Combined Approach for the Structural Characterization of Alkali

Jan 22, 2018 - Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X-ray Diffraction, and Density...
0 downloads 15 Views 4MB Size
Download PDF
Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Combined Approach for the Structural Characterization of Alkali Fluoroscandates: Solid-State NMR, Powder X‑ray Diffraction, and Density Functional Theory Calculations Aydar Rakhmatullin,*,† Ilya B. Polovov,‡ Dmitry Maltsev,*,‡ Mathieu Allix,† Vladimir Volkovich,‡ Andrey V. Chukin,§ Miroslav Boča,∥ and Catherine Bessada† †

Conditions Extrêmes et Materiaux: Haute Température et Irradiation, CEMHTI, UPR 3079, CNRS, Université Orleans, 45071 Orléans, France ‡ Department of Rare Metals and Nanomaterials, Institute of Physics and Technology, and §Department of Theoretical Physics and Applied Mathematics, Ural Federal University, 19 Mira str., 620002 Ekaterinburg, Russia ∥ Department of Molten Systems, Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, SK-845 36 Bratislava, Slovakia S Supporting Information *

ABSTRACT: The structures of several fluoroscandate compounds are presented here using a characterization approach combining powder X-ray diffraction and solid-state NMR. The structure of K5Sc3F14 was fully determined from Rietveld refinement performed on powder X-ray diffraction data. Moreover, the local structures of NaScF4, Li3ScF6, KSc2F7, and Na3ScF6 compounds were studied in detail from solidstate 19F and 45Sc NMR experiments. The 45Sc chemical shift ranges for six- and seven-coordinated scandium environments were defined. The 19F chemical shift ranges for bridging and terminal fluorine atoms were also determined. First-principles calculations of the 19F and 45Sc NMR parameters were carried out using plane-wave basis sets and periodic boundary conditions (CASTEP), and the results were compared with the experimental data. A good agreement between the calculated shielding constants and experimental chemical shifts was obtained. This demonstrates the good potential of computational methods in spectroscopic assignments of solid-state 45Sc NMR spectroscopy.

■

F4:Er3+/Yb3+ exhibited specific recognition of H1299 lung cancer cells overexpressed with a tumor marker. LiScF4 doped with 1% erbium exhibited strong far-infrared luminescence.5 Very recently, NaScF4 and KSc2F7 were used to catalyze the silylcyanation, and they demonstrated excellent catalytic activity with outstanding recyclability.6 The improvement of these different materials for their specific applications requires a precise knowledge of the scandium fluoride local structure and a better description of their structural configuration. Solid-state NMR spectroscopy is a powerful tool in elucidating the local environment of atoms in complex crystalline or disordered materials. There are many reports on solid-state 45Sc NMR devoted to various complex oxides,7−15 but relatively little is known about the fluoride compounds. When it comes to the combination of 19F and 45Sc NMR, the only data available in the literature are for ScF3,7,16,17 NaScF4,18 and [C4H14N2][ScF5].19 In addition, solid-state 19F NMR studies were performed on oxofluoroscandates.17,20−22

INTRODUCTION Scandium-based materials and scandium fluorides in particular have a wide range of industrial applications. The main application field of scandium is the aerospace industry, where scandium-containing alloys, which demonstrate great mechanical properties, are employed.1−3 Scandium in aluminum alloys is used as a precipitation strengthener, a grain refiner, a recrystallization inhibitor, and an additive for enhancing superplastic properties. Scandium-containing alloys are produced by adding an aluminum−scandium master alloy containing 2−5 wt % scandium to aluminum or an aluminum−magnesium base alloy. The master alloy is usually manufactured by alumothermic reduction of scandium oxide or fluoride in molten alkali halide media. An aluminum−scandium alloy prepared by the reduction of scandium fluoride is used for to produce very pure materials free of oxygen contamination. Knowledge of scandium speciation is required to optimize the master alloy synthesis. Alkali-metal fluoroscandates showed a number of potential applications. Recently, lanthanide-doped NaScF4 was studied as a potential luminescent bioprobe.4 The biotinylated NaSc© XXXX American Chemical Society

