Ba₂Si₃P₆: 1D Nonlinear Optical Material with Thermal Barrier Chains

A novel barium silicon phosphide was synthesized and characterized. Ba₂Si₃P₆ crystallizes in the noncentrosymmetric space group Pna2₁ (No. 33) and exhibits a unique bonding connectivity in the Si-P polyanion not found in other compounds. The crystal structure is composed of SiP₄ tetrahedra connected into one-dimensional double-tetrahedra chains through corner sharing, edge sharing, and covalent P-P bonds. Chains are surrounded by Ba cations to achieve an electron balance. The novel compound exhibits semiconducting properties with a calculated bandgap of 1.6 eV and experimental optical bandgap of 1.88 eV. The complex pseudo-one-dimensional structure manifests itself in the transport and optical properties of Ba₂Si₃P₆, demonstrating ultralow thermal conductivity (0.56 W m^-1 K^-1 at 300 K), promising second harmonic generation signal (0.9 × AgGaS₂), as well as high laser damage threshold (1.6 × AgGaS₂, 48.5 MW/cm²) when compared to the benchmark material AgGaS₂. Differential scanning calorimetry reveals that Ba₂Si₃P₆ melts congruently at 1373 K, suggesting that large single crystal growth may be possible.