pH-Controlled Synthesis\nof Mercury Cyanamides/Carbodiimides\nand Piezocatalytic Studies of the Noncentrosymmetric Ones

An entire series of mercury cyanamides/carbodiimides,\nincluding\ntwo new materialsHg<sub>2</sub>(NCN)Cl<sub>2</sub> (<i>Pcnb</i>, <i>Z</i> = 4, <i>a</i> = 9.109(1)\nÅ, <i>b</i> = 15.386(1) Å, <i>c</i> =\n8.017(3) Å) and Hg<sub>4</sub>(NCN)<sub>3</sub>Cl<sub>2</sub> (<i>Pccn</i>, <i>Z</i> = 4, <i>a</i> = 11.330(3) Å, <i>b</i> = 12.905(9) Å, <i>c</i> = 6.844(6) Å)has been synthesized by controlling\nthe pH of the parent solution, and their crystal structures were solved\nand refined based on synchrotron PXRD data. To determine the symmetry\nof the new phases with high certainty, experimental second-harmonic\ngeneration measurements and theoretical DFT calculations were used.\nEvaluation of band gaps was performed using Tauc plots and complemented\nby periodic DFT calculations to verify the type of indirect/direct\nsemiconductors of these cyanamides/carbodiimides. These calculations\nwere also utilized to obtain further information on chemical bonding\nabout Hg–N, Hg–Cl, and C–N from first principles.\nMoreover, the noncentrosymmetric cyanamide Hg<sub>3</sub>(NCN)<sub>2</sub>Cl<sub>2</sub> was first applied to a piezocatalytic field,\nresulting in a 50% efficiency in degrading a common organic dye. This\nwork provides fresh insight in the synthesis of inorganic nitridocarbonates\nby the pH adjustment and highlights the interplay between the experiment\nand theory, and it also offers a promising approach to applying noncentrosymmetric\ncyanamides into the degradation of environmental pollutants.