Article pubs.acs.org/JPCC
Description of the Intermediate Length Scale Structural Motifs in Sodium Vanado-phosphate Glasses by Magnetic Resonance Spectroscopies Gregory Tricot*,† and Hervé Vezin‡ †
UCCS UMR-CNRS 8181, ENSCL-C7, Université Lille 1Sciences et Technologies, 59655 Villeneuve d’Ascq Cedex, France LASIR UMR-CNRS 8516, C4, Université Lille 1Sciences et Technologies, 59655 Villeneuve d’Ascq Cedex, France
‡
ABSTRACT: For the first time, the local and medium range orders in sodium vanadophosphate glasses have been investigated by advanced magnetic resonance spectroscopy methods. One- and two-dimensional 31P/51V magic angle spinning nuclear magnetic resonance techniques (31P(51V) REAPDOR and 51V(31P) D-HMQC) have been used to monitor the formation of P−O−V5+ bonds and to provide the first accurate description of the intermediate length scale structural motifs in these glasses. The structural model has been completed by the investigation of the chemical environment of the V4+ ions (produced through the partial reduction of V5+ during the melting stage of the glass preparation) using standard continuous wave and advanced pulsed electron paramagnetic resonance techniques (HYSCORE). Finally, the combination of both sets of data leads to the first complete and precise structural model of the alkali vanadophosphate glass system.
1. INTRODUCTION Since their discovery in the 50s by Denton et al.,1 vanadophosphate glasses have been extensively studied because of their particular properties deriving from the association of two glass network formers oxides (P2O5 and V2O5) and from the presence inside the glass matrix of vanadium under two different oxidation states.2−4 The association of P2O5 and V2O5 is supposed to give rise to a mixed glass network, formed by P− O−V bonds as suggested by Raman and nuclear magnetic resonance (NMR) spectroscopy.5,6 The presence of P−O−V bonds rules the topology and reticulation of the glass network and thus governs the macroscopic properties like the glass transition temperature (Tg), the viscosity, or the thermal expansion coefficient. Another particularity of vanado-phosphate glasses comes from the presence within the glass matrix of vanadium under two distinct oxidation states (V5+ and V4+). The presence of the latter results from complex oxidoreduction and oxo-basicity reactions leading to partial reduction of V5+ to V4+ during the melting stage. The presence of both vanadium oxidation states is finally kept in the glass structure during the melt-quenching stage. The redox (defined as V4+/ Vtotal) can be controlled by many parameters like the glass composition or the melting temperature/atmosphere. The following equilibrium V2O5 ↔ 2VO2+ + 2O2− + 1/2O2 has been used to model the reactivity and to explain the evolution of the proportion of V4+ with the glass composition, the temperature, and the melting atmosphere.6 Owing to these special characteristics, vanado-phosphate based glasses have been developed to be used as low temperature sealing glasses or as a component for batteries.7 The latter application benefits from the electronical con© 2012 American Chemical Society
ductivity of these glasses occurring by polaron hopping between the V4+ and V5+ sites. In order to understand the impact of the structure on the macroscopic properties and to prepare materials with tailored characteristics, many structural investigations have been made using Raman, infrared, or magic angle spinning (MAS) NMR spectroscopies.5,6 The mixing between the phosphate and vanadate units, giving rise to the mixed glass network, has been suggested by Raman and NMR investigations,5,6 but there is no detailed information about the exact architecture of the vanado-phosphate units forming the glass structure. The environment of the paramagnetic V4+ ions also needs to be accurately described. Many studies using the EPR technique have been carried out.8,9 If all the studies show that reduced vanadium exist under the vanadyl VO2+ form, clear information about the chemical environment of vanadyl ion is still missing. The structural model of the vanado-phosphate glasses has thus to be considered as incomplete. In this paper, glasses formulated with a constant proportion of Na2O oxide (50 mol %) and different P/V proportions were prepared and characterized. One-dimensional 31P and 51V MAS NMR are used to probe the local order around phosphate and vanadate species. The mixing between the vanadate and phosphate species has been qualitatively and quantitatively investigated by 2D dipolar heteronuclear multiple quantum correlation10 (DHMQC) and rotational echo adiabatic passage double resonance11 (REAPDOR) NMR techniques. The results allow describing the phosphate speciation with the Qnm,VOx Received: July 30, 2012 Revised: December 24, 2012 Published: December 26, 2012 1421
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notation where n is the number of connected phosphorus, m the number of attached vanadium, and VOx the coordination state of the connected vanadium. In order to complete the structural model, electron paramagnetic resonance (EPR) techniques were used to describe the local order around the V4+ ions and to investigate the interactions between V4+ and the P5+/V5+ elements. The continuous wave (CW) experiments give access to the local symmetry of V4+ ions by both measuring the Landé factor (g) and the 51V hyperfine coupling. Then, the chemical environment was probed using advanced pulsed EPR experiments (2D-HYSCORE).12 The obtained spectrum provides information about the nature and the numbers of nuclei coupled to the V4+ paramagnetic center.
