Design, characterization, and insights theoretical on (NH4)2Fe0.11Ni0.89(SO4)2(H2O)6 crystal: A novel Tutton salt for UV-B optical filters and thermochemical heat storage batteries

This study presents a comprehensive analysis of the structural, geometric, morphological, electronic, thermal, and vibrational properties of a novel mixed Tutton salt (NH₄)₂Fe_0.11Ni_0.89(SO₄)₂(H₂O)₆. This compound marks the first Tutton-type crystal incorporating Fe²⁺ and Ni²⁺ cations at the divalent sites, offering unexplored possibilities. The Fe/Ni elemental ratio was determined by energy-dispersive X-ray spectroscopy. Structural and geometric parameters obtained from X-ray powder diffraction and Rietveld refinement reveal that the material crystallizes in a monoclinic symmetry within the P2₁/a space group, exhibiting structural isomorphism typical of Tutton salts. SEM images show that the crystal surface grows through the spreading of single layers or the lateral advancement of growth steps. First-principles calculations using density functional theory support the structural and geometric data, experimental Raman and FTIR spectra, and predict an electronic bandgap of approximately 4 eV, derived from the band structure and projected density of states. A complementary theoretical study using 3D Hirshfeld surfaces and 2D fingerprint mappings identifies the H⋯O/O⋯H intermolecular contacts as the primary interactions stabilizing the crystal. The unit cell features a low void percentage (7.42 %), implying high lattice energy between the molecular fragments [NH₄], [SO₄], and [Fe/Ni(H₂O)₆]. Thermally, the crystal remains stable between 300 and 360 K, with phase transformation occurring at higher temperatures, involving full dehydration and the formation of anhydrous salt, followed by a solid-solid phase transition (recrystallization) and material decomposition. A high energy storage density (2.11 GJ/m³) was calculated from dehydration reaction enthalpy. Based on these findings, the mixed crystal (NH₄)₂Fe_0.11Ni_0.89(SO₄)₂(H₂O)₆ is a promising candidate for UV-B optical filters and thermochemical heat storage applications. Overall, the results suggest that occupying the divalent sites of a Tutton structure with different cations allows for tuning electronic, optical, and thermochemical parameters, offering the potential for developing tailored materials.