Kinetic aspects of the synthesis of Ln6−хMoO12−δ (Ln = Sm, Ho -Yb; x = 0, 0.5) rare-earth molybdates using mechanical activation of oxides

Abstract

The synthesis of the Ln6MoO12 (Ln = Sm, Ho - Yb) (3:1) rare-earth molybdates from binary oxides at room temperature has been studied by XRD and electron spin resonance spectroscopy (ESR). The mechanical activation of 3Ln2O3 + MoO3 (Ln = Sm, Ho, Yb) mixtures containing unmilled or premilled MoO3 initiates the formation of Ln6MoO12 (Ln = Sm, Ho - Yb) at room temperature, which is accompanied by a reduction in the ESR signal from Mo5+ paramagnetic ions located on the surface of the activated MoO3 and, hence, by a reduction in the amount of MoO3 in the mixture, due to reaction with Ln2O3 (Ln = Sm, Ho, Yb)·The major phase resulting from the mechanochemical synthesis is a cubic phase with the bixbyite structure (Ia¯3, no. 206) for Ln6−xMoO12−δ (Ln = Dy –Yb; x = 0, 0.5) and the Sm2O3 type structure (C 2/m, no. 12) for Ln6MoO12 (Ln = Gd, Sm).The high-temperature synthesis of Ln6−xMoO12−δ (Ln = Ho – Yb; x = 0, 0.5) and Ho10Mo2O21 from precursors prepared at room temperature has been studied at 1200 °C and heat treatment times of 4, 40, 80, and 160 h. The bixbyite phase disappears because it is metastable and is formed due to kinetic factors in the stability field of lower symmetry phases: tetragonal (T) and rhombohedral (R3¯). We have located the stability range of the rhombohedral (R3¯) phase as a function of Ln below 1200 °C using different heating time.We have found conditions for the synthesis of the tetragonal phase T, which exists only in the case of the intermediate lanthanides. The tetragonal phase in phase-pure form has been synthesized for the first time via prolonged (160 h) heat treatment of Ho10Mo2O21 at 1200 °C.The electrical conductivity of the samples with different % of tetragonal phase has been measured by impedance spectroscopy in dry and wet air. The Arrhenius plot of conductivity for pure tetragonal phase has the form of straight line over the entire temperature range studied, 440–900 °C (Еа = 1.37 eV, σ 600°C = 1 × 10−5 S cm−1) in dry and wet air. The absence of electrode dispersions at low frequencies and conductivity growth in a wet air makes us assume the predominantly electronic conductivity type of the Ho10Mo2O21 tetragonal phase in the temperature range 440–900 °C. In multiphase samples surface electronic conductivity is observed up to 620–650 °С in wet air, which is associated with the presence of defects at grain boundaries of crystallographically related phases (bixbyite, rhombohedral and tetragonal phases).