Solid State Sciences 101 (2020) 106109 Available online 30 December 2019 1293-2558/© 2020 Elsevier Masson SAS. All rights reserved. Thermal stability and ionic conduction characteristics of lithium germanate glasses Prashant Dabas a, b, f, * , R.C. da Silva c , L.C. Alves d , M� arcia Vilarigues b , T.S. Natarajan a, e a Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India b Research Unit VICARTE, Faculty of Science and Technology, Universidade NOVA de Lisboa, Caparica, 2829-516, Portugal c IPFN-IST/UL, Instituto de Plasmas e Fus~ ao Nuclear, Instituto Superior T� ecnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066, Bobadela, Portugal d C 2 TN, Instituto Superior T� ecnico, Universidade de Lisboa, Estrada Nacional 10, 2695-066, Bobadela, Portugal e Department of Physics, Indian Institute of Technology Tirupati, Tirupati 517506, India f Department of Physics, Banasthali Vidyapith, Banasthali, Rajasthan 304022, India ABSTRACT A report is given of the systematic investigation on the glass stability and conduction characteristics of binary lithium germanate glasses in the series, mol% xLi 2 O- (100-x)GeO 2 , 5 � x � 30. The glass stability (against devitrifcation) parameters along with density and glass transition temperature reveal an extremum around 15 mol% of Li 2 O, a signature of the germanate anomaly. The analysis of impedance data in conductivity and modulus formalisms refect different aspects of the structural changes as the amount of Li 2 O increases. The lithium ion conductivity increases with the increase in the Li 2 O concentration while the activation energy for ionic motion does not change much from mol% 5 to 10 Li 2 O and decreases monotonically from mol% 10 to 30 Li 2 O indicating the absence of germanate anomaly. However, the data in modulus formalism does not scale to a master curve for all the glasses suggesting the germanate anomaly. Modeling of the modulus data using the Kohlrausch-Williams-Watts (KWW) function depicts the germanate anomaly in the stretched exponential parameter (β) at 15 mol% Li 2 O. Raman spectra of the glasses reveal the preferential formation of Q 2 units at low alkali concentration along with the formation of ring structures of several Ge tetrahedra and/or the formation of higher coordinated Ge species around the anomalous composition region. A correlation of these structural changes with the conduction characteristics of the glasses is also discussed. 1. Introduction Multicomponent tellurite, germanate and gallate glasses have been widely studied for various optical applications (fber amplifers, super- continuum generation, tunable solid-state laser sources, windows in high-energy laser systems and fbers for medical lasers) and recently as anode materials for lithium ion batteries [1–8]. Among these, germanate glasses depict higher glass transition temperatures making them more versatile for applications. Another reason for germanate glasses to attract considerable interest for several decades is the peculiarity in their basic physical properties such as density, refractive index and glass transition temperature etc. [9–28]. These properties show extrema in binary alkali germanate glasses as a function of alkali oxide concentra- tion whereas no such extrema is observed in corresponding silicate glasses. This phenomenon has been commonly referred to as Germanate Anomaly in the literature [12–14]. The thermal properties of these glasses have not received much attention [15–17]. Importantly, their stability against devitrifcation has not been reported to the best of our knowledge. The inhibition of a glass to devitrify on heating determines its thermal glass stability (GS) which has been probed for several glass systems using parameters such as Hrubÿ (K H ), Weinberg (K W ) etc. [18–22]. These GS parameters use various combinations of glass tran- sition (T g ), crystallization (T c ) and melting (T m ) temperatures to estab- lish the stability of a glass against devitrifcation. Moreover, Nascimento et al. [20] and Ferreira et al. [22] had demonstrated that GS parameters which involve at least three thermal parameters provide a correlation between the glass stability and glass forming ability (GFA). Correlation of the physical property variation with the structural changes occurring in germanate glasses as a function of alkali oxide concentration has been well investigated [12,23–36]. Two different structural models are presently available in the literature regarding the germanate anomaly, one model considers the change in coordination environment of Ge without the formation of non-bridging oxygens (NBOs) as the reason for the anomaly while the second attributes the anomaly to the formation of higher membered rings made up of GeO 4 tetrahedral units with the formation of NBOs [see 12 and references therein, 37]. Ionic conduction and electrical relaxation processes in oxide glasses are dictated by the various structural units present in the * Corresponding author. Department of Physics, Banasthali Vidyapith, Banasthali, Rajasthan 304022, India. E-mail address: dprashant@banasthali.ac.in (P. Dabas). Contents lists available at ScienceDirect Solid State Sciences journal homepage: http://www.elsevier.com/locate/ssscie https://doi.org/10.1016/j.solidstatesciences.2019.106109 Received 24 May 2019; Received in revised form 26 September 2019; Accepted 29 December 2019