Network Dynamics and Species Exchange Processes in

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J. Phys. Chem. B 2009, 113, 416–425

Network Dynamics and Species Exchange Processes in Aluminophosphate Glasses: An in situ High Temperature Magic Angle Spinning NMR View Sebastian Wegner,† Leo van Wu¨llen,*,† and Gregory Tricot†,‡ Institut fu¨r Physikalische Chemie, Westfa¨lische Wilhelms-UniVersita¨t Mu¨nster, Corrensstrasse 30-36, D-48149 Mu¨nster, Germany, and UCCS-Unite´ de Catalyse et de Chimie du Solide, CNRS UMR 8181, USTL-ENSCL, BP 108 59652 VilleneuVe d’ascq Cedex, France ReceiVed: July 10, 2008; ReVised Manuscript ReceiVed: NoVember 1, 2008

In this contribution, we present an in situ high temperature 27Al and 31P magic angle spinning (MAS) NMR study of binary and ternary phosphate glasses at temperatures above the glass transition temperature TG. For binary phosphate glasses, xK2O-(1 - x)P2O5 and ternary aluminophosphate glasses 30K2O-xAl2O3-(70 x)P2O5 with 7 < x < 15 dynamic exchange processes between the various phosphate species (and aluminate species) present in the glasses could be identified in the temperature range between TG and the maximum achievable temperature Tmax of our high temperature MAS NMR setup, TG < T < Tmax. This observation indicates rapid P-O-P and P-O-Al bond formation and bond breaking in the (alumino)phosphate glasses. From a modeling of the temperature dependence of these exchange processes, the activation energy EA for the corresponding process could be determined. These local bond breaking and making processes are ultimately linked to the macroscopic viscous flow and may indeed form the basic microscopic local step of viscous flow. Introduction Phosphate based glasses present an interesting class of amorphous material with a wide range of applications including biomaterials,1 laser hosts,2 nuclear waste storages,3 or antioxidation coatings. The poor overall durability of binary phosphate glasses can be drastically improved by the addition of co-oxides, such as Al2O3 or B2O3, which especially improves the resistance against moisture attack.4,5 A wealth of studies has been devoted to the structural characterization of aluminophosphate glasses, especially with the aim of characterizing the structural role of aluminate species within the phosphate network. On the basis of an advanced solid state NMR protocol, we were able to quantify the phosphate and aluminate units present in these glasses and to identify the structural environments on a medium length scale.6,7 Since in some of the applications the glasses are exposed to elevated temperatures, the evolution of the glass structure (and dynamics) with temperature constitutes a rather important issue of material performance in these glasses. The evolution of glass structure and the dynamic processes at elevated temperatures can be studied employing two different approaches: In the ex situ approach, the glass sample is first annealed at a given temperature; then, the structural evolution (governed by the temperature dependence of the equilibria) is monitored in subsequent experiments, performed at ambient temperature. Utilizing this approach however restricts the analysis to permanent nonreversible changes induced by the annealing process. The in situ approach offers the opportunity to study the dynamic processes and network (re)organization at the annealing temperature as they happen, which especially enables the investigation of reversible processes which are not accessible employing the ex situ approach. In silicate and aluminosilicate systemssthese systems were chosen mainly because of their role as mimics for natural magmasthis in situ † ‡

Westfa¨lische Wilhelms-Universita¨t Mu¨nster. CNRS UMR 8181, USTL-ENSCL.

approach has successfully been employed to study the nature and relative abundance of different silicate Qn species and to access the underlying equilibria using Raman scattering techniques.8 In alkali silicate and aluminosilicate melts, these equilibria are described by 2Q(3)(Si) T Q(2)(Si) + Q(4)(Si) and 2Q(2)(Si) T Q(1)(Si) + Q(3)(Si). Apart from these Raman studies, especially high temperature solid state NMR studies on silicate systems, mainly by the Stebbins group,9-15 have contributed to our understanding of the evolution of glass structure at high temperatures. Employing static 17O line shape analysis, 29Si magic angle spinning (MAS) NMR and two-dimensional exchange NMR experiments on potassium tetrasilicate glasses, these authors could identify the breaking and reforming of Si-O-Si bonds within the glass melt from TG to the melt. From the temperature dependence of the exchange frequencies, a strong coupling of the Si-O-Si bond breaking and the macroscopic viscous flow was concluded.16 These studies, linking microscopic features of the glass structure to the macroscopic viscosity, present an important issue of glass science, since viscous flow plays an important role in the manufacturing steps of a commercial glass and in its high temperature behavior. Contrasted to the situation for the silicate based systems, only sparse in situ data are available on phosphate based glasses. Apart from an in situ high temperature X-ray diffraction study on calcium phosphate glasses17 and an in situ Raman study on aluminosilicophosphate glasses,18 our in situ NMR study on aluminophosphate glasses of composition (50K2O-xAl2O3(50 - x)P2O5) presents the only examination of the structural evolution with temperature on phosphate glasses.19 There, a change in the average aluminum coordination number from six toward four was observed at high temperatures and could be related to structural relaxation processes occurring above TG. In the investigated composition range, however, the strong crystallization tendency of these glasses hindered us to access