Received: October 12, 2017

A

DOI: 10.1021/acs.inorgchem.7b02617 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 1. Experimental (Isotropic Chemical Shifts δiso, Chemical Shift Anisotropies Δ, Asymmetry Parameters ηcs, Relaxation Time T1, at 20 T) and Calculated (Isotropic Magnetic Shielding σiso) 19F NMR Parameters for the Studied Fluoroscandates (Subscripts: t, Terminal; b, Bridging) compound

fluorine site

δiso, ppm (±0.1 ppm)

ηcs (±0.1)

Δ, ppm (±10 ppm)

σiso, ppm

integral intensity, % (±1%)

ScF3 Li3ScF6

1b 1t 3t 2t 1t 2t 3t 1b 3b 2b 1b 2t 3t

−33.9 −100.2 −102.0 −108.6 −106.8 −112.1 −114.2 −23.8 −44.0 −47.1 −58.6 −63.3 −68.0

0.16 0 0.4 0.41 0.04 0.15 0.35 0.3 0 0.53 0.33 0.43 0.44

−196 −116 −99 −101 −109 −110 −101 −176 −145 −63 −143 −128 −115

97.5 209.9 210.6 217.7 214.6 222.0 226.7 74.6 119 123.2 141.8 153.6 158.7

100 33 35 32 32 36 32 14 29 57 28 58 14

Na3ScF6

KSc2F7

K5Sc3F14

In this work, we collected solid-state 19F and 45Sc NMR spectra of scandium fluoride and several alkali fluoroscandates (ScF3, Li3ScF6, NaScF4, Na3ScF6, KSc2F7, and K5Sc3F14) at high magnetic field. The crystal structure of K5Sc3F14 has been determined from powder X-ray diffraction (PXRD). For all of the compounds studied, 45Sc chemical shift and quadrupolar coupling parameters were determined from the analytical simulation of the NMR powder spectra. Correlations between the local structural data and the 19F and 45Sc NMR parameters were also demonstrated. In addition, we carried out density functional theory (DFT) calculations of the electric-field-gradient (EFG) tensor with the CASTEP code. Previous studies demonstrated that DFT methods were capable of calculating quadrupolar interaction parameters for 45Sc from structural data to a sufficiently high degree to be meaningful and also aided site assignment.11,12,14,23,24 This knowledge about fluoroscandates can be used to better describe more complex systems: scandium-based nanocrystals, catalysts, or high-temperature molten salts.