correlation map can be used to trace the POV connectivity scheme. The rotational echo adiabatic passage double resonance (REAPDOR)11,14−16 sequence has been applied in order to quantitatively probe the heteronuclear dipolar interaction between the 31P and 51V nuclei and to determine the number of vanadate units connected to the phosphate species. The application of the REAPDOR sequence on inorganic materials has been extensively described in literature.17−21 This sequence allows building curves expressing the evolution of dephasing signal (S0 − S)/S0 versus an echo delay. The S0 signal is obtained after a 90°−τ−180°−τ 31P spin−echo sequence during which the heteronuclear 31P/27Al dipolar interaction has been averaged out. The S signal is acquired with a similar 31P spin−echo during which the dipolar interaction is reintroduced by irradiating the 27Al nuclei. The (S0 − S)/S0 versus τ curves can then be related to the dipolar interaction intensity that depends on the P−V distance and the number of vanadium ions around phosphorus. By assuming a constant P−V distance, the READPOR results presented here can be used to determine the number of vanadium attached to each phosphate species (m in the Qnm notation) as it has previously been used in the case of alumino-phosphate or boro-phosphate glasses.18−21 More details about the acquisition parameters have been inserted in the figure captions. 2.3. CW and Pulsed EPR Analyses. EPR experiments were performed with an X-band Bruker ELEXYS E580 spectrometer. CW and pulsed experiments were performed at 4 K on powder samples. CW spectra were recorded using 1 mW of microwave power and 5 G of modulation amplitude. For pulsed experiments, 2-pulses echo field sweep detection was performed using standard Hahn echo sequence π/2−τ−π− echo with respectively π/2 and π pulses set to 16 and 32 ns. The τ value was set to 200 ns. For hyperfine sublevel correlation spectroscopy (HYSCORE)12 experiments, we used the pulse sequence π/2−τ−π/2−t1−π−t2−π/2−τ−echo, HYSCORE) whereby an echo is generated at time τ after the last π/ 2 pulse, with τ representing the delay between the first and second π/2 pulses. The echo intensity is measured at each t1 and t2 values, which are varied stepwise at constant τ. This twodimensional set of echoes gives, after Fourier transform along t1 and t2, a two-dimensional HYSCORE spectrum. The length of the π/2 and π pulses were 16 and 32 ns, respectively, and a delay τ = 132 ns between the first two π/2 pulses gave the best sensitivity and resolution for the detection of the phosphorus nuclei. This value was determined by performing electron spin echo envelop modulation (ESEEM) 2D-3Pulses-ESEEM22 experiments versus τ value. HYSCORE spectra are acquired with 256 × 256 data points for both t1 and t2 time domains. Second-order polynomial background subtraction was performed to remove the unmodulated part of the echo. Twodimensional Fourier transformation of the spectra was performed using a Hamming apodization window function and magnitude calculated.
