Mixed-Cation Orthophosphates

Many orthophosphate minerals contain more than one kind of metal atom. However, mixed-metal orthophosphates can be synthesised in great variety - with sodium and nickel, for example, there are at least six different compounds  

Solid solution formation between isostructural salts provides many examples of mixed-cation orthophosphates. In Hopeite, Zn3(P04)2, for example, the Zn atoms can be partially replaced by Mn or Ni. The isostructural trihydrates of Mg and Mn hydrogen phosphates will form mixed crystals of composition Mg, 4Mn HP04-3H20. 

Orthophosphates containing mixed-valency cations are not uncommon, for example, in certain Fe, Cr, Mn and V phosphates aheady quoted above. Mixed-valency salts of type CZn3 e +Fe +(P04)3-2H2O are known [57]. Some of these may be produced in metal surface treatmeuts (Chapter 12.7). 

Potassium titauyl phosphate, KTi0P04, is au importaut electro-optic material (Chapter 12.20). Characterised crystalliue salts iuclude [59-62] 

Syuthesis of mixed-metal orthophosphates (double salts) cau ofteu be effected by straightforward laboratory methods such as mixiug aqueous solutious (5.94), or heatiug a metal ammouium phosphate with alkali carbouate to 800°C (5.95, 5.96). 

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Orthophosphate salts are generally prepared by the partial or total neutralization of orthophosphoric acid. Phase equiUbrium diagrams are particularly usehil in identifying conditions for the preparation of particular phosphate salts. The solution properties of orthophosphate salts of monovalent cations are distincdy different from those of the polyvalent cations, the latter exhibiting incongment solubiUty in most cases. The commercial phosphates include alkah metal, alkaline-earth, heavy metal, mixed metal, and ammonium salts of phosphoric acid. Sodium phosphates are the most important, followed by calcium, ammonium, and potassium salts. 

Ambient aqueous routes and hydrothermal routes The synthesis of rare earth orthophosphate NCs could be carried out in aqueous solutions by the precipitation of rare earth cations by phosphate anions, due to the low aqueous solubility of RPO4. A typical synthesis begins with soluble rare earth salts in aqueous solution, such as nitrates or chlorides, and a phosphate source like phosphate M H3 P04, triphosphate M5P3O10 (M = Na, K, or NH4), or PI3PO4. The two solutions are mixed at room temperature or elevated temperature to form gel-like precipitation as precursor. Then the mixture of precipitation and mother liquor is... 

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