Journal of Physics and Chemistry of Solids xxx (xxxx) xxx Please cite this article as: Saswata Halder, Journal of Physics and Chemistry of Solids, https://doi.org/10.1016/j.jpcs.2019.109265 Available online 8 November 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved. Exploring the intricacies in the conduction mechanism of the perovskite series Ba 2 HoSb 1x Ru x O 6 : A conductivity scaling approach Saswata Halder * , Ram Awdhesh Kumar, Alo Dutta, T.P. Sinha Department of Physics, Bose Institute, 93/1, Acharya Prafulla Chandra Road, Kolkata, 700009, India A R T I C L E INFO Keywords: X-ray diffraction Complex perovskites Time–temperature superposition Electrical conductivity Electric modulus ABSTRACT A composition-dependent investigation of the electrical conductivity and electric modulus formalisms was performed for complex perovskites of the form Ba 2 HoSb 1-x Ru x O 6 (x ¼ 0.25, 0.50, 0.75, 1.0). The combination of X-ray diffraction and Rietveld refnement methods confrmed the structure of all the synthesized samples to be cubic. The lattice parameters and cell volumes of the synthesized materials satisfy the linear Vegard law, con- frming the formation of complete solid solutions. The strength of bonding in the materials was analysed through the bond valence sum approach as a function of changing composition. The electrical conductivity trends were investigated as a function of Ru substitution. A scaling formalism satisfying the time–temperature superposition principle as functions of temperature and composition was sought. The activation energies extracted from the dc conductivity and hopping frequency show positive correlation. The scaled conductivity and modulus curves do not form a master curve in the grain and grain-boundary regions because of the dissimilar activation kinetics in the microstructures. Thus, the scaling remains inconsistent in these microstructural domains. However, a perfect scaling of the conductivity is found as a function of composition. Thus, the conduction mechanism follows a mobility scaling rather than a temperature scaling. A perfect scaling of the electric modulus as a function of composition is not found, showing the pronounced compositional dependence of the relaxation mechanism. Thus, the scaling behaviour has threefold implications: (i) there is no single universal scaling parameter for the conductivity spectra in the different electroactive microstructural domains, (ii) there is a unique scaling function over composition which can account for both temperature-independent and composition-independent conduc- tion processes and (iii) it points towards a composition-independent conduction mechanism in contrast to a composition-dependent relaxation mechanism. 1. Introduction The functionality of a material is inherently correlated to the conformational as well as the compositional fexibility of the host perovskite architecture [1–6]. Over the last few decades, knowledge of the crystal structure as well as the electronic structure has been realized to be essential in rationalizing the different mechanisms which have an underlying effect on the transport properties [7–10]. Ru-based double perovskite oxides, infuenced by the discovery of spin-glass behaviour in Sr 2 FeRuO 6 , have emerged as a lucrative target to study novel physical phenomena [11–13]. The mechanisms infuencing such unique physical properties are complemented by the presence of strong spin–orbit coupling and Coulombic correlation (U) of Ru along with changes in the crystal feld interactions inside the octahedral environment around Ru [14–25]. The present article focusses on the charge carrier dynamics in the radio-frequency region of solid solutions of the form Ba 2 HoSb 1-x Ru x O 6 (x ¼ 0.25, 0.50, 0.75, 1.0) having an ordered cubic perovskite structure. The end members, Ba 2 HoSbO 6 [26,27] and Ba 2 HoRuO 6 [28], have been shown to possess contrasting magnetic ground states. Ba 2 HoSbO 6 pos- sesses an uncomplicated paramagnetic ground state, whereas Ba 2 HoRuO 6 has a long-range antiferromagnetic ordering (type I arrangement) of both Ho 3þ ions and Ru 5þ ions at 22 K and 50 K, respectively. The ac electrical properties of A 2 HoMO 6 (A ¼ Ba, Sr, Ca; M ¼ Sb, Ru) have also been investigated in detail [29,30]. The vast difference in the electrical and magnetic properties of the end members is caused by the sensitivity of the narrow itinerant 4d band of Ru, which is mostly infuenced by the degree of hybridization occurring between the Ru t 2g and O 2p orbitals. Knowledge of the charge carrier dynamics of a material, which is * Corresponding author. E-mail address: saswata.h88@gmail.com (S. Halder). Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs https://doi.org/10.1016/j.jpcs.2019.109265 Received 17 June 2019; Received in revised form 6 November 2019; Accepted 7 November 2019