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 Ln_6−xMoO_12−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 Ln_6−xMoO_12−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 (Ia, no. 206) and rhombohedral (R, no. 148) structures in the family of Ln_6−xMoO_12−δ (Ln = La, Gd–Lu; x = 0, 0.5) molybdates. The heavy rare-earth molybdates Ln_6−xMoO_12−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 (Ia). The stability range of the rhombohedral phase (R) increases with decreasing Ln ionic radius across the Ln_6−xMoO_12−δ (Ln = Er, Tm, Yb, Lu) series. We have detected a proton contribution to the conductivity of the rhombohedral La_5.5MoO_11.25 (2 × 10⁻⁴ S cm⁻¹ at 600 °C in wet air) and high-temperature polymorph Yb₆MoO_12−δ bixbyite structure, (Ia) below 600 °C. At these temperatures, rhombohedral (R) Yb₆MoO₁₂ seems to be an oxygen ion conductor (E_a = 0.53–0.58 eV). The total conductivity of rhombohedral (R) Yb₆MoO₁₂ exceeds that of bixbyite Yb₆MoO_12−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.