10.1021/jp8061064 CCC: $40.75  2009 American Chemical Society Published on Web 12/18/2008

Dynamics in Aluminophosphate Glasses

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TABLE 1: Melting Temperatures (Tm), Weight Loss (∆m/m), and Glass Transition Temperatures (TG ( 10 °C) for the Binary and Ternary Phosphate Glassesa glass

Tm/°C

bin_20 bin_35 ter_7 ter_10 ter_15

900 900 1150 1150 1250

(∆m/m)/%

TG/°C

rel. area Q2

rel. area Q3

19.1 53.8

80.9 46.2

1.0 5.0 0.4

215 242 251 365 451

a For the binary glasses, the fractions of Q2 and Q3 units at ambient temperature as determined from a fit including all spinning sidebands employing DMFIT36 is reported as well.

the temperature regime in which the viscosity of the glass melt becomes so low that the glass is able to flow. In this work, we present the results of an in situ high temperature NMR study of binary potassium phosphate and ternary potassium aluminophosphate glasses in the compositional range 30K2O-xAl2O3-(70 - x)P2O5 (7 < x < 15), exploring the network dynamics in the glasses between the glass transition temperature TG and the melting temperature Tm, to analyze the linkage between the microscopic network dynamics and viscous flow. The network dynamic is evaluated from an analysis of the temperature dependence of 27Al and 31P MAS NMR spectra, obtained in the temperature range 20 °C < T < 650 °C. From the results, we conclude a strong coupling between local P-O-P and P-O-Al bond breaking and forming and viscous flow in these phosphate based glasses. Experimental Details Transparent and colorless samples in both the binary and the ternary glass systems were prepared employing the standard melt quenching method. Two binary phosphate glasses were prepared by mixing reagent grade K4P2O7 and P2O5 in appropriate amounts in a glass ampule within an argon glovebox. The ampoules were afterward flame sealed and the glasses molten in a furnace. After heating the mixture for 30 min, the sealed ampule was put back into the glovebox. For the ternary phosphate glasses reagent grade K2CO3, Al(OH)3, and (NH4)2HPO4 in appropriate quantities (for 10 g of glass) were thoroughly grounded and slowly heated (1 °C/min) in a Pt crucible up to 600 °C to remove H2O, NH3, and CO2. Then, the mixtures were heated to 1000-1200 °C for 20 min depending on composition with subsequent quenching between two copper plates. All samples were stored in a glovebox. As the weight losses (∆m/m) always proved to be less than 5%, glass compositions given in the text correspond to the batch compositions. The binary glasses xK2O-(1 - x)P2O5 are referred to as bin_x (with x denoting the molar percentage of K2O), and the ternary glasses in the system 30K2O-xAl2O3-(70 - x)P2O5 (7 < x < 15) are referred to as ter_x. Differential scanning calometry (DSC) measurements on the glasses were performed employing a heating rate of 10 °C/min to determine the glass transition (TG) temperatures with an estimated error of ( 10 °C. None of the glasses studied in this work exhibited any tendency to crystallize. Table 1 summarizes the melting temperatures (Tm), weight losses (∆m/m), and TG values for the glasses. All in situ MAS NMR experiments were performed employing a Doty HT-MAS probe. The 31P and 27Al MAS NMR experiments were performed at 4.7 T for 31P (ν0 ) 81.1 MHz) and at 7.04 T for 27Al (ν0 ) 78.2 MHz) employing a modified (TecMag upgrade) Bruker CXP console. The applied MAS frequency was 4.50 ( 0.03 kHz using dry N2 for sample

