Phase equilibria in the Nd2O3-V2O3-V2O5 system at 1200.DEG.C.
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Phase equilibria in the Nd<SUB>2</SUB>O<SUB>3</SUB>–V<SUB>2</SUB>O<SUB>3</SUB>–V<SUB>2</SUB>O<SUB>5</SUB> system were established at 1200 °C by changing the oxygen partial pressures from －0.68 (air) to －12.50 in terms of log<I>P</I><SUB>O<SUB>2</SUB></SUB>. In this system, V<I><SUB>n</SUB></I>O<SUB>2<I>n</I>－1</SUB> (<I>n</I>=2–7), VO<SUB>2</SUB>, Nd<SUB>2</SUB>O<SUB>3</SUB>, NdVO<SUB>3</SUB>, NdVO<SUB>4</SUB>, 0.81Nd<SUB>2</SUB>O<SUB>3</SUB>·0.19V<SUB>2</SUB>O<SUB>5</SUB> (Nd<SUB>1.62</SUB>V<SUB>0.38</SUB>O<SUB>3.38</SUB>, A), and 3Nd<SUB>2</SUB>O<SUB>3</SUB>·V<SUB>2</SUB>O<SUB>5</SUB> (Nd<SUB>3</SUB>VO<SUB>7</SUB>, B) were stable under the present experimental conditions. Compounds with Nd<SUB>2</SUB>O<SUB>3</SUB>/V<SUB>2</SUB>O<SUB>5</SUB> mole ratios of 6/1 and 2/1 were not found under the present experimental conditions. Compounds A, Nd<SUB>3</SUB>VO<SUB>7</SUB>, NdVO<SUB>3</SUB>, and NdVO<SUB>4</SUB> have nonstoichiometric compositions. The pattern of the phase diagram is different from those of other Ln<SUB>2</SUB>O<SUB>3</SUB>–V<SUB>2</SUB>O<SUB>3</SUB>–V<SUB>2</SUB>O<SUB>5</SUB> systems that have previously been established by us. On the basis of the phase diagram, the Gibbs energies of the reactions:<BR>(1) 31⁄50Nd<SUB>2</SUB>O<SUB>3</SUB>+19⁄50NdVO<SUB>3</SUB>+19⁄100O<SUB>2</SUB>=Nd<SUB>1.62</SUB>V<SUB>0.38</SUB>O<SUB>3.38</SUB><BR>(2) 50⁄31Nd<SUB>1.62</SUB>V<SUB>0.38</SUB>O<SUB>3.38</SUB>+12⁄31NdVO<SUB>3</SUB>+6⁄31O<SUB>2</SUB>=Nd<SUB>3</SUB>VO<SUB>7</SUB><BR>(3) NdVO<SUB>3</SUB>+1⁄2O<SUB>2</SUB>=NdVO<SUB>4</SUB><BR>were determined to be －61.4, －47.4, and －128.9 kJ respectively. In the Nd<SUB>2</SUB>O<SUB>3</SUB>–V<SUB>2</SUB>O<SUB>3</SUB> system, only NdVO<SUB>3</SUB> is stable under the present experimental conditions.