Coordination numbers perovskite

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. 

Oxide ratio, 18 815 Oxides, 16 598 acidic, 22 190-191 bond strengths and coordination numbers of, 22 570t diorganotin, 24 819 glass electrodes and, 14 28 gold, 22 707 iron, 14 541-542 lead, 14 786-788 manganese, 15 581-592 nickel, 27 106-108 niobium, 27 151 plutonium, 29 688-689 in perovskite-type electronic ceramics, 14 102... 

The coordination number of chloride can be rather high in many sohds. Thus, iV = 8 in CsCl, iV = 6 in NaCl and the perovskite CsNiCla, iV=4 in CuCl, W = 3 in NiCl2 etc. It might be argued that the values of N higher than four correspond to exclusively electrovalent bonding. This is not an easy position to defend the NaCl-type MgS, BaS, LaN,... 

The geometric relaxation described in Section 12.3.1 occurs by redistributing the bond valence between the bonds until GII and BSI both have acceptable values, but in some cases this relaxation is restricted by symmetry. In the case of per-ovskite, the cubic symmetry of the archetypal ABO3 structure (Fig. 10.4) does not allow any of the bonds to relax unless the symmetry is lowered. Thus true cubic perovskites are rare since they can only exist if the A and B ions are exactly the right size. Most perovskites have a reduced symmetry that allows the bonds to relax. For compounds in which the A-O bonds are stretched, the relaxation takes the form of a rotation of the BOg octahedra and results in a reduction of the coordination number of A. The various relaxed structures based on different expected coordination numbers were modelled in Section 11.2.2.4. 

Fig. 13.1. The lattice parameter of a perovskite layer as a function of its cation coordination number. Cations in the BO2 layers are shown on the left, those in the AO layers are shown on the right.

LaCrC>3 is one of the family of lanthanide perovskites RTO3, where R is a lanthanide and T is a period 4 transition element. In the cubic unit cell R occupies the cube corners, T the cube centre and O the face-centre positions. The coordination numbers of T and R are 6 and 8 respectively. LaCrC>3 loses chromium at high temperatures, leaving an excess of O2- ions. The excess charge is neutralized by the formation of Cr4+ which results in p-type semiconductivity with hole hopping via the localized 3d states of the Cr3+ and Cr4+ ions. The concentration of Cr4+ can be enhanced by the substitution of strontium for lanthanum. A 1 mol.% addition of SrO causes the conductivity to increase by a factor of approximately 10 (see Section 2.6.2). 

Consequently, in order to explain our results, that is, the high absorption magnitudes measured, we advance the following hypothesis at a high temperature, as is our case here, the proton can have a high coordination number, and can be interstitially located in tetrahedral and octahedral sites. Therefore, the reaction of hydrogen with the perovskite is given by [32]... 

In this case, we have introduced more hydrogen than those incorporated in the previously referred paper [66], Therefore, more electrons were introduced during the H2 incorporation into the perovskite. Consequently, we can conclude that the studied material at 1023-1273 K behaves as a small band gap semiconductor, because of the increase of electrons with temperature in the conduction band, due to the shifting to the conduction band of the Fermi level and the hydrogen-induced level. Then, there will be a sufficient number of electrons in the conduction band to screen the proton and allow it to have a high coordination number, and be interstitially located in the tetrahedral and octahedral sites. 

Another simplification consists in the fact that among the various structural types of fluorides, the involved d-transition elements generally possess the coordination number 6. The crystallographic features can be deduced from the arrangement of (MF6) octahedra5,6. Besides in three-dimensional (3-D) networks such as found in perovskite, rutile or pyrochlore types for instance, fluorides crystallyze in two-dimensional (2-D) layer structures, one-dimensional (1-D) chain structures and isolated unit arrangements. 

It should be noted the coordination number, N for trivalent lanthanides does not bear the same relevance and context as the transition elements, Fe(3d), Pd(4d) and Pt(5d). The 4/" electron population does not influence N in moving along the lanthanides series while the d electron population has considerable influence on N in the transition metals. Taking nickel as an example we have compounds of Ni(IV), Ni(III), Ni(II) and Ni(0). The majority of the compounds are of Ni(II) such as Ni(H20) +, NiiNH- ) 4" which have N = 6 and an octahedral disposition. The compound KNiF3 is a cubic perovskite with N = 6 and also paramagnetic. When Ni(II) forms diamagnetic complexes N = 4 with a square planar disposition. Tetrahedral NiCl - with N = 4 tetragonal-pyramidal Ni(CN)j- with N = 5 are also known. 

In the orthorhombic perovskite-like compounds, LaFe03, the standard deviation of the average cation-oxygen bond distances has been used as a criterion for obtaining the coordination number [158]. For lanthanides Sm-Lu, there are eight oxygen atoms in the coordination sphere, and for lanthanum the coordination number is nine in the compound. 

The rare earth titanates crystallize in an orthorhombically distorted form of the perovskite structure, commonly known as the GdFeOs type, found widely for other LnT03 materials and described in Pnma (Pbnm in earlier literature) (Figure 16). As the Ln + ions are too small for 12-fold coordination, the TOe octahedral tilt to accommodate a lower coordination number, which is near 9-fold. 

Hydrolysis and condensation rates depend on the molecular structure of metal alkoxides and alkoxide precursors have to be chosen as a function of the desired material final product. In the case of Ti02, for instance, monomeric precursors such as Ti(OPF)4, in which Ti is fourfold coordinated, react very quickly with water leading to the uncontrolled precipitation of polydispersed Ti02. The reaction is much slower with oligomeric precursors such as [Ti(OEt)4] in which Ti has a higher coordination number. Spherical monodispersed Ti02 powders can be produced via the controlled hydrolysis of diluted solutions of Ti(OEt)4 in EtOH. On the contrary, monomeric precursors are more convenient for the sol-gel synthesis of multicomponent oxides. The perovskite phase BaTiOs is formed upon heating around 800 °C when [Ti(OEt)4] is used as a precursor. This temperature decreases down to 600 °C with the monomeric precursor Ti(OPT)4 which favors the formation of Ti-O-Ba bonds. ... 

The coordination numbers of metal ions range from I. as in ton pairs such as Na CI in the vapor phase, (o 12 in some mixed metal oxides. The k>v limit, 1. Is barely within the realm of coordination chemistry, since the Na Cr km pair would not normally be considered a coordination compound, and there are few other examples. Likewise, the upper limit of 12 is not particularly important since k is rarely encountered in discrete molecules, and the treatment of sohd crystal lattices such as hexagonal BaTiOj and perovskite> as coordination compounds is not dc ie frequently. The lowest and highest coordination numbers found in typical coordination compounds are 2 and 9 with the intermediate number 6 being the most important. 

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