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.
Highlights
Borates are among a class of compounds that exhibit rich structural diversity and find wide applications
Exceptions are only met in borates synthesized under high pressures (HP) and high temperatures by Huppertz and coworkers recently
Examination of its crystal structure reveals that BO4 tetrahedra in KZnB3O6 are nearly identical in B-O bond lengths and O-B-O angles to the previously reported HP borates[1,2,3,4]
Summary
Borates are among a class of compounds that exhibit rich structural diversity and find wide applications. 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. The phonon dispersions and partial density of states (PDOS) calculations reveal a soft mode at 1013 K, due to transverse vibrations of the bridge oxygen whereby BO3 and B2O6 are connected, develops to cause the structure to dismantle at around its melting point These results shed light on why KZnB3O6 can be stabilized over such a wide temperature range
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