ARTICLE pubs.acs.org/JPCC
Mixed-Alkali Effects in Aluminophosphate Glasses: A Re-examination of the System [xNa2O(1 x)Li2O]0.46[yAl2O3(1 y)P2O5]0.54 Frederik Behrends and Hellmut Eckert* Institut f€ur Physikalische Chemie, WWU M€unster, Corrensstrasse 30, D48149 M€unster, Germany ABSTRACT: Mixed-alkali effects in the system [xNa2O(1 x)Li2O]0.46[yAl2O3(1 y)P2O5]0.54 (0 e x e 1; y = 0, 0.08, 0.16) have been examined using dc-conductivity measurements, thermal analysis, Raman spectroscopy, and various complementary solid-state NMR approaches. In contrast to the results of an earlier study reporting results on mixed-alkali aluminophosphate glasses with very similar chemical compositions (Faivre, A.; Viviani, D.; Phalippou, J. Solid State Ionics 2005, 176, 325), ionic conductivities and Tg values exhibit composition dependences typical of “normal” mixed-alkali effect behavior. The 31P MAS NMR spectra have been quantitatively analyzed in terms of Q(n)m units, where “n” denotes the number of P O P linkages and “m” is the number of Al atoms connected to P. The data suggest that the number of P O Al linkages is maximized, and that no Al O Al linkages occur. 23Na triple-quantum MAS NMR studies indicate a monotonic trend in isotropic chemical shifts, consistent with intimate Na/Li mixing. In agreement with this conclusion, 23Na spin echo decay measurements reveal a linear dependence of dipolar second moments M2(23Na 23Na) on atomic number concentrations, indicating the absence of “like-cation clustering”, both in the pseudobinary phosphate and in the ternary aluminophosphate systems. Information about the cation dynamics can be obtained from 27Al-MAS NMR first-order quadrupolar spinning sideband patterns involving the |(1/2æ T |(3/2æ Zeeman transitions. These data give clear evidence for a reduced cationic mobility in the mixed-alkali glasses. No significant differences in general behavior are observed between the binary alkali phosphate and the ternary alkali aluminophosphate systems.
’ INTRODUCTION Glasses containing two different types of network modifier cations have gained considerable interest because of their unusual properties concerning the ionic mobility. This phenomenon, called the mixed-alkali effect (MAE),1 5 denotes a dramatic decrease of ionic conductivity if one type of the mobile cations is replaced by one of its homologues at a constant overall cation content. Conceptually, this effect has been explained by the dynamic structure model (DSM) invoking a mismatch concept:6,7 each type of cation generates its own characteristic local environment corresponding to an energy minimum. Cation transfer can easily proceed between well-adapted sites created by cations of the same kind. In contrast, ions cannot easily migrate to a site formerly occupied by a homologue, because the geometry of this vacated site is poorly adapted to the site requirements of the mobile ion considered. As a result, the jump remains unsuccessful, and a correlated forward backward motion is more probable. If the cation distribution in a mixed-alkali glass is random, the probability for one type of ion to be near a well-matched site of its own kind decreases with decreasing concentration. Thus, as this type of cation is successively substituted by one of its homologues, these cations are being increasingly immobilized, because the pathways for facile ion transfer are being blocked. The MAE has been observed for a large number of glass systems, suggesting that the site mismatch effect between the two unlike cations is a universal concept. r 2011 American Chemical Society
Nevertheless, a detailed inspection of the effect in different glass systems reveals pronounced composition and temperature dependences, which are of structural origin, but have as of yet to be explained at a quantitative level. Also, a recent manuscript by Faivre et al. reported that the MAE in phosphate glasses on the electrical conductivities can be dramatically reduced by adding aluminum oxide to the glasses and almost completely vanishes for aluminophosphate glasses with 8 mol % aluminum oxide.8,9 Mixed-alkali glasses of nominal composition [xNa2O(1 x)Li2O]0.46[(Al2O3)0.16(P2O5)0.83]0.54 were reported to show no MAE at all according to ionic conductivity measurements. On the other hand, these glasses did show strong mechanical loss peaks below Tg, as was previously observed for other mixed-alkali systems, whereas no such loss peaks are present in the corresponding single-alkali glasses. The results of Faivre et al. therefore raised some questions regarding the connection between the presence of such mechanical loss phenomena and the presence of a mixed-alkali effect on the ionic conductivities. On the basis of these findings, a detailed structural investigation would provide a unique opportunity to identify the key structural features important for a fundamental understanding of the MAE. According to the DSM theory, the absence of a MAE might be explained if Received: February 18, 2011 Revised: August 2, 2011 Published: August 09, 2011 17175
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the two types of alkali ions are not randomly mixed but rather segregated into separate domains. In this case, the ion conduction pathways comprising well matched sites would not penetrate each other, and one should just observe an ionic conductivity comparable to that of a mixture of both single-alkali glasses. For this reason, we decided to reinvestigate the glass system by Faivre et al. and carry out a detailed structural study on it. To this end, this Article reports the preparation, the bulk and electrical characterization of glasses with the composition [xNa2O(1 x)Li2O]0.46[yAl2O3(1 y)P2O5]0.54 (x = 0, 0.25, 0.5, 0.75, 1; y = 0, 0.08, 0.16), as well as their detailed structural study by Raman spectroscopy and a range of complementary one- and two-dimensional solid-state NMR experiments.
