Abstract
Negative linear compressibility (NLC), a rare and important mechanical effect with many application potentials, in a crystal of α-BiB3O6 (BIBO) is comprehensively investigated using first-principles calculations and high-pressure synchrotron X-ray diffraction experiments. The results indicate that the BIBO crystal exhibits the second largest NLC among all known inorganic materials over a broad pressure range. This unusual NLC behaviour is due to the rotation and displacement of the rigid [BO3] and [BO4] building units that result in hinge motion in an umbrella-like topology. More importantly, the parallel-polar lone-pair electrons on the Bi3+ cations act as “umbrella stands” to withstand the B-O hinges, thus significantly enhancing the NLC effect. BIBO presents a unique example of a “collapsible umbrella” mechanism for achieving NLC, which could be applied to other framework materials with lone-pair electrons.
Highlights
IntroductionBIBO has been previously observed to show negative linear compressibility (NLC) effect along its a-axis by Haussuhl et al.[20] and Dinnebier et al.[21]
It is revealed that the monoclinic bismuth triborate BiB3O6 (α-BiB3O6, or BIBO)[18] possesses significant framework anisotropy which results in the anisotropic thermal, piezoelectric and mechanical properties[19,20,21]
The linear compressibility βl along the principal axes (l = x, y, and z) and the volume compressibility βV transformed by the experimental Cij (0 K) are in good agreement with the calculated values. All of these results demonstrate the validity of the first-principles studies on the mechanical properties in the BIBO structure and confirm that our computational methods are sufficiently accurate for the purposes of this study
Summary
BIBO has been previously observed to show NLC effect along its a-axis by Haussuhl et al.[20] and Dinnebier et al.[21]. Their experiments revealed that the NLC behaviour is extraordinarily large (~ –12.5 TPa−1). It seems that the “wine-rack” mechanism alone is unlikely to produce such large NLC effect; the detailed investigation on the origin of NLC in BIBO is highly necessary. The underlying mechanism for NLC in BIBO is elucidated and proposed as a so-called “collapsible umbrella” model, which would provide useful guidance for pursuing other NLC materials
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