Distorted perovskites

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. 

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). 

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... 

Before dealing with these structure t es in detail, the clear-cut dependence of the occurence of these distorted perovskites on the radius ratio of the ions in question should be mentioned. The tolerance factor defined by Goldschmidt [115),... 

A special case of distorted perovskite structures is reported of compounds KxFeF3 of the bronze type (243). There appear close relationships to the similar bronzes of tungsten, KxW03, which will not be discussed further in this paper. 

A group of 8 ternary fluorides containing the transition metal ions Cr2+ and Cu + crystallizes in a tetragonedly distorted perovskite lattice. This distortion is caused by the Jahn-Teller effect displayed by the configurations d% d (Cr +) and d d (Cu2+) resp., rather than by geometrical reasons. As for their space requirements the ions Cr + and Cu + are very close in size to Mn2+ and Co + resp. and as a consequence the corresponding compounds do not differ in their tolerance factors. 

The syntheses of several simple AB03 compounds by Art Sleight led him to the first example of oxide superconductors possessing the ordered perovskite structure. It was already known that BaPbOg, a distorted perovskite with pseudo-orthorhombic symmetry, exhibited metallic conducting properties to very low temperatures, but... 

Structure at high temperatures. A distorted perovskite would be expected to transform to the cubic structure at high temperatures. The Born model of ionic solids with the appropriate repulsive and van der Waals parameters can explain the relative stabilities of crystal structures in partly covalent solids, an ionicity parameter would have to be used to predict the preferred crystal structure (see Chapter 1, Section 1.3). 

The salts KCrF3 and RbCrF3 have distorted perovskite structures, and from analysis of powder data the former contains tetragonally compressed CrFe octahedra in which two Cr—F bond distances are 2.00 A and four 2.14 A.244 The ionic radius of Cr23- in fluoride perovskites has been estimated at 0.73 A as a weighted average from lattice constants.245... 

ABOs compounds containing lanthanum are closer to the ideal perovskite than those containing smaller rare earth ions. Compounds of the smaller rare earth ions appear to have a distorted perovskite structure of lower symmetry. When, however, the relationship between the radii is very far from being ideal (eq. 31), strong distortion may result giving an entirely different structure. The Goldschmidt tolerance factor, t, for the perovskite structure is related to the ionic radii by... 

The manganites with general formula Laj vSrvMn03 have structures of distorted perovskite. Depending on the types of distortions, the structure could be called orthorhombic O with space group Pnma, orthorhombic O with space group Pnma and rhombohedral phase with the space group R3c. The first structure... 

Fava et al. (16) prepared Balh03 from stoichiometric amounts of BaO and TTT02 and observed an ideal perovskite, whereas Nakamura (17) found that an excess of BaO is necessary and identified a distorted perovskite structure. It is also not obvious why BaO is not taken up in solid solution with Balh03 (15) BaU03 is... 

Tetragonal (P4wn)—distorted perovskite, see (/) Cubic-perovskite [MnCl6]—formed by three-dimensional comer-sharing Monoclinic P2Jc)—infinite chains of face-sharing octahedra, like (a)... 

LuRh03 2.2 Distorted perovskite structure with p-type semiconductor behavior. 429... 

X > 0.28, this results in metalhc conductivity (Goodenough, 1965 Greenblatt, 1996). Nonstoichiometric sodium tungsten oxide (x < 1) has a distorted perovskite stmcture with unequal W-O bond lengths and tilted WOe octahedra. 

By solid-state reactions of Am02 with AI2O3 at 1250°C. in H2 no compound formation could be observed. In order not to increase the reaction temperature, a 1 1 mixed hydroxide precipitation of americium and aluminium reacted in H2 for 2-8 hrs. at 1250°C. This resulted in the quantitative formation of rose-colored AmAlOa having the hexagonal distorted perovskite structure of the aluminates of La, Pr, Nd (JJ), and Pu (17,35). 

While the compounds quoted of the tetravalent actinides show a tetrago-nally distorted perovskite structure, the symmetry of the Me (III) compounds is orthorhombic-pseudotetragonal. 

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