less symmetrical surrounding of Pb by fluoride [28]. 

The sharp peak around OV in Fig. 4 may be caused by the insertion of protons into the empty A sites of the poorly crystalline WO3 with a distorted perovskite structure. Amorphous... 

The magnetic properties of the new solid solution series SrFe Rui 3 3, (0 < X < 0.5) with distorted perovskite structure, where iron substitutes exclusively as Fe(in) thereby causing oxygen deficiency, has also been studied by Greenwood s group [147] using both u and Fe Mossbauer spectroscopy. Iron substitution was found to have little effect on the magnetic behavior of Ru(IV) provided that X remains small (x < 0.2). 

The tolerance factor t for perovskites AMX3 is a value that allows us to estimate the degree of distortion. Its calculation is performed using ionic radii, i.e. purely ionic bonding is assumed ... 

Geometry requires a value of t = 1 for the ideal cubic structure. In fact, this structure occurs if 0.89 < t < 1. Distorted perovskites occur if 0.8 < t < 0.89. With values less than... 

In sodium nitrite the ferroelectric polarization only occurs in one direction. In BaTiOs it is not restricted to one direction. BaTiOs has the structure of a distorted perovskite between 5 and 120 °C. Due to the size of the Ba2+ ions, which form a closest packing of spheres together with the oxygen atoms, the octahedral interstices are rather too large for... 

The parent perovskite-type structure (Fig. 4.13A>) is composed of corner-linked BOe octahedra surrounding large A cations and is conveniently idealized to cubic symmetry (Fig. 4.27a). (The real structures have lower symmetry than the idealized structures, mainly due to temperature-sensitive distortions of the BOft octahedra.) In the phases related to Ca2Nb2C>7 the parent structure is broken into slabs parallel to 110 planes. The formula of each slab is A B 03 +2, where n is the number of... 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Figure 11.6 AMF3 crystal structures, (a) Ideal cubic perovskite structure, (b) Tilting of MXg octahedra in orthorhombically distorted AMF3 perovskites. (c) RbNiF3 CSC0F3 and CsNiF3 crystal structures, (d) Crystal structure of lithium niobate.

The perovskite structure and its variant and derivative structures, and superstructures, are adopted by many compounds with a formula 1 1 3 (and also with more complex compositions). The ideal, cubic perovskite structure is not very common, even the mineral CaTi03 is slightly distorted (an undistorted example is given by SrTi03). 

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