2. EXPERIMENTAL PROCEDURES 2.1. Preparation of the Glass Series and Measurement of the Macroscopic Properties. Glasses were prepared along the 50Na2O−xV2O5−(50 − x)P2O5 (0 < x < 30) composition line with the standard melt-quenching method. Batches of NaPO3 and NaVO3 were mixed in appropriate proportions and directly melted at 650−800 °C during 20 min in a Pt−Rh10% crucible, before being poured on a brass plate. The melting temperature was optimized for each composition to avoid P2O5 volatilization. The glasses are labeled with the NaVP_x notation where x denotes the V2O5 batch content. For example, a sample containing 10% of V2O5 (50Na2O− 10V2O5−40P2O5) will be referred to as NaVP_10. The ZrVPO7 crystalline compounds were synthesized according to ref 7 and used as reference to calibrate the NMR experiments. The glass transition temperatures have been determined by differential scanning calorimetry on a SETARAM EVO131 using a heating rate of 10°/min. The proportion of reduced vanadium (V4+), from which the redox (r = V4+/Vtot) has been calculated, has been measured with the wet chemistry method following the procedure described in ref 6. 2.2. 31P and 51V MAS NMR Analyses. The local orders around vanadate, phosphate, and sodium ions were investigated by 1D MAS NMR. The 51V and 23Na MAS NMR experiments have been performed at 105.2 and 105.8 MHz (Bruker 9.4 T spectrometer) with a 2.5 mm measurement probe operating at a spinning frequency of 30 kHz. The 31P MAS NMR experiments have been performed at 161.9 MHz (9.4 T) with a 3.2 mm probe operating at a spinning frequency of 20 kHz. More details about the acquisition parameters are given in the figure captions. The 51V, 31P, and 23Na chemical shifts are referred to VOCl3, H3PO4, and NaCl solutions, respectively. The interactions between the phosphate and vanadate species have been quantitatively and qualitatively investigated by correlation NMR techniques on a 9.4 T spectrometer equipped with a 3.2 mm HXY channel operating at a spinning frequency of 20 kHz. Two-dimensional correlation spectra, directly showing the interactions between phosphate and vanadate moieties, have been produced with the recently developed 51V( 31P) D-HMQC technique.10 This NMR sequence produces a 2D spectrum showing correlation signals between spatially close phosphate and vanadate units. More details about the acquisition parameters are given in the figure captions. However, it is worthy to note here that our experiment has been performed with a very short contact time (800 μs). This value allows for the creation of a through space correlation signal only in the case of connected P−O−V atoms, as previously shown in the case of the ZrVPO7 crystalline sample.10 As a consequence, the through space
3. RESULTS 3.1. Glasses Preparation and Properties. Homogeneous glasses were obtained up to a V2O5 content of 30%. The obtained glasses exhibit a metallic dark violet color and are opaque, as expected for glasses containing a transition metal under two oxidation states. Glasses with vanadium contents higher than 30% suffered from partial or complete devitrification when cooling. Since weight losses (Δm/m) were always 1422
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the α-NaVO3 structure for which the same chemical shift has been measured.23 This assignment is also supported by the chemical shift anisotropy (CSA) parameters extracted from the spectrum simulation (ΔCSA = 420 ppm and ηCSA = 0.3).23 It is noteworthy that the VO4 signal remains identical all along the composition line, suggesting that the quadrupolar parameters of this signal do not change with the composition. The second signal has been assigned to octahedral vanadium owing to its very large chemical shift anisotropy (ΔCSA = 900 ppm). It is noteworthy that this latter parameter has not been directly determined on the presented samples due to the very low amounts of VO6 units but on samples from the NaPO3−V2O5 composition line6). The octahedral signal is present in glasses with low amounts of V2O5 and disappears when V2O5 molar content reachs 10%. The 31P MAS NMR spectra are illustrated in Figure 1b. Three resonances can be observed all along the composition line. For pure NaPO3 sample (x = 0), one single resonance at −20 ppm, characteristic of the Q20 site in the Na2O−P2O5 system, can be noticed. In samples containing less than 15% V2O5, a second signal, centered at −2 ppm, appears at the expense of the Q20 resonance. Finally, for glasses containing large amounts of V2O5, a third resonance raises at 8 ppm. In the rest of the text, the three signals centered at −20, −2, and 8 ppm will be respectively denoted as P(1), P(2), and P(3). The 23 Na spectra are reported in Figure 1c and show a broad and asymmetric signal centered around −10 ppm. The signal line shape indicates an important distribution of chemical environments resulting in the observed nonsymmetric Czjzek line shape.24 No significant change can be observed with the increase of V2O5 in the formulation.
