Synthesis and spectroscopic characterization of P-doped Na4Si4

Na4Si4 is a Zintl salt composed of Na+ cations and Si44− tetrahedral anions and is a unique solid-state precursor to clathrate structures and nanomaterials. In order to provide opportunities for the synthesis of complex materials, phosphorus was explored as a possible substituent for silicon. Phosphorus doped sodium silicides Na4Si4−xPx (x≤0.04) were prepared by reaction of Na with the mechanically alloyed Si4−x:Px (x=0.04, 0.08, 0.12) mixture in a sealed Nb tube at 650 °C for 3 days. Energy dispersive X-ray spectroscopy confirms the presence of P in all products. Powder X-ray diffraction patterns are consistent with the retention of the Na4Si4 crystal structure. As the amount of P increases, a new peak in the diffraction pattern that can be assigned to black phosphorus is apparent above the background. Raman and solid-state NMR provide information on phosphorus substitution in the Na4Si4 structure. Raman spectroscopy shows a shift of the most intense band assigned to the Si44− ν1 (A1) mode from 486.4 to 484.0 cm−1 with increasing P, consistent with P replacement of Si. Differential nuclear spin-lattice relaxation for the Si sites determined via 29Si solid-state NMR provides direct evidence for Si–P bonding in the (Si1−xPx)4− tetrahedron. The 23Na NMR shows additional Na…P interactions and the 31P NMR shows two P sites, consistent with P presence in both of the crystallographic sites in the (Si4)4− tetrahedron.

P-doped Na4Si4 has been prepared and Raman spectroscopy and 29Si, 23Na, and 31P multinuclear NMR show that P dopes into both crystallographic sites for Si.Figure optionsDownload as PowerPoint slide