Effect of urea on arrangement of novel Mg(II) perrhenate crystal structures and their optical properties: Experimental and theoretical insight

Abstract

Three Mg(II) perrhenate complexes were synthesized in aqueous solution and the crystal structures of neutral Mg(H2O)2(ReO4)2 (1) and two novel ionic complexes with urea (OC(NH2)2, (U)), [Mg(H2O)2(U)4](ReO4)2 (2) and [Mg(U)6](ReO4)2 (3) were determined by single crystal X-ray diffraction. The compounds were examined by DTA-TG-MS analysis, FT-infrared and UV–Vis diffuse reflectance spectroscopy. Detailed analysis of bond order, vibrational and optical spectra, and density of states was performed by periodic PAW/PBE calculations. The calculated free energies showed increased stability of 1, 2 and 3 with a higher number of U, which correlates with the largest covalent contribution in Mg-Ou bond, the highest melting point and optimal thermal resistance of 3. The inclusion of urea expanded UV absorption range up to 395 ​nm, as a result of higher energy valence bands and reduced band gap energy. Above this range, the crystals are characterized by a wide transparency window and improved optical properties.