Solid Solutions CaF2âYF3 with Fluorite Structure ... - ACS Publications

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Solid Solutions CaF2−YF3 with Fluorite Structure Prepared on the Sol−Gel Route: Investigation by Multinuclear MAS NMR Spectroscopy Thoralf Krahl,†,‡ Gudrun Scholz,† and Erhard Kemnitz*,†,‡ †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Str. 2, D-12489 Berlin, Germany Nanofluor GmbH, Rudower Chaussee 29, D-12489 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: Nanoscaled Ca1−xYxF2+x (x = 0...0.40) was prepared on the new fluorolytic sol−gel route using organic precursors and anhydrous HF. These nonstoichiometric fluorite-type phases were investigated using 19F MAS and 19 F−89Y CP MAS NMR. Excess fluoride ions occur as point defects for x ≤ 0.01. For x > 0.01 excess fluoride ions start clustering. The 19F spectra of these phases can be fully explained by assuming a distribution of Ca2+ and Y3+ around fluoride ions. Y3+ is distributed within the crystal lattice of CaF2, with higher concentrations near fluoride clusters. Two different coordination polyhedra for Y3+ are observed. The correlation between 19F and 89Y signals was established. the resulting equilibrium phase adopts the cubic fluorite structure (space group 225 Fm3̅m).11,16,17 The cubic lattice parameter a increases with increasing x (from 5.46 Å for CaF2 to 5.53 Å for Ca0.62Y0.38F2.38). Calcium and yttrium ions share a regular lattice site. The excess charge is compensated by fluoride ions occupying interstitial positions. These positions are labeled F′ at (1/2, u, u) (48i site with u ≈ 0.37) and F″ at (v, v, v) (32f site with v ≈ 0.41). Additionally, the normal fluoride site Fn at (1/4, 1/4, 1/4) may not be fully occupied, and hence, regular anion vacancies can also occur. Most authors agree with the fact that the excess anions exist as point defects for small values of x and form clusters for larger x-values. The value of x, at which the anion defects start to form clusters, is given as around x ≈ 0.01. The nature of the clusters has been discussed for many years. It is now widely accepted that two types of clusters named (i) 8:12:1 (8 anionic vacancies, 12 F′ interstitials, and 1 F″ interstitial) and (ii) 1:0:3 are formed. Some authors deny the occurrence of F″ interstitials for case (i), resulting in the formation of clusters 4:6:0 or 8:12:0.17 However, it is commonly accepted that F′ interstitials (and thus 8:12:1 and similar clusters) mainly occur for small rare earth ions (Ho3+...Lu3+ and Y3+), F″ interstitials (and thus 1:0:3 clusters) mainly for large rare earth ions (La3+...Eu3+).12 Most of the crystalline nonstoichiometric phases studied in the literature were prepared by classic high-temperature synthesis starting from solid alkaline earth and rare earth fluorides.2 Nanoparticles of the same systems were prepared by

1. INTRODUCTION Nonstiochiometric inorganic fluoride phases containing rare earth elements have gained interest in the recent years.1−4 Especially luminescent properties of materials doped with rare earth ions are a recent field of research.5,6 This includes photon up- and -down conversion. Metal fluorides have some advantages over metal oxides as host materials. Their mainly lower phonon energies result in higher quantum yields. The most widely used host materials for these ions are LnF3 (Ln = Y, La, Gd, Yb) and hexagonal or cubic NaLnF4 (Ln = Y, Gd, Yb). Doping of these host materials with luminescent trivalent rare earth ions (e.g., Eu3+, Tb3+) delivers stoichiometric phases. Recently, also cubic divalent metal fluorides came into the focus of interest, especially the alkaline earth fluorides CaF2 and SrF2.7−10 Using them as host materials generates nonstoichiometric fluoride phases. Typical rare earth concentrations range from a few % up to 60% and more. In fact, such materials can be seen as nonstoichiometric compounds AE1−xRExF2+x (AE = Ca, Sr, Ba, Cd, Pb; RE = trivalent rare earth ion, 0 ≤ x ≤ 1), rather than as doped materials. This applies especially to the CaF2, SrF2, and also CdF2, while BaF2 and β-PbF2 have a higher tendency to form ordered stoichiometric phases.2 Numerous investigations have been published to investigate the short-range and long-range order of the defect structures of these nonstoichiometric materials, using mainly X-ray scattering, neutron scattering, EXAFS, and EPR.11−14 The paramagnetism of most rare earth ions prevents them from being investigated by NMR spectroscopy; however, there is one publication probing the anion defect structure of diamagnetic Ca1−xYxF2+x (x = 0.03...0.32) using 19F MAS NMR and, very recently, also of La1−xBaxF3−x.4,15 Especially the system CaF2−YF3 (Ca1−xYxF2+x) is interesting for NMR investigation due to its diamagnetism. For x ≤ 0.38, © 2014 American Chemical Society

