NaCl type

Stmctures are fee of the NaCl type, unless otherwise noted. 

There is a lively controversy concerning the interpretation of these and other properties, and cogent arguments have been advanced both for the presence of hydride ions H" and for the presence of protons H+ in the d-block and f-block hydride phases.These difficulties emphasize again the problems attending any classification based on presumed bond type, and a phenomenological approach which describes the observed properties is a sounder initial basis for discussion. Thus the predominantly ionic nature of a phase cannot safely be inferred either from crystal structure or from calculated lattice energies since many metallic alloys adopt the NaCl-type or CsCl-type structures (e.g. LaBi, )S-brass) and enthalpy calculations are notoriously insensitive to bond type. 

The carbides of the lanthanoids and actinoids can be prepared by heating M2O3 with C in an electric furnace or by arc-melting compressed pellets of the elements in an inert atmosphere. They contain the C2 unit and have a stoichiometry MC2 or M4(C2)3. MC2 have the CaC2 structure or a related one of lower symmetry in which the C2 units lie at right-angles to the c-axis of an orthogonal NaCl-type cell. They are more reactive than the alkaline-earth metal... 

Another valuable reagent is KSiH3, a eolourless erystalline eompound with NaCl-type... 

Planar 4-coordinate O occurs uniquely in NbO which can be considered as a defect-NaCl-type structure with O and Nb vacancies at (000) and (555) respectively, thereby having only 3... 

Schematic representation of defect clusters in Fei- jO. The normal NaCl-type structure (a) has Fe (small open circles) and O (large dark circles) at alternate comers of the cube. In the 4 1 cluster (h), four octahedral Fe" sites are left vacant and an Fe" ion (grey) occupies the cube centre, thus being tetrahedrally coordinated by the 40. In (c) a more extended 13 4 cluster is shown in which, again, all anion sites are occupied but the 13 octahedral Fe sites are vacant and four Fe occupy a tetrahedral array of cube centres.

The predominantly ionic alkali metal sulfides M2S (Li, Na, K, Rb, Cs) adopt the antifluorite structure (p. 118) in which each S atom is surrounded by a cube of 8 M and each M by a tetrahedron of S. The alkaline earth sulfides MS (Mg, Ca, Sr, Ba) adopt the NaCl-type 6 6 structure (p. 242) as do many other monosulfides of rather less basic metals (M = Pb, Mn, La, Ce, Pr, Nd, Sm, Eu, Tb, Ho, Th, U, Pu). However, many metals in the later transition element groups show substantial trends to increasing covalency leading either to lower coordination numbers or to layer-lattice structures. Thus MS (Be, Zn, Cd, Hg) adopt the 4 4 zinc blende structure (p. 1210) and ZnS, CdS and MnS also crystallize in the 4 4 wurtzite modification (p. 1210). In both of these structures both M and S are tetrahedrally coordinated, whereas PtS, which also has 4 4... 

Finally, many disulfides have a quite different structure motif, being composed of infinite three-dimensional networks of M and discrete Sj units. The predominate structural types are pyrites, FeSa (also for M = Mn, Co, Ni, Ru, Os), and marcasite (known only for FeS2 among the disulfides). Pyrites can be described as a distorted NaCl-type structure in which the rodshaped S2 units (S-S 217 pm) are centred on the Cl positions but are oriented so that they are inclined away from the cubic axes. The marcasite structure is a variant of the rutile structure (Ti02,... 

Consider next the experimental points for Csl, CsBr, and CsCl which, as already pointed out, lie quite near the diagonal in Fig. 57. At room temperature neither of those substances crystallizes in the NaCl type of lattice, but in a lattice where each ion has eight neighbors, instead of six. From the observed distance between the cesium nucleus and the halogen nucleus in each of these three crystals, one can easily calculate what... 

In order to construct an MeX-type compound (X Me = 1) using only octahedral elements, the octahedrons must be linked via their faces i.e. by sharing of three angles. This arrangement of the octahedral polyhedrons yields a rock-salt type structure (NaCl type structure). 

In the case of lithium orthoniobate, Li3Nb04, no meta-stable phase was found that had a rock-salt crystal structure with disordered cation distribution [268]. Nevertheless, solid solutions Li2+xTii-4xNb3x03, where 0 < x < 0.22, have a monoclinic structure at low temperatures and undergo transformation to a disordered NaCl type structure at high temperatures [274]. 

