OBTENÇÃO E CARACTERIZAÇÃO DE SAIS KRÖHNKITE X2Mn(SO4)2(H2O)2 (X= Na e K) PARA POSSÍVEIS APLICAÇÕES EM SISTEMAS ÓPTICOS E TERMOQUÍMICOS

Abstract

In this dissertation, X2Mn(SO4)2(H2O)2 (X= Na, K) crystals with kröhnkite structure were synthesized and studied. The structural, thermal, vibrational, and optical properties are investigated and discussed. X-ray diffraction and Rietveld refinement analyses show that at room temperature, Na2Mn(SO4)2(H2O)2 crystallizes in a monoclinic system with space group P21/c (No. 14), a crystal belonging to the kröhnkite family type-D. Calculations based on the density- functional perturbation theory (DFPT) were performed to determine inter- and intra-molecular vibrational modes. One hundred and fourteen modes (111 optical + 3 acoustic) in the 30-3070 cm−1 range are inferred. The optically active modes correspond to 54 Raman and 57 IR. Experimental IR and Raman spectra with their respective assignments are presented and compared with the calculation. Na2Mn(SO4)2(H2O)2 showed thermal stability from 300 K up to about 410 K, followed by structural changes due to transitions and phase transformations with increasing temperature. A dehydration reaction enthalpy of 169.4 kJ/mol was estimated from the DSC curve, where H2O molecules were released from the crystal in three stages. The corresponding total energy density value of ≈ 1.40 GJ/m3 is very promising for domestic thermochemical heat device applications. However, after heat treatment at 583 K, a hygroscopicity assay revealed partial rehydration after 24 h at T = 299 K and RH = 58%. The H2O molecules were not reincorporated into the structure but adsorbed on the crystal surface. In other words, there was no structural reversibility, making such an application unfeasible. X-ray diffraction patterns as a function of temperature show a complex structural evolution involving anhydrous Na-Mn phases. In the range between 300 and 400 K, the thermal expansion of the Na2MnS2H4O10 crystal showed a noticeably anisotropic behavior, typical of monoclinic phases. The expansion coefficient is higher on the [100] crystallographic axis compared to the [010] and [001] axes. On the other hand, UV-Vis optical absorbance and transmittance spectra of the crystal at 300 K exhibited seven bands originating from intraconfigurational transitions of Mn2+ ions. An optical gap of 5.67 eV was valued and attributed to the wide bandgap nature of the crystal. A proof-of-concept experiment for optimal white light generation was carried out using combined light emitted by the crystal and a blue light-emitting diode. Approximately ideal white light is obtained. The crystal-field strength and the B and C Racah parameters for the crystal were also estimated. The other kröhnkite crystal studied, the C-type K2Mn(SO4)2(H2O)2, also proved to be thermally stable from 300 to ≈ 400 K. Endothermic events observed in the TG-DTA and DSC thermograms, associated with phase transition and transformation, were investigated by X-ray diffraction and Raman spectroscopy as a function of temperature. The crystal presented three well-defined phases with increasing 12 temperature: phase 1 of triclinic symmetry and space group1P (No. 2) (up to 360 K), phase 2 of monocline symmetry with space group P2 (No. 3) (between 370- 403 K) and, and, phase 3 of same monoclinic symmetry but with space group PC (No. 7) (between 413-653 K). Between 300 and 360 K, the thermal expansion of triclinic K2MnS2H4O10 exhibited an anisotropic behavior similar to the monoclinic Na2MnS2H4O10 crystal. There appears to be a tendency for kröhnkite structures independent of symmetry. However, unlike the Na2MnS2H4O10 crystal, the H2O molecules of the K2MnS2H4O10 structure are released from the crystal in a single dehydration step. A reaction enthalpy of 159.6 kJ/mol, corresponding to a total energy density of ≈ 1.16 GJ/m3 was obtained, the value being slightly below the value pre-established in the literature of 1.3 GJ/m3. The rehydration test showed that potassium kröhnkite dehydrated at 413 K does not return to its dihydrate structure within 24 h, a similar atmospheric H2O adsorption behavior to sodium kröhnkite. These results suggest that further studies, mainly dehydration/hydration cycles at low water vapor pressure and relative humidity ≈ 100%, should still be carried out before assuming they are promising for applications in thermochemical energy storage devices.