Nanosized powders in the LaPO 4 -YPO 4 (-H 2 O), LaPO 4 -LuPO 4 (-H 2 O) and YPO 4 -ScPO 4 (-H 2 O) systems have been synthesized to increase the mutual solubility of initial components and to prepare effective ceramic matrices by sintering them. Ceramic matrices were characterized by high isomorphic capacity, thermal and chemical resistance. Ln" x PO 4 ·nH 2 O nanopowders; Ceramic matrices; Thermal; Keywords:Chemical stability technique with inverse precipitation and further dehydrated upon high heating rate to retain nanoscale. From this point of view the aim of this study was to investigate peculiarities of formation of nanopowders in the systems where components belong to the same or different structural groups, to determine their isomorphic capacity, to study thermal behavior, and thereby to develop physical and chemical approaches to prepare ceramic samples (as matrices) based on them. In this case the second component plays a role of immobilized ion (isotope).The hydrated La orthophosphate crystallizes in hexagonal rhabdophane-type system upon precipitation and contain 0.5-3 moles of H 2 O per formula unit. At temperature higher than 600 °C nanosized crystals loose water molecules and transform into monoclinic monazite-type form [9]. Yttrium, lutetium and scandium orthophosphates, hydrated and dehydrated, crystallizes in tetragonal xenotime-type form [9,10]. In this instance it was expected formation of limited solid solutions in the LaPO 4 -YPO 4 (- Hydrated rare earth orthophosphate nanopowders were synthesized using sol-gel method at stoichiometric ratio of interacting components Ln(NO 3 ) 3 and NH 4 H 2 PO 4 in an aqueous solution in the form of stable colloids that coagulate at pH ≈ 7 [9-11]. The use of reverse precipitation as it was shown allows improving further physico-mechanical properties of ceramic samples (matrices) obtained by sintering of nanopowders preliminary calcined to remove water from crystalline lattice [10,11]. ExperimentalThe initial reagents for rare earth orthophosphate systems were REE 2 O 3 (special-purity grade, 99.999%), mono substituted ammonium phosphate (special-purity grade, 99.5%), nitric acid (special-purity grade, 70%), aqua ammonia (special-purity grade, 25%), and distilled water. Synthesis of Ln' 1-x Ln" x PO 4 nH 2 O nanopowders was performed by sol-gel technique according the reactions described [11].The proper pH ≈ 7 for complete precipitation was reached by NH 4 OH addition. The precipitates were kept in the mother liquor for 24 h, washed after that by decantation, filtered off, and air-dried at 110 °C for 24 h [9,11]. For X-ray powder diffraction analysis DRON-3 diffractometer (Russia) was used. Recording parameters were as follows: Ni-filtered CuK α radiation, 38 kV, time constant 1, scanning speed 1 degree per minute. The thermal behavior of the samples was studied by means of stepwise heating of powders compacted into tablets under a pressure of 8-10 MPa within the temperature range of 850-1700 °C with further analysis by XRD or differential scannin...