pH-Controlled Synthesis of Mercury Cyanamides/Carbodiimides and Piezocatalytic Studies of the Noncentrosymmetric Ones

An entire series of mercury cyanamides/carbodiimides, including two new materials─Hg₂(NCN)Cl₂ (<i>Pcnb</i>, <i>Z</i> = 4, <i>a</i> = 9.109(1) Å, <i>b</i> = 15.386(1) Å, <i>c</i> = 8.017(3) Å) and Hg₄(NCN)₃Cl₂ (<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 the pH of the parent solution, and their crystal structures were solved and refined based on synchrotron PXRD data. To determine the symmetry of the new phases with high certainty, experimental second-harmonic generation measurements and theoretical DFT calculations were used. Evaluation of band gaps was performed using Tauc plots and complemented by periodic DFT calculations to verify the type of indirect/direct semiconductors of these cyanamides/carbodiimides. These calculations were also utilized to obtain further information on chemical bonding about Hg-N, Hg-Cl, and C-N from first principles. Moreover, the noncentrosymmetric cyanamide Hg₃(NCN)₂Cl₂ was first applied to a piezocatalytic field, resulting in a 50% efficiency in degrading a common organic dye. This work provides fresh insight in the synthesis of inorganic nitridocarbonates by the pH adjustment and highlights the interplay between the experiment and theory, and it also offers a promising approach to applying noncentrosymmetric cyanamides into the degradation of environmental pollutants.