spinning. The magic angle proved to be stable over the entire experimental temperature range (room temperature (RT) < T < 650 °C). The radiofrequency field (rf) strength used was about 27.7 kHz, corresponding to a π/2-pulse length of 9 µs, for the acquisition of the 31P MAS NMR spectra with 24-48 transients and a repetition time of 90 s. Longer repetition times did not change the appearance of the spectra. 27Al MAS NMR spectra were obtained with 1024 to 2048 accumulations, applying a recycle delay of 0.5 s. The rf field strength employed for 27Al was approximately 30 kHz (obtained on a liquid sample); pulses of length π/6 were used for the acquisition. Chemical shifts are referenced to 85% H3PO4 and 1 M Al(NO3)3 for 31P and 27Al, respectively. Results and Discussion The DSC curves for the glasses studied in this work are collected in Figure 1; Table 1 lists the obtained TG values (heating rate 10 K/min) with an estimated error of (10 °C. For none of the glasses was a crystallization peak observed. In Figures 2 and 3, the temperature dependent 31P MAS NMR spectra for the binary glasses bin _20 and bin_35 are collected. Each figure shows the complete spectra (a) and a zoomed view on the isotropic signal component (b). Due to the limited MAS max ) 5.5 kHz), it proved advantageous to record frequency (νMAS the spectra employing a relatively low magnetic field (i.e., 4.7 T) to prevent a considerable overlap of the spinning sidebands due to the spread in chemical shift. The signals at -27.5 and -44 ppm which remain well-resolved up to temperatures of approx. 300 °C can be assigned to Q2 and Q3 units, i.e. a phosphate tetrahedron connected to two more PO4 tetrahedra and three PO4 tetrahedra, respectively, as expected for binary phosphate glasses with a potassium content below the metaphosphate composition.20,21 For both glass compositions, the isotropic positions of the phosphate signals are experiencing an upfield shift with increasing temperature, as observed in a previous work.19 Both signals are accompanied by a set of spinning sidebands. Up to a temperature of approximately 300 °C, no significant changes in the spectra are observable. For temperatures above 300 °C, however, two distinct changes in the spectra for both compositions are obvious. At temperatures starting from T ) 340 °C (glass bin_20) and T ) 325 °C (glass bin_35), a gradual decrease in the intensity of the outer spinning sidebands can be observed, accompanied by a smearing of the two center signals into a single signal at T ) 340 °C for glass bin_20 and T ) 350 °C for sample bin_35. The observed reversal in the intensity of the Q2 and Q3 species in glass bin_30 at T ) 325 °C might principally indicate an increase in the amount of Q3 species, initiated by the speciation reaction 2Q2 T Q1 + Q3. Since however, we do not observe any indication for the presence of Q1 species (δiso ≈ 0 ppm) in glass bin_30, the observed intensity change is most probably just a consequence of the beginning chemical exchange between the two sites. At temperatures above T ) 460 °C for glass bin_20 and T ) 400 °C for sample bin_35, all spinning sidebands have completely vanished, both spectra exclusively exhibit a single isotropic signal. The complete absence of any spinning sideband intensity corroborates the findings from the DSC measurements that no crystallization occurred during the heat treatment of the samples. This was independently confirmed performing a control 31P MAS NMR experiment after cooling down to ambient temperatures, in which no changes compared to the spectrum taken at the beginning of the temperature cycle could be detected. The coalescence behavior of the isotropic signals for the Q2 and Q3 species constitutes a clear indication for a dynamic

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Figure 1. DSC curves for the studied binary (a) and ternary (b) glasses.

Figure 2. 31P MAS NMR spectra for glass bin_20 for the indicated temperatures. (a) Full spectra including all spinning sidebands. (b) Detailed view of the isotropic signals. (c) Simulations of the isotropic signals employing a two-site jump model as outlined in the text with the indicated exchange frequencies.

exchange process between these units, necessarily indicating local P-O-P bond breaking and making. In addition to this dynamic process, reorientations of the different Q-species induce an exchange between different orientations of a given Q-species relative to the magnetic field (i.e without any P-O-P bond breaking).8 Both types of dynamics may contribute to the observed changes in the line widths and intensities of the spinning sidebands and the isotropic signals. Contrasted to the straightforward and unequivocal signal assignment in the spectra for the binary potassium phosphate glasses, the interpretation of the 31P MAS NMR data for the alumina containing ternary glasses is much more involved. Apart from the pure Q2 and Q3 species present in the binary glasses a variety of phosphate units with connections to phosphate and aluminate polyhedra may occur.22 As shown in refs 6 and 7, especially the dipolar based solid state NMR techniques such as rotational echo double resonance (REDOR), rotational echo adiabatic passage double resonance (REAPDOR), and crosspolarization heteronuclear correlation (CP-HETCOR) are well suited to unravel the network connectivity in aluminophosphate glasses. Here we employed 31P{27Al} REAPDOR NMR and

