Synthesis–dependent structural and optoelectronic properties of semicrystalline LiNbO3:SiO2 hybrid silicates

Abstract

Polycrystalline ceramics of lithium niobate (LNO) were prepared via standard hydrothermal and solid–state chemistry reactions, and by the sol–gel method implementing acrylamide polymerization. Congruent LiNbO3 trigonal perovskite phases crystallized in the R3c (161) space group [3 m (C3v) polar point group] were obtained in all cases. Depending on the implemented synthetic route, the average grain and crystallite sizes were estimated to 127–895 nm and 35–40 nm, respectively. As prepared LNO powders were embedded in a–SiO2 mesoporous sonogel networks at different doping rates to conform LiNbO3:SiO2 semicrystalline hybrid silicates with differentiated structural and optoelectronic (OE) properties. Intensive morphological, structural, spectroscopic, and nonlinear optical (NLO) characterizations were performed to explore these novel composites’ structural and photophysical properties for lead–free ferroelectric and optical applications. Results show that the massive porosity of the sonogel glasses and related defective bonding environment promote generalized host–guest molecular interactions influencing the oxidation states and radiative transitions of the dopant compounds via surface–assisted mechanisms and chemical charge–transfers. Additionally, due to the ferroelectric nature of LNO and average macroscopic alignment attained along the polycondensation stage of the solid solutions, SHG activity was observed from the hybrid sonogel (HSG) phases. Bulk NLO susceptibilities and molecular hyperpolarizabilities were estimated within the 9.62 × 10−11 and 10−23 esu range, respectively. Finally, depending on the synthesis methodology and in–gap hybridized levels, energy gap (Eg) narrowing was also observed for all LNO/HSG glasses, showing Eg values as low as 3.05 eV.