Borates, structural units

Borates can be viewed as complex salts in which the Lewis basicity of the structural unit must match the Lewis acidity of the interstitial complex to produce a stable structure. The relationship between basicity of the borate structural unit and its BO4/BO3 ratio was mentioned above, indicating that each specific structural unit has an associated basicity. It was recently shown that the Lewis basicity of borate structural units correlates with the average coordination number of the oxygen atoms they contain, counting H-bonds... 

Many borates are known that contain non-metal cations. Examples include the ammonium borates [39], such as the commercial products ammonium pentaborate, NH4[B506(0H)4] 2 H2O, and ammonium tetraborate, (NH4)2 [6405(011)4] 2 H2O, both of which contain isolated borate anions shown above (Figs. 5a and 6a, respectively) [40]. Isolated borate anions containing more than six boron atoms are rare. However, the mineral ammonioborite [(NH4)3[Bi502o(OH)g] 4 H20 = (NH4)20 5 B2O3 2 2/3 H2O), which may be prepared synthetically by prolonged reflux of a saturated solution of ammonium pentaborate, contains the unusual isolated anion [Bi502o(OH)g] , shown in Fig. 10 [41]. This anion is not known to occur in any metal borate, its formation is apparently directed exclusively by the ammonium cations which can donate H-bonds to interstitial water and the borate structural unit. 

L. Nichelson, B. J. Murray, A. Fletscher, D. R. M. Brew, K. J. D. Mackenzie and M. Schmiicker, Novel Geopolymer Materials Constaining Borate Structural Units, in Geopolymer, Green Chemistry and Sustainable Development Solutions, ed. by J. Davidovits, Saint-Quentin, 2005, p. 31-33... 

The phase relations, stoichiometry and structural chemistry of the metal borates have been extensively studied because of their geochemical implications and technological importance. Borates are known in which the structural unit is mononuclear (1 B atom), bi-, tri-, tetra- or penta-nuclear, or in which there are polydimensional networks including glasses. The main structural principles underlying the bonding in crystalline metal borates are as follows " ... 

Glass-forming systems other than silica have been examined. The fraction of three- and four-coordinated boron in borate glasses can be determined by nmr (29). Both nmr and x-ray diffraction (30) results led to the suggestion that the boroxyl ring is the structural unit of vitreous B203 (22,29). The intermediate-size boroxyl ring represents a compromise between the crystallite and the random-network theory (29) (see Analytical methods). 

This review is concerned with the hydrated metal borates, their interactions in solution, and their synthesis. Section III outlines the known structures of the hydrates and includes details of a few anhydrous borates to show the relationship of their basic structural units to those of the partially and completely hydrated forms. Section IV re-... 

The most common units found in borate structures are shown in Fig. 1. Many of the known borate structures can be rationalized by assigning appropriate charges, protons, or hydroxyl groups (which change the coordination of boron from trigonal to tetrahedral) to these basic units. Thus the B03 unit can become BOj -, B(OH)3, or B(0H)4, respectively. Polymerization by elimination of one water molecule between two hydrated units results in chain formation, and further water elimination gives sheets or networks. These operations are illustrated in Fig. 2 for the triborate ring unit. 

Fig. 1. Basic units of borate structures. All trigonally coordinated boron atoms shown can theoretically become tetrahedrally coordinated.

There are more than 50 types of boron-oxygen anions and polyanions in anhydrous borate, borosilicate, boroaluminate and boroberylate structures. However, all of them consist exclusively of three basic types of structural units of different composition (Fig.l). The first set of these basic types identify with BO3 triangles (A) and B04 tetrahedra ( ) as fundamental (elementary) structural units (FSU) which are shown in Fig. 1, I. They are isolated in orthoborate structures and can be merged in pyro-, meta-, and polyborates. 

Figure 1 Configurations of some basic B-O structural units in anhydrous borates ...

Thus, there are numerous structural types of anhydrous borates. However, it is possible to reduce some of these complex structures to various combinations of the three types of the basic structural units mentioned above FSU, CSU, and CRP. 

Numerous anhydrous borate structures can be reduced to various combinations of three types of basic structural units 1) fundamental structural units, 2) combined basic structural units, and 3) full radicals of polyanions. The stability of the FSU, CSU and CRP units in solid and liquid borates is one of the most important issues, and will provide a better understanding of, and control of, phase formation in complex borate systems. 

A theoretical analysis has been made of the miscibility gaps in the alkali-metal borates, based on the concept of regular solutions. In each system the structural units which control the entropy of mixing are thought... 

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