’ EXPERIMENTAL SECTION Sample Preparation and Characterization. All of the glasses were prepared by a conventional melt-quenching method, using NaPO3 (>99%, Merck), Li3PO4 (>99%, Aldrich), Al2O3 (>99.9%, Aldrich), H2NH4PO4 (>99.5%, Riedel de Haen), and Na2CO3 (>99.5%, AppliChem) as starting materials. Stoichiometric mixtures of the dry educts were heated in a Pt crucible for 1 h at 400 C to remove gaseous decomposition products. The mixtures were melted in air for 20 min at 900 1300 C (depending on the composition) and cast into a cold steel mold. The samples used for electrical conductivity measurements were annealed for 48 h just below Tg and cooled to room temperature at a cooling rate of 0.5 K/min. All of the samples were stored under anhydrous conditions (glovebox). Evaporation losses were regularly checked and found to be minimal, indicating that batch and target compositions are in good agreement; for the Al-free glasses, the same conclusion was reached from the analysis of the 31P MAS NMR spectra (see below). Chemical analysis data obtained on the glasses of the y = 0.16 series are summarized in Table 1. While the results suggest that the P contents are slightly higher than batched, whereas the alkali and alumina contents are
Table 1. Nominal and Analytical Chemical Compositions of the Glasses Studied with y = 0.16 Al
Li
x
exp
nom.
1.00
7.6
8.6
0.75
7.7
8.6
0.50
7.8
8.6
0.25
7.8
0.00
7.7
exp
Na
P
nom
exp
nom
exp
nom
0
0
43.3
46.0
49.1
47.5
10.5
11.5
31.6
34.5
50.3
47.5
20.2
23.0
21.8
23.0
50.2
47.5
8.6
31.0
34.5
11.4
11.5
49.8
47.5
8.6
41.6
46.0
0.3
0.0
50.3
47.5
slightly lower than batched, the overall compositions are quite close to those reported in refs 8 and 9. The experimental and nominal x-values were found to be identical within the limits of experimental error ((0.02). The glassy state was verified by X-ray powder diffraction (Guinier method) and differential thermal analysis, using a NETZSCH STA409 thermal analyzer at a heating rate of 10 K/min. The density of the glasses was determined using the Archimedes method. Raman spectra were recorded on a HORIBA Raman spectrometer produced by JOBIN YVON using a green laser with a wavelength of 532.18 nm. A NOVOCONTROL ALPHA high resolution dielectric analyzer was used to measure the dielectric response of 1 mm thick samples mounted between steel electrodes. Measurements were performed at fixed temperatures (from room temperature to Tg 50 K) using a frequency range from 10 2 Hz to 3 MHz. To obtain nonblocking electrodes, the samples were polished and coated in a vacuum with a thin gold layer. NMR Studies. All of the NMR measurements were carried out on Bruker DSX-400 and DSX-500 spectrometers equipped with 4 mm single and double resonance NMR probes. At the magnetic field strength of 9.4 T (11.7 T), the resonance frequencies were 105.8 MHz (132.3 MHz) for 23Na, 104.2 MHz (130.3 MHz) for 27 Al, and 161.9 MHz (202.4 MHz) for 31P. Chemical shifts are referenced to 1 M NaCl aqueous solution, 1 M Al(NO3)3 aqueous solution, and 85% H3PO4, respectively. Single pulse experiments were carried out using pulse lengths and relaxation delays, respectively, of 0.9 μs and 5 s for 23Na, 1.5 μs and 5 s for 27 Al, and 3.5 μs and 50 s for 31P. These conditions corresponded to flip angles of 30 for all of the quadrupolar nuclei on liquid samples and to 90 flip angles for 31P. All of the single pulse experiments were done under MAS conditions at a spinning frequency of 12 kHz. Average 23Na (I = 3/2) and 27Al (I = 5/2) quadrupolar coupling and isotropic chemical shift parameters were measured at a spinning speed of 14 kHz using the threepulse triple-quantum sequence10,11 with z-filtering.12 For 23Na TQMAS NMR, the first two hard pulses were 3 and 1 μs in length, respectively, at a nutation frequency (liquid sample) of 96 kHz, while the length of the soft detection pulse (nutation frequency 11 kHz on a liquid sample) was 10 μs. In the case of 27 Al TQMAS NMR, the lengths of the hard pulses were 2.2 and 0.8 μs (nutation frequency 142 kHz), and a soft (nutation frequency 7 kHz) pulse of 10 μs length was used for detection. The evolution time t1 was incremented in 64 (23Na) or 120 (27Al) steps, and for each increment 144 (23Na) or 504 (27Al) scans were recorded using a recycle delay of 1 s. All of the MQMAS data were processed and analyzed as described in refs 11 and 12 (including data shearing), yielding the interaction parameters isotropic chemical shift values δCS and second-order quadrupolar effect SOQE = CQ(1 + η2/3)1/2, where CQ and
