Ceramics International xxx (xxxx) xxx Please cite this article as: Komal K. Jani, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.10.138 Available online 20 October 2020 0272-8842/© 2020 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Ca 2+ - substitution effect on the electronic structure of CaCu 3 Ti 4 O 12 studied by electron spectroscopy for chemical analysis Komal K. Jani a , Divyesh V. Barad a , Pooja Y. Raval b , Monika Nehra c , Nimish H. Vasoya d , Narendra Jakhar e , Kunal B. Modi a, * , Sandeep Kumar f , Dong-Kwon Lim g , Rishi Kumar Singhal e a Department of Physics, Saurashtra University, Rajkot, 360005, India b Department of Physics, C. U. Shah University, Wadhwan, Surendranagar, 363030, India c Department of Mechanical Engineering, University Institute of Engineering & Technology, Panjab University, Chandigarh, 160014, India d Department of Balbhavan, Children’s University, Sector – 20, Gandhinagar, 382021, India e Department of Physics, University of Rajasthan, Jaipur, 302004, India f Department of Bio and Nanotechnology, Guru Jambheshwar University of Science and Technology, Hisar, 125001, India g KU-KIST Graduate School of Converging Science and Technology, Korea University, 145, Anam –ro, Seongbuk-gu, Seol, Republic of Korea A R T I C L E INFO Keywords: CaCu 3 Ti 4 O 12 Electron spectroscopy for chemical analysis Electronic structure ABSTRACT A detailed electronic structure study of quadruple perovskite series, Ca 1+x Cu 3-x Ti 4 O 12 (for x = 0.0, 0.1, 0.2, 0.5, 1.0), using electron spectroscopy for chemical analysis (ESCA) has been carriedout. The Ti2p ESCA spectra imply that Ti ions remain in the tetravalent state and show the presence of Ti clusters, TiO 6 and TiO 5 . The Cu2p spectra indicate the dominance of the divalent state of Cu ions. The Ca2p spectra show that Ca atoms are divalent but spectral features are suggestive of the presence of Ca superfcial atoms in the system which is possibly due to the precipitation of Ca atoms in the grain boundary regions. The Ca2p in conjunction with O1s spectra shows that there is a strong possibility of hybridization of Ca2p states with the O1s and/or the Cu3d orbitals. The remarkable shift in O1s spectra towards the lower energy side for the samples with x ≥ 0.5 is mainly due to the substitution of large cation Ca 2+ (1.0 Å), for small cation Cu 2+ (0.73 Å) in the series. That results in bond length enhancement and weakening of the bond strength. It is proposed that the shift of O1s peaks can also be partially caused by oxygen vacancies; nevertheless, more experiments are required to substantiate this. 1. Introduction Over the last two decades pure and substituted quadruple perovskite calcium-copper-titanate (CaCu 3 Ti 4 O 12 ) are subject of interest due to their fascinating physical properties suitable for a diverse range of ap- plications [1–5]. The cationic substitution on the Ca-site (icosahedral environment) and Ti–site (octahedral interstitial site) are quite usual and a hundred percent replacement by other cation is possible without affecting the single-phase nature of the compositions. However, the cationic substitution limit at the Cu-site (square-planar site) is very small without distorting crystal structure [6,7]. Electron spectroscopy for chemical analysis (ESCA) or X-ray photo- electron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) i. e., Extended X-ray absorption fne structure (EXAFS)/X-ray absorption near-edge spectroscopy (XANES) are referred to as the most authentic and powerful surface-sensitive spectroscopic techniques. Due to involvement of core-electron excitation in these techniques, the details regarding the electronic and local structures around the discrete atomic site in a system can be obtained. The particulars regarding the unoc- cupied electronic states are provided by XAS while the particulars about the occupied state density of a system are given by XPS. Thus, both these techniques are complementary to each other. Further, in the multi- cationic systems, ESCA furnishes knowledge of iconicity, charge trans- fer, orbital symmetry, valence state, and cationic vacancies and anionic vacancies. To comprehend surface-related issues (like adhesion, surface defects, strains, the surface composition of high-performance materials, and structure of thin-flm) in manufacturing industries ESCA is widely employed in order to control the quality of the product [8,9]. The selection of certain metallic cations for the substitution purpose should be done carefully. For example, during the solid-state reaction process of oxide systems synthesis, the high temperature (≥950 ◦ C) prolong sintering results in loss of cations such as Ca 2+ , Cd 2+ , Li 1+, and * Corresponding author. E-mail address: kunalbmodi2003@yahoo.com (K.B. Modi). Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint https://doi.org/10.1016/j.ceramint.2020.10.138 Received 30 September 2020; Received in revised form 11 October 2020; Accepted 19 October 2020