Unidirectional thermal expansion in edge-sharing BO(4) tetrahedra contained KZnB(3)O(6)

Borates are among a class of compounds that exhibit rich structural diversity and find wide applications. The formation of edge-sharing (es-) BO(4) tetrahedra is extremely unfavored according to Pauling’s third and fourth rules. However, as the first and the only es-borate obtained under ambient pressure, es-KZnB(3)O(6) shows an unexpected high thermal stability up to its melting point. The origin of this extraordinary stability is still unclear. Here, we report a novel property in KZnB(3)O(6): unidirectional thermal expansion, which plays a role in preserving es-BO(4) from disassociation at elevated temperatures. It is found that this unusual thermal behavior originates from cooperative rotations of rigid groups B(6)O(12) and Zn(2)O(6), driven by anharmonic thermal vibrations of K atoms. Furthermore, a detailed calculation of phonon dispersion in association with this unidirectional expansion predicts the melting initiates with the breakage of the link between BO(3) and es-BO(4). These findings will broaden our knowledge of the relationship between structure and property and may find applications in future.