Received: June 6, 2014 Revised: August 20, 2014 Published: August 21, 2014 21066

dx.doi.org/10.1021/jp505616f | J. Phys. Chem. C 2014, 118, 21066−21074

The Journal of Physical Chemistry C

Article

Table 1. List of the Investigated Samplesa

hydrothermal synthesis,7−9 high-energy ball milling,1,4 or a method called liquid−solid−solution strategy.10 During the last years, the fluorolytic sol−gel chemistry was established as a new, fast, and soft method for the synthesis of metal fluorides, using metal precursors and anhydrous HF.18 In this present publication we report for the first time on the preparation of solid solutions Ca1−xYxF2+x of the fluorite type over a composition range x = 0.0−0.40 using sol−gel chemistry. The diamagnetic character of this system makes it especially suitable for NMR investigations. 19F is a well-established nucleus in solid state NMR. Additionally, although 89Y is a lowγ nucleus, it has a nuclear spin of I = 1/2 and 100% natural abundance, which makes it also suitable as a structure probe. There is only one very recent publication about multinuclear NMR investigations on pure and Sc3+-doped (H3O)Y3F10· xH2O.19 To the best of our knowledge, 89Y solid state NMR was never applied to nonstiochiometric compounds like Ca1−xYxF2+x. Therefore, local structures were studied here using both 19F MAS NMR and 19F−89Y CP MAS NMR.

a

number

shortcut

nominal composition

1 2 3 4 5 6 7 8 9 10

CaF2 CaF2-s CaF2:Y00.1 CaF2:Y00.5-s CaF2:Y01-s CaF2:Y05-s CaF2:Y10-s CaF2:Y20-s CaF2:Y40-s YF3-s

CaF2 Ca0.999Sm0.001F2.001 Ca0.999Y0.001F2.001 Ca0.994Y0.005Sm0.001F2.006 Ca0.989Y0.01Sm0.001F2.011 Ca0.949Y0.05Sm0.001F2.051 Ca0.899Y0.1Sm0.001F2.101 Ca0.799Y0.2Sm0.001F2.201 Ca0.599Y0.4Sm0.001F2.401 Y0.998Sm0.002F3

Samples labeled with “-s” include 0.1% Sm3+.

frequencies of 376.5 MHz (19F) and 19.6 MHz (89Y), respectively. Two double resonance probes of Bruker were used: a 2.5 mm probe for 19F MAS NMR high speed experiments and a 7 mm low-γ probe for 19F−89Y CP measurements. All experiments were performed in ZrO2 rotors. The 19F MAS NMR spectra were recorded with a π/2 pulse duration of 3.3 μs, a spectrum width of 400 kHz and a recycle delay of 5 s. Up to 30 rotor periods (L0 = 30) were added before echo detection in rotor synchronized echo experiments. The MAS frequencies are directly given in the figures. The isotropic chemical shifts δiso of 19F resonances are given relative to the CFCl3 standard using α-AlF3 as a secondary standard (δ19F: −172.6 ppm). Existent background signals of 19F were suppressed with the application of a phase-cycled depth pulse sequence according to Cory et al.20 19 F−89Y CP MAS measurements were performed applying a rotation frequency of 3 kHz, a 19F π/2 pulse length of 10.5 μs, and a contact time of 12 ms. The number of accumulations was dependent on the sample and ranges from about 2000 up to 15 000 for the samples with the lowest content of 89Y. For the calibration of the 89Y spectra we used crystalline YF3 (Aldrich) as a secondary standard. We determined its 89Y chemical shift value of −108.7 ppm by 19F−89Y CP MAS measurements. We referenced this value by measuring against 1 M YCl3 solution in 80% H2O/20% D2O (δi = 0.0 ppm).

2. EXPERIMENTAL SECTION 2.1. Synthesis. All samples were synthesized using fluorolytic sol−gel chemistry. To shorten NMR T1 relaxation times, 0.1% of Sm3+ was added. For comparison, some samples were synthesized without samarium. Trifluoroacetates of Ca, Y, and Sm were synthesized by dissolving CaCO3, Y2O3, and Sm2O3, respectively, in aqueous 50% trifluoroacetic acid. The water free salts were obtained by evaporation of the solvent, followed by drying in vacuum at 150 °C. The salts are hygroscopic and were treated under argon gas. A water-free methanolic solution of HF was produced by dissolving gaseous HF in predried methanol under argon. Its concentration was determined by titration with 1.0 M NaOH vs phenolphthalein (the concentration is usually 20−25 mol/L). The typical synthesis procedure of CaF 2 , YF3 , and Ca1−xYxF2+x (x = 0.001−0.40) for 20 mmol: (20−20x) mmol of water free Ca(CF3COO)2 and 20x mmol of Y(CF3COO)3 in the desired molar ratio were dissolved in 100 mL of predried methanol. If desired, 20 μmol (0.1%) of Sm(CF3COO)3 was added. A stoichiometric amount (= (40 + 20x) mmol) of water free HF dissolved in methanol was added under vigorous stirring. A turbid solution containing gel particles was formed. It was stirred for 4 days. The gel particles dissolved during this time. An opaque colloidal solution of low viscosity was obtained. The solution was dried in vacuum resulting in a white powder. The powder was annealed in an open porcelain crucible at 400 °C for 2 h. During this treatment, the color of the powder changed to slightly gray. A list of the studied samples is given in Table 1 (samples labeled with “-s” include 0.1% Sm3+). To ensure that the small signals observed in 19F NMR are not due to impurities, synthesis of samples 1−5 was done twice. 19 F and 89Y NMR spectra were the same within experimental errors. 2.2. XRD. X-ray diffractograms were obtained with a Seiffert 3003TT diffractometer (Seiffert & Co. Freiberg, Germany, Bregg-Brentano geometry) using Cu Kα (1.542 Å) radiation. The 2ϑ step width was 0.05°. Peak maxima and widths were recalculated to Cu Kα1 (1.5406 Å) using Rachinger correction. The size of the coherent scattering region was calculated using the Debye−Scherrer equation. 2.3. NMR. MAS NMR spectra were recorded with a Bruker Avance 400 MHz spectrometer, operating at Larmor

