Croconic Acid Doped Triglycine Sulfate: Crystal Structure, UV-Vis, FTIR, Raman, Photoluminescence Spectroscopy, and Dielectric Properties

Abstract

Triglycine sulfate (TGS) single crystals doped with croconic acid (CA) were grown by evaporation from aqueous solutions. X-ray diffraction analysis shows a slight reduction in unit cell volume in TGS:CA compared to pure TGS crystals. The polarized Raman and near-infrared absorption spectra show that the positions of most lines resulting from inter- and intramolecular vibrations are in good agreement with those in spectra of undoped TGS crystals. The inclusion of CA in TGS is confirmed by the presence of bands characteristic of CA in the infrared-Fourier transform spectra. The ultraviolet-visible absorption spectra of TGS:CA are characterized by the presence of additional absorption bands (compared to the spectra of pure TGS) located in the transparent region of pure TGS. In the photon energy region 1.6–3.6 eV, a strong “green” luminescence band is present in TGS:CA upon excitation at λ = 325 nm. The position of the emission band depends on the wavelength of the exciting light. Doping of TGS with CA causes pinning of domain walls, which is accompanied by a decrease in amplitude and frequency dispersion of the dielectric anomaly at the phase transition, a decrease in the switchable polarization and an increase in the coercive field of hysteresis loops.

Croconic Acid Doped Glycine Single Crystals: Growth, Crystal Structure, UV-Vis, FTIR, Raman and Photoluminescence Spectroscopy

“…Thus, doping of crystalline Gly with CA significantly affects the shape of the absorption edge of Gly, which manifests itself in smearing of the sharp absorption edge and the appearance of noticeable absorption at frequencies hν < E g . Note that the same changes of absorption spectrum have been observed earlier in CA-doped triglycine sulfate crystals [31].…”

“…However, in the case of α-Gly:CA (90:10) plates, additional lines in the photon energy range E ph = 0.5-0.6 eV (4000-5000 cm −1 ) were observed in the FTIR absorption spectra measured in transmission geometry. Note that similar lines have been observed in the FTIR spectra of TGS crystals doped with CA [31] (Figure 6). No additional lines were observed in the reflection geometry, most likely due to their low intensity.…”

“…Other parameters of powder XRD measurements, WHP and SSP analysis, LB, and Rietveld fittings are the same as detailed in [31,41,50] and briefly discussed in Section S2 of the Supplementary Materials.…”

“…Thus, CA doping of αand γ-modifications of glycine single crystals does not lead to a change in symmetry and noticeable changes in the lattice parameters of the corresponding nominally pure glycine isomorphs. Like TGS:CA crystals [31], the incorporation of CA molecules into glycine crystals leads only to a slight modification of the unit cell parameters. In contrast to TGS:CA, the presence of twins is observed in noncentrosymmetric γ-Gly:CA crystals.…”

“…It was recently discovered that doping above RT ferroelectric TGS crystals with CA molecules results in appearance of a strong green luminescence band in region of photon energy E ph = 1.6-3.5 eV with the band maximum at E ph = 2.55 eV. The doping also leads to the appearance of absorption bands related to local optical transitions in the transparency region of nominally pure TGS crystals and change of dielectric hysteresis loops [31]. CA doping does not change significantly TGS crystal structure, does not result in appearance of new phases, and does not influence frequencies of molecular vibrations measured by Raman and Fourier-transform infrared spectroscopy (FTIR).…”