NH4Ca[B5O7(OH)4] H2O: A Biaxial Mixed‐Cation Pentaborate with Moderate Birefringence. - Lin, Ming‐Sui; Li, Wen‐Hui; Wu, Hong‐Yan; Liu, Xia; Lai, Wen‐Zhong; Zhang, Jian‐Han (ChemistrySelect, 2024)