3. RESULTS 3.1. XRD. Powder diffractograms of the selected samples are given in Figure 1. The diffractograms 1−9 (CaF2 and Y0.1− Y40) can be indexed as face centered cubic. With increasing Y content, the 2θ values are shifted to lower values, indicating an increase of the lattice parameter. Lattice parameters shift from 5.463 Å for 1 CaF2 to 5.533 Å for 9 CaF2:Y40-s (more data in Table S1, Supporting Information). This agrees with the values found previously.16 Note furthermore the increase in relative intensity of the (0 0 2) reflection at 32−33° with increasing Ycontent. The size of the coherent scattering region (i.e., crystallite size) according to the Debye−Scherrer equation is always between 10 and 20 nm. 3.2. NMR. 19F MAS NMR spectra and 19F−89Y MAS NMR of the Y-containing samples 3−10 are given in Figure 2. The 19 F NMR spectra of samples with no or very low Y-content 1 and 3−5 are shown magnified in addition in Figure 3. The 19F spectrum of 1 CaF2 shows one signal at −109 ppm. At very low Y-content (≤1%), additional very narrow sharp resonances occur. A signal of low intensity at ca. −95 ppm develops. To study the nature of these additional signals, rotorsynchronized 19F MAS NMR echo spectra were recorded 21067



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Figure 3. Detailed view of the 19F MAS NMR spectra from Figure 2 in the foot range with no or low Y-doping concentration.

(Figure 4). Applying dipolar evolution times between 1.2 and 1.6 ms leads to the distinct occurrence of small and sharp signals, whereas the signals at −95 and −109 ppm almost disappear. Figure 1. Powder diffractograms of selected samples. Reflection positions of pure CaF2 are marked with dashed lines. Note the increase of the (2 0 0) reflection at 32−33° with increasing Y-content. Refer to Table 1 for sample labels.

Figure 4. Detailed view of the 19F MAS rotor-synchronized echo-MAS NMR spectra of samples with no or low Y-doping concentration (rs echo, L0 = 30). MAS frequency (in kHz): (1) 19, (3) 20, (4) 19, (5) 25, and (6) 20.

With higher Y-contents (from 5% to 40%) also most of the narrow signals disappear in the echo spectra. The 19F signal at −95 ppm increases with increasing Y-content, and a new broad signal at ca. −75 ppm develops (see Figure 2). These 19F spectra were simulated using the program DMfit.21 The broad intense resonance at −70 ppm could only be simulated using two lines at ca. −70 and −80 ppm (see Table 2). The 19F spectrum of 10 YF3-s is different from the spectra of 1−9. Two signals at −57 and −68 ppm in the ratio 2:1 are seen, which is in accordance with the literature.22 The 19F spectra of 2 (CaF2-s = Ca0.999Sm0.001F2.001) resemble those of 3

Figure 2. MAS NMR spectra of the Y-containing samples. Left: 19 F−89Y CP (νrot = 3 kHz). Right: 19F (νrot = 25 kHz). 21068



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Table 2. Fit Results of the NMR Spectra of Samples 6−9a 19

sample

line 1

line 2

line 3

6 CaF2:Y05-s

−108.9 88.2% −109.5 62.6% −110.2 45.4% −111.9 11.2%

−93.5 8.8% −94.8 29.3% −95.8 42.0% −96.8 33.5%

−78.5 3.0% −79.8 3.4% −80.3 6.0% −86.5 8.9%

7 CaF2:Y10-s 8 CaF2:Y20-s 9 CaF2:Y40-s a

19

F−89Y CP MAS NMR

F MAS NMR line 4

−69.9 4.7% −69.7 6.6% −68.5 34.8%

add. signals

line 1

line 2

add. signals

−118.0 11.6%

−33.8 70.3% −34.7 70.7% −35.4 64.9% −40.6 46.9%

−56.0 29.7% −57.9 29.3% −60.3 35.1% −67.8 44.9%

−106.0 8.2%

Upper values: chemical shift in ppm, lower values: relative intensity.