■

T1, s (±5%) 11 2−3

105−108

2.1

53 42 44

X-ray diffraction (XRD) patterns of all prepared compounds are presented in Figure S1. In view of the complexity of the synthesis of LiScF45 and KScF4,26 these compounds were not studied here. PXRD. PXRD patterns were measured using an X’Pert PRO MPD diffractometer with Cu Kα radiation in Bragg−Brentano geometry and β-filter (nickel) in the secondary beam (Ekaterinburg). A solid-state PIXcel detector was used for recording PXRD spectra with an active length of 3.347°. The measurements were carried out at room temperature with 2θ varied from 10° to 100° at a step of 0.023°. The time per one step was 200 s. PXRD data for K5Sc3F14 were recorded on a Bragg−Brentano D8 Advance Bruker diffractometer (Cu Kα radiation) equipped with a LynxEye XE detector over an angular range of 5° < 2θ < 130° (Orléans, France). Structural elucidation of the K5Sc3F14 material was realized using the Rietveld method27 with JANA200628 software. NMR Spectroscopy. Solid-state 7Li, 19F, 23Na, and 45Sc NMR spectra were obtained using Bruker AVANCE III 400 (9.4 T), 750 (17.6 T), and 850 (20 T) NMR spectrometers. 7Li, 19F, 23Na, and 45Sc chemical shifts were referenced to a 1 M LiCl solution, CFCl3, a 0.1 M NaCl solution, and a 0.11 M ScCl3 solution in 0.05 M HCl, respectively. Room temperature magic-angle-spinning (MAS) NMR spectra were acquired using MAS probes from Bruker, with 1.3-mmand 2.5-mm-diameter rotors. Data were collected at rotor frequencies of 20−67 kHz; multiple spinning speeds were used to discern the spinning sidebands (ssbs) from the isotropic peaks. 7Li, 19F, and 23Na MAS NMR data for the crystalline compounds were collected with a single-pulse experiment. For 19F, the π/2 pulse widths were between 1.0 and 1.2 μs with a recycle delay of 7T1. For 7Li and 23Na, the pulse width and recycling delay were 0.2 μs and 0.5 s, respectively. The NMR parameters (chemical shifts, chemical shift anisotropies, asymmetry parameters, line widths, and quadrupolar parameters) were fitted for several different rotor frequencies and for two magnetic fields to the experimental spectra by means of the DMfit program.29 45 Sc MAS NMR spectra were also acquired using a spin−echo pulse sequence with a rotor-synchronized delay. Spin−lattice relaxation times in the laboratory frame (T1) were obtained using a saturation− recovery pulse sequence, and the data were fitted to a stretched exponential of the form 1 − exp[−(τ /T1)α], where τ is the variable delay and α is the stretch exponential coefficient (between 0.85 and 1). In order to improve the resolution for all samples, we used continuouswave (10−20 kHz for 19F and 5 kHz for 45Sc) and PISSARRO30 45Sc and 19 F decoupling. Multiple-quantum magic-angle-spinning (MQMAS) experimental details are given in the Supporting Information. CASTEP Calculations. First-principles calculations of NMR parameters were performed using the CASTEP NMR program31 running in the Materials Studio 7.0 environment using the experimental geometries determined via diffraction methods, pseudopotentials calculated “on-the-fly”, and the Perdew, Burke, and

EXPERIMENTAL SECTION

Synthesis. Scandium fluoride was obtained by precipitation from a solution of scandium chloride using hydrofluoric acid. A scandium chloride solution was prepared by reacting scandium oxide (99.999%, Sigma-Aldrich) with hydrochloric acid (analytical purity). The precipitate was centrifuged and then air-dried at 130 °C. At the final stage, ScF3 was dried under vacuum at 300 °C. The residual water content in the ScF3 samples was below 0.01 wt %. Anhydrous alkali fluorides LiF, NaF, and KF (all of analytical purity) were used for the synthesis of fluoroscandates. All alkali fluorides were preliminarily dried in a dynamic vacuum system running at 300 °C to remove possible absorbed moisture. MF (where M = Li, Na, and K) and ScF3 were mixed in stoichiometric ratios to obtain the desired composition and further ground together in an agate mortar in an argon-filled drybox. The mixture was then transferred to a nickel crucible, which was placed in a silica cell located in a vertical tube furnace mounted inside the drybox. Solid-state synthesis was carried out in a high-purity argon atmosphere in the inert glovebox (below 0.1 ppm moisture and below 1.4 ppm oxygen). The temperature was selected on the basis of the data reported by Champarnaud-Mesjard and Frit.25 The mixture was heated from room temperature to the synthesis temperature (400 °C for Li3ScF6; 350 °C for NaScF4 and Na3ScF6; 600 °C for KSc2F7; 650 °C for K5Sc3F14) at a heating rate of 3 °C/min, held for 100 h, and then cooled to room temperature at an approximate rate of 2 °C/min. B

DOI: 10.1021/acs.inorgchem.7b02617 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 2. 45Sc Isotropic Chemical Shifts (δiso), Quadrupolar Constants (CQ), and Asymmetry Parameters (ηQ) Obtained from the Simulation of the 45Sc MAS NMR at 9.4, 17.6, and 20 T and Calculated 45Sc NMR Parameters (Isotropic Magnetic Shielding σiso, Quadrupolar Constant |CQcalc|, and Asymmetry Parameter ηQcalc) for Fluoroscandates compound

atom

crystallographic site

δiso, ppm (±0.2 ppm)

