Antimony oxyhalides

Antimony oxide is known as a flame retardant synergist when used in combination with halogen compounds. Volatile antimony oxyhalide (SbOX) and/or antimony trihalide (SbX3) are formed in the condensed phase and transport the halogen into the gas phase (3). It has been suggested that antimony is also a highly active radical trap (4). 

Other oxyhalides, mostly oxychlorides and oxybromides, result from the controlled hydrolysis of the trihalides, and are of interest for two main reasons. First, they are quite unrelated to the oxyhalides of bismuth. Although both antimony and bismuth form compounds MOX the structures of the antimony compounds are quite different from those of the compounds BiOX, which have been described on p. 408. The more complex oxyhalides of Sb have no analogues among Bi compounds. Second, a feature of the published structures of the antimony oxyhalides is the coordination of Sb by either three or four O atoms. It should perhaps be remarked here that the investigation of the structures of these complex compounds is difficult, and the precise positions of the O atoms are by no means certain. However, it appears that a feature of these compounds is the formation of extended Sb—O systems, generally layers, interleaved with halogen... 

The maximum flame-retardance is obtained with an antimony to halogen atomic ratio of 1 3 161). An intermediate product of the reaction of Sb203 with a halogen-containing polymer substrate, or with a flame retardant, is an antimony oxyhalide. Pitts found that other oxides may either promote (Fe203, CuO, Ti02) or hinder (ZnO, CaO, MgO) the decomposition of antimony oxyhalides. This, in turn, enhances or diminishes the overall flame-retardant effect161). 

The formation of volatile antimony derivatives when antimony trioxide reacts with hydrogen halide occurs through the intermediate stage of antimony oxyhalide formation ... 

Antimony trioxide (SbaOj). It is produced from stibnite (antimony sulphide). Some typical properties are density 5.2-5.67 g/cm- pH of water suspension 2-6.5 particle size 0.2-3 p,m specific surface area 2-13 m-/g. Antimony trioxide has been the oxide universally employed as flame retardant, but recently antimony pentoxide (SbaOs) has also been used. Antimony oxides require the presence of a halogen compound to exert their fire-retardant effect. The flame-retarding action is produced in the vapour phase above the burning surface. The halogen and the antimony oxide in a vapour phase (above 315 C) react to form halides and oxyhalides which act as extinguishing moieties. Combination with zinc borate, zinc stannate and ammonium octamolybdate enhances the flame-retarding properties of antimony trioxide. 

When the phosphorus halide is PI3, this reaction is a convenient way to produce HI. Arsenic trihalides hydrolyze in an analogous way but the trihalides of antimony and bismuth react to produce oxyhalides. 

Antimony forms polymeric oxyhalides, and not metallic as in BiOCl. The fluoride, SbOF, has been prepared in two forms V, with a ladder structure, and iM which has a layered structure. Both forms have a trigonal bipyramidal structure about antimony with three oxygens, one fluorine and one lone pair. Structural parameters are given in Table 15, from which it can be seen that L-SbOF heads the table as the nearest to an ideal fit for trigonal bipyramidal geometry. 

The sulfur analog, SPC13, and several mixed oxyhalides such as OPCI2F and OPCI2Br are known, but they are not widely used compounds. Oxyhalide compounds of arsenic, antimony, and bismuth are of much less importance than are those of phosphorus. 

All the elements Al, Ga, La, Ti, V, and the 4f and 5f metals form an oxychloride MOCl and most of them also form MOBr and MOI. Antimony and bismuth also form more complex oxyhalides, which are described in Chapter 20, and Bi forms BiOF which belongs in this group. Other oxyfluorides MOF of the above elements were included in group (a), ionic oxyfluorides. 

Many theories attempt to explain the mechanism of flame retardancy. The flame retarding action is thought to take place in the vapor phase above the burning surface. For antimony oxide to work, the halogen and antimony oxide must be found in a vapor phase which will occur at temperatures above 315°C. At these temperatures, antimony halides and oxyhalides are formed and act as flame extinguishing moieties by quenching radicals as they form. 

In its function as a flame retardant, antimony trioxide, in combination with an organic halogen compound, forms antimony trihalide and oxyhalide at flame temperatures. 

As the metallic properties of the central atom increase, as is the trend on proceeding down a group, hydrolysis is incomplete and oxyhalides are formed, as shown for antimony(III) chloride ... 

The electrical conductivities of arsenic, antimony, bismuth, vanadium, selenium and tellurium oxides and of antimony, bismuth and vanadium oxyhalides have been determined in chlorosulfonic acid. Arsenic, antimony and bismuth oxides in chlorosulfonic acid yield the corresponding oxychlorosulfonates as shown (Equation 9). 

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