31

P{27Al} CP-MAS NMR techniques to study the network organization of the ternary glasses ter_7, ter_10, and ter_15. The 31P MAS and 27Al MAS spectra for the studied ternary glasses are compiled in Figure 4a and b, the results from the dipolar NMR experiments are collected in Figure 5. For details of REAPDOR spectroscopy and analysis, the reader is referred to the literature.6,7,23-25 In short, the results from a rotorsynchronized 31P MAS NMR spin-echo experiment (cf. Figure 5b, top spectrum), defining the full echo intensity S0, are compared to spectra resulting from an experiment in which the heteronuclear dipolar coupling between 31P and 27Al nuclei has been reintroduced by the action of a 27Al pulse of length onethird of a rotor period in the middle of the pulse sequence, leading to a reduced signal intensity for those 31P signals which originate from phosphorus sites with spatial proximity to aluminate species (cf. Figure 5b, middle spectrum). The difference of the spectra from the two experiments (Figure 5b, bottom spectrum) then only contains contributions from 31P nuclei experiencing a dipolar coupling to 27Al nuclei. Varying the dipolar evolution time then produces the REAPDOR evolution curves (cf. Figure 5c) which may then be analyzed


Figure 3.


31

P MAS NMR spectra for glass bin_35 for the indicated temperatures. The arrangement of the spectra resembles that of Figure 2.

Figure 4. Compilation of 31P (a) and 27Al MAS NMR data (b) for the glasses ter_x (x ) 7, 10, and 15).

to obtain the number of connected aluminate species to a given phosphorus site employing the SIMPSON software.26 For the simulations presented here we assumed a single P-Al dipolar interaction, i.e a PAl two-spin system (solid lines in Figure 5c), and a PAl2 three-spin interaction (dashed line in Figure 5c). More details of the simulations are given in the figure caption. The composition of glass ter_10 corresponds to the metaphosphate composition 30(K2O-P2O5)-10(Al2O3-3P2O5). The 31 P MAS signal (cf. Figure 4a) mainly consists of a single, slightly asymmetric resonance line centered at about -28.5 ppm. From an analysis of the dipolar evolution curves resulting from a 31P{27Al} REAPDOR NMR experiment on this glass (cf. Figure 5c), an average number of 1 P-O-Al connection per phosphate polyhedron can be concluded. This corroborates the findings of Schneider27 who found a Q2 phosphate species connected to two further phosphate tetrahedra and one aluminate polyhedra as the dominating species in this glass system. Increasing the P/Al ratio (30K2O-7Al2O3-63P2O5) entails the formation of pure Q3 units at the cost of the abovementioned

species. The corresponding 31P MAS NMR spectrum (cf. Figure 4a) exhibits two different signals at chemical shift values of -26.6 and -37.5 ppm. The signal at -37.5 ppm can be assigned to pure Q3 phosphate species, based on a comparison of the 31P MAS NMR spectrum (solid line in Figure 5a) to the 31P{27Al} CP-MAS NMR spectrum (dotted line in Figure 5a). Since only those phosphate species in close proximity to aluminate polyhedra, i.e. experiencing P-O-Al connectivity, can contribute to the 31P{27Al} CP-MAS signal, the absence of the -37.5 ppm signal in this experiment identifies the corresponding phosphate species as a pure Q3 species. Decreasing the P/Al ratio on the other hand (30K2O-15Al2O3-55P2O5) triggers the formation of Q22 units, i.e. phosphate tetrahedra with a connection to two more phosphate and two aluminate polyhedra. Again, this is corroborated by the results of a 31P{27Al} REAPDOR NMR experiment, the results of which are shown in Figure 5c. The simulations indicate the presence of an average number of 1-2 Al per phosphate polyhedron. In the 27Al-MAS NMR spectra for the three different glasses, collected in Figure 4b,


