Valence NaCl structure

The carbides with the NaCl structure may be considered to consist of alternating layers of metal atoms and layers of semiconductor atoms where the planes are octahedral ones of the cubic symmetry system. (Figure 10.1). In TiC, for example, the carbon atoms lie 3.06A apart which is about twice the covalent bond length of 1.54 A, so the carbon atoms are not covalently bonded, but they may transfer some charge to the metal layers, and they do increase the valence electron density. 

An ordered cation-deficient derivative of the NaCl structure was found in the orthorhombic SC2S3 type (168). The unit cell contains twelve rocksalt units (a = 2 ao, b = ]/2 ao, c = 3J/2 o) The cation vacancies are three-dimensionally distributed without pronounced directional preference. In each sulfur-centered octahedron two scandium atoms are missing. SC2S3, ScaSe3, Y2S3 and Y2Se3 (213) Eire normal valence compounds with non-metallic properties. 

To elucidate the nature of chemical bonding in metal carbides with the NaCl structure, the valence electronic states for TiC and UC have been calculated using the discrete-variational (DV) Xa method. Since relativistic effects on chemical bonding of compounds containing uranium atom become significant, the relativistic Hamiltonian, i.e., the DV-Dirac-Slater method, was used for UC. The results... 

When subjected to high pressure certain phosphides and arsenides of metals of Groups IIIB and IVB adopt either the cubic NaCl structure (InP, InAs) or a tetragonal variant of this structure (GeP, GeAs) shown in Fig. 6.1(c) in which there are bonds of three different lengths and effectively 5-coordination. The compounds of Ge and Sn, with a total of nine valence electrons, are metallic conductors. This property is not a characteristic only of the distorted NaCl structure, for SnP forms both the cubic and tetragonal structures, and both polymorphs exhibit metallic conduction. (IC 1970 9 335 JSSC 1970 1 143.)... 

In Fig. 15 we present both XPS and UPS data for TmSe. In UPS, emission from the Tm(5conduction band and the Se(4p) valence band dominate that from the Tm(4/) shell. A Fermi cutoff is detected, analogous to that of TmS, confirming the metallic nature of TmSe. The spectrum of the Se (4p) emission can be related to that expected for the valence band of NaCl structure compounds (52) and will be discussed in a forthcoming publication (53). At XPS energies these contributions are overwhelmed by the much more intense emission from the Tm(4/12 and 4/13) levels (35), whose final states multiplets have been identified using the calculations of Cox (7). As pointed out by Jorgensen (5), the signal from Tm2+ represents the first observation of a well-resolved 4/13 - 4/12 spectrum. Similar results have since been obtained in preliminary... 

The Born-Haber cycle also enables us to understand why most metals fail to form stable ionic compounds in low valence states (e.g., compounds such as CaCl, AlO and ScCl2). Let us consider a metal with 1st and 2nd ionization energies of 600 and 1200 kJ mol-1, which are fairly typical values (of Ca, for example). Let us suppose that this metal forms a +2 ion with a radius of 1.00 A and that its dichloride would have the fluorite structure (as does CaCl2). The M+ ion would have to be appreciably larger than the M2+ ion and a radius of 1.20 A is a fair estimate. With a radius ratio of 1.5, MCI may be expected to have the NaCl structure. For the two compounds, MCI and MC12 then, the Born-Haber cycles are as shown in Table 2-6. 

Many of the oxides, carbides, and nitrides with the NaCl structure tend to be nonstoichiometric. Titanium monoxide exists over the range Tio.ssO to TiO, while FeO never occurs it is always nonstoichiometric with a composition ranging from Feo.goO to Feo.geO. As a consequence of these vacancies, the transition metal exists in two valence states, causing the oxide to exhibit semiconductor properties (as for NiO). 

We shall first discuss the results of Morke et al. (1986) obtained on a series of stable valence CeSj+j compounds. CeS has the NaCl structure and the first-order Raman effect is forbidden by symmetry at the zone center. However, defects can break the translational symmetry of the lattice leading to a relaxation of the... 

