Kröhnkite-type Na2Mn(SO4)2(H2O)2: first-principles analysis, thermal evolution, and application prospects

This paper presents an extensive investigation into Na₂Mn(SO₄)₂(H₂O)₂ crystal, combining experimental techniques, first-principles calculations based on the density-functional perturbation theory (DFPT), and computational tools using CrystalExplorer software to examine structure, intermolecular interactions, thermal stability, chemical transformations/phase transitions, normal vibration modes, possible applications, among others. The double salt crystallizes in a monoclinic symmetry (P2₁/c), characteristic of D-subtype kröhnkite-family salts. Ion–dipole interactions and hydrogen bonding stabilize the crystal structure and feature a void volume of ≈ 3.7%. Thermal analysis and temperature-dependent X-ray diffraction reveal that the structure remains thermally stable between 300 and ≈ 400 K, beyond which dehydration occurs. Although Na₂Mn(SO₄)₂(H₂O)₂ exhibits a high dehydration enthalpy (79.8 kJ/H₂O mol), it shows no structural reversibility (rehydration) after 24 h under open system conditions (H₂O vapor was supplied by atmospheric air), indicating limitations for thermochemical heat storage applications. At high temperatures (> 480 K), complex phase transitions give rise to predominantly anhydrous crystalline phases, including vanthoffite-type Na₆Mn(SO₄)₄ (P2₁/c), Na₂Mn₃(SO₄)₄ (Cmc2₁), and Na_2.74Mn_1.86(SO₄)₃ (P2₁/c). Several optical phonon modes from Na₂Mn(SO₄)₂(H₂O)₂ were identified through FT-IR and Raman spectroscopy and accurately assigned using DFPT calculations. Optical measurements depict a wide energy gap of ≈ 5.67 eV (≈ 219 nm), demonstrating an insulating nature of the crystal. Conversely, fluorescence spectra show a dual-band emission at 567 and 617 nm, corresponding to Mn²⁺ ions in four- and sixfold coordination, respectively. Crystal-field strength, as well as the Racah parameters B and C, were also determined via Tanabe–Sugano energy-level diagram, offering insights into the ligand-field environment of Mn²⁺ ions in the structure. A near-ideal white light (CCT = 5230 K) is achieved by combining the emission of the crystal with a blue LED (λ = 406 nm), which is also used as the excitation source. While this fluorescent behavior is promising, further studies are needed to assess the internal quantum efficiency for phosphor applications. Nevertheless, the findings underscore the potential of double salt structure as a host matrix for light-emitting materials (emissive dopant incorporation), expanding the functional perspectives of kröhnkite-based compounds.