Short and Medium Range Order in Photothermal Refractive Glass

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Short and Medium Range Order in Photothermal Refractive Glass Revealed by Solid-State NMR Techniques Lena Marie Funke, Oliver Janka, Rainer Pöttgen, Leonid Glebov, Michael Ryan Hansen, and Hellmut Eckert J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02143 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry

1

Short and Medium Range Order in Photothermal Refractive Glass Revealed by Solid-State NMR Techniques

Lena Marie Funke,1 Oliver Janka,2 Rainer Pöttgen,2 Leonid Glebov,3 Michael Ryan Hansen1*, and Hellmut Eckert1,4*

1Institut

2Institut

für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany

für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany

3Department 4Instituto

of Chemistry, University of Central Florida, Orlando, FL, USA

de Física São Carlos, Universidade de São Paulo, CEP 369, São Carlos, SP 13566590 Brazil

ABSTRACT Photo-thermo-refractive (PTR) glass is an optically transparent photosensitive Na2OZnO-Al2O3-SiO2 glass, containing NaF and KBr additives, along with cerium, silver, tin, and antimony oxide dopants. After heating above 500°C, UV-exposed regions of this glass produce permanent refractive index changes, resulting from precipitation of NaF nanocrystals. Shortand medium-range order of this glass system is studied via multinuclear single- and doubleresonance solid-state NMR spectroscopy in regular PTR glass and in model glasses with simplified compositions. The results, when combined with data from energy dispersive X-ray spectroscopy (EDX), indicate that the NaF component modifies the standard aluminosilicate framework, producing small amounts of F-bonded five- and six-coordinated aluminum species. The fluoride speciation is obtained from

19F

magic-angle spinning (MAS) NMR spectra,

supported by 19F{27Al} and 19F{23Na} dipolar recoupling experiments. The majority of fluoride within the PTR glass is found within Na-dominated local environments, which also interact strongly with the aluminum. 23Na{19F} rotational echo double resonance (REDOR) reveal that about 1/3 of the Na+ ions have fluoride ions in their first coordination spheres.

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INTRODUCTION Photo-thermo-refractive (PTR) glass is a photosensitive silicate glass with the chemical composition 15Na2O - 4Al2O3 -5ZnO - 70SiO2 - 5NaF - 1KBr, plus trace amounts (~0.01 mol%) of Ce, Ag, Sb and Sn. Its optical properties can be modified in a controlled fashion by UVexposure and subsequent annealing above the glass transition temperature, Tg.1 The photothermo-induced refractive index decrement was explained by precipitation of NaF crystals and the presence of this crystalline phase in treated glasses was confirmed by X-ray diffraction2 and NMR3 measurements. By applying holographic techniques, optical elements with different spatial profiles of refractive index can be created, including volume Bragg gratings,4 phase masks,5 and complex holograms, e.g. lenses.6 Such holographic elements are widely used for narrow band optical filtering7 and laser design.8 While many experimental studies have been devoted to elucidating the mechanism of NaF crystallization in this glass system, e.g. a survey9 a fundamental structural investigation of the base glass has so far not been carried out. This is understandable in view of the glasses’ complex chemical composition, which can make it rather difficult to relate spectroscopic results to structural information. In the present work, we address this problem in two different ways: (a) by preparing model glasses based on simplified compositions through removal of certain glass constituents and tracking the resulting spectroscopic changes and (b) by exploiting the potential of advanced solid-state nuclear magnetic resonance (NMR) methodology. While conventional magic-angle spinning (MAS) is well suited for quantifying short-range order environments, information on the medium-range order can be obtained by combining MAS with specifically designed dipolar recoupling techniques.10 Using advanced techniques, such as rotational echo double resonance (REDOR) and related methods, new quantitative information is available about network former bonds and the spatial distributions of individual glass components.11

EXPERIMENTAL Sample Preparation and Characterization. The precise batch compositions are given in Table 1. The PTR glasses were melted in a platinum crucible at 1450°C for 120 min. The glass melts were subsequently cooled to room temperature at a rate of 0.1 K/s. While the samples B and C have a typical PTR glass composition, the glass matrix A was synthesized only with the main glass components without adding Ce2O3, Ag2O, Sb2O3, and SnO2. Samples A and B were not exposed to UV radiation; sample C was subjected to radiation of a He-Cd ACS Paragon Plus Environment

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laser at 325 nm with dosage of 10 J/cm2. Samples A, B, and C are identical to the samples B1, B5 and B6 studied in the work of Magon et al.12 X-ray powder diffraction confirmed that all the samples were amorphous, as expected. Samples were stored in a non-transparent container to avoid exposure to ambient laboratory lighting. PTR Model Systems. To facilitate an interpretation of the spectroscopic observations made in this rather complex system, we have also studied model glasses in which the composition was modified by omitting or adding selected components (samples M1-M6). For the synthesis of different model compositions, the batch components Na2CO3 (Acros, 99.95 %), SiO2 (Fluka, purum p.a.), ZnO (ABCR, 99.7%), Al2O3 (Merck,), NaF (Merck) and KBr (Merck, 99.5%) were pre-dried for 48 hours at 120°C. The compounds were carefully homogenized in a mortar and melted for 90 min at 1450°C after degassing for 30 min at 800°C. A lid on the crucible was used to minimize evaporation losses during the high-temperature treatment. The glass melts were quenched by placing the crucible onto a copper plate kept at ambient temperature. The glass samples were all transparent and the mass losses were less than 3 %. A sample of crystalline nepheline, NaAlSiO4, used as a standard for calibrating the

