Mixed tutton salts K2Mn0.15Co0.85(SO4)2(H2O)6 and K2Mn0.16Zn0.84(SO4)2(H2O)6 for applications in thermochemical devices: experimental physicochemical properties combined with first-principles calculations

Abstract

In this paper, two hydrated double salts, whose chemical formulas are K₂Mn_0.15Co_0.85(SO₄)₂(H₂O)₆ (KMn/Co) and K₂Mn_0.16Zn_0.84(SO₄)₂(H₂O)₆ (KMn/Zn), were crystallized via solvent slow evaporation. The structural, thermal, and vibrational properties were explored and discussed. Furthermore, a study of geometric, electronic, and thermodynamic (entropy, enthalpy, and free energy) parameters was performed based on computational calculations. Single-crystal X-ray diffraction indicated that the compounds belong to the family of mixed Tutton salts with monoclinic symmetry, space group P2₁/c (\({C}_{2h}^{5}\)), and occupancy factors of Mn_0.15/Co_0.85 (KMn/Co) and Mn_0.16/Zn_0.84 (KMn/Zn). Additionally, the intermolecular contacts and total empties in the unit cells were determined by Hirshfeld surfaces and crystal voids, respectively. The two crystals exhibited thermal stability around 333 K (Mn–Co) and 348 K (Mn–Zn). Above these temperatures, endothermic and exothermic events characteristic of dehydration, crystallization, solid–solid transition, and melting are recorded. Furthermore, KMn/Co and KMn/Zn exhibited significant energy densities, making them promising candidates for thermochemical energy storage systems. An energy band gap of 4.13 eV and 4.49 eV was predicted for KMn/Co and KMn/Zn, respectively. These findings are characteristic of insulating materials with major contributions from p orbitals in the valence and conduction bands. Using group theory and density functional theory (DFT), it was suggested that the two compounds have a total of 183 optical modes in the spectral region from 30 to 3600 cm⁻¹. Furthermore, experimental and calculated Raman and infrared spectra showed a good correlation. Our data suggest that the KMn/Co and KMn/Zn salts present promising thermo-structural results, showing that they can be used in heat-storing thermochemical devices due to their low dehydration temperatures, high dehydration enthalpies, and good energy density.

Graphical Abstract