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Zeman, Otto E. O.; Moudrakovski, Igor L.; Hoch, Constantin; Hochleitner, Rupert; Schmahl, Wolfgang W.; Karaghiosoff, Konstantin; Bräuniger, Thomas (2017): Determination of the P-31 and Pb-207 Chemical Shift Tensors in Pyromorphite, Pb-5(PO4)(3)Cl, by Single-Crystal NMR Measurements and DFT Calculations. In: Zeitschrift für Anorganische und Allgemeine Chemie, Vol. 643, No. 21: pp. 1635-1641
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The chemical shift (CS) tensors for P-31 and Pb-207 in the natural mineral pyromorphite, Pb-5(PO4)(3)Cl, 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 P6(3)/m, the NMR parameters derived from the tensor eigenvalues are delta(iso) = -2172 ppm,eta(cs) = 0.08 for the Pb-207 at Wyckoff position 6h, and delta(iso) = -2810 ppm,eta(cs) = 0.49 for position 4f. For the P-31, which are also located at positions 6h, delta(iso) = -1.74 ppm and eta(cs) = 0.21 was found. A multi-parameter fit was utilized to extract the tensors from spectraacquired with one crystal rotation axis. The fit was sufficiently over-determined 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 eigen-vectors in the crystal ab-plane of the pyromorphite system could only be resolved by using the results of density functional theory (DFT) calculations.