CQ, MHz (±0.1 MHz)

ηQ (±0.1)

σiso, ppm

|CQcalc|, MHz

ηQcalc

ScF3 Li3ScF6

Sc1 Sc2 Sc1 Sc1 Sc1 Sc1 Sc2

1a 4d 2b 2a 4h 4c 2a

−52.5 −7.6 14.0 11.1 −53.5 −2.7 −10.7

0.7 4.4 17.6 4.6 3.6 18.1 14.0

0.81 0.1 0 0.39 0.95 0.3 0

858.0 769.3 790.1 766.2 839.6 788.9 805.4

0 3.9 18.02 5.04 1.81 18.11 12.65

0 0 0 0.84 0.81 0.39 0

Na3ScF6 KSc2F7 K5Sc3F14

Figure 1. Experimental (blue) and simulated (red) 19F MAS NMR spectra at 20 T of ScF3 (a), Li3ScF6 (b), KSc2F7 (c), and Na3ScF6 (d). Ernzerhof32 functionals in the generalized gradient approximation. The gauge-including projector-augmented-wave33−35 method implemented in CASTEP provides accurate computed values of the periodic system properties, and its use is well-established in the solid-state NMR literature.33,36 The cutoff energies were 600 eV. In all cases, calculations were carried out after geometry optimization of all atom positions, keeping the experimental cell parameters and symmetry constraints. Optimizations of the geometry were used to ensure that the lowest-energy configuration of atoms was used (except for ScF3). Crystallographic data used in the calculations were taken from the literature, with the corresponding references given.

■

ScF3. ScF3 adopts a cubic structure37 that contains a single scandium crystallographic site and a single fluorine site in the unit cell (ICSD 77071). ScF3 has been previously studied by 19 F and 45Sc NMR.7,16,17 The 19F NMR spectrum of ScF3 recorded at 20 T with scandium decoupling and with a rotation frequency of 30 kHz is shown in Figure 1a. Applying the decoupling decreases the line width (full width at half-maximum, fwhm) from 3800 to 3400 Hz. Increasing the rotation frequency further up to 60 kHz did not improve the spectral resolution. The value of the chemical shift of −33.9 ppm obtained in the present study is in good agreement with the values reported in the literature, i.e., −30,17 −35.9,7 and −36 ppm.16 Our experimental spectrum can be nicely reconstructed with a single set of 19F chemical shift parameters as expected from the structural data. The measured anisotropy parameters Δ = −195 ppm and ηcs = 0.16 differ significantly from those given by Lo et al.7 (305 ppm and 0.53). It should also be noted that slightly different 19F chemical shift anisotropy parameters were also obtained from a

RESULTS

Solid-state 19F and 45Sc NMR spectra were acquired for ScF3, Li3ScF6, Na3ScF6, and KSc2F7. Tables 1 and 2 summarize the 19 F and 45Sc MAS NMR data, respectively. 19F and 45Sc MAS NMR spectra are presented in Figures 1 and 2, respectively. 45 Sc MAS NMR spectra at a second magnetic field are shown in Figures S2 and S3. Polyhedra and crystal structures of the studied compounds are summarized in Table S1. C

DOI: 10.1021/acs.inorgchem.7b02617 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 2. Experimental (blue) and simulated (red) 45Sc MAS NMR spectra at 9.4 T of ScF3 (a), Na3ScF6 (c), and KSc2F7 (d) and at 17.6 T of Li3ScF6 (b). For parts b−d, the insets show expanded 45Sc MAS NMR spectra in the region of the central transition (left) and an expanded satellite transition sideband revealing their complex line shape (right).