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Figure 5. (a) Comparison of the 31P MAS (solid line) and 31P{27Al} CP-MAS NMR spectrum (dotted line) for glass ter_7. The experimental conditions for CP are as follows: νRF (31P) 6.5 kHz; ramped contact pulse for 27Al, centered around νRF ) 6 kHz; νRF (π/2-pulse 27Al) 50 kHz; contact time 5 ms. (b) 31P{27Al} REAPDOR NMR spectra for glass ter_7: (top spectrum) 31P spin echo MAS NMR; (middle spectrum) 31P{27Al} REAPDOR NMR; (bottom spectrum) difference of the two, dipolar evolution time 1.856 ms. (c) REAPDOR evolution curves for the three ternary glasses. The solid lines correspond to SIMPSON26 simulations assuming a two-spin interaction, i.e. a single P-O-Al connection per phosphate tetrahedron, and the dashed line corresponds to a PAl2 three-spin interaction. The experimental details are as follows: νRF (31P) ) 100 kHz; νRF (27Al) ) 45 kHz (ter_7/ter_10) and 35 kHz (ter_15); length of REAPDOR pulse 23.8 µs. A P-O-Al distance of 3.2 Å6,7 was used for the simulations.

the signal at -17 ppm indicates the presence of octahedrally coordinated AlO6 units as the dominating Al species in these glasses. Only in the Al-rich glass containing 15 mol% Al2O3, minor contributions of tetra- and pentacoordinated Al, i.e. AlO4 and AlO5 units, can be detected, which can be readily explained by the increasing O/P ratio.21 Thus, although the overall spectral resolution is somewhat limited, the analysis clearly shows that the glass structure of the studied ternary glasses is dominated by a mixed aluminophosphate network containing P-O-P and P-O-Al linkages. The temperature dependent 31P MAS NMR spectra in the temperature range 20 °C e T e 650 °C for the glasses ter_7, ter_10, and ter_15 are collected in Figures 6, 7, and 8, respectively. As for the binary glasses, the complete MAS spectra including all spinning sidebands are plotted in Figures 6-8a, whereas Figures 6-8b exhibit a magnified view of the isotropic signals; Figure 6c contains the simulated isotropic

signals, vide infra, The corresponding 27Al MAS NMR spectra are compiled in Figure 9. For glass ter_7, a significant reduction in the spinning sideband intensity is observed in the 31P MAS NMR spectra for temperatures >360 °C, indicating the onset of mobility on a time scale of the inverse of the MAS frequency. Again, this is accompanied by a gradual smearing of the isotropic signals into a broad line at T ) 450 °C and into a single narrow line at T ) 600 °C. For the other two ternary glasses studied, only the changes in spinning sideband intensities and line widths with increasing temperature can be observed (with onset temperatures of 560 and 625 °C for glasses ter_10 and ter_15, respectively). Here, due to the lack of spectral resolution in the 31P MAS NMR spectra, no distinct changes in the isotropic signals can be observed. The shift of the onset temperatures to higher values follow the observed trend for the glass transition temperatures as observed in the DSC curves. The increasing network mobility with temperature also manifests


Figure 6.


31

P MAS NMR spectra for glass ter_7 for the indicated temperatures. The arrangement of the spectra resembles that of Figure 2.

itself in the 27Al MAS NMR spectra. Whereas the 27Al MAS NMR spectra for the three different glasses at room temperature exhibit the typical asymmetric line shape (for the octahedrally coordinated Al species) indicative of a distribution of the principal values of the tensor of the electric field gradient (EFG), the line shape becomes symmetric at temperatures > 400 °C for ter_7, > 500 °C for ter_10, and >625 °C for ter_15, respectively, indicating an averaging of the quadrupolar interaction of second order due to the network dynamics. These temperatures correlate well with the onset temperatures of the reduction in the spinning sideband intensity for the 31P MAS NMR spectra. Apart from the changeover from an asymmetric to a symmetric line shape monitored for all studied glasses, for glass ter_15 a gradual merging of the 27Al signals for AlO6 and AlO5 species can be observed at temperatures g600 °C, indicating a dynamic exchange process between these two species. To check for a possible correlation between the observed dynamic exchange processes (between the different phosphate units, i.e. P-O-P and P-O-Al bond breaking and reforming) and the macroscopic viscous flow, we modeled the 31P MAS NMR spectra to obtain correlation times and activation energies for the dynamic exchange. Since it seems impossible to separate the two types of dynamics (i.e., dynamic exchange between different Q-sites and dynamic exchange between different orientations of the same Q-site),8 we restricted our analysis to a simulation of the temperature dependent line shape changes of the isotropic signals employing a simple two site exchange model, thus neglecting any effect of the exchange processes on the spinning sidebands. The time domain signal is then calculated according to28,29

