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

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.