α-, β-Pb4B2O7 and α-, β-Pb4B6O13: Polymorphism drives changes in structure and performance

Abstract

Introducing Pb2+ cations with lone pair electrons in borates is efficient to form multiple crystalline forms. Here, we report two new compounds, α-Pb4B2O7 and β-Pb4B6O13, which exhibit different crystal forms from the previously reported lead borates, β-Pb4B2O7 and α-Pb4B6O13, respectively. Two sets of polymorphs: α-, β-Pb4B2O7 and α-, β-Pb4B6O13, exhibit completely different crystal structures and diverse optical properties. Thermal gravimetric and differential scanning calorimetry (TG-DSC) and variable-temperature powder X-ray diffraction (XRD) analyses were performed to study their thermodynamic stabilities. Structure-property relationships were discussed through first-principles calculation. Notably, the new phases, α-Pb4B2O7 and β-Pb4B6O13, have larger birefringence than their corresponding polymorphs due to the rearrangement of the functional groups in their structures.