Figure 1. The dc-conductivity for the phosphate and aluminophosphate glasses at 373 K (left) and 453 K (right). 17176
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Figure 2. Glass transition temperatures Tg for the phosphate and aluminophosphate glasses.
Table 2. dc-Conductivity at 373 K, Its Activation Energy, and Tg Values for the Glasses under Study x
y
σDC ( 2%/S cm
1
EA ( 0.2/kJ mol
1
Tg ( 2/K
0
0
8.09 10
8
78.8
561
0.25
0
1.63 10
10
103.2
518
0.5
0
2.79 10
11
113.2
503
0.75
0
8.54 10
11
92.0
508
1
0
1.25 10
8
78.4
548
0
0.08
635
0.25 0.5
0.08 0.08
593 570
0.75
0.08
570
1
0.08
0
0.16
8.70 10
7
0.25
0.16
3.46 10
9
96.4
630
0.5
0.16
1.85 10
10
113.6
620
0.75
0.16
3.50 10
10
92.7
627
0.16
7.20 10
8
66.2
650
1
602 69.0
661
η are the nuclear electric quadrupole coupling constant and the electric field gradient asymmetry parameter, respectively. 23 Na Hahn spin echo decay experiments were carried out at 200 K at static conditions using a π-pulse length of 14.5 μs. 64 scans with a recycle delay of 1 s were recorded for each evolution time. 23Na{31P}constant-time rotational echo double resonance (CT-REDOR) studies were done at a spinning speed of 6.25 kHz using the pulse sequence reported in ref 13. The π-pulse lengths were 8.2 μs (23Na) and 8.6 μs (31P). For each t1 value, 32 scans were taken with a recycle delay of 5 s. The experiment was performed for an evolution time of two and three rotor periods. Raw experimental data were corrected using the compensation scheme published in ref 14.
’ RESULTS, DATA ANALYSIS, AND INTERPRETATION dc-Conductivity. Figure 1 shows the dc-conductivity of the glasses with y = 0 and y = 0.16, plotted as a function of x. These values, as well as the activation energies of the dc-conductivity, that were obtained by applying an Arrhenius fit to temperaturedependent dc-conductivities are listed in Table 1. Both series show significant mixed-alkali effects that are comparable in magnitude and result in a σDC minimum around x = 0.5. For all of the compositions studied, the overall dc-conductivity is higher in the aluminophosphate glasses. This effect is not
unexpected due to the higher number density of the mobile ions in the aluminophosphate glasses. This increase is of the same magnitude across the entire mixed-alkali composition range. Both for the phosphate glasses and for the aluminophosphate glasses, the σdc values measured for the lithium rich glasses are somewhat higher than those of the sodium-rich samples. While the MAE decreases with increasing temperature, its manifestations are still clearly visible at 452 K, the highest investigated temperature of the present study. Overall, neither the σDC values nor the activation energies give any evidence for a decreased MAE in aluminophosphate glasses as compared to the pseudobinary phosphate glasses. For the binary alkali phosphate glasses and the single-alkali aluminophosphate glasses, our results agree well with those reported in ref 8, while for the mixed-alkali aluminophosphate glasses, the electrical conductivities measured in our samples are lower by several orders of magnitude. Glass Transition Temperatures. The glass transition temperatures of the glasses dependent on x and y are shown in Figure 2 and listed in Table 2. For each single x value, the Tg is increased, when aluminum is present in the glass, as previously observed.8,15 The effect can be rationalized by considering that the phosphate chains are cross-linked by the addition of aluminum. When taking into account the actual compositions of the glasses, the Tg values are in excellent agreement with those reported in