Neutron Powder Diffraction, Multinuclear, and Multidimensional NMR Structural Investigation of Pb₅Ga₃F₁₉

Abstract

The room temperature structure of Pb5Ga3F19 is investigated by combining neutron diffraction and multinuclear 19F, 71Ga, and 207Pb one-dimensional and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments. Two models built in space group I4cm are reported for the description of the crystalline structure of Pb5Ga3F19. The structure is built from a network of both opposite corner-sharing Ga2F6 3− octahedra forming infinite chains along the c-axis and isolated Ga1F6 3− octahedra. The two models present two slightly different views of the strong static disorder of the fluorine ions belonging to the Ga2F6 3− octahedra. 71Ga NMR results show that the local environment of all Ga2 ions is identical, which indicates a tilt of the Ga2F6 3− octahedra within the chains. 207Pb NMR experiments confirm that the environment of only one of the two lead sites, Pb1, is strongly affected by the disorder, which gives rise to three broad distinct 207Pb NMR lines for this site. All 19F NMR lines are assigned using the 19F DQ-SQ MAS experiment. 19F−207Pb through-bond and through-space heteronuclear correlation experiments are carried out for the first time, supporting assignment of both the 19F and 207Pb NMR spectra. These correlation experiments also show that both models correctly describe the medium-range order of the structure of Pb5Ga3F19.

SMARTER crystallography of the fluorinated inorganic–organic compound Zn3Al2F12·[HAmTAZ]6

“…Cross-polarization polarization inversion (CPPI) 48 curves were recorded at a spinning frequency of 8 kHz, using CP 49 conditions that fulfill the n = +1 Hartmann-Hahn 50 condition (50 kHz RF pulse on 13 C) and 1 ms contact time. 13 C Hahn-echo spectra were recorded at a MAS frequency of 20 kHz using various inter-pulse delays synchronized with 1 to 4 rotor periods, 3.3 μs 90°pulse length, 400 s recycle delay and 160 transients for each spectrum. The 15 N cross-polarization (CP) MAS (5 kHz) NMR spectrum was recorded on an Avance II 300 Bruker spectrometer (B 0 = 7 T, Larmor frequency = 30.4 MHz) using a 7 mm probe.…”

“…The contact time was set to 7 ms, the recycle delay to 15 s and ∼16 000 transients were accumulated. In all 13 C and 15 N NMR spectra, 1 H SPINAL-64 decoupling with a nutation frequency of 80 kHz was applied during the acquisition period.…”

“…63 For the calculation of the 27 Al and 67 Zn electric field gradient (EFG) tensor values, a 2 × 2 × 2 Monkhorst-Pack k-point grid was used to sample the Brillouin zone, with a plane-wave basis set expanded to kinetics energy lower than 500 eV. For the calculation of the 19 F, 13 C, 15 N and 1 H shielding tensor components, a 2 × 2 × 3 Monkhorst-Pack kpoint grid was used to sample the Brillouin zone, with a planewave basis set expanded to kinetics energies lower than 500 eV.…”

“…Because all atoms have NMR active nuclei ( 1 H, 13 C, 15 chemical shift δ iso = −1.5 ppm, characteristic of an Al atom in six-fold fluorinated coordination, a small quadrupolar coupling constant C Q ∼ 250 kHz and the asymmetry parameter η Q ∼ 0 characteristic of only a slightly distorted AlF 6 octahedron. A single set of parameters has been used for the reconstruction of this NMR spectrum, indicating the absence of distribution of the 27 Al quadrupolar parameters and therefore the absence of F/OH substitution in the compound.…”

“…The use of NMR also allows getting insights in various sub-networks that are usually difficult to access from diffraction measurements, like ionic mobility, the localization of organic templates in organic-inorganic hybrid compounds, the distribution of iso-electronic atoms or group of atoms (Si/Al, F/OH…). SMARTER crystallography has provided structural models of a wide variety of material classes, ranging from small organic molecules [3][4][5] and pharmaceuticals [6][7][8] over semiconductors for optical applications, 9,10 inorganic fluorides, [11][12][13][14][15] to porous systems like zeolites, [16][17][18][19][20][21] or metal-organic-framework (MOFs), [22][23][24][25][26][27][28][29] which have been described with a high degree of accuracy.…”