Effect of rare earth ions on the phase transition of Na 2SO 4 crystals

Abstract

The V ⇄ I phase transition of Na 2SO 4 crystals was investigated on a sample of pure Na 2SO 4 and on rare-earth-ion ( Ln 3+ = La 3+, Eu 3+, Tm 3+)-doped Na 2SO 4 samples in various ambient gases (O 2, N 2, NH 3) with high temperature X-ray diffraction and differential thermal analysis. On heating in N 2 flow, the initiating temperature for the V → I transition was lowered by doping with Ln 3+ ion and the doping effect was enhanced by an increase in the ionic size ratio r Ln 3+ r Na + . The low temperature form of the solid solution (LSS) Na 2SO 4 and rare earth sulfate, which was a by-product in the preparation of the Ln 3+-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 Eu 3+-doped sample ( r Ln 3+ r Na + ⋍ 1 ). On the other hand, on cooling in N 2 flow, the HSS was stable until room temperature in the La 3+- or Eu 3+-doped sample ( r Ln 3+ r Na + ≧ 1 ), but transformed to LSS in the Tm 3+-doped sample ( r Ln 3+ r Na + < 1 ). The results obtained in other ambient gas flows (O 2, NH 3) were considerably affected by the sorption of ambient gas into Na 2SO 4 crystals and/or the reaction between ambient gas and Na 2SO 4 and were different from those obtained in N 2 flow.