reference.8 Overall, the Tg values for the aluminophosphate glasses are slightly higher than those reported, because of the slightly higher amount of aluminum in our glasses. All three glass series show a distinct minimum of Tg around x = 0.5 as a result of the MAE that is present and of comparable magnitude in all glass series investigated. Raman Spectra. Additional information about the glass network can be obtained by Raman spectroscopy. The Raman spectra of the glasses with y = 0 and y = 0.16 are shown in Figure 3. Of major interest are the signals of the symmetric P O P vibration (∼680 cm 1) and the symmetric P NBO (non bridging oxygen) vibrations of the different Qn phosphate units, which are easily differentiable by Raman spectroscopy because of their different average P O bond orders (Q1/Q2/Q3, 1050 cm 1/1170 cm 1/1290 cm 1).16,17 In all series, the scattering peak corresponding to the P O P vibration is monotonically shifted to lower energy when the sodium content is increased. This is consistent with a lengthening of the P O bonds that occurs when the smaller lithium is replaced by sodium, which demands larger sites in the network. Furthermore, the signal corresponding to the P NBO vibration is shifted monotonically to lower energies as x is increased. This is in agreement with previously reported data on this system and is a consequence of the changed electronegativity of the cations that are coordinated by the NBOs.16 The evolution of the signals is comparable in all of the glass series, and there is no signal broadening for the x = 0.5 glasses. The latter would be expected in case of like-cation segregation. The addition of aluminum to the glass network does not affect the P NBO vibrations, but leads to a shift toward higher energy for the vibrations involving the bridging oxygen atoms. The intensity of this signal is increased as compared to the P NBO signal, suggesting that P O Al units contribute to this scattering peak. The P NBO signals experience an asymmetric broadening toward lower energies as a result of a greater number of possible variations in the second coordination sphere of the P-atoms. The results and conclusions from our Raman data are in good agreement 17177
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Figure 3. Top: Raman spectra of the glasses with y = 0 (left) and y = 0.16 (right). Bottom: Wavenumber corresponding to the scattering peak maxima of the vibrations involving the bridging oxygens (BO, left) and the nonbridging oxygen atoms (NBO, right) as a function of x and y.
Figure 4.
31
P MAS NMR spectra of the glasses with y = 0 (left) and y = 0.16 (right) and exemplary decompositions.
with those already reported on related aluminophosphate glasses in the literature.18 No signal attributable to a symmetric Al O Al vibration can be found, consistent with the absence of such linkages in these glasses. 31 P Solid-State NMR Spectra. The 31P MAS NMR spectra are of major interest because they yield information about the different phosphate units that are present in the glass. These building units are classified using the Qnm nomenclature introduced by Grimmer and Wolf.19 The index “n” is the number of phosphorus atoms and “m” is the number of aluminum atoms that are connected with the central phosphorus atom via a bridging oxygen. Figure 4 shows typical spectra, and the corresponding fit parameters are summarized in Tables 3 and 4. The aluminum free glasses show two distinct signals at around 22 ppm (Q20) and 36 ppm (Q30). The quantitative Q(2)/Q(3) ratios are found in good agreement with the expected values from the compositions, using van Wazer’s predictions based on a binary network modification model.20 Similar good agreement has previously been found in many other NMR studies of alkali phosphate glasses.21 Both the Q(2) and the Q(3) signals are monotonically shifted to lower frequencies when sodium is replaced by lithium. This result agrees with the data reported by Sato et al. and corresponds to the expected electronegativity effect.22 Samples more rich in lithium tend to have larger linewidths, indicating a