At ambient conditions, the lanthanide monopnictides RX (X = N, P, As, Sb, Bi) and monochalcogenides RX (X = 0, S, Se, Te, Po) crystallize in the NaCl structure. Given the combination of an electropositive R-ion with an electronegative pnic-tide or chalcogenide, one might assume that an ionic picture can be applied here. But based on the observed properties, Rhyne and McGuire (1972) proposed a classification distinguishing between the so-called valence balanced compoimds... 

Fig. 12. Demonstration of typical chemical shifts in lanthanide Lm absorption. The energy calibration of the spectra recorded from the Pr absorption in the compounds is accurate within 0.2 eV with respect to o fixed at the intersection point of the high-energy absorption with the absorption line in (dhcp) Pr metal (dashed-dotted line). The intersection point shifts to higher energies with decreasing metallic character of the compounds. The maxima of the prominent main lines, however, remain unshifted, just as the onsets of the lines. PrCu crystallizes in orthorhombic FeB structure (a = 7.343 A, 6 = 4.584 A, c = 5.604 A). The semi-metals PrSb, PrBi have fee (NaCl) structure a = 6.366 and 6.463 A, respectively). Pr Oi] is a nonstoichiometric modification of nominally tetravalent Pr02 (fluorite type, Cap2). The bar diagram indicates ligand-field-split absorption lines for both, the tri- and tetravalent valence states in Pr Oij (cf. section 14).

A pressure-induced transition from an NaCl to a CsCl structure is observed for EuSe at about 145 kbar, but there was none for SmSe up to 200 kbar and for YbSe up to 250 kbar, Jayaraman et al. [3] also see the individual sections pp. 140,185, and 399. A pressure-induced valence transition is observed for SmSe and a compositionally induced valence transition in the case of TmSe (see p. 324), both retaining the cubic NaCl structure. 

SmSe exists, like SmS (see Rare Earth Elements C 7, 1983, p. 221), in a semiconducting form with divalent Sm and a metallic form with Sm in the intermediate valence state between two and three. Both forms have a cubic NaCl structure, but the metallic SmSe has smaller lattice constants. In contrast to SmS, the pressure-induced phase transition of SmSe is continuous. Only Singh et al. [1, 2] report the existence of hexagonal SmSe films. 

Tm Se samples with x 0.87 to 1.05 have a cubic NaCl structure. Phases with x O.87 around ImgSeg are characterized by a superstructure of defects in the NaCl structure, see p. 322. Nearly stoichiometric TmSe is in the intermediate valence state. The lattice constant decreases with decreasing x in Tm Se and the Tm valency increases and reaches the trivalent state near x = 0.87. Therefore, the determination of the lattice constants is a common method to characterize the Tm valency or the stoichiometry of the specimens, see p. 324. 

A completely calculated energy band structure scheme of TmSe did not exist up to now. The models proposed for other rare earth monochalcogenides with NaCl structure, for example those of EuS, or SmS (see Rare Earth Elements C7, 1983, pp. 252 and 259, respectively) have been used to describe the properties of TmSe. Accordingly, the valence band is derived predominantly from the 4p states of Se and the conduction band from the 5d and 6s states of Tm. The cubic crystal field splits the 5d states into lower energy tsg (triplet) and... 

The Tmi xEUxSe mixed crystals have a cubic NaCl structure. The lattice constants for samples with x = 0 to -0.25 are shown in Fig. 211, together with the Vegard-law lines for a variety of valences, Kaldis etal. [2, p. 137], also see Boppart, Wachter [8, p. 36], [9]. Lattice constant and density measurements, as well as X-ray fluorescence and chemical analysis, indicate the existence of Schottky defects, particularly in the Tm-rich part of the system. The vacancy concentration changes smoothly with x in Tmi xEUxSe. The following table presents the lattice constant a, measured and calculated densities Dexp and Dcaic. respectively, number of Schottky pairs nsch. and vacant lattice sites n as a function of x in Tmi xEUxSe ... 

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