27Al{29Si}

REDOR experiment, was prepared according to reference.13 Details can be found in the Supporting Materials Section. Energy Dispersive X-ray (EDX) Spectroscopy. Semiquantitative EDX analyses on all bulk samples were carried out on a Zeiss Evo MA 10 scanning electron microscope, equipped with an Oxford Instruments EDX detector. The glass pieces were mounted on Al stubs using non-conductive tape and investigated in the variable pressure (VP) mode of the instrument under 60 Pa N2. For each sample, four spot investigations and one area scan were conducted. The compositions of each measurement were in good agreement and therefore averaged. The experimental error of these EDX measurements is ±1 at.%, for elements lighter than sodium even higher (±2 at.%), as listed in Table 2. Table 1: Batch compositions of the PTR glasses A, B, C and of the model glasses M1-M6. A PTR glass matrix 71.1SiO2-13.0Na2O-6.4NaF-5.1ZnO2.9Al2O3-1.5KBr B,C PTR glass 71.1SiO2-13.07Na2O-6.4NaF-5.1ZnO2.9Al2O3-1.5KBr-0.062Ag2O-0.007CeO20.021Sb2O3-0.008SnO2 M1 PTR glass matrix 70SiO2-15Na2O-5NaF-5ZnO-4Al2O3-1KBr M2 PTR glass matrix – ZnO 70SiO2-15Na2O-5NaF-4Al2O3-1KBr M3 PTR glass matrix – ZnO + Na2O 70SiO2-20Na2O-5NaF-4Al2O3-1KBr M4 PTR glass matrix – Al2O3 70SiO2-15Na2O-5NaF-5ZnO-1KBr M5 PTR glass matrix – Al2O3 – Na2O 70SiO2-11Na2O-5NaF-5ZnO-1KBr M6 PTR glass matrix – NaF 70SiO2-15Na2O-5ZnO-4Al2O3-1KBr



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Solid State NMR. All NMR spectroscopic measurements were carried out on Bruker Avance III 300 MHz, Avance DSX 400 MHz, DSX 500 MHz spectrometers, and a Bruker DSX 500 spectrometer interfaced with a 4.7 T magnet. Additional

19F

MAS NMR spectra were

measured on an Agilent DD2 600 MHz console interfaced with a 5.7 T magnet. To minimize the effect from radio-frequency inhomogeneities, all NMR experiments involving quadrupolar nuclei were conducted with rotors that were only filled in the middle 1/3 of their total volumes. The 29Si MAS NMR spectroscopic and saturation recovery experiments were conducted in a 9.4 T magnet using  excitation pulses of 4.6 µs and a MAS rotation frequency of νR = 8.0 kHz. To assure full relaxation the recycle delays for glass samples A, C, and M1-M6 were set to 21350 s. The spectrum of the sample B was measured at a recycle delay of 4600 s. 23Na MAS NMR measurements were performed at 11.7 T and a MAS rotation frequency of νR = 14.0 kHz. Pulse lengths of 0.75 μs (22.5° flip angle) and recycle delays of 1.0 s were used and pulse length of 3.0 μs (90° flip angle) and recycle delays of 50.0 s were also tested.

23Na

triple-

quantum (TQ)MAS NMR experiments, were performed at 9.4 T using the three-pulse z-filtered sequence14 and a MAS rotation frequency of 12.5 kHz. Typical pulse lengths were 4.3 μs, and 1.5 μs for the hard excitation and reconversion pulses, respectively. The length of the soft detection pulse was 10 μs and a recycle delay of 1.0 s was used. 27Al single pulse experiments were conducted at 11.7 T with 0.5 µs pulses (flip angles < 22.5°) at a MAS rotation frequency of 13.0 kHz and a recycle delay of 0.8 s. 27Al TQMAS experiments were performed at 11.7 T and νR =14.0 kHz with typical pulse lengths for the of 1.3 µs and 3.5-3.7 µs for the hard excitation and reconversion pulses, and a 10 µs soft detection pulse. The recycle delay was set to 0.5 s. The isotropic chemical shift iso and the Second Order Quadrupolar Effect (SOQE) = CQ(1 + Q2/3)1/2 (CQ and Q representing the nuclear electric quadrupolar coupling constant and the electric field gradient asymmetry parameter) were obtained from the centers of gravity in the F1 and F2 dimensions of all TQMAS spectra as previously described.15

19F

MAS NMR

spectra were measured at low magnetic fields (B0 = 4.7 T and 5.7 T) and high spinning speed νR = 25.0-27.0 kHz and 35.0 kHz, using 2.5 and 1.6 mm rotors, respectively. All measurements were performed using an NMR probe with a low fluorine background. However, as the fluorine concentrations in the PTR glasses are relatively low, additional efforts for probe background suppression turned out to be essential. Using the Elimination of Artifacts in NMR spectroscopy (EASY) sequence16 or a rotor synchronous Hahn echo turned out to be more effective than π/2 pulse excitation and subtraction of the empty probe background signal. The EASY sequence was tested with different pre-excitation delays. The spin-spin relaxation times, T2 were checked ACS Paragon Plus Environment

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with a rotor-synchronous echo decay to test the reliability of the Hahn echo sequence as a quantification approach. With no changes of the spectral line shape at different inter-pulse delays T > 2TR (equal spin-spin relaxation times for different signal components) rotorsynchronous Hahn spin echo experiments turned out to be the method of choice for quantitative distinction of chemically different fluorine species despite low fluorine concentrations and long spin-lattice relaxation times.

