Zr-doped samarium molybdates — potential mixed electron–proton conductors S.N. Savvin a , A.V. Shlyakhtina b, ⁎, I.V. Kolbanev b , A.V. Knotko c , D.A. Belov b,c , L.G. Shcherbakova b , P. Nuñez a a Department of Inorganic Chemistry, University of La Laguna, 38200 La Laguna, Tenerife, Spain b Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991, Russia c Faculty of Chemistry, Moscow State University, Leninskie gory 1, Moscow 119991, Russia abstract article info Article history: Received 16 May 2013 Received in revised form 8 January 2014 Accepted 18 January 2014 Available online xxxx Keywords: Rare-earth Sm molybdate Fluorite Oxide ion conductivity Proton conductivity Electron conductivity Impedance spectroscopy Two Zr-doped samarium molybdatesSm 6-x 7 Zr x 7 Mo 1 7 O 12 7 þ x 24 -δ corresponding to x = 0.6 and 1 (SZMO) have been synthesized at 1600 °C for 3 h using mechanically activated mixtures of starting oxides. Fluorite-like Sm 0.771 Zr 0.086 Mo 0.143 O 1.739 - δ (06SZMO) and Sm 0.714 Zr 0.143 Mo 0.143 O 1.756 - δ (10SZMO) have similar total conductivity of about 4 × 10 -4 S/cm at 800 °C in air. Below 600 °C, the total conductivity of 06SZMO in air exceeds that of 10SZMO. An increase in bulk and grain boundary conductivity of 06SZMO observed at low temperate under wet conditions suggests there may be a proton contribution to the total conductivity. Under reducing conditions (5% H 2 –Ar) 06SZMO becomes essentially an electronic conductor. Its conductivity reaches 0.25 S/cm at 800 °C and the activation energy decreases to 0.3 eV. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Among the Ln 6 WO 12 family of rear-earth tungstates, La 6 WO 12 has attracted particular interest in the last decade because it was found to possess proton conductivity reaching 5 × 10 -3 S/cm at 900 °C in wet hydrogen [1–4]. The oxygen ion conductivity (in air) and proton conductivity (in wet hydrogen) of the Ln 6 WO 12 (Ln = La, Nd, Gd, Er) compounds decrease as the ionic radius of the lanthanide decreases. The highest conductivity is thus offered by La 6 WO 12 . The crystal structure of the Ln 6 WO 12 tungstates ranges from the cubic fluorite-like and highly disordered La 6 WO 12 to pseudotetragonal (for Ln = Nd–Gd) and finally to rhombohedral (for the heavy lantha- nides Ln = Tb–Lu). Detailed structural studies [3–5] revealed that the crystal structure of La 6 WO 12 is probably more complex than it was previously thought. A model suggesting partial substitution of W 6+ for La 3+ has been put forward by Magrasó el al. [4] in order to explain apparent under-occupancy of the La sites in the crystal structure of the lanthanum tungstate. According to this model the W 6+ species that reside on the La sites behave as donors and the excess charge associated with them is compensated by filling the inherent anion vacancies with oxygen ions. Therefore, the overall stoichiometry of the lanthanum tungstate can be conveniently represented by the formula La 28 - x W 4+x O 54+δ v 2 - δ , where v 2 - δ stands for vacant oxygen sites. Moreover, structural studies of solid solutions spanning the range of La/W ratios from 4.8 to 6 and prepared by the freeze drying method showed that the samples slightly deficient in La (La/W = 5.3–5.7) were single-phase, whereas those approaching the ideal stoichiometry of La 6 WO 12 , always contained small amount of La 2 O 3 impurity. Recently, molybdenum-substituted lanthanum tungstates La 28 - y (W 1 - x Mo x ) 4+y O 54+δ (x = 0–1; y = 0.923) were investigated [6]. For x ≤ 0.4 these solid solutions adopt a cubic structure and for x ≥ 0.6 they show rhombohedral superstructure. A strong increase of electronic conductivity (n-type) under reducing conditions and high levels of proton and oxide-ion conductivity were observed in these materials at moderate Mo concentrations (x ≤ 0.4). These electrochem- ical properties make them attractive for such applications as hydrogen separation membranes [7]. In recent years, much attention has been paid to the industrial use of lanthanides because of their unique optical properties. The lanthanides can be used in a variety of applications, in particular as components of inorganic pigments, tunable lasers, and organic light-emitting diodes [8–11]. Vishnu et al. [11] studied the structural and optical prop- erties of a new series of nontoxic yellow dyes with the general formula Sm 6 - x W 1 - y Zr x Mo y O 12+δ (x = 0–0.6, y = 0–1). Both Sm 6 WO 12 (JCPDS PDF, # 16–414) and Sm 6 MoO 12 (# 24–1121) as well as the zirconium-containing solid solutions were shown to have ordered, defect- fluorite structure. Sm 5.4 Zr 0.6 MoO 12+ δ demonstrated the best chromatic properties among the Sm 6 - x W 1 - y Zr x Mo y O 12+δ (x = 0–0.6, y = 0–1) solid solutions [11]. Solid State Ionics xxx (2014) xxx–xxx ⁎ Corresponding author. Tel.: +7 499 9397950; fax: +7 499 2420253. E-mail addresses: annash@chph.ras.ru, annashl@inbox.ru (A.V. Shlyakhtina). SOSI-13276; No of Pages 6 0167-2738/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ssi.2014.01.031 Contents lists available at ScienceDirect Solid State Ionics journal homepage: www.elsevier.com/locate/ssi Please cite this article as: S.N. Savvin, et al., Solid State Ionics (2014), http://dx.doi.org/10.1016/j.ssi.2014.01.031