Polymorphism of lead oxoborate

• Pb4B2O7 melt undergoes statistical undercooling.
• Orthorhombic nonlinear optical crystal Pb4O(BO3)2 is the metastable γ-polymorph.
• Temperature of metastable melting of γ-Pb4O(BO3)2 is equal to 530 °C.

The study of lead borate melt crystallization by differential thermal analysis (DTA) and X-ray diffraction analysis has shown that, for Pb4O(BO3)2 (or 4PbO·B2O3) stoichiometric compound, its well-known orthorhombic modification (non-centrosymmetric Aba2 space symmetry group (SSG), a = 15.472(1), b = 10.802(1), c = 9.9486(6) Å unit cell parameters) is metastable. It forms from the undercooled melt and has a melting point of 530 ± 5 °C.