Phase Selectivity in Borate Flux Synthesis Using Refractory Additives

Abstract Here, we report a novel approach to borate flux synthesis in which refractory oxide additives alter reaction phase outcome. In the absence of refractory additives, reactions of copper oxide and boron oxide flux yielded single crystals of the previously reported Cu15(BO3)6(B2O5)2O2 (Cu3B2O6), containing solely trigonal planar-coordinated borate monomers and dimers. Addition of TiO2 led to the discovery of a novel structural prototype Cu7–x+yTix(BO3)(B2O5)(B3O7)O (Cu7–x+yTixB6O16), which crystallizes in space group P1̅, with a = 3.31989(4) Å, b = 10.54906(13) Å, c = 17.9143(2) Å, α = 75.5076(10)°, β = 88.8619(10)°, and γ = 86.7381(10)° for x = 0.276(7), y = 0.0135(15). It consists of quasi-2D layers of Cu2+ cations enclosing trigonal planar borate monomers, dimers, and the uncommon (B3O7)5– trimer. Adjacent layers are connected by oxygen and disordered Cu/Ti sites. In contrast, both ZrO2 and HfO2 grew single crystals of the previously reported material CuB2O4 containing solely tetrahedrally coordinated boron in a 3D framework. Ex situ X-ray μCT imaging of the ZrO2 reaction shows CuB2O4 crystals growing at the interface of ZrO2 and the flux, suggesting that ZrO2 (and by extension, HfO2) acts as a heterogeneous nucleation substrate that stabilizes the four-coordinate boron tetrahedra. Magnetization of Cu7–x+yTixB6O16 is reported, showing an antiferromagnetic transition at TN = 12.8 K. We then differentiate refractory additives into two distinct classes, flux-resistant and flux-susceptible, with ZrO2 and HfO2 as B2O3-flux-resistant refractory additives and TiO2 as a B2O3-flux-susceptible refractory additive. These results demonstrate that refractory additives can provide a useful handle for directing phase formation and accessing new materials in flux synthesis, and provide challenge cases to benchmark emerging AI models in unconventional materials stabilization and crystal growth methods.