Micro-structure, optical properties and ac conductivity of rare earth double perovskite oxides: Sr 2 ErNbO 6 Rajesh Mukherjee a,n , Sadhan Chanda b , Chandrahas Bharti c , P. Sahu d , T.P. Sinha b a Department of Physics, Ramananda College, Bishnupur, Bankura 722122, India b Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata 700009, India c Sensor and Actuator Division, CSIR-Central Glass and Ceramic Research Institute, 196, Raja SC Mullick Road, Kolkata 700032, India d Department of Physics, Jadavpur University, Kolkata 700032, India article info Article history: Received 14 February 2013 Received in revised form 11 April 2013 Accepted 12 April 2013 Available online 22 April 2013 Keywords: Ceramics X-ray diffraction Dielectric properties AC conductivity abstract A polycrystalline rare earth double perovskite oxide strontium erbium niobate, Sr 2 ErNbO 6 (SEN) is synthesized by the solid state reaction technique. Rietveld analysis of X-ray diffraction pattern reveals that SEN has monoclinic phase with space group P2 1 /n. Fourier transform infrared spectrum shows two phonon modes of the sample due to the antisymmetric NbO 6 stretching vibration. The compound shows significant frequency dispersion in its dielectric properties. The frequency dependent ac conductivity spectra follow the Jonscher's universal power law. The conductivity at 100 Hz varies from 1.38  10 -6 Sm -1 to 2.34  10 -4 Sm -1 with the increase of temperature from 303 K to 653 K respectively. The increase of conductivity can be explained from the Correlated barrier hopping (CBH) model. The relaxation mechanism of the sample in the framework of electric modulus formalism is modelled by Davidson–Cole equation. The activation energy of the sample, calculated from both conductivity and modulus spectra is found to be 0.8 eV which indicates that the conduction mechanism for SEN is due to ion hopping. The scaling behavior of imaginary electric modulus suggests that the relaxation describes the same mechanism at various temperatures. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Double perovskite oxides (DPOs) having chemical formula A 2 B′B″ O 6 are widely studied due to the diversity in the crystal structure, composition and physical characteristics [1,2]. These DPOs have found wide applications in a variety of technological devices, in particular, in microelectronics, telecommunications, fuel cells, superconductors, charge storage devices, memory devices and in the area of spintronics [3–6]. Doi and Hinatsu have investigated the crystal structure and magnetic properties of ordered perovskites Sr 2 LnRuO 6 (Ln¼ Eu–Lu). Below the transition temperatures, there is a large difference in the temperature dependence of the magnetic susceptibility measured under the zero-field cooled condition and under the field cooled condition, which shows the existence of a weak ferromagnetic moment associated with the antiferromagnetism [7]. Kanaiwa et al. have investigated the structural chemistry and magnetic properties of the series of double perovskite Sr 2 LnTaO 6 (Ln¼ lanthanides) [8]. They have shown by Rietveld refinement of X-ray diffraction data that all the compounds in the series are monoclinic with an ordered arrange- ment of Ln and Ta ions. From the magnetic susceptibility measure- ment they concluded that these compounds are paramagnetic down to 5 K. Corredor et al. have performed the experimental and theore- tical studies of crystalline structure, surface morphology and electronic structure of Sr 2 GdRuO 6 oxides material [9]. Dias et al. have synthesized chemically substituted double perovskite Sr 2 LnTaO 6 ceramics and investigated the vibrational properties using Raman and IR spectro- scopy together with group theoretical calculation [10]. Though the various properties of rare earth based DPOs have been investigated, the investigation of ac electrical properties of rare earth based DPOs is scarce. As most of the devices are operated in the ac electrical mode, the investigations of ac electrical properties of these oxides are interesting. Khalam and Sebastian have first synthesized Sr(Er 1/2 Nb 1/2 )O 3 and have studied its microwave dielectric properties [11]. We have synthesized the SEN and performed the Rietveld refinement of X-Ray diffraction data. It has been observed that the SEN has monoclinic structure with space group P2 1 /n. In this paper we have investigated in detail the ac electrical properties of rare earth DPO: Sr 2 ErNbO 6 (SEN) using alternating current impedance spectroscopy (ACIS) in the frequency range from 100 Hz to 1 MHz and in the temperature range from 303 to 673 K. 2. Experiment The conventional solid state reaction technique is used for the synthesis of SEN. The starting materials SrCO 3 (Loba Chemie, Extra pure), Er 2 O 3 (Alfaaesar, 99.99%) and Nb 2 O 5 (Loba Chemie, 99.9%) Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physb.2013.04.031 n Corresponding author. Tel.: +91 33 23031189; fax: +91 33 23506790. E-mail address: rajeshxrd@gmail.com (R. Mukherjee). Physica B 422 (2013) 78–82