Abstract
A very recent study demonstrated that the KBe2BO3F2 (KBBF) family of crystals, including KBBF, RbBe2BO3F2, and CsBe2BO3F2, are the only known borates exhibiting a rarely occurring isotropic area negative thermal expansion (NTE) behavior, over a very large temperature range. In the present work, the NTE mechanism in these crystals is comprehensively investigated using the first-principles calculations. It is revealed that the area NTE behavior mainly originates from the concerted distortion of [BeO3F] tetrahedra in the two-dimensional [Be2BO3F2]∞ framework with respect to temperature, while the [BO3] triangles remain almost rigid. Moreover, the different magnitude of NTE effect in the three crystals is attributed to the interaction difference between the alkali metal atoms (K, Rb, or Cs) and the [Be2BO3F2]∞ layer.
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