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

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.