wider chemical shift distribution. A similar influence of the atomic number of the alkali ion was previously observed for mixed K Na metaphosphate glasses23 and is consistent with a previously established correlation for metaphosphate glasses; ions with weaker electric potential cause smaller distortions in the phosphate chains, narrowing the distribution of structural environments for the phosphate tetrahedra. The observed chemical shift and fwhm of both signals change monotonically with composition. This finding argues against like-cation segregation, as in the latter case a line width maximum for x = 0.5 would be expected, corresponding to a superposition of the peaks for the lithium and sodium phosphate glasses in equal proportions. The 31P MAS NMR spectra of the aluminophosphate glasses are more complex and can be deconvoluted into multiple contributions. Our peak deconvolutions differ from those suggested in ref 9 and are instead guided by those proposed for potassium aluminophosphate glasses, which show significantly improved resolution.24 In addition to the Q20 and Q30 units, we identify spectral components near 2 ppm (Q10 units), 16 ppm (Q12 units), and 26 ppm (Q21 units). In addition, a component assignable to Q11 units can be identified, whose chemical shift depends on whether it is connected to fourcoordinated aluminum (δCS = 12 ppm) or to 5- or 6-fold aluminum (δCS = 8 ppm). From these deconvolutions, we can 17178
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Table 3. Fitting Parameters of the 31P MAS NMR Spectral Deconvolution for Q10, Q20, and Q30 in the Glasses under Study Q10 x
δ ( 0.5/ppm
y
Q20
fwhm ( 0.5/ppm
Arel ( 1/%
δCS ( 0.5/ppm
Q30
fwhm ( 0.5/ppm
Arel ( 1/%
δCS ( 0.5/ppm
fwhm ( 0.5/ppm
Arel ( 1/%
0
0
24.2
9.9
84
38.6
14.8
16
0.25
0
23.2
9.3
85
36.7
13.6
15
0.5
0
22.4
8.9
90
35.9
12.6
10
0.75
0
22.0
8.5
88
35.6
12.6
12
35.5
12.1
15
1
0
10.5
4.7
0
21.1
8.3
85
0
0.08
4.6
7.3
3
23.2
9.1
62
0.25
0.08
4.1
6.7
2
22.5
9.0
62
0.5
0.08
22.0
8.0
67
0.75
0.08
21.2
7.8
69
1
0.08
1.0
6.7
1
19.9
7.8
63
0
0.16
3.6
5.7
5
23.5
9.8
30
0.25
0.16
3.0
5.8
4
22.7
9.8
29
0.5
0.16
2.2
5.8
3
21.9
9.2
29
0.75
0.16
0.5
5.4
2
21.1
8.6
31
1
0.16
1.5
4.5
2
19.9
7.9
34
Table 4. Fitting Parameters of the 31P MAS NMR Spectral Deconvolution for Q11(4), Q11(5/6), Q12, and Q21 in the Glasses under Study Q11(4) δCS ( 0.5/ x 0
y
ppm
Q11(5/6)
fwhm ( 0.5/ Arel ( 1/ δCS ( 0.5/ ppm
%
ppm
Q12
Q21
fwhm ( 0.5/ Arel ( 1/ δCS ( 0.5/ fwhm ( 0.5/ Arel(1/ δCS( 0.5/ ppm
%
ppm
ppm
%
ppm
fwhm 0.5/ Arel( 1/ ppm
%
0.08
11.9
7.2
7
20.3
9.1
12
31.6
12.6
16
0.250.08 0.5 0.08
11.1 10.8
7.1 6.6
6 3
19.0 18.6
8.5 7.5
15 15
29.1 27.0
11.0 10.5
16 15
0.750.08 12.2
4.2
0
9.9
6.9
3
17.4
7.2
16
26.8
9.8
12
8.1
6.7
8
16.2
7.4
13
25.8
9.6
14
1
0.08
0
0.16
14.1
7.3
5
10.4
8.3
18
19.1
9.7
31
28.6
12.9
11
0.250.16
13.9
7.2
4
10.1
8.2
22
18.6
9.3
33
26.1
12.5
10
0.5 0.16
13.7
7.0
4
9.5
8.0
22
18.2
9.0
32
25.9
12.1
11
0.750.16
13.5
5.5
2
8.9
7.8
23
17.1
8.7
32
25.3
11.5
11
1
11.9
5.0
2
7.8
7.4
25
15.9
8.2
27
24.9
10.9
10
0.16
extract the average number of P O Al linkages per P atom (see Table 4). We can compare these experimental values with approximate ones predicted from the glass compositions, taking into consideration that the average Al coordination number is near six (as indicated by 27Al-MAS NMR, see below), and assuming the absence of Al O Al linkages. As Table 5 illustrates, the agreement is generally excellent and supports the above assumption. The systematic deviation toward slightly smaller numbers of P O Al linkages as compared to the calculated values can be attributed to the presence of small amounts of four- and fivecoordinate Al species. For the y = 0.16 glasses, the concentrations of these Al(4) and Al(5) units are a little larger than for y = 0.08, and thus larger deviations from the above model are observed. In view of the chemical analysis results, part of these discrepancies might also be due to slightly lower aluminum contents as compared to the batch composition. Table 5 also lists the numbers of P O Al linkages deduced from the fit given in ref 9; in this case, the results show large fluctuations from sample to sample for y = 0.08, while for the y = 0.16 glasses the concentrations of P O Al units deduced from this deconvolution are unrealistically low and