fit of the 19F NMR spectrum of a different ScF3 sample synthesized in our laboratory. The determined 45Sc chemical shift value of −52.5 ppm is in good agreement with the previously reported results of −51.816 and −52 ppm.7 As previously mentioned by Lo et al.,7 because of the small deviation of the local 45Sc environment from perfect spherical symmetry, we observed a set of ssb patterns in the 45Sc NMR spectra, indicating that the quadrupolar frequency deviated from zero; the estimate gave CQ ∼ 730 kHz. Sadoc et al.16 explained the nonzero quadrupolar frequency of 45Sc by dynamic and static disorders. The presence of structural defects in the ScF3 structure, giving rise to a nonzero 45Sc quadrupolar coupling constant, could also explain the difference of the 19F chemical shift anisotropy parameters reported for different ScF3 samples (i.e., with different amounts of structural defects). Li3ScF6. The structure of Li3ScF6 contains three fluorine, two lithium, and two scandium crystallographic sites with multiplicities of 12:12:12 for fluorine, 6:12 for lithium, and 2:4 for scandium atoms (ICSD 415135).5 Each Sc3+ is coordinated by six terminal fluorine atoms at the corners of an octahedron. 45 Sc MAS NMR spectrum at 17.6 T contains two typical quadrupolar lines with relative intensities of the signals of 1:2. The “narrow” resonance (CQ = 4.7 MHz; ηQ = 0.1) at an isotropic chemical shift δiso of −7.6 ppm in the MAS NMR spectrum is assigned to a scandium site with a multiplicity of 4 (Sc2) and the broad line caused by the second-order quadrupolar interaction (CQ = 17.6 MHz; ηQ = 0; δiso = 14 ppm) to the scandium site with a multiplicity of 2 (Sc1). The 19 F MAS NMR spectrum at 34 kHz and 20 T reveals only two

signals with integral intensities of 68:32. The intense signal has a nonsymmetrical line shape, and it can be fitted with two lines with integral intensities of 1:1. The signal at −204.7 ppm and a very weak signal at −33.7 ppm correspond to unreacted starting materials: LiF and ScF3, respectively. 7Li NMR spectrum at 17.6 T and 60 kHz contains only one resonance with Lorentzian−Gaussian line shape and with a maximum at −1.1 ppm and fwhm = 240 Hz (Figure S4a). Because diamagnetic lithium has a very small chemical shift range, it is not possible to resolve the resonances of LiF and Li3ScF6. Na3ScF6. According to the literature data,38 the structure of Na3ScF6 contains three crystallographically nonequivalent terminal (nonbridging) fluorine sites with relative multiplicities of 1:1:1, two sodium sites with relative multiplicities of 1:2, and one scandium site in a slightly distorted octahedral environment (ICSD 401761). There are only minor variations in the Sc−F bond lengths (∼0.015 Å), and the small deviations from ideal octahedral angles are observed in the F−Sc−F bond angles, which are 90.0 ± 1.9°. Solid-state 45Sc MAS NMR at 20 T shows a strong and sharp peak. However, determining the NMR parameters from a single high-field spectrum remains difficult because of the ambiguity interplay between them. Examination of the spectrum at the low field (9.4 T) reveals a quadrupolar line shape. From the fitted spectrum, the chemical shift and quadrupolar parameters could be obtained. As noted above, the observation of a small CQ is not surprising because of the small distortions of the octahedra. As expected, the 19F NMR spectrum at 60 kHz and 20 T shows three well-resolved peaks and their ssb pattern with relative intensities of 1:1:1 for Na3ScF6. This is in agreement with the presence of three D

DOI: 10.1021/acs.inorgchem.7b02617 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Table 3. 23Na Isotropic Chemical Shifts (δiso), Quadrupolar Constants (CQ), and Asymmetry Parameters (ηQ) Obtained from the Simulation of the 23Na MAS NMR at 17.6 T and Calculated 23Na NMR Parameters (Isotropic Magnetic Shielding σiso, Quadrupolar Constant |CQcalc|, and Asymmetry Parameter ηQcalc) for Fluoroscandates (n.m. = Not Measured) compound

atom

crystallographic site

δiso, ppm (±0.2 ppm)

CQ, MHz (±0.1 MHz)

ηQ (±0.1)

σiso, ppm

|CQcalc|, MHz

ηQcalc

Na3ScF6

Na2 Na1 Na1 Na2 Na3 Na4 Na5 Na6

2b 4e 3a 3a 3a 3a 3a 3a

−0.3 −8.1 n.m. n.m. n.m. n.m. n.m. n.m.