29Si, 23Na, 27Al

and

19F

chemical shifts are referenced to TMS,

aqueous solutions of NaCl (1M) and Al(NO3)3 (1M), and CFCl3, respectively, using Tris(trimethylsilyl)silane (TTMS, δ(29Si) = -9.75 ppm) and solid aluminum fluoride (δ(19F) = 172 ppm) as secondary standards. The Fourier transformed and phased NMR signals were analyzed using the DMfit 2015 and 2011 program packages.17 23Na and 27Al MAS NMR spectra were fitted assuming a Czjzekbased distribution of quadrupolar interaction.18 The specific measurement conditions selected for the various heteronuclear X-Y double resonance experiments are summarized in Table 2. The REDOR experiments used the standard sequence of Gullion and Schaefer,19 employing XY-4 phasecycling20 of the  pulses on the recoupling channel. The 29Si π pulse lengths for 27Al{29Si}

REDOR experiments were initially optimized using a

29Si-enriched

NaAlSiO4

(nepheline) glass sample and then subsequently fine-tuned by maximizing the measurable REDOR effect for the studied samples. All double-resonance NMR experiments with 19F on the observed channel were conducted with a saturation comb preceding the REDOR protocol, ensuring reproducible initial conditions. For the 19F{27Al} rotational echo adiabatic passage double resonance (REAPDOR) experiments, an adiabatic pulse lasting for 1/3 of the rotor period was used in the middle of the evolution period.21 The latter serves to recouple the 19F27Al

magnetic dipole-dipole interaction by adiabatic mixing of 27Al spin states during the rotor

period. To extract dipolar coupling information from the REDOR and REAPDOR curves simulations were carried out using the SIMPSON 4.1.1 program.22 In the case of 27Al{29Si} and 23Na{19F}

REDOR, simple two-spin simulations were performed, whereas in the case of

19F{27Al}

REAPDOR, the two-spin simulation used needs to consider all the experimental

conditions B0, νR, and the quadrupolar coupling parameters of

27Al

as extracted from

27Al

TQMAS NMR. In these simulations, the effects of the 19F chemical shift anisotropy and the asymmetry parameter η(19F) on the shape of the REAPDOR curve were examined as well. The 19F{23Na} REDOR curves, which likely reflect multi-spin interactions, were analyzed by fitting the initial data range (S/S0 ≤ 0.3) to a parabolic function,23 ACS Paragon Plus Environment


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∆𝑆 𝑆0

4

= 3𝜋2(𝑁𝑇R)2𝑓𝑀2(𝑆 - 𝐼)

(1)

in terms of dipolar second moments fM2. Here NTR, the number of rotor cycles times the rotor period defines the dipolar mixing time, and f is a calibration factor determined by a measurement on the crystalline model compound Na2PO3F. A detailed analysis of the REDOR curves obtained on this compound has been recently given.24 The theoretical second moment M2(S-I)theor is computed from the van Vleck equation,25 4 𝜇0 2

( ) 𝐼(𝐼 + 1)𝛾 𝛾 ℏ ∑

𝑀2(𝑆 - 𝐼) = 15

4𝜋

1 2 2 2 . 𝐼 𝑆 𝐼𝑟6𝑆𝐼

(2)

1

based on all the S....I inter-nuclear distances in the crystal structure. The lattice sum ∑𝐼𝑟6 usually 𝑆𝐼

converges at four times the shortest inter-nuclear distance. Table 2: Measurement conditions for the double-resonance REDOR and REAPDOR experiments. Experiment 27Al{19F}

REDOR 19F{27Al} REAPDOR 23Na{19F} REDOR 19F{23Na} REDOR 27Al{29Si} REDOR

B0 / T

R / kHz

7.05 11.7 7.05 11.7 11.7

14.0 26.7 14.0 27.0 10.0

 pulse length / µs (observed nucleus) 4.3 3.4-5.2 4.9 3.5 10.9

 pulse length / µs (non-observed nucleus) 4.7 1/3 TR 4.7 2.6 19.8

RESULTS AND DISCUSSION Sample Compositions. Table 3 compares the nominal chemical compositions (in atomic %) with results from a chemical analysis data reported earlier12 and new results obtained on samples A, B, and the model glasses M1-M6 by energy-dispersive X-ray (EDX) spectroscopy. For the model glasses the results show some deviations from the expected values. The Na contents tend to be lower than batched in the majority of the samples, while the Si and F contents appear elevated. Elevated F contents are not plausible in view of the well-known volatility of fluoride in glass melts, however, it is well-known that EDX cannot be considered truly quantitative for elements lighter than Na. For this reason, no use will be made here of the F contents obtained by EDX analysis and our conclusions will be discussed based on “upperlimit” F contents as batched (most likely the F contents are lower). The X-ray fluorescence


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results obtained from the PTR glasses A and B differ significantly in Si content, which might be attributed to surface inhomogeneities within the investigated specimens. Table 3: Atomic percentages of the elements as calculated from the batch compositions of the glass samples (Table 1) compared to those measured by Energy Dispersive X-Ray Spectroscopy. Errors are given in parentheses. O* 57.76 57(2) 60(2) 57.91

F* 2.17 3(2) 4(2) 1.68

Na 11.04 10(1) 10(1) 11.78

Al 2.00 2(1) 1(1) 2.69

Si 24.28 26(1) 22(1) 23.57

K 0.50 1(1) 1(1) 0.34

Zn 1.74 2(1) 1(1) 1.68

Br 0.50 1(1) 1(1) 0.34

Ag 0.004 -

Ce 0.003 -

Sb 0.014 -

Sn 0.003 -

expected

58(2) 58.19

2(2) 1.74

10(1) 12.20

2(1) 2.79

25(1) 24.39

1(1) 0.35

1(1) -

1(1) 0.35

-

-

-

-

M2 expected M3 expected M4 expected M5 expected M6

58(2) 56.95 57(2) 57.76 59(2) 58.87 58(2) 59.93 57(2)

2(2) 1.66 2(2) 1.81 1(2) 1.89 1(2) -

10(1) 14.90 13(1) 12.64 12(1) 10.19 11(1) 10.45 11(1)

2(1) 2.65 3(1) 2.79 2(1)

26(1) 23.18 23(1) 25.27 26(1) 26.42 27(1) 24.39 27(1)

1(1) 0.33 1(1) 0.36 1(1) 0.38 1(1) 0.35 1(1)

