Controlled synthesis processes of cobalt containing borates in a multi-component flux system based on Bi2O3–MoO3–Na2O–B2O3

Abstract

Cobalt-containing borates are emerging as multifunctional materials for optical devices, Li-ion and Na-ion battery anodes, and rare-earth-free permanent magnets. In particular, the phases Co₂B₂O₅, Co₃B₂O₆ and Co₃BO₅ – which incorporate Co²⁺ and Co³⁺ centers – can all be crystallized from closely related Bi₂O₃–MoO₃–Na₂O–B₂O₃ fluxes with only minimum compositional adjustments. In this study, we elucidate (1) high-temperature crystallization pathways in these multicomponent flux systems and (2) factors that stabilize Co in oxidation states +2 versus +3. By systematically varying the Na₂O/MoO₃/B₂O₃ ratio, we tune relative populations of Co²⁺- and Co³⁺-bearing species in the flux. We monitor the competitive formation of intermediate phases – Na₂MoO₄, CoMoO₄, Na₂B₄O₇ and putative NaCoO₂ – and show how these intermediates direct the ultimate borate phase. Moreover, by incrementally introducing NiO into the flux, we estimate solid solutions of the kotoite type (Co_3−xNi_xB₂O₆) and ludwigite type (Co_3−xNi_xBO₅) and map their phase boundary in unprecedented detail. This compositional “switch” controls whether Co³⁺-rich borates are formed. We report the first synthesis of Co_3−xNi_xB₂O₆ solid solutions within the range of 0 < x < 1, which has not been previously explored, revealing a magnetic ordering transition near 35 K. Crystals of Co₂B₂O₅ (pyroborate) and Co_3−xNi_xBO₅ ludwigites have also been grown. Comprehensive structural refinements and magnetic measurements of all the phases are presented to substantiate our mechanistic insights.