Table 3. Used Parameter Sets of Ca2+ (from Reference 25) and Y3+ (This Work) for the Calculation of the 19F NMR Chemical Shift Applying the Superposition Modela

(CaF2:Y00.1 = Ca0.999Y0.001F2.001). They are given in Figure S1 (Supporting Information). The 89Y spectrum of the 3 CaF2:Y00.1 shows only one narrow signal at −33 ppm. At Y-contents of 1% and higher, this signal is shifted slightly to more negative parts per million values. With increasing Y-content, a second signal at ca. −50 to −60 ppm gains intensity. These two signals are the main features for the spectra from 5 CaF2:Y01-s to 9 CaF2:Y40-s. Two of the spectra show additional features: 5 CaF2:Y01-s shows a small narrow resonance at −132 ppm. For 9 CaF2:Y40s, a broad shoulder at −100 ppm is observed. The 89Y spectra 6−9 (5−40% Y-doping) were also simulated using DMfit.21 The spectrum of 10 YF3-s has one single narrow line at −108 ppm, which is in accordance with the literature.23 This spectrum is clearly different from the others.

atom 2+

Ca Y3+ a

dl0 (Å) (ppm)

σl0 (ppm)

2.976 1.240

2.366 2.281

−46.3 −74.9

δ0,iso(CCl3F) = −293.7 ppm.

The tetrahedral cavities are occupied by F− (lattice site 8c). The octahedral holes are empty. For the phases Ca1−xYxF2+x with low rare earth content, the structure model is quite simple: Y3+ substitutes Ca2+ on a regular lattice site 4a. The additional fluoride ions occupy different possible sites in the octahedral holes. These ions are statistically distributed over the solid and do not interact with each other. Hence, they are true point defects. 19 F MAS NMR Spectra. The 19F NMR chemical shift of these additional fluoride ions can be estimated using the superposition model. The ions are not situated in the center of an octahedron. Instead, three sites of different symmetry are possible, named as 24e (C4v symmetry), 32f (C3v symmetry), and 48i (C2v symmetry). Structure models for point defects located in Ca6 octahedra and the calculated chemical shifts depending on the position are given in Figure 5. Calculation details are given in the Supporting Information (Table S2). The exact lattice position of the point defects is not known, but the trend is quite clear. Their 19F chemical shift is expected somewhere between −90 and −150 ppm. This is in good agreement with the resonances between −120 and −150 ppm in spectra 3−5. The dephasing behavior of the spins of these fluoride ions has been checked by echo-MAS NMR experiments (see Figure 4). It was found that it is slower than that of regular fluoride ions Fn on 8c lattice positions, represented by narrow 19F signals at longer dipolar evolution times (L0 = 30). Appyling a rotation frequency of 25 kHz, L0 = 30 corresponds to a dipolar evolution time of 1.2 ms. This shows clearly that the excess fluoride ions have a different chemical environment from the regular lattice fluoride ions. For the phases 3−5, there occur also additional signals in the echo-NMR spectra between −50 and −90 ppm. They can also be assigned to point defects, which are not located in a Ca6 octahedral cavity but in a Ca5Y unit (which is nominally a distorted octahedron). Replacing one Ca2+ ion in the local environment of F− by Y3+ leads to a shift of the chemical shift toward more positive values around −20 to −40 ppm (according to the superposition model). All these narrow signals can unambiguously be assigned to point defects created by F− in octahedral cavities (Ca6 or

4. DISCUSSION 4.1. Superposition Model of 19F NMR Chemical Shifts. The 19F chemical shifts of inorganic salt-like fluorides can be empirically calculated using the superposition model.24 The chemical shift δiso is the difference of a constant δ0,iso and the shielding σ. σ is the sum of the influences of the n surrounding metal cations. l=1

δiso(d1...dn) = δ0,iso −

αl0 (Å−1)

∑ σl n

with σl = σl0 exp{−αl0(dl − dl0)} and δ0,iso(CCl3F) = −293.7 ppm

For each type of metal cation a parameter set (σl0, αl0, dl0) is given, and hence, the overall chemical shift only depends on the M−F distances dl. A parameter set for Ca2+ has been published already.25 Unfortunately there is no such set for Y3+. Therefore, we optimized the parameters for Y3+ from the 19F MAS NMR spectrum of YF3 in a similar way as described for the case of ZrF4 and CeF4, relating the measured NMR data to the known crystal structure of YF3.26,27 The optimized parameters are given in Table 3. Using these parameters, a chemical shift of −109 ppm is calculated for CaF2 and of −56 and −68 ppm for YF3. However, the data basis for the calculation of the parameters for Y3+ is quite limited. Therefore, the chemical shifts calculated from this parameter set should be interpreted with care and, hence, rather treated as estimations. 4.2. Point Defects: Phases with Low Y-Doping (up to 1%). The regular fluorite structure of CaF2 can be seen as a cubic closest packing of Ca2+ ions (space group 225 Fm3̅m). 21069