1.81 2(1) 1.89 2(1) 1.74 2(1)

1(1) 0.33 1(1) 0.36 0.2 0.38 1(1) 0.35 1(1)

-

-

-

-

expected A B expected M1

*not

quantitative for elements lighter than Na. 29Si

MAS NMR. Figure 1 shows the 29Si MAS NMR spectra of the glass matrix A, the

doped PTR glass B, and the PTR glass after UV irradiation C, as well as the model glass samples M1-M6 described in Table 1. The spectra are poorly resolved and asymmetrically broadened towards higher frequencies. While in principle these spectra can be described by many fitting models, we note that the chemical shift region of the observed 29Si signal corresponds to that of the three species that can be considered most probable in silicate glasses with the given chemical composition, namely Si(4)0Al, Si(4)1Al and Si(3)0Al units.26-29 Based on this observation we analyze the

29Si

MAS NMR spectra in terms of these three components. Each Si(m)nAl

component is represented by a Gaussian line shape reflecting a distribution of isotropic

29Si

chemical shifts. Table 4 summarizes the results of a constrained deconvolution, keeping the 29Si signal positions and linewidths of the three components fixed to the maximum extent possible, allowing only adjustments to these parameters if a constrained fit did not succeed. Some alternative fitting variants leading to similar results within experimental error are shown in the Supporting Materials Section (Figure S2). For the PTR matrix A the Si(n) distribution is 32% Si(4)0Al, 34% Si(4)1Al and 34 % Si(3)0Al. For the model glass M1 the distribution (30% Si(4)0Al, 28% Si(4)1Al and 42 % Si(3)0Al) is the same within experimental error as may be expected from the very similar elemental contents shown in Table 4. Table 4 also compares the ACS Paragon Plus Environment

29Si


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deconvolution parameters with a simplified structural scenario based on the following three assumptions: (1) Al2O3 forms {AlO4/2}- units, which require an equivalent amount on network modifier for charge compensation, (2) an amount of modifier equivalent to the F content is needed to charge compensate these anions, (3) the remaining Na2O and ZnO act as network modifiers so that each O2- transforms two Si(4) units into two Si(3) units and decreases the number of bridging oxygen atoms, according to the traditional network modification model.26 More complicated aspects of the structure such as, Si(3)mAl (m > 0) and Si(n)mAl (m < 2) units are neglected here; they are considered electrostatically unfavorable as they imply spatial proximity of negatively charged {AlO4/2-} groups and negatively charged Si(3) groups. Furthermore, we neglect Si(4)mAl (m ≥ 2) groups due to the relatively low Al content, assuming the absence of aluminosilicate clustering. Considering the simplifications made by this model and the uncertainties in the chemical analysis, the agreement can be considered satisfactory. The isotropic 29Si chemical shifts δiso (see Table 4) are in good agreement with former results on sodium silicate and sodium aluminosilicate glasses.26-29 The binary distribution of Si(3) and Si(4) observed in the Al-free glass samples M4 and M5 indicates maximum dispersion of the charge balancing Na and Zn cations in the aluminosilicate network. Comparison of the 29Si

MAS NMR spectra for glass samples A-C indicates that the local structure remains

unaffected by doping and photonic treatment. To test the influence of the components Al2O3, ZnO and NaF on the aluminosilicate glasses’ network structure, 29Si MAS NMR spectra of the model glasses were recorded. The spectra of glasses M1, M2, M3, and M6 can be fitted with the same parameters as discussed above. Glass M1, with a very similar nominal composition as the glasses A-C, should be built up from the same building units, as found experimentally. The presence or absence of NaF (glass M6) has no influence on the connectivity of the aluminosilicate network (cf. Figure 1). Likewise, the substitution of ZnO by Na2O (glass M3), has no effect on the distribution of units in the aluminosilicate network, supporting the role of ZnO as a network modifier. No “Zn-specific” effect upon the

29Si

chemical shift is seen in

agreement with the literature, i.e. the spectra are affected only by the total network modifier to network former ratio.27,28 Glass M2 has the same composition as the glass matrix A, however, without any ZnO. As ZnO is not substituted by an equivalent amount of Na2O, the expected amount of Si(3)0Al units is lower in this case, which is consistent with the experimental result. The fraction of Si(4)1Al is approximately constant in all the Al-containing model glasses, supporting a homogenous distribution of {AlO4/2-} units in the silicate network. The 29Si MAS NMR spectra of glasses M4 and M5, which both do not contain Al2O3, can be fitted by two


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components only, with the Si(4)1Al component near -99.0 ppm being absent. The two-component fit is consistent with ZnO and Na2O acting exclusively as network modifiers.

Figure 1: 29Si MAS NMR of the PTR model glasses M1-M6 and glasses A, B, and C recorded at a spinning frequency νR = 8.0 kHz and B0 = 9.4 T. Table 3 summarizes the deconvolution parameters for all samples. Table 4: Deconvolution results from 29Si MAS NMR spectra, 29Si Gaussian line shape parameters for the glasses A, B, C and model glasses M1-M6.*

Si(4)0Al

Si(4)1Al

FWHM ±2/ ppm 11.5

Area fraction ± 4/% 32 [44]

δiso ± 0.5 / ppm

A

δiso ± 0.5 / ppm -106.8

B

-106.0

11.5

C

-106.0

M1

Si(3)0Al

-98.7

FWHM ±2/ ppm 12.0

Area fraction ± 4/% 34 [23]

δiso ± 0.5 / ppm -91.6

FWHM ±2/ ppm 12.0

Area fraction ± 4 /% 34 [33]

25 [42]

-99.0

12.0

30 [24]

-91.4

12.0

45 [34]

11.5

27 [42]

-99.0

12.0

31 [24]

-91.4

12.0

42 (34]

-106.0

11.5

30 [30]

-99.0

12.0

28 [36]

-91.4

12.0

42 [34]

M2

-106.0

11.5

30 [42]

-99.1

12.0

31[37]

-91.4

12.0

39 [21]

M3

-106.0

11.5

27 [10]

-99.0

12.0

32 [59]

-91.4

12.0

42 [31]

M4

-105.0

12.9

48 [43]

0 [0]

-91.5

11.5

52 [57]

M5

-105.0

12.9

52 [49]

0 [0]

-91.5

10.5

48 [51]

M6

-106.0

11.5

24 [20]

31 [32]

-91.4

12.0

45[48]

-99.0

12.0

*Values given in brackets represent expected values for the scenario described in the text based on the results from the EDX analyses.

27Al

MAS,

27Al

TQMAS,

27Al{29Si}

REDOR and

27Al{19F}

REDOR. Figure 2

summarizes the 27Al MAS NMR spectra obtained for all glasses. All 27Al MAS NMR spectra (with the exception of the one measured on glass M6) include three distinct ACS Paragon Plus Environment

27Al

signal


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components, which are asymmetrically broadened towards lower frequency as is typically observed in MAS NMR spectra of the central transition for many half-integer quadrupolar nuclei in glasses.17 The specific line shapes are characteristic for broad electric-field gradient distributions caused by local variation of the coordination environments. Average isotropic chemical shifts and SOQE values as extracted from 27Al TQMAS NMR (Figure S3) are listed in Table 5. Relative area fractions have been obtained using the DMFIT software assuming a Czjzek distribution.18 The three 27Al signals at isotropic chemical shifts near 60, 27, and 0 ppm are assigned to four-, five- and six-coordinated aluminum species, Al(4), Al(5), and Al(6), respectively. While Al(5) and Al(6) are usually not observed in sodium aluminosilicate glasses with Na2O/Al2O3 ratios larger than unity, the introduction of fluoride is known to increase the coordination number of Al in such systems.30-32 The formation of higher coordinated Al is attributed to the strong affinity of aluminum towards fluoride ions. On this basis, we attribute the signals near 27 and zero ppm to {AlO4/2F2-} and {AlO4/2F23-} entities. This conclusion is supported by the spectrum of the NaF-free model glass M6, in which these higher-coordinated Al species are almost absent (only a small amount of five-coordinate aluminum species, attributed to {AlO5/22} units is seen). Our conclusions are further strongly confirmed by the

27Al{19F}

REDOR

experiments summarized in Figure 3c and d. Four-coordinated Al only shows a very weak REDOR effect (arising from long-range 27Al-19F interactions), while the Al(5) and Al(6) signals are strongly affected by nearby 19F nuclei. The convergence of the Al(5){19F} REDOR curve to a normalized difference signal of ΔS/S0 smaller than unity suggests the presence of some {AlO5/22-} units as both 19F and 27Al isotopes have 100% natural abundance. The formation of such species can indeed be promoted by the presence of doubly charged ions (such as Zn2+ in the present case, see also Figure 2). Overall, in the PTR glasses no more than 15 % of Al are coordinated to fluorine. The dominant Al(4) species are assumed to be linked to Si atoms via Al-O-Si bridges. Their average number can be estimated with the help of a 27Al{29Si} REDOR experiment, see Figure 3b. Note that the dephasing effect is indistinguishable from that of a crystalline nepheline sample, which includes four Si-O-Al linkages at an Si…Al distance of 3.2 Å. The natural abundance of 29Si of 4.67 % implies that in glass C only ~ 4 × 4.67% = 18.8% of the aluminum species produce a 27Al{29Si} REDOR difference signal from a two-spin interaction, whereas the remaining 81.2% are not dipolarly coupled. The corresponding simulation is shown in Figure 3b. The small deviations between the simulation and the experimental result obtained for nepheline likely arises from experimental imperfections.


Page 11 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11

Figure 2: 27Al MAS NMR of the PTR glasses and the model glasses recorded at a spinning frequency νR = 13.0 kHz, B0 = 11.7 T. Deconvolution parameters for all glass samples are summarized in Table 5. Table 5: 27Al MAS NMR deconvolution and 27Al quadrupolar coupling parameters for the glasses A, B, C and model glasses M1-M6. Al(4) Al(5) Al(6) Sam-

δiso /

ple

ppma,b

A

3.1 (4.6) 3.2 (4.6) 3.0 (4.1) 4.5

85

M1

60.2 (60.2) 60.5 (60.2) 60.3 (60.5) 59.6

M2

60.3

M3 M6

B C

δiso /

SOQE/

Area

δiso /

SOQE/

Area

ppma,b

MHza,b

fract.c/%

ppma,b

MHza,b

fract.c/ %

2.8 (4.6) 2.9 (4.6) 3.0 (4.7) 4.6

7

6

0.8 (1.9) 0.9 (1.9) 0.3 (1.7) 0.2

1.9 (3.3) 2.2 (3.3) 2.1 (2.8) 3.0

8

92

26.5 (27.4) 26.7 (27.4) 26.0 (28.7) 28.0

4.5

92

27.6

4.6

6

0.2

3.0

2

61.0

4.6

91

27.7

4.6

6

0.6

3.0

3

61.1

4.6

99

27.6

4.6

1

-

-

-

adetermined

Czjzek

SOQE

Area

/MHza,b fract.c/ %

85 85

8 8

7 7 2

from 27Al TQMAS NMR, SOQE ± 0.1 MHz, δiso ± 0.4 ppm, bvalues in parentheses are derived from CQ ± 0.3 MHz , δiso ± 0.4 ppm carea fraction ± 2 %,

fits,18



12

(a)

(b)

(c)

(d)

