Electronic properties and vibrational spectra of (NH 4 ) 2 M″(SO 4 ) 2 ·6H 2 O (M = Ni, Cu) Tutton's salt: DFT and experimental study Santunu Ghosh a, ⁎, Saif Ullah b, ⁎, João P.A. de Mendonça b , Luciano G. Moura a , Marcos G. Menezes d , Leonã S. Flôres c , Tiago S. Pacheco b , Luiz F.C. de Oliveira c , Fernando Sato b , Sukarno O. Ferreira a a National System of Laboratories in Nanotechnologies (SisNANO), Department of Physics, Federal University of Viçosa, Viçosa 36570-000, MG, Brazil b Department of Physics, Federal University of Juiz de Fora, Juiz de Fora 36036-330, MG, Brazil c Department of Chemistry, Federal University of Juiz de Fora, Juiz de Fora 36036-330, MG, Brazil d Institute of Physics, Federal University of Rio de Janerio, Rio de Janeiro 21941-909, RJ, Brazil abstract article info Article history: Received 9 January 2019 Received in revised form 12 April 2019 Accepted 14 April 2019 Available online 15 April 2019 The complex crystals of the family of the Tutton's salt have been investigated through the numerous experimen- tal and theoretical studies to understand their physical properties and their potential technological applications. In spite of the more than 60 years of research, there are very few studies about the electronic properties of Tutton's salt. In our present work, we have calculated the stability, electronic properties and the first theoretical study of band structure of the three different crystals of the Tutton's salt, ammonium nickel sulfate hexahydrate ((NH 4 ) 2 Ni(SO 4 ) 2 ·6H 2 O), ammonium nickel‐copper sulfate hexahydrate ((NH 4 ) 2 Ni 0.5 Cu 0.5 (SO 4 ) 2 ·6H 2 O) and am- monium copper sulfate hexahydrate ((NH 4 ) 2 Ni(SO 4 ) 2 ·6H 2 O) with the help of periodic ab-initio calculations based on density functional theory (DFT). In addition to this, the internal Raman and FTIR modes of the ionic frag- ments [Ni(H 2 O) 6 ] 2+ , [Cu(H 2 O) 6 ] 2+ NH 4 + and SO 4 2- of the sample crystals were obtained by employing the ab initio and the orientation of the molecular vibrations of the ionic fragments have been presented in picturized form. Furthermore, the Raman and FTIR spectroscopy of the sample crystals were measured in the range 100–4000 cm -1 and 400–4000 cm -1 respectively, and the internal vibrational modes obtained from experimen- tal measurement have been compared with those obtained from DFT calculations. © 2019 Elsevier B.V. All rights reserved. Keywords: Tutton's salt Electronic band structure Density functional theory Band gaps Raman spectroscopy FTIR spectroscopy 1. Introduction The crystals of the Tutton's salt exhibits remarkable physical proper- ties and during the past few decades these complex inorganic compounds have been extensively investigated in a number of notable articles [1–10], with the purpose to use them as ultraviolet (UV) light filters and UV sensors in solar-blind technology. Solar-blind UV light filters can locate and track the sources of UV energy emission, as a result, they can be used in modern missile approach warning systems, aircraft protection, sniper shoot detection, as well as in nuclear reactor. In addi- tion, these types of dielectric crystals with hydrogen bonds are also of interest for nonlinear optics, optoelectronic and laser materials. The family of the Tutton's salts are isomorphic hydrated complexes that crystalize in the monoclinic system with centrosymmetric space group P2 1 /c [Z = 2] [4,6] having empirical formula M 2 M'(SO 4 ) 2 .6H 2 O or M 2 M′(SeO 4 ) 2 ·6H 2 O, where M is a univalent cation which may be Cs + , K + , NH 4 + , Rb + , Tl + , and M′ is a bivalent cation of the first series of the transition metals Co +2 , Cr +2 , Cu +2 , Fe +2 , Mn +2 , Ni +2 ,V +2 , Zn +2 . The unit cell of Tutton's salts contains one or more octahedral hexahydrate complexes [M′(H 2 O) 6 ] 2+ in the crystal unit cell where, M′ is situated at the inversion center and surrounded by six water molecules through a slightly distorted octahedral formed by the Jahn–Teller effect [11–14]. The Jahn-Teller theorem states that, if the ground state electronic configuration of complex inorganic compounds is orbitally degenerate the complex will distort to remove the degeneracy or in an alternative way we can say that, when orbitals in the same level are occupied by dif- ferent numbers of electrons, this will lead to a geometric distortion of the molecule/ion. The octahedral complexes of the divalent Cu(II) ion with the configuration (t g ) 6 , (e g ) 3 , provides typical examples [15,16] of a Jahn-Teller system with strong vibrionic coupling [17] for which quasi-stationary distortions in the nuclear configuration are separated by small energy barriers. If the site symmetry is lower than the octahe- dral symmetry, then e g ground state of Cu +2 ion splits into two nonde- generate states d x 2 -y 2 and d z 2 . In this case, the ground state will be either d z 2 or d x 2 -y 2 depending upon whether the distortion of the octahedral is elongated or compressed. Recently, there has been a Shakespearean variety of DFT studies of the electronic band structures of inorganic Ni and Cu compounds [18–26] directed towards prospective applications in the fields of Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 218 (2019) 281–292 ⁎ Corresponding authors. E-mail addresses: santunug@gmail.com (S. Ghosh), sullah@fisica.ufjf.br (S. Ullah). https://doi.org/10.1016/j.saa.2019.04.023 1386-1425/© 2019 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa