Dissolution of Liquid B2O3 in Molten CaCl2−CaO

Electrodeposition of boron and borides in molten CaCl 2 −CaO−B 2 O 3 offers a versatile route to boron-based materials, yet the dissolution chemistry of B 2 O 3 in molten CaCl 2 −CaO and the associated phase relationships remain insufficiently understood. Here, we investigate B 2 O 3 dissolution in molten CaCl 2 −CaO between 1073 K and 1223 K. The solubility of B 2 O 3 increases with temperature and demonstrates a linear dependence on CaO concentration. Dissolution proceeds via the formation of the solid intermediates Ca 2 BO 3 Cl and Ca 2 B 5 O 9 Cl, followed by dissociation of Ca 2 B 5 O 9 Cl into soluble ions. Raman-based speciation indicates that dissolved boron occurs predominantly as metaborate species ( \({\text{BO}}_{2}^{ - }\) ) present as short-chain units and six-membered rings. Additionally, phase relationships in the CaCl 2 -rich region of the CaCl 2 −CaO−B 2 O 3 ternary system are established across the studied temperature range. These findings provide a thermochemical foundation for designing and optimizing fluoride-free CaCl 2 −based molten salt systems for boron electrodeposition.