Figure 3: (a) 27Al MAS NMR spectra of the PTR glass matrix A, doped PTR glass B and the doped and UV exposed PTR glass C (plotted on top of each other) and the PTR glass matrix without NaF (glass M6); spinning frequency νR = 13.0 kHz, B0 = 11.7 T. (b) 27Al{29Si} REDOR curves (plots of the normalized dipolar recoupled signal intensity difference S/S0 versus the dipolar evolution time NTR, νR = 10.0 kHz, B0 = 11.7 T) of crystalline NaAlSiO4 (red triangles), Al(4) in PTR glass sample C (black squares). Dotted curves correspond to a two-spin simulation based on an Al….Si distance of 3.2 Å as described in the text. (c) 27Al rotor-synchronous spin-echo spectrum S0 and 27Al{19F} REDOR difference spectrum ΔS at NTR = 0.57 ms and B0 = 7.05 T. (d) Site-selective 27Al{19F} REDOR curves for sample B. 19F 19F

MAS NMR, 19F{27Al} and 19F{23Na} REDOR experiments. Figure 4 shows the

MAS NMR spectra (obtained using a rotor-synchronized Hahn echo (/2 - tr -  - tr) with

detection triggered after 4 rotor cycles) of the model and PTR glasses. Table 6 summarizes the results from the peak deconvolutions. All spectra include three principal 19F signals near -220 (species F1), -175 (species F2) and -144 ppm (species F3), for which preliminary assignments can be proposed based on 19F chemical shift considerations and the results obtained from the model glasses. The

19F

chemical shift of species F1 is identical to that of crystalline NaF,

suggesting an assignment to an F species predominantly surrounded by sodium ions.32,33 The ACS Paragon Plus Environment

Page 12 of 24

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13 19F

chemical shift of species F2 is identical to that observed for many AlF3–based fluoride

glasses, suggesting an Al-bound fluoride species.32,34 Consistent with this assignment, the 19F signal near -175 ppm is absent in the Al-free model glasses M4 and M5. The latter glasses also show substantially reduced intensities for species F3, suggesting that Al also contributes to the coordination of that particular species. Furthermore, the zinc-free samples M2 and M3 feature diminished intensities of the -144 ppm signal, suggesting that Zn-bonded species make a contribution to this

19F

resonance as well. These findings suggest that the signal F3 may

comprise different signal contributions due to either Zn-bound or Al-bound F atoms. These preliminary assignments are fully confirmed by MAS-NMR dipolar recoupling experiments following an approach previously developed in our laboratory.35-37 The 19F{23Na} and

19F{27Al}

REAPDOR data of our glasses are summarized in Figure 5. The strongest

19F{23Na}

REDOR effect can be observed for the signal at -220 ppm and the strongest effect in

19F{27Al}

REAPDOR is detected for the signal near -175 ppm, in agreement with the

assignment above based on the 19F chemical shift. The latter 19F signal also shows a 19F{23Na} REDOR effect, which can be explained by the fact that the {AlO4/2F2-}and {AlO4/2F23-} species require charge compensation, placing Na+ ions into the first coordination sphere of the F2 species. The PTR glasses A-C show higher quantities of 19F at -175 ppm consistent with the higher amounts of five- and six-fold coordinated Al that is detected via these glasses. Furthermore, we note that the

19F

27Al

MAS NMR in

signal at -220 ppm also shows a

19F{27Al}

REAPDOR effect. A comparison of the experimental 19F{27Al} REAPDOR data with two-spin SIMPSON simulation in Figure 5a shows that the F2 species interacts with one Al atom at an internuclear Al-F distance of ~1.8 Å. In contrast, the

19F{27Al}

REAPDOR curve of the

19F

signal at -220 ppm can be simulated by a two-spin system with a hypothetical 27Al…19F distance of 3.0-3.2 Å. While indicating the absence of direct Al-F bonds for this species, this result still signifies F…Al spatial proximity. For the F3 species the 19F{27Al} REAPDOR signal is hard to analyze because of the low signal-to-noise ratio. However, the effect can be increased by summation of 19F{27Al} REAPDOR difference spectra ΔS over a range of dipolar mixing times (see Figure 5b). The sum over the difference spectra ΣΔS contains average distance information when compared to the sum over the echo spectra ΣS0 in this region of interesting dephasing times NTR (2 to 12 rotor cycles). We find that this sum is about half as intense as the echo signal ΣS0, indicating significant 27Al-19F dipole-dipole interactions. Finally, the 19F{23Na} REDOR data of Figure 5c and d for the PTR glass sample B have been analyzed in terms of second moments M2(F-Na) for F1 and F2 species using Eq. (1). The corresponding 19F{23Na} REDOR measurement for the model compound Na2PO3F33 yielded a



14

calibration factor of f = 0.22. Although the calibrated M2(F-Na) values listed in Table 7 for the different 19F signals observed in glass B are subject to large experimental errors, a definitive trend can be noticed. The F1 fluorine species features the largest M2(F-Na) values, which even exceeds the theoretically expected M2 value of crystalline NaF (1490·106 rad2s-2). The second moment M2(F-Na) of 785·106 s-2 for fluorine species F2 is a strong indication that this Al-bonded fluorine species has Na in its first coordination sphere. The fluorine species F3 features an M2(FNa)

of 495·106 rad2s-2 that is well comparable with that of the model compound Na2PO3F

(458·106 rad2s-2). A further sharp 19F signal at δiso(19F) = -224 ppm (marked as F1a in Figure 5b and d) can be seen having a 19F{23Na} REDOR curve that appears even steeper than that of the fluorine signal F1 (Figure 5c). This signal is assigned to crystalline NaF (formed by a small extent of crystallization) based on its 19F chemical shift, and the absence of dephasing in the 19F{27Al}

REAPDOR experiments (Figure 5b). No reliable M2 value can be deduced from the

19F{23Na}

REDOR data point shown (Figure 5c).