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(0.1−1% Y). It is very different from that of YF3 at −109 ppm, in which Y3+ has a coordination number of 9. At 1% Y-content a shoulder at around −50 ppm occurs, which is most probably the result of the beginning clustering of the defects (see next section). The nature of the signal at −132 ppm in the spectrum 5 CaF2:Y01-s is not clear. This signal can be observed in different lot numbers of the same nominal composition. According to the hypothesis of the authors this may be Y3+ on a regular lattice site with an additional fluoride ion in a neighboring octahedron and, thus, resulting in the coordination number of 9. 4.3. Defect Clusters: Phases with High Y-Doping (More than 1%). At higher doping concentrations, the structure of the solid solution cannot be treated as an ensemble of point defects any more. Instead, the excess fluoride ions start clustering, and now also vacancies on regular lattice fluoride sites 8c occur. Some authors find the additional fluoride ions on 32f and 48i positions,11 while others claim only the 48i positions to be occupied.17 However, it is accepted that the fluoride ions can only form clusters around unoccupied regular 8c positions. The most accepted cluster models for small rare earth ions (Ho3+−Lu3+ and Y3+) are named 8:12:0 and 8:12:1 (8 Fn vacancies, 12 F′ ions, 0 or 1 F″ ions). A structure model of such a 8:12:0 cluster is given in Figure 6. The metal ions located around these clusters have mainly the coordination number 8 (square antiprism (slightly distorted, local symmetry C4v), Figure 6B) or 10 (Figure 6C). The F′ fluorides in these clusters are formally coordinated by four metal ions (Ca2+ or Y3+) forming a strongly distorted tetrahedron.

Figure 5. Calculated 19F NMR chemical shifts for fluoride point defects in the octahedral cavities of CaF2 depending on the lattice position. (A) C4v: 24e site (1/2,1/2, x); (B) C3v: 32f site (x, x, x); and (C) C2v: 48i site (1/2, x, x). The red octahedron indicates the formal borders of the octahedral cavity.

Ca5Y). They show a slow spin dephasing behavior, which can be clearly seen by echo-MAS NMR. The broad signal at around −95 ppm, which increases with increasing Y-content, has the same fast spin dephasing behavior as the main signal. It is caused by fluoride Fn on a regular lattice site 8c. In pure CaF2 these fluoride ions are coordinated by four Ca2+ ions. In the doped samples, a small number of fluoride ions coordinated by three Ca2+ ions and one Y3+ ion exist. Using the superposition model, the chemical shift of such fluoride ions can be estimated to be ca. −87 ppm (see Table 4). Table 4. Calculated 19F Chemical Shift (in ppm) of Fluoride Ions in Tetrahedral Holes of Y-Doped CaF2a bond length (Å)

FCa4

2.366

−109

−87

−66

−45

−24

2.390

−121

−99

−77

−54

−32

FCa3Y FCa2Y2 FCaY3

FY4

comment regular metal− fluoride bond length in CaF2 bond length increased by 1% due to local relaxation effects

a

Note that an increase of the bond length by only 1% shifts the signals by 10−15 ppm.

Due to local relaxation effects, this signal may be shifted slightly to more negative parts per million values. Hence, the small shoulder between −90 and −100 ppm is caused by regular fluoride Fn having one Y3+ ion in the first coordination sphere. 89 Y NMR Spectra. The signal at −33 ppm in 3 CaF2:Y00.1 is caused by Y3+ occupying a regular metal lattice site 4a, being coordinated by eight fluoride ions forming a cubic coordination polyhedron. This signal can also be observed in the spectra 3−5

Figure 6. Structure view of the 8:12:0 cluster (8 vacancies, 12 atoms F′, 0 atoms F″). The 8:12:1 cluster has an additional F″ in the middle octahedron. (A) 8:12:0 cluster in an elementary cell. (B) and (C): Coordination polyhedra of the metal atom adjacent to a cluster (B) square antiprism, cn = 8, and (C) cube, in which one corner is substituted by a triangle, cn = 10. The amount (B):(C) in this structure is 4:1. 21070



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F NMR Spectra. The spin dephasing behavior of fluoride ions located in such clusters is much faster than that of true point defects discussed in the last section. Therefore, the narrow signals in the 19F NMR spectra 6−9 are not seen anymore. However, the rotor-synchronized echo spectrum of 6 CaF2:Y05-s still shows a small signal at −86 ppm. In this sample, a small amount of point defects still occurs besides fluoride clusters. The estimation of the 19F chemical shift of these clustered fluoride ions according to the superposition model is not as simple as in the case of point defects. The interstitial F− atom positions in the elementary cell, i.e., the long-range order, are known by neutron diffraction experiments (F″ 32f at (0.41, 0.41, 0.41), F′ 48i at (0.37, 0.37, 0.37)).11 However, local relaxation effects occur, changing the local bond lengths. A change of the bond length by only 1% results in a deviation of the calculated 19F chemical shift by 10−15 ppm (see Table 4 second row). Hence, the chemical shift can only be estimated with a large error bar. The majority of interstitial fluoride ions exists as F′ on the 48i site. Instead of exact calculations of the 19 F chemical shift, the following assumptions will be taken: (i) Local relaxation occurs in such a way that the chemical shift of F′ coordinated by four Ca2+ is nearly the same as that of regular Fn coordinated by four Ca2+; and (ii) each of the metal sites around the fluoride ion can be occupied by either Ca2+ or Y3+. Substitution of Ca2+ by Y3+ changes the 19F NMR chemical shift by 15−25 ppm toward more positive values. Each different coordination sphere (FCa4, FCa3Y, FCa2Y2, etc.) causes an extra signal in the 19F NMR spectrum. With this assumption, the relative intensity of the different observed signals should follow a discrete binomial distribution. The distribution for four atoms in dependence to the doping amount is given in Table 5. A comparison of these values with the relative intensities obtained by a fit (lines 1 to 4) in Table 2 shows a good agreement in first approximation. 19