inferior to 2%, the glass compositions given in the text are referred to the batch compositions. The glass transition temperature exhibits a nonlinear evolution. Tg increases at low V2O5 content and decreases when V2O5 content is higher than 10%. If the redox (V4+/Vtot) decreases with the glass composition (in good agreement with previous results6), the absolute molar content of V4+ does not change significantly and does not exceed 2% all along the composition line. The melting conditions, the proportion of reduced vanadium, and the weight losses are reported in Table 1. Table 1. Melting Temperature (Tm), Weight Losses (Δm/ m), Glass Transition Temperature (Tg), Redox (r = V4+/ Vtotal), and the Absolute Molar Content of V4+ of the 50Na2O−xV2O5−(50 − x)P2O5 Glass System x
Tm (°C)
Δm/m (%)
Tg (°C ± 3)
r (% ± 1)
V4+ (mol % ± 0.5)
0 2.5 5 7.5 10 15 20 25 30
750 800 800 800 800 700 700 650 650
1.2 0.7 1.4 1.1 1.7 1.2 0.4 1.2 0.9
285 290 300 307 312 296 280 257 242
64.0 48.1 37.3 32.4 16.2 14.1 11.2 8.0
0 0.8 1.2 1.4 1.6 1.2 1.4 1.4 1.2
3.2. 51V and 31P 1D MAS NMR. The 51V MAS NMR spectra, shown in Figure 1a, are composed of two different resonances at −580 and −680 ppm. The first signal has been assigned to tetrahedral vanadium close to sodium ions like in
Figure 1. 51V (a), 31P (b), and 23Na (c) MAS NMR spectra obtained at 9.4 T on the 50Na2O−xV2O5−(50 − x)P2O5 composition line. The 51V spectra have been recorded with a pulse length of 1 μs, a radio frequency (rf) field strength of 35 kHz (measured on a liquid sample), 1024 transients and a recycle delay (rd) of 1 s. The spinning sidebands are denoted as *. The 31P spectra have been acquired with a pulse length of 1.5 μs, a rf strength of 80 kHz, 32 transients and a rd of 60 s. The 23Na spectra have been obtained with a pulse length of 1 μs, a rf strength of 40 kHz (measured on a liquid sample), 128 transients and a rd of 3 s. 1423
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3.3. 51 V/31P Correlation MAS NMR. The sample NaVP_30 has been investigated with the D-HMQC technique in order to edit a 2D correlation map directly showing the P/V interactions and tracing the presence of P−O−V bonds. The 2D spectrum, reported in Figure 2a, clearly shows a correlation
Figure 3. ΔS/S0 curves obtained on the P(1) (full triangles), P(2) (empty circles), and P(3) (full squares) components. The data have been obtained from the 51V(31P) REAPDOR NMR experiment performed on the x = 2.5, x = 12.5, and x = 30 samples for P(1), P(2), and P(3), respectively. Dotted lines represents the ΔS/S0 curves obtained on a Q11,VO4 environment using the crystalline ZrVPO7 compound. The REAPDOR experiments were conducted at a spinning frequencies of 20 kHz with rf fields of 60 and 50 kHz for 51 V and 31P, respectively. The 20 sets of S0 and S signals were recorded with 16 scans separated by a rd of 60 s. Each transient was preceded by a saturation pulse scheme.
