The Fundamental Disorder Unit in (Si, P)-(O, N) Networks

This study presents the synthesis and characterization of oxonitridosilicate phosphates Sr₃SiP₃O₂N₇, Sr₅Si₂P₄ON₁₂, and Sr₁₆Si₉P₉O₇N₃₃ as the first of their kind. These compounds were synthesized under high-temperature (1400 °C) and high-pressure (3 GPa) conditions. A unique structural feature is their common fundamental building unit, a vierer single chain of (Si, P)(O, N)₄ tetrahedra. All tetrahedra comprise substitutional disorder which is why we refer to it as the fundamental disorder unit (FDU). We classified four different FDU motifs, revealing systematic bonding patterns. Including literature known Sr₅Si₂P₆N₁₆, three of the four patterns were found in the presented compounds. Common techniques like single-crystal X-ray diffraction (SCXRD), elemental analyses, and ³¹P nuclear magnetic resonance (NMR) spectroscopy were utilized for structural analysis. Additionally, low-cost crystallographic calculations (LCC) provided insights into the structure of Sr₁₆Si₉P₉O₇N₃₃ where NMR data were unavailable due to the lack of bulk samples. The optical properties of these compounds, when doped with Eu²⁺, were investigated using photoluminescence excitation (PLE), photoluminescence (PL) measurements, and density functional theory (DFT) calculations. Factors influencing the emission properties, including thermal quenching mechanisms, were discussed. This research reveals the new class of oxonitridosilicate phosphates with unique systematic structural features that offer potential for theoretical studies of luminescence and band gap tuning in insulators.