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

The formation of Ln(6-x)MoO(12-delta) (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 degrees C. The morphotropy and polymorphism (thermodynamic phase and kinetic (growth-related) transitions) of the Ln(6-x)MoO(12-delta) (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 (R (3) over bar, no. 206) and rhombohedral (R (3) over bar, no. 148) structures in the family of Ln(6-x)MoO(12-delta) (Ln = La, Gd-Lu; x = 0, 0.5) molybdates. The heavy rare-earth molybdates Ln(6-x)MoO(12-delta) (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 degrees C, and we have obtained compounds and solid solutions with the bixbyite structure (Ia (3) over bar). The stability range of the rhombohedral phase (R-3) increases with decreasing Ln ionic radius across the Ln(6-x)MoO(12-delta) (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 degrees C in wet air) and high-temperature polymorph Yb6MoO12-delta bixbyite structure, (R (3) over bar) below 600 degrees C. At these temperatures, rhombohedral (R (3) over bar) Yb6MoO12-delta seems to be an oxygen ion conductor (Ea = 0.53-0.58 eV). The total conductivity of rhombohedral (R (3) over bar) Yb6MoO12 exceeds that of bixbyite Yb6MoO12-delta by more than one order of magnitude and is 3-10(-5) S cm(-1) at 500 degrees C. According to their high-temperature (T > 600 degrees C) activation energies, the lanthanum and ytterbium molybdates studied here are mixed electron-ion conductors.