With a closer look on the comparison between the signal intensities in spectra (Table 2) and the binomial distribution (Table 5), it can be seen that the 19F NMR signals of the units [FCaY3] are much larger than expected for a binomial distribution. The reason for that behavior is that the Y3+ ions are not fully statistically distributed through the solid but form Y-rich regions besides regions with a statistic distribution of Y3+. This interpretation is supported by the 89Y spectra and will be discussed in the last section of this chapter. Note the shift of all 19F signals of the same coordination sphere to lower parts per million values with increasing Ycontent (Table 2). This is caused by the increase of the lattice parameter with increasing Y-content. Local bond lengths around F− will also increase, resulting in a shielding of the fluoride ions and, hence, a shift to more negative parts per million values. These effects (binomial distribution of the coordination sphere and local relaxation) have been also observed in the mixed crystals Ca1−xSrxF2 and Ca1−xBaxF2.28,29 89 Y NMR Spectra. In 89Y NMR two signals are observed. The signal at −33 ppm is the main signal for low Y-content. With increasing Y-content, a second signal at ca. −55 ppm gains intensity, and both signals are shifted toward more negative parts per million values. At 40% Y-doping, the signals are situated at −42 and −67 ppm. The low-field signal (−33 to −42 ppm) is caused by Y3+ occupying a normal lattice site coordinated by eight fluoride ions, and the the coordination polyhedron is a cube [YF8]cub. This is essentially the same signal already observed in the case of point defects. The high-field signal is caused by Y3+ occupying positions near a fluoride cluster. The more Y3+ is doped into the lattice, the more clusters are formed and, hence, the more Y3+ that is situated near a cluster. The coordination number is also 8, but the coordination polyhedron is a slightly distorted square antiprism [YF8]sqap (see Figure 6). Additionally, some Y3+ might have the coordination number 10, but no separate signals are observed for them. This interpretation of the 89Y signals is strongly supported by the recently published results of 89Y MAS NMR investigations of nano-(H3O)Y3F10·xH2O.19 In the crystal structure there occurs one unique Y3+ ion, which is coordinated by eight fluoride ions forming a square antiprism [YF8]sqap. For the bulk atoms inside the particles, a 89Y NMR chemical shift of −54 to −56 ppm was observed. In the 89Y spectrum of 9 (40% Y-doping), an additional broad signal at ca. −118 ppm occurs. It correlates with the broad signal at ca. −69 ppm in the corresponding 19F spectrum. These signals are caused by Y-rich phases (either pure YF3 or YF3 doped with small amounts of Ca2+). Hence, this sample is not phase pure, although the XRD shows pure fluorite. The Yrich phases are amorphous, which also explains the broadness of the signals in the NMR spectrum. Correlation between 19F and 89Y NMR Signals. To show the correlation and connectivity between the signals in 19F and 89 Y NMR spectra, it would be ideal to measure a 2D 19F−89Y HETCOR MAS NMR spectrum of each sample CaF2:Y01− Y40 under the precondition that the CP conditions for all species are identical. However, such spectra could not be measured in a reasonable way. The 7 mm low-γ probe for 19 F−89Y CP measurements does not allow the high rotation frequency above 20 kHz needed for a suitable resolution of the 19 F axis. Therefore, a correlation is constructed manually relating the local structures of Y3+ and F− ions to each other.

Table 5. Expected Probabilities (in %) of Tetrahedrally Coordinated Fluoride Ions [FCa4−zYz] (z = 0, 1, 2, 3, 4) in Cubic Ca1−xYxF2+x (x = 0−0.04) Following a Binomial Distribution x

FCa4

FCa3Y

FCa2Y2

FCaY3

FY4

0.05 0.10 0.20 0.40

81.5 65.6 41.0 13.0

17.1 29.2 41.0 34.6

1.4 4.9 15.4 34.6

0.0 0.4 2.6 15.4

0.0 0.0 0.2 2.6

Hence, the interpretation of the 19F NMR spectra 6−8 (5 to 20% Y-content, Figure 2) is as follows: with increasing Ycontent, the signal at ca. −95 ppm (already observed in the case of point defects only) increases rapidly. This signal is caused by [FCa3Y]. Additional signals at ca. −80 and −70 ppm also gain intensity with increasing Y-content caused by [FCa2Y2] and [FCaY3]. For spectrum 9 (40%Y-content) in principle the same applies. The line at −69 ppm is far too intense, and an additional line at −118 ppm is necessary to fit the signal. Thus, this sample seems not to be phase-pure fluorite, but another phase is present. This phase is amorphous and cannot be seen in the XRD but in 19F and 89Y NMR (see also Discussion below). These findings are in accordance with the phase diagram of CaF2−YF3. The limit of solubility of YF3 in CaF2 is given as 38%.16 21071



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Figure 7. Proposed correlation between 19F and 89Y NMR signals. Local coordination structures are shown at the side. Signals derived by point defects have been omitted. Coordination polyhedra cub: cube, sqap: square antiprism. (*) One Y−F bond is significantly longer than the others. These structures should better be written as [FY2+1] and [YF8+1].