confirming thus the vanadyl structure of the V4+ ion. Owing to the 99.75% natural abundance of 51V, hyperfine coupling (hf) arising from electron−nuclear interaction can be observed. Quantification of the hf coupling leds to A∥ and A⊥ values of 537.6 and 210 MHz, respectively. All the glass samples exhibit the same g and A values indicating that the glasses exhibit the same V4+ environment independently of the glass formulation. In order to go further in the description of the nuclear environment of the V4+ centers, pulsed EPR experiments were performed using two-dimensional HYSCORE experiments. Each hyperfine line observed in the CW spectrum results from various mI 51V (I = 7/2) transitions but the superhyperfine coupling cannot be resolved using CW spectroscopy. Information about the nuclear environments of V4+ and about the nuclear superhyperfine coupling values can be obtained using spin−echo spectroscopy such as 2D-HYSCORE experiments. Figure 4b exhibits the spectrum detected by spinecho experiments recorded as a function of the magnetic field. The spectrum reported in Figure 4b is similar to those measured with CW techniques. As using such techniques, we can make an orientation selection of the nuclear environments. As HYSCORE suffer from a blind spot effect, the τ value was previously optimized by performing 3P-ESEEM varying τ and was found to be optimum at 132 ns. Figure 5 displayed the results for the three selections of orientation recorded. The mI = −7/2∥ transition shows in the (+,+) quadrant a pair of cross peaks centered at 31P nuclear Larmor frequency indicating that Aiso < 2ν(31P) (Figure 5a). The A value measured for this phosphorus is 10 MHz. It is noteworthy that a peak is observed on the diagonal. This signal has been assigned to 51V nuclear frequency due to the presence of V5+ in the vicinity. Nevertheless, the weak dipolar coupling associated to this peak rules out direct connectivity between 51 5+ V and V4+ moieties. The spectrum recorded for the mI = 3/ 2 transition where the magnetic field is perpendicular to the V4+ axis (Figure 5b) displays a pair of cross peaks in the (−,+) quadrant arising for the strong coupling case (Aiso > 2ν(31P)). The cross peaks are separated by 12.5 MHz (twice the 31P nuclear frequency) and split at A/2. The Aiso determined from experimental spectrum is 16 MHz. Additionally, in the (+,+)
Figure 2. Two-dimensional 51V{31P} D-HMQC spectrum obtained on the NaVP_30 sample (a). The 51V and 31P slices extracted from the 2D spectrum (b,d) are compared to the 1D 51V and 31P MAS NMR spectra (c,e). The 2D spectrum has been collected using 1928 × 120 data points. The 51V selective 90°−τ−180°−τ spin−echo was performed with pulse lengths of 8 and 16 μs, the 31P channel was irradiated with 90° pulses of 5 μs length. The dipolar interaction was introduced with the SFAM2 pulse scheme during 800 μs by irradiating the 31P channel with a 60 kHz amplitude and 20 kHz frequency modulated rf field. Each t1 slice was recorded with 512 scans and a rd of 0.5 s.
signal between P(3) and the tetrahedral vanadate species and definitively proves the presence of P−O−V in the glass structure. It is worthy to note that samples with low V2O5 content have also been studied with the same technique in order to investigate the interaction between the phosphate and the VO6 units. However, all the attempts were unsuccessful, not because of the low V2O5 amount, but more likely because of the important relative proportion of paramagnetic reduced vanadium that leads to very short 51V T2′ values. The 31P(51V) REAPDOR results are summarized in Figure 3. Typical results obtained for the three components P(1), P(2), and P(3) have been reported, accompanied by the REAPDOR curve obtained on the crystalline reference ZrVPO7 in which the phosphate site is connected to one tetrahedral vanadium species resulting in a P−V distance of 3.42 Å.13 P(1) presents an almost inexistent dephasing curve (full triangles), suggesting a very weak interaction between this phosphate species and the vanadate moieties. The REAPDOR curve of P(2) exhibits a higher dephasing curve than P(1). This dephasing is similar to the one obtained on the ZrVPO7 crystalline sample, suggesting that both phosphates experience the same VOP connectivity scheme, i.e., a connectivity with a single vanadate unit. Finally, the most intense dipolar interaction (and thus the most important dephasing) is observed for P(3). 3.4. EPR Results. The CW EPR spectrum of the NaVP_30 sample is displayed in Figure 4a. The spectrum feature is typical of V4+ (d1 electronic configuration, S = 1/2) species in an axial symmetry with g⊥ > g∥ with respective values of 1.98 and 1.94, 1424
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Figure 4. (a) CW EPR spectrum recorded at 4 K (left) and (b) 2-pulses echo field sweep recorded at 4 K on the NaVP_30 sample. Arrows shows the transition positions used for 2D-HYSCORE experiments.
coupling constant that are observed in the (+,+) quadrant (10 and 5.7 MHz, respectively).
4. DISCUSSION The information about the vanadate network is in a good agreement with previous NMR investigations obtained on the NaPO3−V2O5 glass system.6 At low V2O5 amounts (
