Triborate

Only certain types of crystalline materials can exhibit second harmonic generation (61). Because of symmetry considerations, the coefficient must be identically equal to zero in any material having a center of symmetry. Thus the only candidates for second harmonic generation are materials that lack a center of symmetry. Some common materials which are used in nonlinear optics include barium sodium niobate [12323-03-4] Ba2NaNb O lithium niobate [12031 -63-9] LiNbO potassium titanyl phosphate [12690-20-9], KTiOPO beta-barium borate [13701 -59-2], p-BaB204 and lithium triborate... 

The barium oxide borate [12007-55-5] BaO -2B202, and dibarium oxide triborate [13840-10-3] 2BaO -3B202, have also been reported. 

The polyions postulated in solution all have known structural analogues in crystalline borate salts. Investigations of the Raman (66) and B nmr (67) spectra of borate solutions have confirmed the presence of three of these species the triborate (3), B202(0H) 4, tetraborate (4), [B40 (0H) 4], and pentaborate (5) B O (OH) 4, polyanions. Skeletal stmctures were assigned based on coincidences between the solution spectra and those soHd borates for which definitive stmctural data are available (52). These same ions have been postulated to be present in alkah metal borate glasses as well. 

A single-crystal x-ray diffraction study has shown that the borate anion in anhydrous borax is polymeric in nature and is formed via oxygen bridging of triborate and pentaborate groups (83). The chemistry of anhydrous borax has been reviewed (73,84). 

Fig. 9. The borate glass stmctural groups where = boron Q — oxygen (a) boroxol (b) pentaborate (c) triborate and (d) diborate (142).

The most important polyborate species observed in solution are the triborate anions [B303(0H)4] and [B303(0H)5] , the pentaborate anion [B506(0H)4] , and the tetraborate anion [B405(0H)4] . The population distributions of these species found at 25 °C in 0.4 molar boric acid equivalent solution as a function of pH is shown in Fig. 2 [8]. Concentrations of these species may vary considerably with temperature and overall boron concentration, and other minor polyborate species not represented are also likely to exist in solution. Fig. 2 should be regarded as a snapshot of a complex system taken under one set of conditions. 

Two triborate moieties may link together by sharing a common tetrahedral boron atom to form the pentaborate anions shown in Fig. 5. The pentaborate monoanion shown in Fig. 5a is observed in solution (see Fig. 2) and occurs in synthetic and mineral borates. Examples are the synthetic commercial ammonium pentaborate, NH4[Bs06(0H)4] 2 H2O [ = (NH4)20 5 B2O3 ... 

Fig. 4. Orthorhombic metaboric acid (a) and triborate mono-, di-, and trianions (b-d)...

Another example of cation control over borate structure is found in the recently elucidated structure of the industrially important synthetic zinc borate Zn[B304(0H)3] [2 ZnO 3 B3O3 3 H2O], shown in Fig. 12 [45]. This compound has a polytriborate structure reminiscent of that found in colemanite (Fig. 7a) [29]. Yet, due to the different coordination requirements of the Ca and Zn cations, the anionic polyborate chains in these two compounds have different spatial arrangements. In the case of colemanite, parallel triborate chains are linked together into extended sheets by coordination with interstitial (and H2O). These sheets are bound... 

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. 2. Structural transformations of the triborate unit. (The use of protons and hydroxyl groups is schematic and does not represent chemical reaction.)...

The most common ion of the triborate unit is B303(0H)i, and it is usually found associated with calcium and magnesium. Triborates of calcium Ca gOn xH20 have been reviewed (85). Modification of triborate groups with attachment of a side tetrahedron leads to B4 species. Uralborite, Ca BsOslOHls OB(OH)a], is an example (367) and should be written as shown rather than with the ambiguous shorter formula of Ca2[B404(0H)8]. Christ and Clark (80) have doubted... 

The so-called metaborates of empirical formula B02 can take several forms. The infinite linear (B02) T ion is found in the low-temperature form of LiB02 (449), Ca(B02)2 (275), and Sr(B02)2 (46). At high temperatures and pressures, the lithium compound possesses a sphaleritelike structure with boron in four-coordination (278). The triborate isolated B3Or group is found in the barium (297), sodium (276), and potassium (361) compounds, and the framework B30 " group, in which all the boron atoms are tetrahedral, exists for a high-pressure calcium metaborate phase (280) and copper metaborate (284). Two other anhydrous calcium metaborates are known to give the series... 

Knowledge of the stability of various polyborate ions is lacking. It is known from structure determinations that the isolated BsO OH) - is the most common ion of the triborates, but not why there are so few examples of B303(0H) and B303(0H)iT. Further, why does borcarite have an eight-membered ring in preference to a tetraborate group ... 

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