Crystalline structure of SrAlF₅investigated by vibrational spectroscopy

Abstract

SrAlF 5 has been considered as one of the rare fluoride ferroelectric crystals, belonging to the polar I 4 group. However, recent x-ray diffraction data suggest that the correct structure is centrosymmetric (I 4 1 /a). Since ferroelectricity is forbidden in this structure, the existence of an inversion centre was investigated by vibrational spectroscopy. Thus, measurements on SrAlF 5 single crystals have been made by polarized Raman scattering and infrared reflectance spectroscopy. The results, discussed on the basis of the factor group analysis of the proposed structures and other members of the ABF 5 family, favour a centrosymmetric structure.

Investigation of the Cr:LiSrAlF6crystal by high-temperature Raman spectroscopy

“…Around the crystal melting point, the Raman peak at 550 cm −1 disappears, which is accompanied with three new Raman peaks appearing at 270, 490, and 600 cm −1 . The similar Raman peaks are observed in SrAlF 5 crystal, [25] of which three characteristic vibrational peaks are located at 290, 515, and 600 cm −1 . It is well known that in the SrAlF 5 crystal, the AlF 6 octahedron forms a chain structure by sharing the api-cal F − ions along the c axis, Sr 2+ ions are distributed in the fluoroaluminate frame.…”

About the Ferroelectricity of SrAlF5

“…Even though the polarization selection rules of the Raman spectra support a centrosymmetric space group, as it was pointed out by Silva et al [12], the number of observed vibrational bands is systematically one quarter of that predicted by the factor group analysis of the Kubel's crystalline structure (Figs. 1 and 2).…”

“…In a recent work, we investigated the existence of an inversion center in SrAlF 5 using polarized Raman and infrared spectroscopies [12]. These results suggest a centrosymmetric crystalline system, but whose unit cell should be one quarter of the one proposed by Kubel.…”

“…However, the presence of an inversion center in the I 4 1 /a space group forbids the ferroelectricity in this compound. In a recent work, the existence of an inversion center in SrAlF 5 was investigated using polarized Raman and infrared spectroscopies [12]. These results suggest a centrosymmetric crystalline system, but whose unit cell should be one quarter of the one proposed by Kubel. In this work, we review the vibrational spectroscopy results and present an impedance spectroscopy analysis of SrAlF 5 at high temperatures.…”

Energy transfer in pure and rare-earth doped SrAlF5crystals

“…It has been shown that SAF is highly transparent down to the cutoff wavelength near 150 nm and its band gap was estimated to be 12 eV [1,2]. Vibrational spectroscopy studies of SAF were performed in [3]. However, the most interesting properties of this compound arise from its complex structure [4,5].…”