Determination of the 31P and 207Pb Chemical Shift Tensors in Pyromorphite, Pb5(PO4)3Cl, by Single‐Crystal NMR Measurements and DFT Calculations

Abstract

The chemical shift (CS) tensors for 31P and 207Pb in the natural mineral pyromorphite, Pb5(PO4)3Cl, were determined from orientation‐dependent NMR spectra of a single crystal, and MAS‐NMR spectroscopic experiments. For the two crystallographically independent lead atoms in the hexagonal crystal lattice with space group P63/m, the NMR parameters derived from the tensor eigenvalues are δiso = –2172 ppm, ηcs = 0.08 for the 207Pb at Wyckoff position 6h, and δiso = –2810 ppm, ηcs = 0.49 for position 4f. For the 31P, which are also located at positions 6h, δiso = –1.74 ppm and ηcs = 0.21 was found. A multi‐parameter fit was utilized to extract the tensors from spectra acquired with one crystal rotation axis. The fit was sufficiently overdetermined to also include the orientation of the crystal rotation axis into the fit, which could not be determined by X‐ray diffraction because of the high absorption coefficient of pyromorphite, caused by the large lead content, and the large crystal size. In contrast to the precisely determined tensor eigenvalues, the orientation of the eigenvectors in the crystal ab‐plane of the pyromorphite system could only be resolved by using the results of density functional theory (DFT) calculations.