Other orthophosphate synthesis methods

A direct precipitation technique for the preparation of the RE orthophosphates has been described by Hikichi et al. (1978) in which a 0.05 mol/1 solution of the rare-earth trichloride was added to a dilute, stirred H3(P04) solution. The mixed solutions were then maintained at 20, 50, and 90°C for 1 to 900 days and were titrated to maintain a constant pH by adding H3(P04) or other phosphates. Both the hydrated low-temperature hexagonal forms of the rare-earth phosphates (as discussed in the previous section on Chemical composition) and the anhydrous RE orthophosphates were produced by Hikichi et al. (1978). 

Hydrothermal methods have been used by Gratz and Heinrich (1997) to synthesize solid solutions of (Ce,Y)P04 at temperatures in the range of 300 to 1000°C and at pressures between two and 15 kbar. These workers reported the formation of two immiscible phases (monazite and xenotime) over a wide range of compositions. 

Complete solid solutions between La(P04) and (Cao.5Tho.5)P04 were also hydrothermally synthesized at 780°C and 200 MPa by Podor and Cuney (1997). Their results show (1) that the substitution of thorium in monazite is not limited by the temperature and pressure conditions experienced by granitic magmas, and (2) that radiogenic lead is stable in natural monazites. 

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In addition to the orthophosphates, there are also a small number of reports on nanomaterials of other rare earth phosphate salts (Tang et al., 2005c). In this section, we will discuss the chemical synthesis of rare earth orthophosphates in aqueous solutions, nonaqueous solutions, and dry methods, together with the brief discussion of the luminescent properties, as well as the applications in biosensing. 

The polyol method could be applied to synthesis LaP04 Ce,Tb and other rare earth orthophosphate NCs. The so-called polyol method is the precipitation of a solid while heating sufficient precursors in multivalent and high-boiling alcohol like diethylene glycol (DEG) with a boiling point of 246 °C. Therefore, spherical NPs of 20-300 nm in size, usually metal... 

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