Based on the results of the chemical analysis, we can make some quantitative estimations based on the analytically determined Al contents and the nominal F contents (serving as upper-limit estimates). If all the Al(5) species were coordinated by one and all Al(6) species by two F species, the

27Al

MAS NMR spectral parameters given in Table 5 would

predict about 14-24% of the total amount of F to be Al-bonded (if only terminal bonds are formed). This number is substantially lower than the area fraction of 43% given in Table 6, summarizing the results from 19F MAS NMR. In other words, the relative area of the 19F signal at -175 ppm is larger than that what is expected from 27Al MAS NMR. This discrepancy can be explained if the actual F contents of the glasses are lower than batched, which may be caused by partial evaporation or F/O exchange of the volatile fluorine species under melting conditions. Other possible sources of errors could be that the concentrations of Al(5) and Al(6) are underestimated in the 27Al MAS NMR spectra of Figure 2 or that the Al(6) species are bonded to more than two F atoms. We note that we can rule out a relative quantification error in the 19F spin-echo MAS NMR spectra of Figure 4, by studying the F1:F2 peak intensity ratio as a function of the number of rotor cycles. These control experiments revealed comparable spinspin relaxation times for both species.


Page 14 of 24

Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15

Figure 4: Rotor-synchronized 19F Hahn echo MAS NMR experiments and their deconvolutions detected at 5.7 T for the PTR glasses and the model samples measured at νR = 35.0 kHz and an inter-pulse delay of 4 TR. Spinning sidebands are labeled by asterisks. The probe background signal (Bg) is shown as well. Deconvolution parameters of all samples are listed in Table 6. Table 6: Fluorine components and their fractions determined from 19F MAS NMR for the glasses A, C and model glasses M1-M5.a F1 F2 F3 Sample

δiso /

FWHM

Area

δiso /

FWHM

Area

δiso /

FWHM

Area

ppm

/ ppm

fraction / %

ppm

/ ppm

fraction / %

ppm

/ ppm

fraction / %

A

-219

21

44

-175

27

50

-144

23

6

C

-219

21

46

-175

27

49

-144

23

5

M1

-222

20

48

-180

40

43

-141

20

9

M2

-220

20

58

-176

38

38

-141

20

4

M3

-218

20

61

-174

38

34

-141

20

5

M4

-219

19

94

-

-

-

-140

16

6

M5

-219

19

91

-

-

-

-141

16

9

aError

bars are: δiso ± 2 ppm, FWHM ± 1 ppm, area fraction ± 2 %.



16

( a )

( b )

( c )

( d )

Figure 5: (a) 19F{27Al} REAPDOR curves for AlF3 (red) and fluorine species F1 and F2 of the PTR glass sample B and two arrays of simulated 19F{27Al} REAPDOR curves at the same experimental conditions. For the fluorine species F2, the simulated greyed array of curves is based on a 19F-27Al twospin system with CQ(Al(6)) = 2.1 MHz, distances d(19F-27Al) = 1.8 Å, 1.9 Å, 2.0 Å, δaniso (19F) = [100130 ppm] and ησ = [0.1;1]; for the fluorine species F1, a 19F-27Al two-spin system was simulated with CQ = 3.1 MHz, d(19F-27Al) = 3.0 Å, 3.2 Å, δaniso (19F) = [100-130 ppm] and ησ = [0.1;1]. (b) Sum of rotorsynchronized 19F spin-echo spectra ΣS0 (black) and sum of 19F{27Al} REAPDOR difference spectra ΣΔS (red) for N = 2-12. The red line shows 19F species with 27Al nuclei nearby. All 19F{27Al} REAPDOR spectra were collected for the glass sample B at νR = 26.667 kHz and B0 = 11.7 T. (c) 19F{23Na} REDOR curves for Na2PO3F (red) and the species F1a, F1, and F2 with parabolic fits (dashed lines) labeled according to the 19F{23Na} REDOR spectra on the right. (d) Sum of rotor-synchronized 19F spin-echo spectra ΣS0 (black) and sum of 19F{23Na} REDOR difference spectra ΣΔS (red) for N = 2-18. The red line shows 19F species with 23Na nuclei nearby. All the 19F{23Na} REDOR spectra were recorded for the glass sample B, at νR = 27.0 kHz and B0 = 11.7 T. Table 7: Second moments (M2(F-Na) ± 10%) for the PTR glass sample B determined from 19F{23Na} REDOR experiments.a Component

M2(F-Na)/106 rad2s-2 Exp.

Norm.

F1

410

1850

F2

175

785

F3b

110b

495

Na2PO3F

100

458c

aSecond

moments were extracted from parabolic fits according to Eq. (2). (see Figure 5 (c)) and calibrated to Na2PO3F (f = 0.22), see Eq. (1). bParabolic fit not shown. cCalculated average second moment M2 according to equation (2) including both F sites in the crystal structure of Na2PO3F.38


Page 16 of 24

Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


17 23Na

MAS,

23Na

TQMAS NMR, and

23Na{19F}

REDOR. The

23Na

MAS NMR

spectra of the 23Na central transition for all PTR and model glasses are illustrated in Figure 6. Their asymmetric central transition line shapes tail towards lower frequency is a typical feature observed for glasses with wide distributions of quadrupolar coupling parameters. The

23Na

MAS NMR spectra of glass C at 235 K and 303 K are identical, suggesting that the spectrum at room temperature is not influenced by Na diffusion. The isotropic 23Na chemical shifts and SOQE values as extracted from the DMFIT software, assuming a Czjzek distribution,18 show small compositional variations and some systematic deviations from the parameters extracted from the 23Na TQMAS NMR spectra (see Figure S4). In addition, glasses M1, M2, M3 and M6 include much more pronounced and intense spinning sideband patterns than the Al-free samples M4 and M5, the PTR matrix A and the PTR glasses B and C before and after UV exposure. These observations do not lend themselves to a straightforward structural interpretation.

