Materials Chemistry and Physics 239 (2020) 122087 Available online 5 September 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved. Barium polyphosphate glasses, from structure to thermochemistry Hounaida Mrabet a , Mohamed Atef Cherbib a, b , Ismail Khattech a, * a Universit� e de Tunis El Manar, Faculty of Science, Chemistry Department, Materials Crystal Chemistry and Applied Thermodynamics Laboratory LR15SE01, Tunisia b Otto Schott Institute of Materials Research, Friedrich-Schiller-University of Jena, Fraunhoferstrasse 6, 07743, Jena, Germany HIGHLIGHTS � The glass forming ability of BaO in polyphosphate glasses is greater than expected. � The terminal tetrahedron of phosphate fragments are preferentially linked to Ba 2þ . � The chemical durability is enhanced by adding Ba 2þ . � ▸ The Ba–O bond dissociation energy infuences the heat of dissolution of glasses. A R T I C L E INFO Keywords: Barium oxide Phosphate glasses structure Chemical durability Glass thermochemistry ABSTRACT In order to understand the effects of BaO addition on the structure and thermochemistry of polyphosphate glasses, a series of vitreous compositions having the formula (100-x)NaPO 3 -xBaO was investigated with x ranging from 0 to 25 mol%. The glasses were synthesized using the melt quenching technique and reveal a limit of vitrifcation of 25 mol% of BaO. Fourier Transform Infrared Spectroscopy and 31 P Magic Angle Spinning Nuclear Magnetic Resonance show the depolymerization of the metaphosphate chains, leading to Q 1 and Q 2 phosphate groups as main structural units together with a smaller average chain length. The substitution of Na 2 O and P 2 O 5 by BaO increases the density, glass transition and crystallization temperatures indicating the shrinking and reticulation of the polyphosphate network. The enhancement of the chemical durability was related to the strengthening of the glasses towards water attack and the low mobility of the Ba 2þ cations. Finally, the micro- calorimetric investigations, pursued in 4.5 wt% H 3 PO 4 solution, show a decrease of the dissolution enthalpy. This trend was correlated with the structural modifcation within the vitreous network and the reduction of the chemical degradability of the glasses. 1. Introduction Various phosphate compositions are investigated for their bioactive, optical and physical properties thanks to their chemical similarity with the mineral part of the bones, high refractive index, large coeffcients of thermal expansion and low melting temperatures [1–3]. Additionally, the large solubility of rare earths oxides in phosphate melts and the transparency of phosphate glasses in the UV region are appropriate in high energy high power solid state lasers [4,5]. Nevertheless, except for some applications such as tissue engineering; in which therapeutic cat- ions are included in the compositions to elicit an anti-infammatory, bactericide or osteogenic effect when dissolved [6–9], the use of phos- phate glasses in common applications is relatively scarce due to their high degradability in severe working conditions. This limitation is inherent to the structure of these materials [3]. Indeed, the P– – O bond in the PO 4 tetrahedron limits the reticulation compared to silica glasses. This hindrance enables the diffusion of water into the surface which promotes the degradation of the glasses. The PO 4 tetrahedron is the basic building block of phosphate glasses. Depending on the amounts of the phosphorus pentoxide and modifer oxides e.g. BaO, Na 2 O or Li 2 O, various structures can be formed. Conventionally, the Q n nomenclature is used to differentiate the phos- phate groups, where n is the number of bridging oxygen per phosphate tetrahedron. Moreover, the oxygen over phosphorus ratio (O/P) can be used to distinguish the regions in which the Q n groups are present depending of the glass composition. For 2.5 � O/P � 3, Q 3 and Q 2 units are expected in the glass composition. For an O/P ranging from 3 to 3.5, Q 2 and Q 1 groups constitute the polyphosphate glasses and fnally Q 1 and Q 0 species are formed for compositions having an O/P between 3.5 and 4 [3,10]. * Corresponding author. E-mail address: ismail.khattech@fst.utm.tn (I. Khattech). Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys https://doi.org/10.1016/j.matchemphys.2019.122087 Received 7 January 2019; Received in revised form 17 July 2019; Accepted 27 August 2019