This effect is most prominent for signals caused by [FCa3Y] and [FCa2Y2] but negligible for [FCaY3]. Additionally, local relaxation effects must also be considered. They will occur with the greatest amplitude for [FCaY3] and will most probably reshift the 19F signal to more positive parts per million values. Correlation between Fluoride Defect Clustering and Distribution of Yttrium. When speaking of “clustering”, two different effects should be viewed. One effect is that the additional fluoride ions start to form anionic clusters. We cannot observe these clusters directly in 19 F MAS NMR spectroscopy, but we have clearly shown by rotor-synchronized echo-MAS NMR that there do not occur point defects anymore. Also we cannot distinguish between the proposed cluster models 8:12:0 and 8:12:1 (number of regular anion vacancies: number of F′ (48i):number of F″ (32f)). But from all work done by different groups in the last 40 years it is quite clear that fluoride defect clustering occurs. Assuming that all fluoride defects concentrate in one of these clusters, it can be calculated from a simple geometric model how much of the total fluoride is clustered and how much occurs on regular lattice sites (see Table 6 for results and Table S3 (Supporting Information) for details). When 40% Y-doping in CaF2 is reached, between 43 and 50% of the total fluoride is clustered. This might be one of the reasons higher amounts of doping do not form thermodynamically stable structures any more. From the 19F spectra it cannot be directly concluded whether the 8:12:0 or the 8:12:1 clusters are predominant. The effects of Y-doping on the spectra are so large that all other effects are suppressed. From the 89Y spectra there is evidence for the

This is straightforward for YF3. The crystal structure contains one unique Y atom (δiso( 89 Y): −108 ppm) and two crystallographically different F atoms in the ratio 2:1 (δiso(19F): −57 and −68 ppm).26 The Y atoms are linked to both F atoms, and hence, two spots in the correlation should occur. For Ca1−xYxF2+x (x = 0−0.40) there occur four distinct signals in the 19F NMR spectrum. They are caused by fluoride ions coordinated in a distorted tetrahedral fashion by four metal atoms (either on a regular lattice site or inside a defect cluster), and the local structures are, namely, [FCa4] (δiso(19F): −109 to −112 ppm), [FCa3Y] (δiso(19F): −93 to −97 ppm), [FCa2Y2] (δiso(19F): −78 to −87 ppm), and [FCaY3] (δiso(19F): −68 to −70 ppm). The first cluster does not contain yttrium, and hence, the signals at −109 to −112 ppm would not correlate with any 89Y signal. The other clusters contain an F−Y bond. This Y3+ ion is coordinated by eight F− ions. There occur two different coordination polyhedra of [YF8], the cube (δiso(89Y): −32 to −41 ppm) and the square antiprism (δiso(89Y): −51 to −68 ppm). As discussed in the section before, all these local structures are linked together, and hence, six spots will occur in a hypothetical HETCOR spectrum. The 19F−89Y correlation manually constructed in this way is shown in Figure 7. It is notable that the signals of 9 CaF2:Y40 are located in a marked distance from the signals of 5−8. The lattice parameter of CaF2:Y40 is significantly larger than that of the other fluorite-type samples. This causes a general increase of bond lengths in the solid, resulting in a better shielding of the F− ions in accordance with the superposition model,24 resulting in a shift to more negative parts per million values. 21072



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For the first time, we present a multinuclear investigation of these solid solutions using 19F and 89Y MAS NMR spectroscopy simultaneously. Our 19F NMR spectra resemble those already published,15 but our interpretation of the spectra is different. Two main structural features were determined. (i) For low Y-doping content (up to 1%), excess fluoride occurs as well as defined point defects. These fluoride ions can be directly observed performing rotor-synchronized 19F echoMAS NMR echo experiments. Even at 5% Y-doping such point defects were still observed. (ii) For high Y-doping content (from 5 to 40%), excess fluoride ions form mainly clusters. These clusters cannot directly be observed by 19F MAS NMR, but only indirect. The fast spin dephasing behavior of clustered fluoride is the same as that of regular fluoride. This has been clearly observed in rotorsynchronized echo-MAS NMR. The complex 19F NMR spectra of these samples can be nearly fully explained with a statistical distribution of Ca2+ and Y3+ ions in the lattice. The effect of Ydoping on the local environment of fluoride is so large that all other effects generated by defect clustering are completely overlapped. The distribution of Y3+ through the CaF2 lattice corresponds roughly to a binominal distribution. Deviations from this distribution occur, where Y3+ ions concentrate near the fluoride defect clusters. This could be doubtlessly shown by 89 Y MAS NMR. (iii) For the reasons explained in (ii), the exact nature of the clusters could not be determined. However, the absence of distinct signals below −100 ppm in the 89Y spectra suggests the absence of F″ and, hence, the absence of 8:12:1 clusters. Clusters of the type 8:12:0 (or 4:6:0) should dominate. For high doping content of 40% Y, small amounts of additional Y-rich phases occur. Investigation of these phases will be the subject of further studies.