would have to be explained in terms of large concentrations of Al O Al linkages. The existence of Q10 units in the aluminum-rich glasses can be understood to arise from the cation to phosphorus ratio >1 in these glasses. Furthermore, the Q11 units bound to Al(4) can be found almost exclusively in the y = 0.16 glasses because the amount of 4-fold coordinated aluminum is too low in the y = 0.08 samples, as indicated by the 27Al MAS NMR spectra. The fwhm increases systematically when y is increased from 0.08 to 0.16 for all signals, which is consistent with the higher variety in the second and third coordination spheres, arising from the presence of multiple species in the network. Nevertheless, we find that for the Q(n)m units with a given number of bridging oxygen atoms m + n, the fwhm tends to be lower for m > 0 than for m = 0 units. This finding may indicate that the more rigid environment around the aluminum polyhedra may reduce the structural flexibility (and hence the width of the chemical shift distribution) for all phosphate units that are linked to Al. The exchange of the mobile cation induces the same trends as in the regular phosphate glass. The signals are shifted monotonically by 3 4 ppm to 17179
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Table 5. Average Number of Al O P Bonds per Phosphorus Atom Based on the Experimental Fit Data of Our Study and the Experimental Fit Data of Reference 9, Both Calculated for an Estimated Average Aluminum Coordination of 6 in the Glasses under Study x
y
experimental
calculated
ref 9
0
0.08
0.47
0.52
0.99
0.25 0.5
0.08 0.08
0.51 0.48
0.52 0.52
0.80 0.26
0.75
0.08
0.47
0.52
0.68
1
0.08
0.49
0.52
0.31
0
0.16
0.96
1.14
0.30
0.25
0.16
0.93
1.14
0.31
0.5
0.16
1.01
1.14
0.37
0.75
0.16
0.99
1.14
0.30
1
0.16
0.91
1.14
0.29
Table 6. Isotropic 23Na Chemical Shift and SOQE Values Obtained by 23Na TQMAS NMR Experiments, Number Density, and M2(23Na 23Na) Values in the Glasses under Study δcs ( 1/ SOQE ( x
y
ppm
M2(23Na
23 6
Na) (
F ( 0.1/
0.2/MHz
0.2 0.3/(10 rad2 s 2)
(mol mm 3)
0.250
7
1.2
1.4
0.6
0.5 0
6
1.3
2.2
1.1
0.750
4
1.3
3.0
1.6
1
0
3
1.5
4.7
2.1
0.250.16 0.5 0.16
8 6
1.2 1.3
1.3 2.1
0.6 1.2
0.750.16
5
1.5
2.8
1.8
1
4
1.6
4.9
2.3
0.16
lower frequencies, and their fwhm is increased when sodium is replaced by lithium. No evidence for a changed behavior or a like cation custering can be found. 23 Na MAS NMR Spectra, Spin Echo Decay, and 23Na{31P} CT-REDOR. Table 6 shows the evolution of the 23Na isotropic chemical shifts as determined by 23Na TQMAS NMR experiments. Both in the aluminum-free and the aluminum-containing glasses the replacement of sodium by lithium ions results in a monotonic decrease of the 23Na isotropic chemical shifts. This trend agrees with the universal trend found for a large variety of mixed-alkali glasses25 and suggests that the sodium sites are slightly expanded as lithium is added to the glass.26,27 The compositional dependence of δCS(23Na) clearly argues against a like-cation segregation scenario because then the isotropic shift should remain constant. Alterations of the sodium sites by the presence of a second alkali ion species are also observed in systematic trends of the SOQE values extracted from the TQMAS experiments. To gain further insights into the spatial distribution of the sodium ions, homonuclear dipole dipole coupling information was gathered from 23Na spin echo decay spectroscopy.28,29 Approximating the initial decays (2t1 < 200 μs) as Gaussians, average dipolar second moments M2(23Na 23Na) were extracted from the data. Figure 5 and Table 6 reveal an approximately linear dependence of these values on the sodium number density, consistent with a random distribution of the sodium
Figure 5. Dipolar second moments measured by 23Na spin echo experiments as a function of the glasses number density.