g(t) ) b g (0) exp(i2πυ + π)t1

in which b g(0) contains the occupancies of the n involved sites, π denotes the exchange matrix, containing the exchange frequencies, 1 denotes the unity vector of length n, and υ represents a diagonal matrix containing the resonance frequencies for the individual sites. Due to mentioned limitations of the model and due to the rather weak dependence of the line shape on the exchange frequencies at very low and very high exchange frequencies, the estimated error margins for the obtained exchange frequencies are quite large ((50% of the obtained values at very high end very low exchange frequencies, (30% of the obtained values in the intermediate region; cf. Figure 10). The simulations for the isotropic 31P MAS NMR signals for the binary glasses bin_20 and bin_35 are compiled in Figures 2c and 3c; the simulations for glass ter_7 are collected in Figure 6c. The spectra were obtained by a Fourier transformation of g(t) after an exponential multiplication was added to account for the line width. A line broadening of ca. 500 Hz was used for most of the simulations, except for the 600 °C spectrum of glass ter_7, for which a line broadening of only 150 Hz was employed. The observed shift of the isotropic chemical shift of the various signals with temperature was accounted for by an extrapolation of the evolution of the shift values taken from the spectra at temperatures at which the lines are not affected by the exchange process (e.g., T < 300 °C) for glass bin_35. For the binary glasses, there are only two sites present, i.e. Q02 and Q03 units. The first indication for a dynamic exchange between these two species can be found in the spectra at T ) 300 °C for glass bin_20 and T ) 325 °C for glass bin_35, respectively, in which a broadening of the two signals can be observed. This broadening is followed by a gradual merging of the two signals into a single broad line indicating coalescence


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Figure 7. 31P MAS NMR spectra for glass ter_10. (a) Full spectra including all spinning sidebands. (b) Detailed view of the isotropic signals.

Figure 8. 31P MAS NMR spectra for glass ter_15. (a) Full spectra including all spinning sidebands. (b) Detailed view showing the isotropic signals.

in the temperature range 380 °C e T e 460 °C. The activation energy for the dynamic Q02 S Q03 exchange process (i.e rapid P-O-P bond breaking and reforming) can be obtained from an Arrhenius plot of the obtained correlation times as a function of the inverse temperature (cf. Figure 10). For glass bin_20, an activation energy EA ) 89 ( 20 kJ/mol (Figure 10a) is calculated, and the analysis of the data for glass bin_35 results in a value of EA ) 105 ( 20 kJ/mol (Figure 10b). For the ternary glasses, the modeling of the dynamic exchange via a simulation of the isotropic 31P MAS NMR signals was only performed for glass ter_7, due to the lack of spectral resolution in isotropic signals of glasses ter_10 and ter_15. As shown above, apart from pure phosphate species (mainly Q03), phosphate species connected to one AlO6 octahedron (i.e., Q12 species) constitute the dominant local phosphate environment. The occurrence of a narrowed single resonance line at high temperatures in the 31P MAS NMR spectra of this glass indicates a rapid dynamic exchange between all involved species. We modeled this exchange assuming a two-site exchange process (between Q21 and Q30 species). The resulting spectra are collected in Figure 6c, the obtained correlation times are plotted as a function of the inverse temperature in Figure 10c. From this, an activation energy of EA ) 112 ( 20 kJ/mol can be deduced. A possible mechanism for the observed dynamic exchange between Q02 and Q03 species in the binary phosphate glasses is sketched in Figure 11a. The initial step involves the attack of a nonbridging oxygen to a Q3 unit accompanied by a (simultaneous or successive) breaking of a P-O-P linkage. From our experiments, we can not decide whether or not these two steps occur simultaneously or successively. If the two steps occur

