A series of 3R2O3.Ta2O3(R3TaO7, R = rare earth) samples were synthesized by the solid state reaction at 1350°C for 96h(l) or at 1700°C for 4h(2), melting and cooling(3), rapid quenching of the melt(4) and annealing of samples obtained by rapid quenching at 1350°C for 96h(5) or at 1700°C for 4h(6). The crystal structures of these samples were examined by powder X-ray diffraction method. X-ray diffraction pattern of sample(2) (R3TaO7 after heating at 1700°C for 4 h) was identical with that of sample(6) (annealing at 1700°C for 4 h). So that, the phase equilibrium was confirmed for the samples heated at 1700°C for 4 h. Similarly, equilibrium states were attained by heating and annealing at 1350°C for 96 h (sampe (1) and sample(5)). With the decrease of ionic radii of R3+ ions from R = La(1.032Å) to Ta5+(0.64Å), the crystal structure of R3TaO7 obtained by solid state reaction became in higher symmetry with the disordered states; weberite type orthorhombic (space group : Cmcm or C2221)>pseudo-cubic>pyrochlore type cubic>fluorite type cubic, as shown in Table 1. The rapid quenching of the melt brought about disordered phases with a little higher symmetry and larger unit cell volume (Fig. 7). Partially disordered phases were obtained by rapid quenching for R3TaO7 with larger R3+ ions. A phase transition seems to occur between 1350°C and 1700°C in Ho3TaO7, and above 1700°C close to the melting point in other R3TaO7 with larger R3+ ions than Ho3+ (Fig. 9). This phase transition seems to be an order-disorder type transition mainly in cation sublattice.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)