PO4 tetrahedra

In many cases they are smaller and will fit into the interstice of the propagation unit, illustrated by the PO4 - tetrahedron mentioned above. In... 

The substitution of oxygen by nitrogen in PO4 tetrahedron has allowed the synthesis of a new family of solids with original properties the nitrided phosphates. These systems (e.g., AlPON, AlGaPON) with tunable acid-base properties are used in a growing number of intermediate and fine chemistry production processes [204] as well as supports in heterogeneous catalysis (e.g., dehydrogenation reactions) [205]. 

Figure 3.30. Heteropoly molybdate catalysts (a) Keggin anion (PMoi204o) (the central PO4 tetrahedron is shaded and the M0O6 octahedra are unshaded) and (b) selectivity (S) against conversion (C) data for acrolein to acrylic acid over KjcH3 PMoi2O40 (with x = 1-3) at " 350 °C show high selectivities. (After Black et al 1987, Gai et al 1983.)...

Figure 1.10 (a) Replacement of a BOg octahedron by a PO4 tetrahedron in an octahedral unit and (b) accommodation of a diphosphate group. 

Some alkali orthophosphates have been well studied. The most important among these are KH2PO4 and NH4H2PO4. In the KH2PO4 structure, hydrogen bond links the PO4 tetrahedron to four others in a continuous three-dimensional network, while the K ion is coordinated 8-fold by oxygen atoms. In the case of NH4H2PO4, the structure is similar, but a system of N-H-0 bonds exists instead of coordination of the K ion. These bonds are mostly ionic, and hence, acid phosphates are soluble and used as such in the acid-base reaction to form CBPCs. 

It may be useful to consider a definite numerical example, a distorted PO4 tetrahedron with internal coordinates ... 

These compounds are built up of anionic units formed of a PO4 tetrahedron and 12 MOg octahedra, whose packing delimits intersecting tunnels where the A ions are located. The ion-exchange properties of the ammonium compounds, which can be partially replaced by alkali ions, are easily explained by the interconnection of the tunnels. The incompleteness of the exchange results of a phase transition during the exchange. 

Figure 1. Drawings of PO4 tetrahedron (left) and Cal and Ca2 polyhedra for the three apatite stmctnres. Each overlay is a superposition of the analogous polyhedra from the fluorapatite, hydroxylapatite, and chlorapatite end-members, drawn to the same scale and with coincident central cations. From Hughes et al. (1989).

Figure 12. (a) Randomly oriented nine-fold coordinated unit of La(P04) with one attached PO4 tetrahedron, (b) the same structural unit as shown in (a) but reoriented to the reveal thecoordination described in the text and the Cs symmetry plane, (c) idealized pentagonal rare-earth-to-oxygen bond arrangement (after Mullica et al. 1984). 

In palermoite, SrLi2[Al(P04)(0H)]4, (Al( )6) octahedra condense by sharing edges to form [Al2share corners to form an [Al2chains link in the b-direction by sharing octahedron-tetrahedron vertices. These chains are seen end-on when viewed in the a-direction (Fig. 45f), cross-linked by (PO4) tetrahedra. The framework has large interstices that are occupied by [8]-coordinated Sr and [5]-coordinated Li. 

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