Table 39. Synthesis conditions and modifications of compounds crystallizing in NaCl type structure (X Me = l).

Magnesia forms solid solutions with NiO. Both MgO and NiO have face-centered cubic lattices with NaCl-type structures. The similarity between the ionic radii of the metals (Ni2+ = 0.69 A, Mg2+ = 0.65 A) allows interchangeability in a crystal lattice, and thus the formation of solid solutions with any proportion of the two oxides is possible. Such solid solutions are more difficult to reduce than NiO alone. Thus Takemura et al. (I) demonstrated that NiO reduced completely at 230°-400°C (446°-752°F) whereas a 10% NiO-90% MgO solid solu-... 

As has been shown by the X-ray diffraction method the parent metals (i.e. Pd or Ni), the a-phase, and /3-phase all have the same type of crystal lattice, namely face centered cubic of the NaCl type. However, the /9-phase exhibits a significant expansion of the lattice in comparison with the metal itself. Extensive X-ray structural studies of the Pd-H system have been carried out by Owen and Williams (14), and on the Ni-H system by Janko (8), Majchrzak (15), and Janko and Pielaszek (16). The relevant details arc to be found in the references cited. It should be emphasized here, however, that at moderate temperatures palladium and nickel hydrides have lattices of the NaCl type with parameters respectively 3.6% and 6% larger than those of the parent metals. Within the limits of the solid solution the metal lattice expands also with increased hydrogen concentration, but the lattice parameter does not depart significantly from that of the pure metal (for palladium at least up to about 100°C). 

The cubic UB, 2-type boride structure with space group Fm3m can be described on the basis of a B,2-cubooctahedron (see Fig. 1) . The association of the B,2-poly-hedra by oriented B—B bonds gives rise to a three-dimensional skeleton with boron cages. Formally, the arrangement of the B,2-units and of the metals atoms is of the NaCl-type. Each metal is located in the center of a B24-cubooctahedron. 

The classical view of the lone pair is that, after mixing of the s and p orbitals on the heavy metal cation, the lone pair occupies an inert orbital in the ligand sphere [6]. This pair of electrons is considered chemically inert but stereochemi-cally active [7]. However, this implies that the lone pair would always and in any (chemical) environment be stereochemically active, which is not the case. For example, TIF [8] adopts a structure, which can be considered as a NaCl type of structure which is distorted by a stereochemically active lone pair on thallium. In contrast TlCl [9] and TlBr [10] adopt the undistorted CsCl type of structure at ambient temperature, and at lower temperatures the (again undistorted) NaCl type of structure. The structure of PbO [11] is clearly characterized by the stereochemically active lone pair. In all the other 1 1 compounds of lead with... 

Thallous halides offer a unique possibility of studying the stereochemistry of the (chemically) inert electron pair, since their structures and their pressure and temperature-dependent phase transitions have been well established. Thallium (1) fluoride under ambient conditions, adopts an orthorhombic structure in the space group Pbcm which can be regarded as a distorted rocksalt structure (Fig. 2.4). In contrast to TIF, the thallium halides with heavier halogens, TlCl, TlBr and Til, adopt the highly symmetric cubic CsCl structure type under ambient conditions [46]. Both TlCl and TlBr, at lower temperatures, undergo phase transitions to the NaCl type of structure [47]. 

The kind of bond between neighboring atoms also has to be considered. For instance, the coordination number for a chlorine atom in the CC14 molecule is 1 when only the covalently bonded C atom is counted, but it is 4 (1 C + 3 Cl) when all atoms in contact are counted. In the case of molecules one will tend to count only covalently bonded atoms as coordinated atoms. In the case of crystals consisting of monoatomic ions usually only the anions immediately adjacent to a cation and the cations immediately adjacent to an anion are considered, even when there are contacts between anions and anions or between cations and cations. In this way, an I- ion in Lil (NaCl type) is assigned the coordination number 6, whereas it is 18 when the 12 I- ions with which it is also in contact are included. In case of doubt, one should always specify exactly what is to be included in the coordination sphere. 

---