Sulfur-doped lithium phosphate glasses ceramics: a detailed exploration of sulfur on the structural, optical, and electrical properties

Abstract

The inclusion of sulfate anions in an appropriate glass matrix has proven to enhance the physical and chemical properties and widen the application of the glass system. In this context, the impact of minor sulfur dopants (0 to 2 mol%) on the structural, optical, and electrical properties of lithium phosphate glasses has been investigated in detail. 3(Li₂O)-y(S)-(1-y) PO₃ (y = 0.0, 1, 1.5, and 2 mol %.) ceramic glasses were synthesized using the melt-quenching technique. The density showed a sulfur-related decrement, whereas the molar volume increased which can be ascribed to the formation of non-bridging oxygens. The structural features of the sulfur-doped lithium phosphate glass was studied by X-ray diffraction (XRD), which established the formation of glass–ceramic nanocomposites. Moreover, the formation of LiPO₃ and LiSO₄ phases was confirmed by XRD. The Fourier transformer infrared spectroscopy (FTIR) represented sharp peaks at 896 cm⁻¹ related to the stretching vibration of P–O–P groups which exhibits a clear shift with the addition of sulfur. The optical bandgap increased from 3.8 to 4.27 eV as the sulfur content was increased from 0.0 to 2.0 mol %. as established by applying Kubelka -Munk combined with Tauc’s relations. Also, the bandgap dependence of refractive index was estimated and discussed by different Moss, Herve, Reddy, and Kumar models. The complex impedance analysis revealed non-Debye-type dielectric relaxation behavior. The ac conductivity exhibited an increase with temperature according to the Arrhenius law, with a double activation energy for the conduction process. As the sulfur content increased, the variation in conductivity and high-temperature activation energy suggested a transition from a predominantly polaronic conductive regime to an ionic conductive regime at approximately 1.5 mol% of sulfur in lithium phosphate glasses. The frequency-dependent behavior of electric conductivity followed a modified power law relation, σ_ac(ω) = σ_dc(0) + A ω^s1 + B ω^s2, where s > 1 and constant value with varying temperature. The hopping mechanism was identified as the dominant electrical transport process in the system. The relaxation character of the frequency and temperature dependence of the electrical modulus, as well as the dielectric loss parameters, was evident. These findings suggest that sulfur-doped lithium phosphate glasses have potential applications in various fields, including solid-state batteries, optoelectronics, and photonics.