Effect of crystallite size on electron spin resonance of Gd 3+ and luminescence of Eu 3+ doped in La 6 WO 12

Abstract

Nano-sized Eu 3+ and Gd 3+ doped lanthanum tungstates of composition La 6-x Ln x WO 12 (Ln = Eu, Gd; 0 ≤ x ≤ 0.2) are prepared by sol-gel complexation method. These samples are characterized by powder X-ray diffraction (XRD). The average crystallite size is calculated from the XRD using Scherer's equation. The electron spin resonance (ESR) spectrum of Gd 3+ doped lanthanum tungstate (La 6 WO 12) gives characteristic "U" type spectrum. The luminescence spectrum of Eu 3+ doped La 6 WO 12 gives characteristic 5 D 0 → 7 F n (n = 0 to 4) bands due to trivalent europium ions. The intensity of 5 D 0 → 7 F 2 transition decreases with increase in the sintering temperature due to depletion of surface Eu 3+ ions. The intensity of 5 D 0 → 7 F 2 sintered at 700°C has highest intensity and may have laser applications. Materials with mixed conductivity (ionic and electronic) at high temperatures and chemically stable are of considerable interest in solid oxide fuel cells (SOFC) and hydrogen gas separation membranes. Oxides with high proton conductivity find potential applications in both hydrogen production and hydrogen separation. A survey of literature data on electrical properties shows that rare-earth tungstate of composition Ln 6 WO 12 (Ln = lanthanide) are promising materials for hydrogen permeable membranes due to their mixed conductivity and remarkable stability in moist CO 2 environments 1-3 . These lanthanide tungstate oxides, Ln 6 WO 12 , have a defect fluorite structure and they can be formulated as Ln 6 WO 12 k 2 or A 4 O 6.85 k 1.15 (k= ϋ) O for a fluorite formulation. The type of lattice adopted by Ln 6 WO 12 oxides depends on the ionic radii of Ln 3+ . The structure of Ln 6 WO 12 is characterized by seven edge-sharing cubes with oxygen ions in the corners. The tungsten ion is in the middle of central cube while the lanthanide ions are in the center of neighboring six cubes. The central cube has two oxygen vacancies located at opposite vertex positions. Each of these six rare earth containing cubes shares one of the vacant corners with central W-containing cube 4 . It is well known that the preparation method influences the properties, microstructure and size distribution of materials considerably. Wet chemical methods are found to be effective to obtain stoichiometric oxides at lower temperatures compared to conventional solid state method. Lower sintering temperatures are desirable for the synthesis of tungstates due to their volatile nature. Studies pertaining to preparation of nanosized Ln 6 WO 12 compositions and/or the influence of crystallite size on ESR and emission profiles, to our knowledge, are not reported. In this paper, we present the preparation of La 6-x Ln x WO 12 (Ln = Gd, (x = 0.02, 0.05, 0.1, 0.2), Eu (x = 0.05)) and the effect of crystallite size on ESR of Gd 3+ and emission of Eu 3+ profiles.