EFFECT OF RARE-EARTH IONS ON THE PHASE-TRANSITION OF NA2SO4 CRYSTALS

The V ⇄ I phase transition of Na2SO4 crystals was investigated on a sample of pure Na2SO4 and on rare-earth-ion (Ln3+ = La3+, Eu3+, Tm3+)-doped Na2SO4 samples in various ambient gases (O2, N2, NH3) with high temperature X-ray diffraction and differential thermal analysis. On heating in N2 flow, the initiating temperature for the V → I transition was lowered by doping with Ln3+ ion and the doping effect was enhanced by an increase in the ionic size ratio rLn rNa. The low temperature form of the solid solution (LSS) Na2SO4 and rare earth sulfate, which was a by-product in the preparation of the Ln3+-doped samples, transformed to a high temperature form (HSS) after the V → I transition, and the initiating temperature for the LSS → HSS transition was highest in the Eu3+-doped sample ( rLn rNa {reversed tilde equals} 1). On the other hand, on cooling in N2 flow, the HSS was stable until room temperature in the La3+- or Eu3+-doped sample ( rLn rNa ≧ 1), but transformed to LSS in the Tm3+-doped sample ( rLn rNa < 1). The results obtained in other ambient gas flows (O2, NH3) were considerably affected by the sorption of ambient gas into Na2SO4 crystals and/or the reaction between ambient gas and Na2SO4 and were different from those obtained in N2 flow. © 1991.