research papers 836 http://dx.doi.org/10.1107/S1600576715006664 J. Appl. Cryst. (2015). 48, 836–843 Received 11 August 2014 Accepted 2 April 2015 Edited by G. Kostorz, ETH Zurich, Switzerland Keywords: chemical routes; Rietveld analysis; small-angle neutron scattering; optical band gaps. Influence of doping on crystal growth, structure and optical properties of nanocrystalline CaTiO 3 : a case study using small-angle neutron scattering Oindrila Mondal, a Manisha Pal, b Ripandeep Singh, c Debasis Sen, c Subhasish Mazumder c and Mrinal Pal d * a MUC Women’s College, Burdwan, 713104, India, b Sarojini Naidu College, Kolkata, 700028, India, c Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai, 400085, India, and d CSIR–Central Glass and Ceramic Research Institute, Kolkata, 70032, India. *Correspondence e-mail: palm@cgcri.res.in The effect of dopant size (ionic radius) on the crystal growth, structure and optical properties of nanocrystalline calcium titanate, CaTiO 3 (CTO), have been studied using small-angle neutron scattering. X-ray diffraction, along with high- resolution transmission electron microscopy, confirms the growth of pure nanocrystalline CTO. Rietveld analysis reveals that the difference of ionic radii between dopant and host ions induces strain within the lattice, which significantly affects the lattice parameters. The induced strain, due to the difference of ionic radii, causes the shrinkage of the optical band gap, which is manifested by the redshift of the absorbance band. Mesoscopic structural analysis using scattering techniques demonstrates that the ionic radius of the dopant influences the agglomeration behaviour and particle size. A high- resolution transmission electron microscopy study reconfirms the formation of pure highly crystalline CTO nanoparticles. 1. Introduction Materials with perovskite structure possess a broad range of physical characteristics, which offer various functionalities in devices including dynamic random access memories, ferro- electric nonvolatile memories, piezoelectric and optical devices, sensors, and catalyst electrodes in fuel cells (Lupina et al., 2009; Zhu et al., 2010; Jung et al., 2011; Allibe et al. , 2010; Wei et al., 2010). Among the perovskite materials, calcium titanate, CaTiO 3 (CTO), is an important member, owing to crucial functional properties such as a high dielectric constant, a large positive temperature corresponding to the resonance frequency, significant luminescent properties and relatively high mechanical strength (Moreira et al., 2009; Stanishevsky & Holliday, 2007). Tuning of properties by partial replacement of Ti 4+ ions by trivalent cations is common practice for CTO (Zhang et al., 2007; Milanez et al., 2009; Zhao et al. , 2013). Trivalent cations create oxygen vacancies, which compensate for the high dielectric loss and induce new electronic states within the band gap, giving rise to interesting photoluminescence properties (Zhang et al., 2007; Milanez et al., 2009). Pr 3+ -doped CTO nanocrystals have been reported to show enhanced photo- luminescence and phosphorescence (Zhang et al., 2007; Diallo et al., 2001). The effect of Y 3+ and Nb 3+ on the transport properties of CTO has also been investigated (Ueda et al., 1997). An insulator to metallic transition has been observed as the result of an increase in dopant concentration from 0 to 10 2 mol. Shifting of the band edge of CTO towards higher energy with an increase in Eu 3+ doping has also been reported (Huong et al., 2012). The radiative f–f transition of Eu 3+ was ISSN 1600-5767 # 2015 International Union of Crystallography