Kröhnkite-type K2Mn(SO4)2(H2O)2 double salt: synthesis, structure, and properties

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 $$Poverline{1}$$ ( $$C_i¹$$ ) 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).