title: Unidirectional thermal expansion in edge-sharing BO4 tetrahedra contained KZnB3O6.
abstract: Borates are among a class of compounds that exhibit rich structural diversity and find wide applications. The formation of edge-sharing (es-) BO4 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-KZnB3O6 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 KZnB3O6: unidirectional thermal expansion, which plays a role in preserving es-BO4 from disassociation at elevated temperatures. It is found that this unusual thermal behavior originates from cooperative rotations of rigid groups B6O12 and Zn2O6, 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 BO3 and es-BO4. These findings will broaden our knowledge of the relationship between structure and property and may find applications in future.
authors.affiliation: Research &Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Yanfang
authors.initials: Y
authors.lastname: Lou
authors.affiliation: Research &Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Dandan
authors.initials: D
authors.lastname: Li
authors.affiliation: Research &Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Zhilin
authors.initials: Z
authors.lastname: Li
authors.affiliation: Research &Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China.
authors.forename: Shifeng
authors.initials: S
authors.lastname: Jin
authors.affiliation: 1] Research &Development Center for Functional Crystals, Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China [2] Collaborative Innovation Center of Quantum Matter, Beijing 100190, China.
authors.forename: Xiaolong
authors.initials: X
authors.lastname: Chen