Tutton salt (NH4)2Fe(SO4)2(H2O)6: A promising crystal for bandpass filter and solar-blind devices

Abstract

Tutton salt (NH₄)₂Fe(SO₄)₂(H₂O)₆ was successfully synthesized by slow solvent evaporation from a saturated aqueous solution. The structural, chemical, vibrational, and optical properties of the compound were thoroughly investigated. Computational studies were performed to support experimental data, utilizing Hirshfeld surface analysis, crystal voids, interaction energy, and density functional theory (DFT) calculations. The (NH₄)₂Fe(SO₄)₂(H₂O)₆ crystallizes in a monoclinic structure of P2₁/a space group, with solid formation occurring at an acidic pH of 3.6. Hirshfeld surface analysis revealed key intermolecular interactions, with H···O/O···H contacts predominating. The low void percentage in the primitive cell suggests high lattice energies between the [Fe(H₂O)₆]²⁺, [SO₄]^2–, and NH₄⁺ molecular layers forming the salt. The stability of crystals was also assessed across different pH environments. The crystals remain physically and chemically stable at neutral pH, whereas acidic and alkaline environments induced mass loss and passive surface oxidation, respectively. Vibrational analysis, carried out using Raman and infrared spectroscopy, was compared with DFT-calculated spectra for more accurate spectral assignments. Several other parameters were also evaluated, including geometric (bond length and angle), thermodynamic (free energy, enthalpy, entropy, and Debye temperature), and electronic (band structure and projected density of states). Optical studies revealed features typical of materials used in solar-blind technology and bandpass filters, with absorbance and transmittance spectra indicating potential applications. (NH₄)₂Fe(SO₄)₂(H₂O)₆ demonstrated promising properties for solar-blind photodetectors operating in the 190–270 nm interval. Furthermore, it can be effectively useful as a bandpass filter within the 418–800 nm spectral range.