Several fundamental studies have been made on glasses composed of silica and with oxides of phosphoric anhydrides [1] [2] [3] [4] . Due to their low chemical durability, phosphate glasses have gained less attention. However, several phosphate glasses with high aqueous corrosion resistance have been reported [5] - [12] . The electrical engineering field constitutes one of the numerous fields of application of glasses. This field is particularly interested in materials with high ionic conductivity that can be used as solid electrolytes, or in materials of very high resistivity capable of playing the dielectric role for miniaturised condensers. Historically, ionic conduction has been studied in crystalline compounds for a long time [13] . The interest in ion-conducting glasses is more recent, but has progressed rapidly over the last thirty years due to the use of these materials as solid electrolytes. The transport properties of these electrolytes have been the subject of numerous works, whose main lines have been grouped together in some publications [14] [15] [16] [17] . The notion of point defects is a very useful tool in the interpretation of conduction in crystallised materials, but it is ill-defined for amorphous materials, because they do not have long-range structural periodicity. However, glasses have many spaces (due to their low density), which are capable of containing cations of different sizes [7] [16] [18] [19] . Ions that are weakly bonded to the network can move in the material under the action of a force resulting from an electric potential gradient. The well-known mobility of alkaline cations, and in particular Li⁺ and Na⁺, in solid electrolytes has prompted us to explore the electrical conductivity of the revealed vitreous phases [2] [3] [4] [5] . Our aim in the present work is to define the glass area in the ternary system Li₂O-P₂O₅-Cr₂O₃ for the first time. A second aim is to undertake an electrical study of glasses and glass ceramics of the ternary system Li₂O-P₂O₅- Cr₂O₃. The study of the conduction mechanisms and their relation to the structure of the phosphorus network illustrate that with increased amounts of the modifier oxide Li₂O in the glass network, the mobility of the Li⁺ carriers is facilitated, due to increased depolymerisation of the phosphate network [7] [18] [19] . On the other hand, the increasing glassy depolymerisation of the network induces an increase in the number of non-bridge oxygens and consequently an increase in the dielectric permittivity [7] [18] [20] [21] .

A glass of composition xLi₂O-yCr₂O₃-(100-(x + y)P₂O₅ (mol %) was obtained by the melting quench method at 1000˚C. Appropriate mixtures of the compounds Li₂CO₃, Cr₂O₃ and (NH₄)₂HPO₄ were initially prepared at various temperatures between 300˚C - 500˚C in order to achieve a preliminary preparation before the final glass preparation. The melts were performed in alumina crucibles for about 20 min at 1000˚C ± 10˚C. The isolated glass samples were approximately 10 mm diameter and 1 to 3 mm in thickness. Their vitreous state was first evidenced by their shiny aspect and confirmed by XRD. Annealing of these glasses was realised at increasing temperatures in intervals of 100˚C. The first structural approach was X-ray diffraction, which allowed the crystallisation of the vitreous domain Li₂O-P₂O₅-Cr₂O₃ to be followed. The dielectric permittivity () and dielectric loss (tnδ) measurements were determined from capacity measurements realised on samples in the form of small cylinders carved with abrasive papers. While depositing a layer of silver lacque on the parallel faces of the samples, we form a condenser plan of which the capacity is measured using a Hewlett Packard 4262 A LCR Meter. The measures of conductivity were determined from resistivity measurements realised on samples. The samples were polished in the form of small cylinders using abrasive papers. Pastilles so obtained have diameters of around 10 mm and thicknesses varying between 1 and 3 mm.

In the Li₂O-P₂O₅ system, transparent and colourless glasses could be prepared with molar fractions of Li₂O between 0 and 0.62. These values are in good agreement with previously published results [18] but are rather superior to those given by the other authors [20] [22] . This is probably caused by different experimental conditions (temperature of melting; speed of tempering, etc.) [23] . In the binary systemCr₂O₃-P₂O₅, the substitution of P₂O₅ by Cr₂O₃ led to very hygroscopic green glasses with a chromium oxide content between 0 and 7 mol%, see Figure 1. Ternary glasses have also a green colour which becomes more clear and green as the lithium oxide content increases and as the Cr₂O₃ content increases between 0 £ y < 5; (mol%). The demarcation of the glassy zone within the ternary diagramLi₂O-P₂O₅-Cr₂O₃ is given by the following limits: 0 £ x £ 62; 0 £ y <7; 36 £ χ £ 100 (mol%) (Figure 1). The localization of analysed compounds is presented inside the ternary diagram given in Figure 2.

