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Smita Acharya1

1, Rashtrasant Tukadoji Maharaj Nagpur University, Nagpur, , India

Abstract
The La2Mo2O9 (LAMOX) is a fast oxy-ion conductor. Its conductivity is subject to a structural phase transition from room temperature non-conductive monoclinic to high temperature conductive cubic phase at about 580oC. The origin of the conductivity in cubic LAMOX has been suggested to be due to a “disorder” in the O sublattice. The order/disorder phase transition of LAMOX is suppressed by most of the substituents which stabilizes the highly conducting cubic phase to room temperature. Various partial substituents are possible on the cationic sublattices of LAMOX: K+, Sr2+, Ba2+, Gd3+, Sm3+, Nd3+, Dy3+ for La3+ and V5+, S6+, Cr6+, W6+, Nb5+ for Mo6+ position. All these substitutions tends to suppress the resistive transition and to stabilize the cubic phase at room temperature. The phase transition is key for origin of fast ion conductivity in LAMOX based system. In spite of that structure of LAMOX is not still well understood. It may be due to (i) slight distortion from two phases, and (ii) the symmetry breaking spontaneous strains associated with the cubic-monoclinic phase transition is extremely small. To better characterize this phase transition, we have studied the structure of LAMOX-based systems by means of X-ray powder diffraction and Raman spectroscopy as a function of temperature. The evolution of structure at high temperature in pure LAMOX system and suppression of phase transition in La2-xRxMo2-yPyO9-d (R = Gd, Sm, Dy, Nd and P = W, V, Nb) are studied; dopants content x and y are varying in between (1 to 50 at%). The compositions (x & y) are dopant sensitive for cubic-phase of LAMOX . Structural study by XRD exhibit splitting of diffraction pattern in pure LAMOX in between 24-27o , 2theta, which are disappeared in doped system. Pure-LAMOX, XRD patterns agrees with monoclinic form (ICSD 172479) while for doped LAMOX system, XRD match with cubic structure (ICSD 420672). Further insight into the structure of pure LAMOX and doped LAMOX are provided by Raman spectroscopy. For LAMOX system, we observed three bands with maxima at around 80, 350 and 870 cm-1; however pure-LAMOX, splitting in vibration mode around 870 cm-1 is detected. According to literature data, modes near 870 cm-1 are associated with symmetric and asymmetric stretching vibrations of tetrahedral MoO4 units generate Raman bands. The structural variations in doped and pure-LAMOX system are confirmed by room temperature Raman bands. The electrical conductivity and dielectric properties are studied in the temperature range 300 to 700oC and frequency range from 0.1 to 1 MHz. The conductivity of this compounds show Arrhenius type behaviour. The random free-energy model has been used to analyze the frequency dependence of the conductivity. The charge carrier relaxation time and activation have been determined from the conductivity spectra using this model. We have observed that the dielectric relaxation peaks arise from the diffusion of oxygen ions via vacancies.

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