Kröhnkite-type K2Mn(SO4)2(H2O)2 double salt: synthesis, structure, and properties
2022
de Oliveira Neto, João G. | Lang, Rossano | Rodrigues, Jéssica A. O. | Gutiérrez, Carlos E. O. | Murillo, Manuel A. R. | de Sousa, Francisco F. | Filho, José G Silva | dos Santos, Adenilson O.
A comprehensive study of the K₂Mn(SO₄)₂(H₂O)₂ double salt crystal with a kröhnkite-type framework is presented. Structural, morphological, thermal, vibrational, and optical properties have been explored and discussed. Moreover, calculations based on the density-functional perturbation theory were performed to accurately analyze inter- and intra-molecular vibrational modes, presenting 111 optical phonon modes in the spectral region of 50–3650 cm⁻¹. The X-ray diffraction studies confirmed that the K₂Mn(SO₄)₂(H₂O)₂ system crystallizes in triclinic symmetry with [Formula: see text] ([Formula: see text]) space group. In addition, the crystal was thermally stable from 300 K up to near 360 K and has an optical band gap of 5.78 eV, typical of insulating material. Nevertheless, when optically excited at 3.1 eV (at 400 nm), i.e., resonantly with the ⁶A₁g(S) → ⁴A₁g(G), ⁴Eg(G) electronic transition of Mn²⁺ ion, a dual-emission was detected: green–yellow (≈ 562 nm) and orange (≈ 598 nm—more intense emission), both corresponding to ⁴T₁g(G) → ⁶A₁g(S) de-excitation. The dual behavior is due to the two different Mn²⁺ luminescent species occupying slightly distorted octahedral (orange emission) and tetrahedral (green–yellow emission) sites. From the optical spectrum and Tanabe-Sugano diagram, the crystal-field strength and the Racah interelectronic-repulsion parameters were also estimated. The findings suggest that changes in the Mn coordination number (four- and six-fold) and in the crystalline field, the latter either by adequate doping or by the growth of mixed crystals, could lead to a tuning of the wavelength of the emitted light (from green to deep red).
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