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

• Published in: Journal of materials science 2022-05, Vol.57 (17), p.8195-8210
• Main Authors: 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.
• Format: Article
• Language: English
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Classical Mechanics; Computation & Theory; Coordination numbers; Crystal growth; Crystal structure; Crystallography and Scattering Methods; Crystals; Field strength; Insulation; Manganese ions; Materials Science; Mixed crystals; Optical properties; Perturbation theory; Polymer Sciences; Solid Mechanics; Thermal stability; Vibration mode
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-022-07188-7

A comprehensive study of the K 2 Mn(SO 4 ) 2 (H 2 O) 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 K 2 Mn(SO 4 ) 2 (H 2 O) 2 system crystallizes in triclinic symmetry with P 1 ¯ ( C i 1 ) 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 6 A 1g (S) →  4 A 1g (G), 4 E g (G) electronic transition of Mn 2+ ion, a dual-emission was detected: green–yellow (≈ 562 nm) and orange (≈ 598 nm—more intense emission), both corresponding to 4 T 1g (G) →  6 A 1g (S) de-excitation. The dual behavior is due to the two different Mn 2+ 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). Graphical abstract