Visnyk of the Lviv University. Series Physics 59 (2022) ñ. 53-60
DOI: https://doi.org/10.30970/vph.59.2022.53

Effect of uniaxial stress on the infrared spectra of (NH4)2BeF4 crystal

B. Horon, M. Rudysh, V. Stadnyk, R. Brezvin, P. Shchepanskyi, ². Matvijishyn

(íå ìåíøå 1800 çíàê³â) Crystals of ammonium fluoroberylate (AFB, (NH4)2BeF4) are typical representatives of a wide group of dielectric crystals of A2ÂÕ4 type, which have two phase transitions and, accordingly, possess three different phases. In the high-temperature region (T > 182 K) AFB is in the paraelectric phase. At room temperature, the crystal belongs to the Pnma spatial symmetry group and has a slight distortion of the BeF42- tetrahedral group. Previously, IR spectra of FBA crystal were studied in the spectral range of 300–4000 cm-1. Bands corresponding to different internal vibrations of NÍ4+ and BeF42- ions (\sim370 cm-1 and \sim100 cm-1) were detected, as well as several bands caused by a combination of these modes. However, studies of the influence of uniaxial loading on the behavior of the IR spectra of these crystals have not been conducted. A preliminary study of the effect of mechanical loading on the birefringent properties of AFB crystals showed that the spontaneous increments of birefringence depend linearly on the load, and hysteresis of changes was not detected. Study of spectral dependences of birefringence of the AFB crystal for pressures of different magnitudes and directions showed that stresses \sigmam lead to changes in \Delta ni of different magnitudes. Dispersion curves of birefringence do not change qualitatively under pressures applied, but their absolute values are altered.In this work, the IR reflection spectra of the AFB crystal were investigated for three polarizations of light in a wide spectral range of 500–4000 cm-1. Six reflection bands corresponding to vibrational deformation and librational modes of BeF42- and NH4+ tetrahedra were detected. The effect of uniaxial pressure on the behavior of the IR reflection spectra of this crystal was investigated. A significant change in the intensities and positions of the reflection bands responsible for the oscillations of the BeF42- tetrahedral group was revealed. The baric displacement of the reflection bands, which correspond to the vibrations of the NH4+ tetrahedral groups, is insignificant, indicating the determining influence of the vibrations of the BeF42- tetrahedron on the physical properties of the crystal.

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