Linear Zero Thermal Expansion in a Deep-Ultraviolet Transparent Crystal of BPO4 with Cristobalite-like Structure

Wang, Naizheng; Jiang, Xingxing; Molokeev, Maxim S.; Song, Gaomin; Guo, Shibin; Huang, Rongjin; Li, Laifeng; Wu, Yicheng; Lin, Zheshuai Source: CRYSTAL GROWTH & DESIGN, 19 (6):3109-3112; 10.1021/acs.cgd.9b00361 JUN 2019

We report the discovery of the zero thermal expansion (ZTE) effect in BPO₄, a famous deep-ultraviolet (DUV) optical material with cristobalite-like structure. It is revealed that BPO₄ has a linear ZTE coefficient of −0.16(5) MK^–1 along the c-axis as temperature increases from 13 to 300 K, which originates from the subtle counterbalance between the rotation-induced expansion and contraction effects among BO₄ and PO₄ groups. BPO₄ is a unique DUV cristobalite-like material exhibiting the linear ZTE behavior.