Table 6. Expected Fractions (in %) of Disordered Fluoride in Ca1−xYxF2+x Assuming That All Defects Occur in WellDefined Clustersa [F′ + F″]/Ftotal x

8:12:0 cluster

8:12:1 cluster

0.05 0.1 0.2 0.4

7.3 14.3 27.3 50.0

6.3 12.4 23.6 43.3

Y in sqap coordination from 19 F−89Y CP NMR 29.7 29.3 35.1 44.9

a

Fraction of Y in square-antiprismatic coordination (from Table 2, not quantitative). See text for further explanation.

occurrence of mainly 8:12:0. In the case of 8:12:1 clusters, at least some Y3+ should have the coordination number 9. This should result in signals between −100 and −135 ppm, which are not observed. The second effect concerning “clustering” is the distribution of Y3+ in the CaF2 matrix. The ions Y3+ and Ca2+ share the same lattice site 4c. Hence, a real clustering is not possible here since the ions positions are fixed. But it is interesting to investigate whether the distribution of Y3+ is of purely statistical nature or not and how it is influenced by the fluoride defect clustering. As already discussed before, from the view of a fluoride ion the distribution of Y3+ roughly follows a statistic binomial distribution (see Table 2 and Table 5). A closer look reveals a local concentration of Y3+ around some fluoride ions, forming building units [FCaY3]. In the 89Y NMR spectra it can be clearly distinguished between Y3+ in a normal CaF2 environment ([YF8]cub δiso(89Y): −32 to −41 ppm) and Y3+ next to a fluoride defect cluster ([YF8]sqap δiso(89Y): −51 to −68 ppm). Integration of the spectra does not give quantitative information here because the magnetization transfer from 19F to 89Y does also not give quantitative information. Experiments with different CP contact times between 5 and 20 ms were performed. Such spectra had a similar relative intensity of both signals. We assume that the relative intensities of the signals show qualitatively the relation between Y3+ in a regular environment and Y3+ next to clusters (see Table 6). Comparing both effects the following can be concluded. At Y-concentrations of 5% and 10%, the fraction of Y3+ located next to clusters is larger than the fraction of fluoride in clusters. This can only be explained with the fact that the distribution of Y3+ through the solid is not purely statistic. The more positively charged Y3+ (compared to Ca2+) concentrates near the more negatively charged clusters (12 or 13 F− per elementary cell in a cluster compared to 8 F− per elementary cell in a regular environment, see also Figure 6). This correlates very well with the increased intensity of the signals of [FCaY3] units in the 19F MAS NMR spectra. At higher Y-concentrations (20% and 40%) the amount of Y3+ next to clusters only slightly increases, while the amount of fluoride in the cluster increases rapidly. This means that the distribution of Y3+ is only slightly influenced by the clustering of the fluoride defects.

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ASSOCIATED CONTENT

S Supporting Information *

19 F MAS NMR spectra of 2 CaF2-s, cubic lattice parameters extracted from XRD, calculation of the metal−fluoride distances needed for Figure 5, and crystallographic site occupation frequencies needed for cluster models. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 30 2093 7555. Fax: +49 30 2093 7277. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Sobolev, B. P.; Sviridov, I. A.; Fadeeva, V. I.; Sul’yanov, S. N.; Sorokin, N. I.; Zhmurova, Z. I.; Khodos, I. I.; Avilov, A. S.; Zaporozhets, M. A. Mechanochemical Synthesis of Nonstoichiometric Nanocrystals La1−yCayF3−y with a Tysonite Structure and Nanoceramic Materials from CaF2 and LaF3 Crystals. Crystallogr. Rep. 2008, 53, 868−880. (2) Sobolev, B. P. Nonstoichiometry in Inorganic Fluorides: I. Nonstoichiometry in MFm-RFn (m < n ≤ 4) Systems. Crystallogr. Rep. 2012, 57, 434−454. (3) Sidorov, A. A.; Popov, P. A.; Aksenov, S. V.; Begunov, A. I.; Fedorov, P. P. Thermal Expansion of Solid Solutions Based on Calcium and Barium Fluorides. Inorg. Mater. 2013, 49, 525−527.

5. CONCLUSIONS We were successful in synthesizing solid solutions Ca1−xYxF2+x (x = 0.001−0.4) as well as CaF2 and YF3 using fluorolytic sol− gel chemistry. The synthesis route is fast and easy and avoids the use of large temperature sintering or high-energy ball milling. The obtained xerogels are nanocrystalline, and the crystallite size is between 10 and 20 nm. 21073



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