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 Co 2 B 2 O 5 , Co 3 B 2 O 6 and Co 3 BO 5 -which incorporate Co 2+ and Co 3+ centers -can all be crystallized from closely related Bi 2 O 3 -MoO 3 -Na 2 O-B 2 O 3 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 Na 2 O/MoO 3 /B 2 O 3 ratio, we tune relative populations of Co 2+ -and Co 3+ -bearing species in the flux. We monitor the competitive formation of intermediate phases -Na 2 MoO 4 , CoMoO 4 , Na 2 B 4 O 7 and putative NaCoO 2 -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-x Ni x B 2 O 6 ) and ludwigitetype (Co 3-x Ni x BO 5 ) and map their phase boundary in unprecedented detail. This compositional "switch" controls whether Co 3+ -rich borates are formed. We report the first synthesis of Co 3- x Ni x B 2 O 6 solid solutions within the range 0 < x < 1, which has not been previously explored, revealing a magnetic ordering transition near 35 K. Crystals of Co 2 B 2 O 5 (pyroborate) and Co 3- x Ni x BO 5 ludwigites have also been grown. Comprehensive structural refinements and magnetic measurements of all the phases are presented to substantiate our mechanistic insights.