Glycine and triglycine sulphate crystals doped with croconic acid: crystal structure, UV-VIS absorption, and dielectric properties

Single crystals of amino acid glycine (Gly) and triglycine sulphate (TGS) doped with organic croconic acid (CA) were synthesized by evaporation from aqueous solutions. Depending on the concentration of CA in the solution, Gly:CA crystalizes in structure of γ- (sp.gr. P3₁ (144)) or α- (P2₁/n (14)) polymorphs of glycine. TGS:CA crystallizes in ferroelectric (P2₁ (4)) TGS structure. X-ray phase analysis of single crystals and powders shows that the introducing of CA does not change the symmetry of the crystal lattices, but only slightly changes the volumes of the unit cells that is possible when small CA molecules enter the pores of crystal structures. Analysis of UV-Vis absorption spectra shows that CA doping leads to the appearance in Gly:CA and TGS:CA of new absorption bands placed in transparency region of pure TGS and Gly crystals. As compared to pure TGS the dielectric hysteresis loops of TGS:CA are characterized by increase of coercive field and decrease of switchable polarization.