SYNTHESIS AND STRUCTURAL STUDY OF SODIUM TITANIUM PHOSPHATE-SODIUM TIN PHOSPHATE SOLID-SOLUTIONS .2. THERMAL-EXPANSION

The structure of NaTi2(PO4)3 shows the space group R3cBAR, whereas that of NaSn2(PO4)3 presents the space group R3BAR at room temperature and it undergoes a second order phase transition at 575-degrees-C from this structure to another with the space group R3BAR. Evolution of the structure of NaTi2(PO4)3-NaSn2(PO4)3 solid solutions with temperature has been studied and temperature phase transition established for the compositions studied. Lattice thermal expansion of NaTi2-xSnx(PO4)3 solid solutions with x = 1, 1.2, 1.5 has been determined from x-ray diffraction data at temperatures ranging from 26 to 1000-degrees-C. DSC and dilatometric measurements have been also carried out on these samples. The composition with x = 1.5, NaTi0.5Sn1.5(PO4)3, presents a second order phase transition at 355-degrees-C, from the phase with R3BAR space group to the phase with R3cBAR space group, similar to that of the compound with x = 2, NaSn2(PO4)3. For the solid solution with x = 1.2, NaTi0.8Sn1.2(PO4)3, this transition seems to occur at a temperature closed to the room temperature, whereas the composition with x = 1, NaTiSn(PO4)3, does not show any phase transition. Thermal expansion of these solid solutions for x < 1.2 is more similar to that of NaTi2(PO4)3 whereas for x > 1.2 is analogous to that of NaSn2(PO4)3.