Structure types arsenic

There is sometimes ambiguity as to which nearest neighbours to assign to the local coordination polyhedron. This is illustrated by the two different puckered layer structure types which are displayed by the most stable polymorphs of phosphorus and arsenic respectively. They may both be thought of as resulting from the breaking of three bonds about each atom on a simple cubic lattice as shown in Fig. 1.5. The layers then distort Black... 

Fig. 1.6 The breaking of three-bonds about each simple cubic site that leads to the black phosphorus and arsenic structure types. After Burdett and Lee (1985).

The 8-N rule states that the number of bonds (or local coordination, x) equals 8 minus the number of the periodic group. This rule is illustrated in Fig. 1.2 where we see that for N — 7 the halogens take dimeric structure types with x = 1, for N = 6 the chalcogenides selenium and tellurium take helical chain structures with x = 2, for N = 5 the pnictides arsenic, antimony, and bismuth take a puckered layer structure with x = 3, and for N = 4 the semiconductors... 

Thus, we expect the puckered graphitic sheet with 90° bond angles to have the smallest normalized fourth moment and shape parameter, s, and hence to be the most stable structure for the half-full p band as is indeed observed in the middle panel of Fig. 8.5. We should also note that if the n bonding is neglected then this three-atom contribution is identically zero for = 90°, so that 5 = 0 and we have the total bimodal behaviour of the p eigenspectrum that is observed in the lower panel of Fig. 8.1 for the arsenic structure type. 

Arsenate. — The arsenates of the rare earths crystallize [263] in two structural types, the huttonite and the zircon. The structural change from huttonite (La—Nd) to zircon (Sm—Lu) occurs at samarium. The lattice parameters of EuAsCU are a = 7.167 and c — 6.374 A. The rare earth arsenates can be prepared by reacting the nitrates with (NEU HAsCU, and heating the product to 700° C. 

With actinides one of the possible compositions found is M As2 like in ThAs2 in which each Th atom has 9 As neighbors. Three types of structures are important with transition metals and arsenic, CoAss, NiAs and those related to FeS2. CoAss is a diamagnetic semiconductor that occurs in the mineral skutterudite. It has a cubic structure related to that of ReOs but with distortions that result in planar AS4 rings. The NiAs structure is one of the most common MX structural types. In the structure each Ni is 8-coordinate and surrounded by 6 As and by 2 Ni atoms. The As atoms form a hep lattice in which the interstices are occupied by Ni atoms in a way that each As is surrounded by a trigonal prims of 6 Ni. ... 

This chapter presents an overview of the crystal structures of inorganic compounds in which actinide polyhedra are directly coordinated by phosphate or arsenate tetrahedra. Arsenates are considered here for comparative purposes, because of the very similar structural roles played by arsenate and phosphate ions. Structural data are taken mainly from the Inorganic Crystal Stmcture Database [6] and the published literature. Entries from the Powder Dififtaction File of the International Centre for Diffraction Data [7] have been consulted in the case of compounds whose structures may reasonably be inferred. Mixed organic-inorganic compounds [8] and polyoxometalates are not addressed phosphites, phosphinates and arsenites also fall outside the scope of this chapter. Data are tabulated for a given structure type, with phosphates separated... 

A veritable arsenal of techniques has been mobilized to provide information regarding the structure of redox molecular sieves [9-14], X-Ray powder diffraction (XRD) provides an immediate check for crystallinity and structural type. X-Ray absorption fine structure spectroscopy (EXAFS) and X-ray absorption near edge spectroscopy (XANES) give fiirther insights into coordination geometry and bond lengths. Infiared and Raman spectroscopy have been used to identify characteristic features, e.g. the 960 cm bond attributed to the Si-OTi stretching vibration in TS-1 [9]. 

This is sometimes described as a trend from covalent, molecular Asia through intermediate SW3 to ionic Bils, but this exaggerates the difference in bond-type. Arsenic, Sb and Bi have very similar electronegativities (p. 550) and it seems likely that the structural trend reflects more the way in which the octahedral interstices in the hep iodine lattice are filled by atoms of gradually increasing size. The size of these interstices is about constant (see mean M-X distance) but only Bi is sufficiently large to fill them symmetrically. 

The condensed compounds of this type so far investigated are mainly phosphate-silicate, sulfate-phosphate, vanadate-phosphate, and arsenate-phosphate. All the component 0x0 acids or the anions of these condensed compounds can form isopoly acids or their anions. As can be seen in silicate minerals, there are several structural types of isopoly silicates which are polymers of interconnected Si04 tetrahedra. For example, pyroxene has a chain structure 1 (13). In the structure of... 

While many different structural types are known for heterocycles containing arsenic, antimony, or bismuth, structural diversity comes from the expanded CN that are available with these heteroatoms. Throughout the Chapter, the letter E, as used in the ring, represents As, Sb, or Bi and its use is explained in the text. Scheme, or Equation. In general, few ring systems exist that contain ring bound heteroatom substituents, and as a result, hydroxy-oxo, amino-imino, and thiol-thioxo tautomerism is nearly nonexistent. 

Arsenic, antimony and bismuth under ambient conditions form layered rhombo-hedral lattices (the structural type A7) in which each atom is connected with three nearest neighbors at distances respectively of 2.516, 2.908 and 3.071 A (internal bonds), and with three atoms of a following layer (external bonds) at much greater distances 3.121 (As), 3.355 (Sb), 3.529 (Bi) A. These features allow to speak of Nc = 3, i.e. of a molecular stmeture. Phase transformations in these elements at high pressures lead to shortening of external bonds and lengthening of internal ones, until... 

The luminescence properties of in RASO4 have been reported already in 1966 but the arsenates have not found use in lamps as luminescent materials (Wanmaker et al., 1966). The IR excited visible luminescence has been obtained with Yb " and Er " in arsenates but the conversion efficiency is much lower than that obtained in some fluorides (Sommerdijk et al., 1971). The rare earth YASO4 host lattice interactions have been reviewed and the crystal field parameters for various R " ions in this site symmetry have been presented (Wortman et al., 1976). Vishwamittar (1974a,b) has studied the crystal field parameters of Er " in zircon-type arsenate structures. The multiphonon relaxation rates of excited states of R ions in yttrium arsenate have been investigated by Reed and Moos (1973a,b). 

Sb LSb, yields the primitive-cubic polonium-type structure, which is S / known as a high-pressure modification of arsenic. Therefore, five electrons... 

The above syntheses constitute an extension of earlier work in which precursors of type (53, X = AsCH3 and PC6H5) were employed for the synthesis of a range of three-donor cyclic systems of general structure (55) (Kyba Chou, 1980). Yields for these reactions ranged from 19 to 40%. The arsenic derivatives of type (55) were the first macrocycles incorporating this heteroatom to be reported. 

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