pH Controlled Pathway and Systematic Hydrothermal Phase Diagram for Elaboration of Synthetic Lead Nickel Selenites

The PbO–NiO–SeO₂ ternary system was fully studied using constant hydrothermal conditions at 473 K. It yields the establishment of the corresponding phase diagram using a systematic assignment of reaction products by both powder and single-crystal X-ray diffraction. It leads to the preparation of three novel lead nickel selenites, α-PbNi­(SeO₃)₂ (<b>I</b>), β-PbNi­(SeO₃)₂ (<b>II</b>), and PbNi₂(SeO₂OH)₂(SeO₃)₂ (<b>III</b>), and one novel lead cobalt selenite, α-PbCo­(SeO₃)₂ (<b>IV</b>), which have been structurally characterized. The crystal structures of the α-forms <b>I</b>, <b>IV</b>, and <b>III</b> are based on a 3D complex nickel selenite frameworks, whereas the β-PbNi­(SeO₃)₂ modification (<b>II</b>) consists of nickel selenite sheets stacked in a noncentrosymmetric structure, second-harmonic generation active. The pH value of the starting solution was shown to play an essential role in the reactive processes. Magnetic measurements of <b>I</b>, <b>III</b>, and <b>IV</b> are discussed