Influence of Ni/Mo ratio on structural and electrical properties of double perovskite system Sr 2 Ni 1+x Mo 12x O 62d Pravin Kumar 1 • Nitish Kumar Singh 1 • Rajesh Kumar Singh 1 • Prabhakar Singh 1 Received: 5 May 2015 / Accepted: 18 August 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstract Technologically important double perovskite system Sr 2 Ni 1?x Mo 1-x O 6-d with x = 0.00, 0.05, 0.10, and 0.15 was prepared by solution combustion method. The structural and the Rietveld analysis of compositions revealed the formation of double perovskite tetragonal phase Sr 2 NiMoO 6 with space group I4/m as a major phase. SrMoO 4 and NiO were also observed as minor phases. Microstructural studies depicted the formation of uniform grains for all the samples. The average grain size was found to lie between the ranges of 1–4 lm. XPS analysis of the synthesized compositions showed the decreasing ratio of Mo 5? to Mo 6? ions in the system with increasing Ni content, which played an important role in the conduction mechanism. The thermal expansion coefficient (TEC) of all compositions indicated that it is more compatible to the TEC of standard electrolytes. The electrical conductivity for all the compositions was studied using impedance spectroscopy in the temperature range 200–600 °C. Com- position with x = 0.05 showed better electrical conduc- tivity with good catalytic activity. 1 Introduction Solid oxide fuel cells (SOFCs) are suitable candidates for new generation of electrical energy, which possess high efficiency, long-term stability, and better fuel flexibility [1– 4]. In the development of SOFCs, perovskite and double perovskite materials are technologically important due to their applications in the fields of renewable energy [5–7]. The double perovskite with general formula A 2 B 0 B 00 O 6 falls under the subgroup of perovskite ABO 3 , where A is an alkaline earth element (A = Sr, Ca, Ba) and B 0 ,B 00 are heterovalent transition metals (B 0 = Fe, Cr, Fe, Co, Ni, etc. and B 00 = Mo, W, Re, Sb, etc.) [8–15]. The double per- ovskites A 2 B 0 B 00 O 6 represent two types of octahedral viz. B 0 O 6 and B 00 O 6 , oriented in three dimensions of crystal lat- tice [16, 17]. Among such kinds, molybdenum-based double perovskites bear exceptional electromechanical properties. Many of them are highly enviable in SOFC for their superior performance as anode materials [6, 18, 19]. Due to these reasons, some of these materials having mixed electronic and ionic conductions (MEICs) such as Sr 2 MgMoO 6-d [6, 8, 10], Sr 2 CoMoO 6 [12], Sr 2 FeMoO 6-d [7, 9], and Sr 2 NiMoO 6-d [11, 19] have been studied at large scale for their high elec- trical conductivity and good catalytic activity under different reducing atmospheres (air/H 2 /H 2 -Ar/CH 4 ). Among them, Sr 2 NiMoO 6-d is an attractive candidate for anode material in SOFC. However, functionality of Sr 2 NiMoO 6-d faces major challenges due to the formation of secondary phases, and these are appeared owing to inadequate heat treatments [18]. Prasatkhetragarn et al. [17] reported that an extra addition of NiO (5 wt%) in Sr 2 NiMoO 6 system eliminates the secondary phase of SrMoO 4 . In Sr 2 NiMoO 6-d , formation of single phase occurs after heat treatment above 1200 °C[17, 20]. Also, Ni-doped double perovskite is important because of its high catalytic activity in comparison with Ni-free double perovskite anode materials [20–23]. These reports motivated the workers to investigate Ni-substituted double perovskites for anode materials in SOFC. Few authors have also reported that Sr 2 NiMoO 6 exhibits a second-order phase transition from tetragonal (s.g.I4/m) to cubic (s.g.Fm  3m) at tempera- tures around 300 °C[11], 280 °C[17], and 277 °C[24]. The & Prabhakar Singh psingh.app@iitbhu.ac.in 1 Department of Physics, Indian Institute of Technology (BHU), Varanasi 221005, India 123 Appl. Phys. A DOI 10.1007/s00339-015-9448-x