Arsenic structures

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).

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. 

In CaSi2, a layered polyanion [Sf] is present which consists of puckered Si rings and which is related to a layer in the a-arsenic structure (Fig. la, [14]). SrSi2 forms a cubic arrangement wdiere the triply silicon atoms [(3b)Si] coimect to a threedimensional framework which is a type of its own (Fig. lb,... 

BertheviUe B, Herrmannsdorfer T, Yvon K (2001) Structure data for K2MgH4 and Rb2CaH4 and comparison with hydride and fluoride analogues. J Alloys Compd 325 L13-L16 Hines J, Cambon O, Astier R et al (2004) Crystal structures of a-quartz homeotypes boron phosphate and boron arsenate structure-property relationships. Z Krist 219 32-37 Feldmann C, Jansen M (1995) Zurkristallchemischen Ahnlichkeit von Aurid- und Halogenid-lonen. Z anorg allgem Chem 621 1907-1912... 

Theoretical studies, on the other hand, have predicted a variety of non-molecular structures at pressures for which only molecular phases have been observed. These include a monatomic simple-cubic phase [206-208], a semi-metallic arsenic structure A7 [206], a metallic, simple-tetragonal phase [206, 209], and the cubic gauche (CG) structure [205,207,210-213], for which recent experimental evidence has been given [203]. Results from theoretical calculations also predict other polymeric forms of nitrogen, as will be described hereafter. 

B. Group V Elements and tsoelectronic compounds a. The arsenic structure... 

As the primitive cell of the arsenic structure contains two atoms the simplest superstructure is realized in the GeTe type. Replacement of As As by GeTe removes the center of symmetry and therefore these phases can be piezoelectric and even ferroelectric. Removal of the inversion symmetry makes the three... 

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). 

Arsenic dissolves in concentrated nitric acid forming arsenicfV) acid, H3ASO4, but in dilute nitric acid and concentrated sulphuric acid the main product is the arsenic(III) acid, HjAsOj. The more metallic element, antimony, dissolves to form the (III) oxide Sb O, with moderately concentrated nitric acid, but the (V) oxide Sb205 (structure unknown) with the more concentrated acid. Bismuth, however, forms the salt bismulh(lll) nitrate Bi(N03)3. 5H,0. 

Alloys of antimony, tin, and arsenic offer hmited improvement in mechanical properties, but the usefulness of lead is limited primarily because of its poor structural qualities. It has a low melting point and a high coefficient of expansion, and it is a veiy ductile material that will creep under a tensile stress as low as 1 MPa (145 IbFin"). 

NaAs03 has an infinite polymeric chain anion similar to that in diopside (pp. 349, 529) but with a trimeric repeat unit LiAs03 is similar but with a dimeric repeat unit whereas /6-KASO3 appears to have a cyclic trimeric anion As309 which resembles the cyc/o-trimetaphosphates (p. 530). There is thus a certain structural similarity between arsenates and phosphates, though arsenic acid and the arsenates show less tendency to catenation (p. 526). The tetrahedral As 04) group also resembles PO4) in forming the central unit in several heteropoly acid anions (p. 1014). 

Figure 13.17 Molecular structure of some sulfides of arsenic, stressing the relationship to the AS4 tetrahedron (point group symmetry in parentheses).

The Structure of AS2S5 is unknown. It is said to be formed as a yellow solid by passing a rapid stream of H2S gas into an ice-cold solution of an arsenate in cone HCl slower passage of H2S at room temperature results in reduction of arsenate to arsenite and consequent precipitation of AS2S3. It decomposes in air above 95° to give AS2S3 and sulfur. 

Dezincification of brasses This may occur, particularly in stagnant or slowly-moving warm or hot waters relatively high in chloride and containing little carbonate hardness. Dezincification of a brasses is inhibited by the usual arsenic addition (see Fig. 4.12), but two-phase brasses are liable to severe attack in some waters . In such waters the use of duplex-structure brass fittings should be avoided. 

BurJ81 Burdett, J. K., McLarnan, T. J. A study of the arsenic, black phosphorus and other structures derived from rock salt by bond-breaking processes. I. Structural enumeration. J. Chem. Phys. 75 (1981) 5764-5773. 

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