Optical, dielectric, and thermal properties of lithium sulphate monohydrate crystals grown by slow evaporation method have been studied. The crystal structure was resolved by direct methods using single crystal X-ray diffraction data collected at room temperature and refined by full-matrix least-squares procedures to a final R-value of 0.0174. Plasma energy, Penn gap, Fermi energy and electronic polarizability of the grown crystal were calculated from single crystal XRD data. The electronic polarizability of lithium sulfate monohydrate was also calculated and compared with the theoretical data using Clausius-Mossotti equation. Optical band gap calculated from optical data for the grown crystal is 4.49 eV. Fourier Transform Infrared Spectroscopy study confirmed the presence of water in the crystal structure. The AC conductivity, dielectric constant and dielectric loss of the grown crystal were systemically investigated, showing a peak at about 130°C which could be attributed to the water molecules in the crystal structure. The anomalous dielectric properties shown by the crystal have been correlated with its thermal behavior. The title crystal obeys Jonscher's power law relation; σ(ω) = σ o + Aω s , with temperature dependent exponent s < 1. The activation energy calculated for the material is 0.24 eV and suggests protonic conduction by hopping mechanism in addition to cationic conduction by lithium ions. The micro-indentation study was also carried out which revealed that the crystal belongs to a category of soft materials.

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Slow evaporation method was used to grow single crystals of lithium rubidium sulphate with high transparency. Single crystal X-ray diffraction analysis shows that at room temperature, the crystal belongs to a monoclinic system with the space group [Formula: see text]/[Formula: see text]. Some physical parameters like plasma energy, Penn gap, Fermi energy and electronic polarizability have been evaluated for the grown crystal. The relation between electronic polarizability and diamagnetic susceptibility for the crystal has been established on the basis of Kirkwood’s semiempirical equation. The ac conductivity and dielectric behavior of the grown crystal were systemically investigated as a function of frequency and temperature. The title compound obeys Jonscher’s power law relations: [Formula: see text], with exponent [Formula: see text]. From the conduction study, it is observed that both small and large polarons take part in conduction process, and at low temperature, both the band and hopping mechanisms are responsible for conduction. From impedance and modulus studies, it is observed that only one conduction relaxation is observed in the crystal at low frequency. The activation energy of the dielectric relaxation (0.41[Formula: see text]eV) and that of the conduction ([Formula: see text][Formula: see text]eV) are in agreement, as a result, there is no difference among the conduction and relaxation species at high temperature. Differential scanning calorimetric analysis of the crystal shows three endothermic peaks at 166[Formula: see text]C, 185[Formula: see text]C, and 202[Formula: see text]C, from which various thermodynamic parameters such as [Formula: see text], [Formula: see text] and [Formula: see text] have been calculated. From Vickers hardness test, we have determined the mechanical strength, and other parameters which include fracture toughness, brittleness index, yield strength, and stiffness constant for the crystal.

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