1.5 1.7 n.m. n.m. n.m. n.m. n.m. n.m.

0.65 0.42 n.m. n.m. n.m. n.m. n.m. n.m.

556.3 566.6 571.5 573.0 571.8 570.2 572.4 568.2

1.4 2.0 2.2 2.0 3.2 2.0 3.1 3.7

0.74 0.37 0.83 0.62 0.23 0.97 0.45 0.43

NaScF4

fluorine sites of the same multiplicities in the structure. The 23 Na NMR spectrum of Na3ScF6 at 17.6 T and 60 kHz contains three peaks (Figure S4b). The small peak with a maximum at 7.1 ppm has Lorentzian−Gaussian line shape and is assigned to NaF.39 Two typical quadrupolar lines with relative intensities of 1:2 could be ascribed to Na2 and Na1 atoms, respectively. The 23 Na quadrupolar parameters are given in Table 3. KSc2F7. The structure of KSc2F7 was determined by Güde and Hebecker40 (ICSD 47227). The orthorhombic structure contains edge- and corner-linked continuous chains of ScF7 pentagonal bipyramids. These chains are linked together equatorially to form sheets in the ab plane, which are further linked apically into a three-dimensional structure. There are one scandium site and three bridging fluorine sites. The multiplicities of the F1:F2:F3 sites are 4:8:2. The 19F MAS NMR spectrum of KSc2F7 contains three peaks. Accurate simulation of the spectrum, including all ssb patterns, yielded an integral intensity ratio of 14:29:57 or 4:2:8 for the resonances at −23.8, −44.0, and −47.1 ppm, respectively. 45 Sc MAS NMR spectrum at 20 T reveals a single site with a Lorentzian line shape with a fwhm of 750 Hz and an isotropic chemical shift of −53.5 ppm. As in the case of Na3ScF6, the 45Sc MAS NMR spectrum was recorded at two magnetic fields (9.4 and 20 T). Analytical simulations of these spectra then allowed one to obtain the value of the quadrupolar parameters. Study of K5Sc3F14. The crystal structure of K5Sc3F14 has not been described up to now. Only the unit cell and space group were suggested on the basis of a basic XRD study.25 The authors suggested a structure isotypic with chiolite Na5Al3F14, but no atomic positions were reported. We therefore decided to undertake an accurate crystal structure determination of K5Sc3F14. Structure Solution and Refinement. First, autoindexing analysis of the room temperature PXRD pattern was performed. Both the Dicvol41 and Treor42 indexing routines led to the same solution with good reliability factors: a quadratic cell with a = 7.86 Å and c = 11.87 Å. These cell parameters are in good agreement with the previous indexation of K5Sc3F14.25 A subsequent ICDD database search using similar cell parameters and chemistry restrictions (only alkali metals, transition metals, and fluorine) resulted in three compounds with similar indexation: K 5 In 3 F 14 (ICSD 248085),43 K5Ti3F14 (ICSD 60243),44 and K5V3F14 (ICSD 419506).45 These three compounds crystallize in the P4/mnc space group symmetry. The proposed tetragonal indexation was then tested via a Lebail fit of the PXRD pattern, and the obtained good reliability factors confirmed the similarity with the Na5Al3F14-type structure.

