Add time:08/16/2019         Source:sciencedirect.com

The infrared and far-infrared (20–4000 cm−1) reflectivities of single crystals of nickel bromide and copper-chloride boracite have been measured in the ferroelectric and paraelectric phases. The spectral features of the boracites above 200 cm−1 are very similar and can be associated with the internal vibrational modes of the boron-oxygen framework. In the ferroelectric phase, an anomalous increase of the line-width of a low-frequency band in the Ni-Br boracite is observed near the transition temperature. A similar temperature dependence is also seen in the line-width of the anti-symmetric stretch mode of the BO3 group in both boracites. This behavior is interpreted as the result of an enhanced coupling to a low frequency soft mode. Comparison of values of the static dielectric constants calculated from the Lyddane-Sachs-Teller relation with those obtained from capacitance measurements shows that ϵ (cap) of both boracites in the paraelectric phase is 1.5–2.0 times larger than ϵ(LST). These observations support the current understanding of the improper ferroelectric phase transition in these materials as both of an order-disorder and displacive nature.

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