Structural phase transitions coupled with prominent dielectric anomalies and dielectric relaxation in [(CH3)3NH]2[KCo(CN)6] and mixed [(CH3)3NH]2[KFexCo1−x(CN)6] double perovskite hybrids

Abstract

The crystals of pure [(CH₃)₃NH]₂[KFe(CN)₆] (TrMAFe) and [(CH₃)₃NH]₂[KCo(CN)₆] (TrMACo) as well as their mixed crystals (TrMAFe_xCo_1−x), with different ratios of x = 0, 0.12, 0.18, 0.49, 0.56, 0.73, 0.81, 1.0, have been grown from aqueous solutions. The structure of TrMACo has been determined at 360 K to be cubic (Fmm). In phase II (100 K), the TrMACo crystal is monoclinic (C2/c). The thermal stability of the pure and mixed crystals has been determined by using both DTA and TGA. Based on the DSC results, we have found a single phase transition (PT) for both pure and mixed crystals. The Fe(III) concentration was estimated by using the SEM technique. We have found a linear relationship between the PT temperature (T_c) and the molar concentration of Fe(III). Based on the obtained results, a phase diagram has been constructed. The mechanism of the structural PT has been discussed based on the results of dielectric relaxation and ¹H NMR and X-ray spectroscopy measurements. The results confirmed that the PT mechanism of both pure and mixed crystals is related to the change in the dynamic state of the TrMA⁺ cations. The dielectric activation energy changes linearly with the mole fraction ranking from 35 to 38 kJ mol⁻¹, for the mixed crystals.