Phase Selectivity in Borate Flux Synthesis Using Refractory Additives

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 Cu_{7–<i>x</i>+<i>y</i>}Ti_<i>x</i>B₆O₁₆ is reported, showing an antiferromagnetic transition at <i>T</i>_<i>N</i> = 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.