Thermochromic behavior of tellurite glasses M. Çelikbilek a , A.E. Ersundu a , E.O. Zayim b , S. Aydin c,⇑ a Yildiz Technical University, Department of Metallurgical and Materials Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul 34220, Turkey b Istanbul Technical University, Physics Engineering Department, Faculty of Science and Letters, Istanbul 34469, Turkey c Istanbul Technical University, Department of Metallurgical and Materials Engineering, Faculty of Chemical and Metallurgical Engineering, Istanbul 34469, Turkey article info Article history: Received 5 December 2014 Received in revised form 16 January 2015 Accepted 1 March 2015 Available online 5 March 2015 Keywords: Tellurite glasses Thermochromism Semiconductor Band gap energy Absorption edge abstract Thermochromic behavior of tellurite glasses in the TeO 2 –WO 3 –A 2 O (A 2 O, A = Li, Na, K) system was inves- tigated. Transmittance in visible region, absorption edge and optical band gap energy were determined at different temperatures below glass transition temperature. Reversibility of thermochromic behavior was tested using repeated heating and cooling measurements. The absorption edge values shifted towards longer wavelengths with increasing temperature and shift rate of absorption edge was found in the range of 3.94  10 4 and 8.86  10 4 eV/K similar to those of conventional semiconductor materials. Band gap energy values were found to decrease with increasing temperature lying in the range 1.81– 2.83 eV. Tellurite glasses showed reversible thermochromic behavior and expected to find use in ther- mochromic applications. Good semiconducting property of tellurite glasses is found to be the cause of thermochromism. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Chromic glasses have recently attracted great attention both scientifically and industrially as a result of an interest on dis- covering new glass systems which give automatic responses to changes in environmental conditions. Studies on chromic glasses have been mainly focused on electrochromic, photochromic and thermochromic glasses which have ability to change their optical properties reversibly with the change in electrical current, light and temperature, respectively. Most of the developments on chro- mic systems have been realized with electrochromic and pho- tochromic glasses, whereas there exist limited number of works on thermochromic glasses. Although studies on thermochromic glasses are mostly concentrated on thin-film coated glasses for energy efficient window applications (especially VO 2 -based coat- ings), there still has been a search for bulk glasses showing ther- mochromism. In this regard, until today few studies on thermochromic bulk glasses have been realized [1–7]. To the best of our knowledge, first study on thermochromic bulk glasses was realized by Abe et al. on phosphate glasses in the K 2 O–B 2 O 3 –Al 2 O 3 –P 2 O 5 and K 2 O–Al 2 O 3 –P 2 O 5 systems. Later Kawashima et al. also studied thermochromic behavior of phos- phate glasses in CaO–P 2 O 5 , CaO–Al 2 O 3 –P 2 O 5 and K 2 O–B 2 O 3 – Al 2 O 3 –P 2 O 5 systems. Both studies reported that the coloring in phosphate glasses is due to the change in the molecular state of colloidal phosphorus formed in the glass [1,2]. In their works on different bismuthate glasses, Sen et al. and Chen et al. found out that the thermochromic property in bismuthate glasses results from the temperature dependence of the expansion of the lattice and mainly due to the electron–phonon interactions [3–5]. Li et al. investigated temperature and compositional dependence of optical absorption edge in glasses containing high amount of PbO and TeO 2 (80 wt.%) and concluded that the shift of absorption edge in these glasses can be attributed to the polarizability of O–Pb and O–Te bonds [6]. In their study on thermochromic property of tell- urite glasses containing transition metal oxides, Inoue et al. reported that transition metal oxides are compound semiconduc- tors and they change their optical properties due to semiconductor to metal phase transition. Therefore, they concluded that tellurite glasses containing transition metal oxides show thermochromic property due to their high thermal expansion rate [7]. Tellurite glasses have drawn much attention as promising can- didates for many opto-electronic applications due to their advanta- geous properties, such as high refractive index, relatively low- phonon energy, good visible and infrared transmissivity, suitability for doping with rare earth elements in a wide range and good elec- trical properties owing to the unshared pair of electrons of the TeO 4 groups that do not take part in bonding. It is known that TeO 2 does not form glass under normal quenching conditions with- out addition of a secondary component. Therefore, addition of sec- ondary components are required to synthesize tellurite glasses. http://dx.doi.org/10.1016/j.jallcom.2015.03.013 0925-8388/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +90 212 285 6864; fax: +90 212 285 3427. E-mail address: saydin@itu.edu.tr (S. Aydin). Journal of Alloys and Compounds 637 (2015) 162–170 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom