Structural stability improvement, Williamson Hall analysis and band-gap tailoring through A-site Sr doping in rare earth based double perovskite La 2 NiMnO 6 Shah Aarif Ul Islam* , Mohd Ikram Received: 31 March 2018 / Revised: 31 May 2018 / Accepted: 9 January 2019 / Published online: 1 March 2019 Ó The Nonferrous Metals Society of China and Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract The structural, morphological and optical prop- erties of single-phase polycrystalline La 2-x Sr x NiMnO 6 (x = 0, 0.3 and 0.5), synthesized by solid state reaction were investigated. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM)/energy dispersive analysis of X-rays (EDAX), Raman spectroscopy and diffuse reflectance spectroscopy (DRS) to elucidate the role of A-site Sr-doping in double perovskite La 2 NiMnO 6 . Rietveld analysis of XRD patterns revealed that all the samples have monoclinic structure with space group P2 1 /n. Positive gradient in the Wil- liamson Hall plots revealed the presence of tensile strain in all the samples. The morphological studies revealed that average grain size increases along with appreciable decrease in porosity with Sr doping. The Ni/Mn antisite disorder was introduced in the La 2 NiMnO 6 by Sr-doping confirmed by an increase in the full width at half maximum (FWHM) and decrease in intensity of the Raman modes at around 540 and 665 cm -1 which correspond to the antisymmetric stretching and symmetric stretching modes, respectively. DRS results reveal that the band gap in La 2 NiMnO 6 can be tuned down by Sr-doping to a value of 1.37 eV (very close to 1.40 eV, considered as optimum value for better efficiency of a solar cell). Thus, Sr-doped La 2 NiMnO 6 may be of prime importance for applications in solar cells. Keywords Double perovskite; Monoclinic; Williamson Hall analysis; Band gap tailoring 1 Introduction Double perovskites are compounds with most common formula A 2 BB 0 O 6 , where A generally belongs to a rare- earth, an alkali metal or an alkaline earth metal group, and B, B 0 belong to the transition metal cation family. Owing to their enriched physics and potential technological appli- cations primarily in spintronic devices [1–5], double per- ovskite oxides are of foremost importance. These oxides possess electric and magnetic ordering simultaneously. However, the simultaneous occurrence of electrical and magnetic ordering depends on the transition metals chosen to occupy the crystallographic sites B and B 0 , as well as the type of exchange mechanism between them mediated by the anions. Because of these special properties, A 2 BB 0 O 6 oxides are among the most probed compounds by the investigators. Furthermore, these double perovskite oxides have extended range of applications as magneto-dielectric capacitors [6, 7], spin filtering tunnel junctions [8, 9], solid- state thermoelectric Peltier coolers [10] and solar cell application [11], etc. The physical properties and the phase formation of these oxides are influenced by various factors, the synthesized material can have oxygen deficiency, it may show the presence of anti-site disorder (imperfect separation of B, B 0 site cations). It also depends on the synthesis technique, which in turn depends on the precursor compounds chosen to produce the material. Anyone of these effects will influence the structure of the crystal and consequently may lead to changes in the physical properties. Other changes in the magnetic response may be expected depending on the degree of the paramagnetic character of the chosen rare earth cation. A report by Dass et al. [12] reveals that the conditions adopted during synthesis do play an important S. Aarif Ul Islam*, M. Ikram Solid State Research Laboratory, Department of Physics, National Institute of Technology, Srinagar, J&K 190006, India e-mail: aarifulislam111@gmail.com 123 Rare Met. (2019) 38(9):805–813 RARE METALS https://doi.org/10.1007/s12598-019-01207-4 www.editorialmanager.com/rmet