Ni octahedra

The Ni octahedra derive their stability from the interactions of s, p, and d electron orbitals to form octahedral sp3d2 hybrids. When these are sheared by dislocation motion this strong bonding is destroyed, and the octahedral symmetry is lost. Therefore, the overall (0°K) energy barrier to dislocation motion is about COCi/47r where = octahedral shear stiffness = [3C44 (Cu - Ci2)]/ [4C44 + (Cu - C12)] = 50.8 GPa (Prikhodko et al., 1998), and the barrier = 4.04 GPa. The octahedral shear stiffness is small compared with the primary stiffnesses C44 = 118 GPa, and (Cn - C12)/2 = 79 GPa. Thus elastic as well as plastic shear is easier on this plane than on either the (100), or the (110) planes. 

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

The anionic substructure of the salt Cs2[Ni(N3)4] H202183 consists of a 2D structure formed by Ni(N3)6 octahedra connected via four EE azido groups placed in a distorted plane, but no magnetic data have been reported. 

Figure 1.9 (a) The perovskite structure. Without the large A atom at the body centre position, the structure becomes that of cubic ReOj (b) The K2Nip4 structure consisting of rocksalt (KF) and perovskite (KNiFj) layers. The NiFg octahedra share equatorial corners restricting the Ni-F-Ni interaction to the xy-plane. 

Nitro-pentammino-chromic Nitrate, [Cr(NI-I3)5NO2](N03)2, is obtained from the chloride by precipitation of a solution of the chloride with concentrated aqueous ammonium nitrate, or by decomposing a solution of aquo-pentammino-salt with half its volume of dilute nitric acid and addition of sodium nitrite. It crystallises in yellow octahedra, is less soluble in water than the corresponding chloride, and decomposes explosively on heating. 

Tetrammino-nickel Nitrate, [Ni(NH3)4](N03)2.H 0, is obtained by the addition of ammonia to a solution of nickel nitrate and the addition of alcohol to the well-cooled liquid. The precipitated ammine crystallises in large transparent octahedra of a blue colour. It decomposes in air with loss of ammonia, and falls to a bluish-white powder. On heating it melts, loses water and ammonia, leaving a residue of 1 Werner, Ber., 1914, 47, 3087. 

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