NH ₄ Ca[B ₅ O ₇ (OH) ₄ ] H ₂ O: A Biaxial Mixed‐Cation Pentaborate with Moderate Birefringence

Abstract Ammonium‐calcium mixed‐cation pentaborate NH 4 Ca[B 5 O 7 (OH) 4 ]·H 2 O has been synthesized for the first time. Single‐crystal X‐ray diffraction analysis reveals that it crystallizes in the triclinic space group P ‐1 with lattice parameters a = 6.7042(3) Å, b = 7.1859(3) Å, c = 10.7447(4) Å, α = 100.304(3), β = 97.774(3), γ = 96.972(3). The structure is characterized by 1D boron–oxygen chains composed of [B 5 O 7 (OH) 4 ] 3− polyanions that are further interspaced by Ca 2+ , NH 4 + cations, along with lattice water molecules. The phase purity and thermal stability of the as‐synthesized sample were confirmed by XPRD and TG‐DSC analyses. Experimental UV–vis diffuse reflectance spectroscopy coupled with Urbach fitting indicates a direct bandgap of 4.50 eV, contrasting with theoretical calculations that suggest an indirect bandgap of 5.53 eV. Additionally, both theoretical computation and polarized light experimental test indicate that NH 4 Ca[B 5 O 7 (OH) 4 ]·H 2 O exhibits moderate birefringence.