Figure 6: 23Na MAS NMR spectra of the PTR and model glasses measured at νR = 14.0 kHz, B0 = 11.7 T, including the optimized fits according to the Czjzek model. Deconvolution parameters of all samples are given in Table 5. Spinning sidebands are marked by asterisks.

Figure 7 summarizes the 23Na{19F} REDOR results for the PTR glasses A, B and C, which reveal a bimodal distribution of sodium ions with respect to the magnetic dipole-dipole coupling with 19F nuclei, i.e., the 23Na{19F} REDOR curves for all glasses A, B, and C give dipolar second moments of M2(Na-F) ~ 55·106 s-2 and M2(Na-F)’ = 0.4·106 s-2 obtained from twoACS Paragon Plus Environment


Page 18 of 24

18

spin simulations (see Table 8). The weakly coupled 23Na species, which make up 64-66% of all sodium in the samples, are assigned to the sodium ions in an aluminosilicate-type environment, whereas the strongly coupled

23Na

species are assigned to sodium ions, having at least one

fluoride ion in their first coordination sphere. Note that even though the REDOR curve for Na2PO3F appears steeper than those measured for the glasses, the M2(Na-F) values are actually larger for the latter samples. This is a result of the fact that only 1/3 of all Na contribute to it while 2/3 of Na are characterized by an extremely weak dipolar coupling, resulting in a substantial flattening of the 23Na{19F} REDOR curve for longer dipolar recoupling times. The difference spectrum ΔS for a long evolution time NTR in the

23Na{19F}

REDOR

experiment and the S0 spectrum happen to be identical, making further differentiation of these Na species based on the 23Na{19F}

23Na

echo line shapes impossible. Similar results were obtained by

cross-polarization experiments under MAS conditions (CP/MAS) with variable

contact times (Figure S5). We further found the 23Na{19F} CP/MAS NMR spectra recorded at 200 μs and 5000 μs CP contact times to be identical within experimental error. Thus, there appears to be no chemical shift discrimination between F bonded Na species and non-F-bonded Na species. This finding highlights the utility of the dipolar analysis based on REDOR for unambiguous species identification and quantification in the present study.

Figure 7: 23Na{19F} REDOR curves for the PTR glass samples A, B and C recorded at νR = 14.0 kHz and B0 = 7.05 T. Dashed lines illustrate the simulation of a two-spin simulation for Na2PO3F and a bimodal distribution of second moments (see Table 8) considering two two-spin systems with d(23Na19F) = 2.2 Å and d(23Na-19F) = 5.0 Å with weighting factors of 0.35 and 0.65, respectively.


Page 19 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


19 Table 8: Isotropic 23Na chemical shifts, 23Na quadrupolar coupling parameters, and second moments M2(Na-F) characterizing the bimodal distribution of effective heteronuclear dipolar couplings between 23Na and 19F in all studied glasses. Sample

δisoa /

SOQEa /

δb /

CQb /

FWHM

M2(Na-F)/106

M2(Na-F)’/106

ppm

MHz

ppm

MHz

CSb / ppm

rad2s-2

rad2s-2

A

-1.1

1.9

-1.9

3.0

20

58c

0.4c

B

-0.8

1.8

-1.4

3.0

20

55c

0.4c

C

-0.5

1.6

-2.0

3.0

20

55c

0.4c

M1

-0.1

1.9

-2.7

2.8

22

-

-

-

M2

1.6

2.0

0.5

3.0

21

-

-

-

M3

2.9

1.8

0.4

3.0

22

-

-

-

M4

2.0

1.8

0.8

3.2

23

-

-

-

M5

1.9

1.8

1.3

3.2

22

-

-

-

M6

1.1

2.0

-2.7

2.9

21

-

-

-

34

45.8d

-

Na2PO3F aDetermined

from

23Na

TQMAS NMR as shown in Figure S2. Error bars are: SOQE ± 0.1 MHz, δiso ± 0.5 ppm. Q ± 0.1 MHz, δ ± 0.5 ppm, FWHM ± 2 ppm. M2’, M2(S-I) ± 10 % dCalculated average second moment M2(S-I),theor according to Eq. (2) over all four Na sites in the crystal structure.38 bDetermined from 23Na MAS NMR as shown in Figure 6 employing the Czjzek model, C

CONCLUSIONS The results of the present study provide detailed information about the different types of NMR parameters for the various elemental components in PTR glasses. The standard aluminosilicate framework is partially modified by the NaF component producing F-bearing Al(5) and Al(6) units. Three distinct F species are detectable, which were assigned to F-species dominated by Na coordination (F1) and different types of Al-bonded F-species (F2 and F3) with the help of dipolar recoupling experiments. Finally, the 23Na{19F} REDOR results indicate that about 1/3 of all the Na+ ions have fluoride within their first coordination spheres. Overall, the strategy taken in this work of interpreting multinuclear single- and double-resonance MAS NMR spectra in the context of analytically determined compositions and parallel investigations on model glasses with modified composition represents a successful approach towards the structural analysis of PTR glasses and potentially other compositionally complex glass systems.



20

ASSOCIATED CONTENT Supporting Information Preparation and XRD characterization of the crystalline nepheline reference compound used, 29Si MAS NMR spectra and alternative simulations, 27Al TQMAS, 23Na TQMAS spectra and 23Na{19F} CP/MAS NMR spectrum.

AUTHOR INFORMATION Corresponding Authors: *E-mail: [email protected]; [email protected]; [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by FAPESP grant 2013/07793-6. L.M.F. thanks the Stiftung der Deutschen Wirtschaft for a personal doctoral stipend. We thank Ms. Hanna Klitzke (WWU Münster) for preparing a sample of crystalline nepheline.

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Short and Medium Range Order in Photothermal Refractive Glass

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