cations. Our results are comparable to those reported by Zwanziger et al. for sodium phosphate glasses.28 The compositional trends for the sodium phosphate and the sodium aluminophosphate glasses are virtually identical, suggesting that in both glass systems analogous relative cation distributions are present. Our results clearly rule out a predominant like cation segregation scenario, which would yield much larger M2 values that would further be independent of x. Potential changes in the internuclear distance distributions involving the sodium cations and the phosphorus atoms were examined via compensated 23Na{31P} CT-REDOR experiments. These experiments are preferred over standard REDOR experiments because of the larger number of relevant data points available at short evolution times that are used for the second moment analysis. The dipolar second moments extracted from these data for the y = 0.16 glasses are shown in Figure 6. They show a slight trend toward increased M2(23Na 31P) values with increasing lithium content, even though the effect is close to experimental error. While at first glance this finding appears to contradict the increased Na 3 3 3 O distance inferred from the chemical shift trend, both observations can be reconciled by considering an increase in the average coordination number of the sodium ions. Such an effect implies a lower degree of bond covalency (decreasing resonance frequency) while bringing more 31P nuclei into the second coordination sphere of the sodium ions. 27 Al MAS NMR. Figure 7 summarizes the 27Al MAS NMR data. The spectra show three distinct signals assigned, as usual, to six- (resonance shift near 15 ppm), five- (resonance shift near 12 ppm), and four-coordinated aluminum (resonance shift near 40 ppm).30,31 Six-coordinated aluminum polyhedra dominate, even though the fractions of four- and five-coordinated aluminum tend to increase with increasing aluminum content. Isotropic chemical shifts δCS and the SOQE parameter extracted from TQMAS spectra (see Figure 8) are summarized in Table 7. The δCS values indicate that all three types of aluminum species are fully connected to phosphorus, and there is a minor shift to lower frequencies when sodium is replaced by lithium. Consistent with ref 9, the four- and five-coordinated species appear somewhat depleted in the mixed-alkali glasses as compared to the single-alkali glasses. However, the significance of this minor structural feature is difficult to relate to the MAE. Closer inspection of Figure 7b reveals another phenomenon that can be linked much more directly to the MAE: There are many distinct spinning sidebands present in the spectra of the mixedalkali glasses. Sideband manifolds of this kind arise from the effect of MAS upon the |(1/2æ T |(3/2æ Zeeman transitions (“satellite peaks”), which are anisotropically broadened 17180
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Figure 6. Exemplary 23Na{31P} CT-REDOR curves (left) and heteronuclear dipolar second moments (right) of glasses with y = 0.16. The bar denotes the estimated experimental error.
Figure 7. (a) 27Al MAS NMR spectra of the y = 0.08 (left) and y = 0.16 (right) aluminophosphate glasses at ambient temperature. (b) Selected 27Al MAS NMR spectra of the y = 0.16 aluminophosphate glasses at ambient temperature (left) and at 200 K (right).
by first-order quadrupolar perturbations. Most interestingly, the room temperature spectra of the single-alkali glasses do not show such sideband manifolds, even though the TQMAS data (Figure 8) do not suggest any differences in magnitudes or distributions of 27Al electric field gradients between single- and mixed-alkali glasses. As the sidebands do appear upon lowering the temperature to 200 K (see Figure 7b), their absence at room temperature in the single-alkali glasses can be attributed to a dynamic effect. At room temperature, the sodium or lithium ion motion is sufficiently rapid to influence the 27Al electric field gradients such that the resonance frequency of a given 27Al nucleus contributing to one of the satellite transitions changes during the rotor period. This process interferes with the coherent averaging by MAS, thus leading to a disappearance of the spinning sideband manifolds. The physical effect is identical to the one previously described by Ashbrook and Wimperis in 27Al MAS NMR spectra of aluminum phosphate inclusion compounds32 and in the 2H MAS NMR spectra of various crystal hydrates and polymers.33 In contrast, the alkali ions are sufficiently immobilized in the mixed-alkali glasses, such that the 27Al resonance frequencies remain constant during the rotor period, resulting in the usual
first-order spinning sideband manifolds. This finding is consistent with the dc electrical conductivity trend shown in Figure 1. While the latter data concern the long-range ion transport, the 27Al MAS NMR data clearly illustrate the decreased ionic mobility also at the local level, as usually found in other mixed-alkali glasses. Again, the effect is present in both the Al-free and the Al-containing glasses, indicating that, contrary to the conclusions in ref 8, the glass system [xNa2O(1 x)Li2O]0.46[yAl2O3(1 y)P2O5]0.54 (y = 0.08 and 0.16) shows just “normal” mixed-alkali effect behavior, both on the macroscopic and on the microscopic scale.
