Orthophosphates and Diphosphates

These crystals are known as aluminium orthophosphate crystals. There are three polymorphous main modifications of these crystals (Kleiber 1969). One of these is berlinite, which is an isotype to quartz as a trigonal-trapezoidal Si02 type. Another is AlPO as a cristobalite structure of cubic to pseudocubic Si02. The third main crystal phase of AlPO is of the tridymite-type of hexagonal to pseudohexagonal structure. In addition to these main phases of AlPO, high-temperature modifications of the Si02-type (e.g., P-quartz, P-cristobalite, etc.) are known. 

It is interesting to note that AlPO crystals of the tridymite-type generally show defects in the lattice structure. AlPQ, however, crystallizes more easily into the ideal two-layer structure of tridymite than does Si02. The reason is the higher polarization of the phosphate tetrahedron compared with that of the (SiO l -tetrahedron. 

Berlinite is the a-quartz modification of AlPO. Its tetragonal structure is characterized y a = 4.9429 A, c = 10.9476 A, V = 231.6 A and space group P3j21 (Schwarzenbach 1966). 

Despite the highly isotypical relationship between phosphates and Si02, the crystallochemical relationship is not close enough to enable mixing of Si(rV) and P(V) in the crystal lattice of AlPO. This principle of diadochy, i.e., in which P(V) is partially replaced by Si(IV) is possible in apatite. This reaction, however, does not occur in AlPO.  

Because of this isotypical relationship of the berlinite modification of AlPQi to quartz, the question now arises Have stuffed derivatives been observed in AlPQj-based crystals as they have been in silica quartz At this 

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Simple salts such as Si3(P04)4 and SiP207 contain discrete orthophosphate and diphosphate anions, respectively. In these salts, the Si atoms are octahedrally coordinated by oxygen and the bonds are considerably more ionic than is the case with tetrahedrally coordinated Si atoms. Many crystalline silicophosphates containing condensed tetrahedra as in Equation 9.259 have also been found, and these are dealt with in Chapter 5.4. 

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