successively, the transition state necessarily involves five coordinate phosphorus, similar to the proposed five coordinate transition state for Q3/Q4 exchange in silicate glasses.14 In addition to a Q02/Q03 exchange process, in the ternary phosphate glasses an additional exchange between mixed aluminophos2 2 and Q2,AlO6 ) and the pure phosphate phate units (i.e., Q1AlO6 species (Q20 and Q30) takes place, indicating rapid P-O-Al bond breaking and reforming. This exchange may be visualized following the mechanism as sketched in Figure 11b. Here, the exchange between a Q30 and a Q21AlO6 site is initiated by the attack of a nonbridging phosphate oxygen to an AlOx species. This exchange process is accompanied by a simultaneous change in the Al coordination and necessarily entails a dynamic exchange process between the involved AlOx species, as observed in the 27 Al MAS NMR spectra for glass ter_15 (cf. Figure 9c; spectra at 600 and 625 °C). The obtained correlation times may now be compared to shear correlation times describing macroscopic viscous flow, which can be calculated using the Maxwell relation τshear ) η/G∞ with G∞ denoting the shear modulus at infinite frequency, 1-3 × 1010 Pa.30,31 From this, shear correlation times in the range of 30 s < τ < 100 s are obtained at the glass transition (η ) 1012 Pa s). Extrapolating our NMR data to the glass transition temperatures (cf. Figure 10) produces correlation times for the local P-O-P and P-O-Al bond breaking and reforming in the range of 1-5 s for glasses bin_20, bin_35, and ter_7. These values are in reasonable agreement with those obtained from the Maxwell relation and clearly indicate a close relation between local phosphate species exchange and the macroscopic viscosity.


Figure 9.


27

Al MAS NMR spectra for the indicated temperatures for glass ter_7 (a), glass ter_10 (b), and glass ter_15 (c).

Figure 10. Arrhenius plots (correlation time plotted as a function of the inverse temperature) for the obtained correlation times for the exchange process observed in glass bin_20 (a), glass bin_35 (b), and glass ter_7 (c). The linear fits correspond to activation energies of 89, 105, and 112 kJ/mol for glass bin_20, glass bin_35, and glass ter_7, respectively.

In addition to the correlation times, the obtained activation energies for the local bond breaking and reforming processes

may be compared to those obtained from macroscopic viscosity measurements. Huang et al.32 studied a set of iron phosphate


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Figure 11. Sketch of possible mechanisms for the observed exchange processes between Q20 and Q30 units (a) and Q30 and Q21 units (b) as observed in the binary and ternary glasses. Note that the scheme in part b also accounts for the exchange between AlO5 and AlO6 units, as observed in the 27 Al MAS NMR spectra for glass ter_15 at high temperatures.

glasses and obtained activation energies for the viscosity of about 82-126 kJ/mol, from the slope of the log η versus 1/T plot. In another study by Kim et al.33 on this glass system, an activation energy of about 100 kJ/mol was found by high temperature viscosity measurements. For the viscosity of a pure P2O5 glass, built exclusively from Q03 units, Doremus finds an activation energy of 175 kJ/mol.34 Thus, the activation energies found in this work for the binary potassium phosphate glasses and the ternary aluminophosphate glass (89-112 kJ/mol) agree quite well with the published viscosity data (82-126 kJ/mol). Together with the correlation times for the local bond breaking and reforming matching those calculated from the Maxwell relation (vide supra), these findings can be taken as an indication that the local P-O-P and P-O-Al bond breaking and reforming constitute the determining microscopic step of the macroscopic viscosity. Conclusion The rapid dynamic exchange between the various phosphate species constituting the network in binary and ternary potassium (alumino)phosphate glassessi.e. the breaking and formation of P-O-P and P-O-Al bondsscould be identified as the fundamental step controlling the macroscopic viscosity in these glasses. The obtained activation energies of about 90-110 kJ/ mol correspond quite well to published values for the viscosities of a range of different phosphate glass systems. These findings are corroborated by the concomitant agreement between the extrapolated correlation times at the glass transition temperatures for the studied glass samples and the shear viscosity correlation times as calculated from the Maxwell relation. In essence, the