In order to determine precisely the structure of K5Sc3F14, a Rietveld refinement of the PXRD pattern was performed. The starting model was based on the K5In3F14 structure using the quadratic cell parameters previously determined. Both the cationic and fluorine positions as well as atomic displacement parameters were refined. Small amounts of a secondary phase identified as K3ScF6 were also clearly visible in the PXRD data refinement. So far, the room temperature structure of K3ScF6 remains unknown, which is why the main reflections were considered as excluded regions. Good reliability factors were obtained (wRp = 9.27%, Rp = 6.90%, and GOF = 2.22). The fit of the PXRD Rietveld refinement is shown in Figure 3. Atomic coordinates, atomic displacement parameters, and K−F and Sc−F interatomic distances are summarized in Tables 4−6.

Figure 3. Experimental (red), calculated (black), and difference (blue) PXRD Rietveld refinement of K5Sc3F14 (green marks). Excluded regions have been used for the K3ScF6 secondary phase (asterisks). Reliability factors are wRp = 9.27%, Rp = 6.90%, and GOF = 2.22.

As proposed by Champarnaud-Mesjard et al.,25 the structure of K5Sc3F14 adopts the chiolite structure type (archetype Na5Al3F14).46 It contains two potassium and two scandium crystallographic sites and three inequivalent fluorine sites with multiplicities of 8:16:4 and consists of alternating layers (perpendicular to the c direction) of corner-linked ScF6 octahedra with one in every four octahedra missing, similar to a perovskite-like sheet. This vacancy is replaced by a larger K+ ion in 8-fold coordination, K2, such that a combined rotation of the Sc1F6 octahedra around the a and b axes occurs to accommodate the deformation (Figure 4). Additional E

DOI: 10.1021/acs.inorgchem.7b02617 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 4. Crystallographic Data for K5Sc3F14 from XRD Rietveld Refinement chemical formula fw (g/mol) temperature, °C Z cryst syst space group unit cell dimens a (Å) b (Å) c (Å) cell volume (Å3) density, calcd (g/cm)

K5Sc3F14 596.3 20 2 tetragonal P4/mnc (No. 128) 7.8577(2) 7.8577(2) 11.8720(2) 733.02(2) 2.702(1)

Table 5. Atomic Coordinates, Occupancy (Occ), and Equivalent Isotropic Displacement Parameters (Ueq × 103Å2) of K5Sc3F14 atom

site

x

y

z

Occ

Ueq

K1 K2 Sc1 Sc2 F1 F2 F3

8g 2b 4c 2a 8h 16i 4e

0.7180(1) 0.5 0.5 0 0.0619(5) 0.4561(4) 1

0.2180(1) 0.5 0 0 0.2486(5) 0.1786(3) 0

0.25 0 0 0 0 0.1192(2) 0.1705(3)

1 1 1 1 1 1 1

0.031(1) 0.040(2) 0.024(1) 0.019(1) 0.046(2) 0.048(1) 0.025(2)

Figure 4. Layer structure of K5Sc3F14 viewed along the x axis.

identified as K3ScF6. The 45Sc MAS NMR spectrum contains two lines with relative intensities of 1:1 (δiso = 9.2 ppm, CQ = 3.8 MHz, and ηQ = 0.6; δiso = 20.4 ppm, CQ = 2.5 MHz, and ηQ = 0.5), and 19F NMR spectrum contains a peak at −77.7 ppm. As mentioned above, the study of K3ScF6 is currently in progress. CASTEP and 19F NMR Data. As mentioned in the Experimental Section, DFT optimization of the atomic positions was performed for all compounds studied, keeping symmetry constraints and fixing the cell parameters to the experimental values. The resulting atomic coordinate sets after geometry optimization compared to the initial structural data are given in Table S2. As expected, the differences between coordinates (Δx, Δy, Δz) were higher for the lighter elements (fluorine, lithium, sodium, and potassium) than for scandium atoms for which the atomic coordinates were determined with a higher accuracy from XRD measurements. The structures of Li3ScF6, NaScF4, Na3ScF6, and KSc2F7 were determined from the single-crystal XRD refinement,4,5,38,47 and the atomic coordinates were very similar before and after geometry optimization with very small differences between the coordinates (