The regular melting-quench method allowed the isolation of a small vitreous domain within the ternary system Li₂O-P₂O₅-Cr₂O₃ at 1000˚C. Conductivity measurements were performed on glasses and vitreous ceramics isolated within the ternary systemLi₂O-P₂O₅-Cr₂O₃. Dielectric permittivity and dielectric loss were measured at a frequency of 10 kHz and in the temperature range of 298 to 573 K. An increase in the Li₂O content in the glass network results at the same time in an increase in (T) and an increase in tgδ. These phenomena result from a increasing depolymerisation of the glass network due to increased numbers of non-bridge oxygens because of the progressive breaking of phosphorus network links as the modifier Li₂O content increases [7] [18] [21] . Indeed, the oxide modifier Li₂O has much lower link energies than those of the formative oxides [24] . As a consequence the mobile lithium ion is weakly connected to the network and will be thermally activated to cross the barriers of potential presented by the network, involving the formation of non-bridge oxygens by the breaking of P-O-P links. This leads to the formation of the - P - O- entities, which are easily polarised, hence the increase in. On the other hand, more the concentration of the modifier oxide increases, the greater the number of lithium-oxygen-phosphorus bonds in the vitreous network more the calorific energy needed to break the potential barriers presented by the network is low, hence, temperature (T_t) decreases. However, an increase in chromium oxide content, even at very low concentration, induces a decrease of the dielectric losses and an increase of the T_t temperature. Hence we can say that an increase in Cr₂O₃ content induces an increase of the rigidity of the glass network. However, this behaviour changes when the system changes from a glass to a ceramic glass. The study of the conductivity as a function of the temperature for various samples shows that σ varies according to Arrhenius’s law. Increase in temperature andLi₂O content both induce an important increase in the conductivity. The mobile lithium ion (Li⁺) is weakly connected to the network and will be thermally activated to cross the potential barriers presented by the network. This leads to the initiation of greater ionic conduction as the number of free Li⁺ ions increases. The effects of Cr₂O₃ on the conductivity and the activation energy in glasses and ceramic glasses are different. We note that the conductivity decreases in the glasses followed by an increase of the activation energy, while the conductivity increases in vitreous ceramics followed by a decrease of the activation energy. However the decrease of σ in the chromium glasses can be explained by the replacement of Li-O-P or P-O-P hydrated bonds by Cr-O-P covalent bonds. Cr³⁺ ions would be trapped in sites of low coordination (£6) and so would be a part of the formative network [27] . The glassy domain bounded in our experimental conditions (T_f = 1000˚C) is relatively small and the vitreous ceramics studied have a composition rather close to the border area separating glassy and crystallised domains inside the ternary structure Li₂O-P₂O₅-Cr₂O₃. Thus, the network defects are probably more numerous there and in a favourable position with regard to the mobile species. We can conclude that the conductivity seems to improve when we pass from the glassy domain to the vitreous ceramic domain [28] . At the same time, the increased Li₂O content in the vitreous network and increased temperature, both induce an increase in the dielectric permittivity. Mean while, the progress of T_t seems to exhibit different behaviour when we change from glasses to glass ceramics. This could also be explained by the border area separating the glassy and crystallised domains.

The regular melting-quench method allowed the isolation of a small vitreous domain within the ternary system Li₂O-P₂O₅-Cr₂O₃ at 1000˚C. Conductivity measurements were realised on glasses and vitreous ceramics isolated within the ternary system Li₂O-P₂O₅-Cr₂O₃. Dielectric permittivity and dielectric loss were measured at a frequency of 10 MHz and in the temperature range 298 to 573 K. An increase in the Li₂O content in the glass network results at the same time in an increase in (T) and an increase in tgδ. These phenomena result from an increasing depolymerisation of the glass network due to increased numbers of non-bridge oxygens because of the progressive breaking of phosphorus network links as the modifier Li2O content increases. Conductivity measurements were realised on glasses and ceramic ceramics isolated within the ternary system Li₂O-P₂O₅-Cr₂O₃. In both glasses and ceramic glasses, the value of σ increases as Li₂O content increases. The increase of σ is probably induced by an increase in the number of Li⁺ carriers whose mobility is facilitated by the depolymerisation of the phosphorus network.

Increase in the Cr₂O₃ content seems to have caused different behaviour from glasses to vitreous ceramics. The conductivity decreases in the chromium glasses while it increases in glass ceramics due to network defects, which are probably more numerous and in a favorable position for mobile species. On the other hand, an increase in the chromium oxide content induces a decrease dielectric loss and an increase of T_t temperature in the glass. Hence we can say that an increase of Cr₂O₃ content induces an increase of the rigidity of the glass network. The highest conductivity obtained at 300˚C was in the order of 2 × 10⁻³ (Wcm)⁻¹ with an activation energy of 0.45 e.v; however, this remains low compared with that of the crystalline compound LiAl₁₁O₁₇ (σ =10⁻² (Ωcm)⁻¹) at 25˚C. The obtained conductivities are probably responsible for large dielectric losses, which mean that our glasses are currently not suitable for possible application as dielectric condensers.

Aqdim, S., Er- rouissi, Y., Cherif, A. and Makhlouk, R. (2017) Elaboration and Characterization of Glasses and Ceramic-Glasses within Theternary Diagram Li₂O-Cr₂O₃-P₂O₅. Advan- ces in Materials Physics and Chemistry, 7, 123-137. https://doi.org/10.4236/ampc.2017.74011