Phase transition peculiarities in LAMOX single crystals

Abstract

The series of oxide-ion-conductingLa2Mo2O9 single crystals, undoped and doped with Ca, Bi, W, Nb, Znand V (LAMOX), was grown by the flux method in the systemLa2O3–MoO3, which has allowed us to use polarization microscopy for the identification of phases. Phasetransition peculiarities in the LAMOX family have been studied by polarizationmicroscopy and calorimetry. The results demonstrate that both the monoclinic phase(α), which is stable at room temperature, and the metastable cubic phase(βms), or a mixture of these phases, may exist at room temperature, depending on thepost-growth cooling rate and the nature of the dopant at low doping level. Onheating, all of the quenched crystals undergo (450 °C) and (500–560 °C) phasetransitions (where β designates the stable cubic phase). At heavy doping levels,the high-temperature transition is suppressed and the crystals(La2Mo1.95V0.05Oy,La2Mo1.84W0.16Oy in our case) are found in the cubic state. The thermal peak near450 °C at high doping level is not associated with a transition and may be the result of defect association/dissociation in the cubic crystals.The thermal history, nature of the dopant and doping level are shown to influence thephase transition sequence and type.