Polymorphism in the family of Ln6−xMoO12−δ (Ln = La, Gd-Lu; x = 0, 0.5) oxygen ion- and proton-conducting materials

Abstract

The formation of Ln6−xMoO12−δ (Ln = La, Gd, Dy, Ho, Er, Tm, Yb, Lu; x = 0, 0.5) rare-earth molybdates from mechanically activated oxide mixtures has been studied in the range 900-1600 °C. The morphotropy and polymorphism (thermodynamic phase and kinetic (growth-related) transitions) of the Ln6−xMoO12−δ (Ln = La, Gd-Lu; x = 0, 0.5) molybdates have been analyzed in detail. As a result we have observed two new types of oxygen ion- and proton-conducting materials with bixbyite (Ia3̄, no. 206) and rhombohedral (R3̄, no. 148) structures in the family of Ln6−xMoO12−δ (Ln = La, Gd-Lu; x = 0, 0.5) molybdates. The heavy rare-earth molybdates Ln6−xMoO12−δ (Ln = Er, Tm, Yb; x = 0, 0.5) have been shown for the first time to undergo an order-disorder (rhombohedral-bixbyite) phase transition at 1500-1600 °C, and we have obtained compounds and solid solutions with the bixbyite structure (Ia3̄). The stability range of the rhombohedral phase (R3̄) increases with decreasing Ln ionic radius across the Ln6−xMoO12−δ (Ln = Er, Tm, Yb, Lu) series. We have detected a proton contribution to the conductivity of the rhombohedral La5.5MoO11.25 (2 × 10−4 S cm−1 at 600 °C in wet air) and high-temperature polymorph Yb6MoO12−δ bixbyite structure, (Ia3̄) below 600 °C. At these temperatures, rhombohedral (R3̄) Yb6MoO12 seems to be an oxygen ion conductor (Ea = 0.53-0.58 eV). The total conductivity of rhombohedral (R3̄) Yb6MoO12 exceeds that of bixbyite Yb6MoO12−δ by more than one order of magnitude and is 3 × 10−5 S cm−1 at 500 °C. According to their high-temperature (T > 600 °C) activation energies, the lanthanum and ytterbium molybdates studied here are mixed electron-ion conductors.