observed behavior for phosphate based glasses resembles that of silicate and borate glasses as studied by Stebbins, in which local dynamic exchange between different silicate tetrahedra or borate species was identified as the decisive step controlling the viscosity of the investigated glasses. Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft is gratefully acknowledged. References and Notes (1) Vogel, J.; Holand, W.; Naumann, K.; Gummel, J. J. Non-Cryst. Solids 1986, 80, 34. (2) Weber, M. J. J. Non-Cryst. Solids, 1990, 123, 208. (3) Donald, I. W.; Metcalfe, B. L. J. Non-Cryst. Solids 2004, 348, 118. (4) Kreidl, N. J.; Weyl, W. A. J. Am. Ceram. Soc., 1941, 24 (11), 372. (5) Brow, R. K. J. Am. Ceram. Soc. 1993, 76 (4), 913. (6) van Wu¨llen, L.; Tricot, G.; Wegner, S. Solid State Nucl. Magn. Reson. 2007, 32, 44. (7) Wegner, S.; van Wu¨llen, L.; Tricot, G. J. Non-Cryst. Solids, 2008, 354, 1703. (8) (a) Mysen, B. Geochimica Et Cosmochimica Acta 1996, 60, 3665. (b) Mysen, B. Contributions to Mineralogy and Petrology 1997, 127, 104. (c) Mysen, B. Physics of the Earth and Planetary Interiors 1998, 107, 23. (d) Mysen, B. O.; Frantz, J. D. Geochimica Et Cosmochimica Acta 1994, 58, 1711. (9) Farnan, I.; Stebbins, J. F. J. Am. Chem. Soc., 1990, 112, 32. (10) Stebbins, J. F. J. Non-Cryst. Solids 1988, 106, 359. (11) Stebbins, J. F.; Sen, S. J. Non-Cryst. Solids. 1998, 224, 80. (12) Stebbins, J. F.; Sen, S.; George, A. M. J. Non-Cryst. Solids 1995, 192 + 193, 298. (13) Stebbins, J. F.; Sen, S.; Farnan, I. Am. Mineral., 1995, 80, 861. (14) Farnan, I.; Stebbins, J. F. Science 1994, 265, 1206. (15) Sen, S.; Stebbins, J. F. Phys. ReV. B 1997, 55, 3512. (16) Stebbins, J. F.; Ellsworth, S. E. J. Am. Ceram. Soc. 1996, 79, 2247.

Dynamics in Aluminophosphate Glasses (17) Dias, A. G.; Skakle, J. M. S.; Gibson, I. R.; Lopes, M. A.; Santos, J. D. J. Non-Cryst. Solids 2005, 351, 810. (18) Mysen, B. O. Contrib. Miner. Pet. 1998, 133, 38. (19) van Wu¨llen, L.; Wegner, S.; Tricot, G. J. Phys. Chem. B 2007, 111, 7529. (20) Brow, R. K. J. Non-Cryst. Solids, 2000, 263&264, 1. (21) Kirkpatrick, R. J.; Brow, R. K. Solid State Nucl. Magn. Reson. 1995, 5, 9. (22) Brow, R. K.; Kirkpatrick, R. J.; Turner, G. L. J. Am. Ceram. Soc. 1990, 73, 2293. (23) Gullion, T. Chem. Phys. Lett. 1995, 246, 325. (24) Chopin, L.; Vega, S.; Gullion, T. J. Am. Chem. Soc. 1998, 120, 4406. (25) Ba, Y.; Kao, H. M.; Grey, C. P.; Chopin, L.; Gullion, T. J. Magn. Reson. 1998, 133, 104. (26) Bak, M.; Rasmussen, J. T.; Nielsen, N. C. J. Magn. Reson. 2000, 147, 296. (27) Schneider, J.; Oliveira, S. L.; Nunes, L. A. O.; Panepucci, H. J. Am. Ceram. Soc. 2003, 86, 317.

J. Phys. Chem. B, Vol. 113, No. 2, 2009 425 (28) Abragam, A. The Principles of Nuclear Magnetism; Clarendon Press: Oxford, 1961. (29) Schmidt-Rohr, K.; Spiess, H. W. Multidimensional Solid State NMR and Polymers; Academic Press: San Diego, 1996. (30) Scherer, G. W. Relaxation in Glass and Composites; Wiley: New York, 1986. (31) Brawer, S. Relaxation in Viscous Liquids and Glasses; American Ceramic Society: Columbus, OH, 1985. (32) Huang, W.; Day, D. E.; Ray, C. S.; Kim, C. W. J. Nucl. Mater. 2005, 346, 298. (33) Kim, C.; Day, D. E. J. Non-Cryst. Solids 2003, 331, 20. (34) Doremus, R. Glass Science; John Wiley: New York, 1994. (35) Sidebottom, D.; Changstrom, J. Viscoelastic Relaxation of Molten Phosphorous Pentoxide, APS March Meeting, New Orleans, Louisiana, 2008. (36) Massiot, Fayon, D. F.; Capron, M.; King, I.; Calve´, S.; Le Alonso, B.; Durand, J.-O.; Bujoli, B.; Gan, Z.; Hoatson, G. Magn. Reson. Chem. 2002, 40, 70.

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