’ DISCUSSION AND CONCLUSIONS As discussed in ref 8, the absence of a mixed-alkali effect on the electrical conductivity in alkali aluminophosphate glasses stands in apparent contradiction to a clear MAE signature in the compositional dependence of Tg and also to the observation of a strong mechanical loss peak that is only observed in mixedalkali glasses, but not in single alkali glasses. According to the dynamic structure model, the origins of these large mechanical loss peaks in the mixed-alkali glasses relate to the relaxation of the mismatch that is created when an ion jumps between mismatched sites in the glass. This principal feature of the 17181
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27
Al TQMAS NMR spectra for the y = 0.16 aluminophosphate glasses with x = 0 (left) and x = 0.5 (right).
Figure 8.
Table 7. Isotropic 27Al Chemical Shift, SOQE Values, and Area for the Three Distinct Aluminum Signals Obtained by 27Al TQMAS NMR Experiments in the Glasses under Study Al(OP)6 x
y
δcs ( 1/ppm
Al(OP)5
Al(OP)4
SOQE ( 0.2/MHz
Arel ( 1/%
δcs ( 1/ppm
SOQE 0.2/MHz
Arel ( 1/%
δcs ( 1/ppm
SOQE 0.2/MHz
Arel ( 1/% 1
0.5
0.08
11
2.4
97
16
4.3
2
47
4.2
1
0.08
10
2.3
95
17
3.9
3
48
3.9
2
0
0.16
11
2.7
72
16
3.9
11
47
3.8
17
0.25 0.5
0.16 0.16
10 10
2.6 2.6
85 90
17 17
4.0 3.9
7 6
46 48
3.6 3.6
8 5
0.75
0.16
10
2.6
88
18
4.0
7
49
3.8
5
1
0.16
10
2.6
85
18
3.3
7
49
3.4
7
dynamic structure model had been thrown into question by the apparent contradictions observed by Faivre et al., casting doubts on the overall validity of the model. In contrast to the results of ref 8, our data indicate that mixed-alkali aluminophosphate glasses show completely “normal” mixed-alkali effect behavior, both with regard to bulk properties and at the microscopic level. Thus, the above perceived contradictions can be considered removed. Our findings bring back into focus some of the ideas that were proposed earlier linking the conductivity MAE to structural effects34 and reassert the ubiquity of the mixed-alkali effect as encountered in oxidebased network glasses. The effect of alumina on the magnitude of the mixed-alkali effect in other types of glasses has been recently discussed by Tomozawa and co-workers.35 As pointed out there, contradictory data regarding the magnitude of the mixed-alkali effect have been previously observed for mixed-alkali aluminosilicate glasses studied in different laboratories.36 38 While these discrepancies were suggested to arise from differences in the measurement technique,35 this explanation cannot apply here, as, aside from the different sample thicknesses used, the measurement techniques applied by us and in ref 8 are essentially the same. It would be of great interest to explore the big differences found between both studies. As the NMR results reported in ref 9 could be largely reproduced by us, there is no hint for any differences in short-range order between both studies. In the present work, further advanced experimentation has resulted in more detailed information about the
connectivity in the network and the spatial distribution of the cations. Our 23Na NMR results clearly rule out large-scale likecation segregation and suggest that the relative Na/Li distribution is random within experimental error. As a matter of fact, some degree of like-cation clustering might be expected to occur as a consequence of the site mismatch effect, and such effects have indeed been observed in some other mixed-alkali glass systems.23,39 It is possible that in the present system the effect eludes detection by 23Na spin echo decay spectroscopy because it is too weak, as the mismatch between Li+ and Na+ is only moderate. Possibly more insights into the above-described differences in the electrical conductivity behavior between this study and ref 8 might be obtained by applying the same advanced NMR experiments used here to the glass samples of Faivre et al. Finally, as an interesting aspect of the mixed-alkali effect, the cation dynamics can be probed indirectly by monitoring the 27Al MAS NMR spinning sideband patterns generated by the effect of MAS upon the |(1/2æ T |(3/2æ Zeeman transitions, which are anisotropically broadened by first-order quadrupolar perturbations. The results clearly document the reduced cationic mobility in mixed-alkali glasses, showing no significant differences between the pseudobinary alkali phosphate and the pseudoternary alkali aluminophosphate systems.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 17182
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