Contents lists available at ScienceDirect Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol Effect of the iron content on the structure and electrical properties of sodium borosilicate glasses: XRD, TEM, Mössbauer, FTIR and DIS spectroscopy study A. Ciżman a, ⁎ , E. Rysiakiewicz-Pasek a , T. Antropova b , M. Krupiński c , O.A. Pshenko b , A. Zarzycki c a Wrocław University of Science and Technology, Wyb. Wyspiańskiego 27,Wrocław, Poland b Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Science, Nab.Makarova 2, Saint Petersburg, Russia c Institute of Nuclear Physics, Polish Academy of Sciences, Radzikowskiego 152, Krakow, Poland ABSTRACT The iron-bearing sodium borosilicate glasses with different contents of iron were prepared by the charge melting method and studied by XRD, TEM, FTIR, Mössbauer and Dynamic Impedance Spectroscopy techniques. An analysis and the transmittance spectra of two-phase sodium borosilicate glasses with different contents of iron oxide were carried out. A usage of the Mössbauer spectroscopy revealed differences between two-phase and porous glasses. In both cases Fe 2+ and Fe 3+ ions in octahedral and tetrahedral coordination were found, while in the two-phase glasses the significant contribution of magnetite was additionally detected. The acti- vation energy associated with the relaxation processes determined from the electric modulus spectra was found to be in a range between 0.986 eV and 0.729 eV, which is close to the activation energy for dc conductivity. Two different conduction mechanisms were observed and their physical origin was explained. The hopping between two valence states of iron and the sodium ion conduction mechanism was additionally discussed. 1. Introduction The interest in borosilicate glasses increased tremendously during the last decade due to their wide applicability ranging from chemically and thermally resistant technical glass to optical, sealing and nuclear waste glasses. This great attention mainly results from the presence of an irregular network, where different ions can be introduced into without a destruction of the atomic structure which can be used to adjust physical and chemical properties of the glass. [1–3] In particular, the electrical conductivity of borosilicate glasses is determined by the glass former B 2 O 3 , which initiates the formation of either BO 4 - tetra- hedra and BO 3 triangle groups with bridging oxygen or borate groups with nonbridging oxygen (NBO). The reported temperature dependence of the electrical conductivity is associated with an increase of the va- lence state of sodium ion and cation diffusion coefficient, while the physicochemical properties and structure of borosilicate glass are strongly determined by the composition, mainly by Na 2 O/B 2 O 3 molar ratio and SiO 2 content. [4] The electrical conductivity of the glass can be easily increased by adding transition metal oxides to the glass network. [3,5] The transition metals in NBO glasses reduce the four-coordinated boron (BO 4/2 ) to the three-coordinated boron (BO 3/2 ) groups. [3] In the case of iron-sodium borosilicate glasses (SBS) glasses the structural reorganization is caused by the competition between FeO 4 and BO 4 groups accompanied by a compensation of the negative charge by the Na + ions. [3] It was shown that the number of NBO groups increased, which weakened the SBS glass structure. [3] These changes can cause an increase in dc con- ductivity as well as suggests that sodium ions could be consumed by the borate network and became the charge carriers. [3] An extra increase in the electrical conductivity can be raised by introducing a transition metal oxide to the network. In such case the electrical property can be described by the sodium ion migration and the electron hopping be- tween localized states. Understanding the structural foundation of the composition and temperature dependence of conductivity in sodium borosilicate glasses doped with iron ions is a major challenge. Although there are many works devoted to the study of structural properties, dielectric oxide glasses containing Fe 2 O 3 as a glass former, there is extensive research investigation on SBS glasses doped with Fe 2 O 3 .[3,6,7] Known publications devoted to the study of iron-con- taining sodium borosilicate glasses by different methods are mainly devoted to the structural aspect. [3,8,9] In most of the known works mainly tempered glasses have been studied, and the compositions are often outside the metastable liquation region. Data on the complex study of independent and complementary methods of phase-separated glasses and porous glasses obtained from them, which serve as matrices of composite multiferroics, are not found in the known literature. Thus, the question of the relation of the elec- trical properties of phase-separated iron-containing sodium borosilicate glasses, as well as the porous glasses obtained from them, with their structural parameters has not been practically studied. It should be https://doi.org/10.1016/j.jnoncrysol.2019.119847 Received 2 July 2019; Received in revised form 15 November 2019; Accepted 4 December 2019 ⁎ Corresponding author. E-mail address: agnieszka.cizman@pwr.edu.pl (A. Ciżman). Journal of Non-Crystalline Solids 531 (2020) 119847 0022-3093/ © 2019 Elsevier B.V. All rights reserved. T