The Na3RE(PO4)(2) phosphates (where RE: La, Ce, Nd, Gd or Er) were obtained by precipitation from sols under mild hydrothermal conditions. Characterization of the powders was carried out by XRD, SEM, ICP/OES and DSC/TG methods. The resulted molar ratio of Na:RE:P in the precipitates was equal 3:1:2 and proved the successful synthesis of Na3RE(PO4)(2) compounds. The powders consisted of non-agglomerated spindle or flower-like shaped grains with the size up to 5 microns. The most of the obtained powders have a trigonal structure, which is a characteristic feature of high-temperature polymorphs of Na3RE(PO4)(2). An exception is the Na3La(PO4)(2) compound, which crystallized in an unidentified type of structure. Under heating, the Na3RE(PO4)(2) phosphates change their structure. Their polymorphic transformation path, i.e. temperatures of transitions and their structural changes, is dependent on RE3+ radius size. For La, Ce and Nd compounds heated at high temperatures, only orthorhombic polymorphs have been observed, whereas monoclinic or orthorhombic polymorphs of Na3Er(PO4)(2) and Na3Gd(PO4)(2) phosphates can form in the course of sintering.
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