Microscopic structure of the glassy ionic conductor x · LiF + (1 − x) · LiPO3 from NMR data

Journal of Non-Crystalline Solids 240 (1998) 79±90

Microscopic structure of the glassy ionic conductor x á LiF + (1 ) x) á LiPO3 from NMR data S.V. Dvinskikh a, I.V. Murin b, A.F. Privalov c, A.A. Pronkin d, E. R ossler e, H.-M. Vieth c,* a

Institute of Physics, St. Petersburg State University, Ulyanovskaya 1, Petrodvoretz, 198904 St. Petersburg, Russian Federation Department of Chemistry, St. Petersburg State University, Universitetskij pr.2, Petrodvoretz, 198904 St. Petersburg, Russian Federation c Institut f ur Experimentalphysik, Freie Universit at Berlin, Arnimallee 14, D-14195 Berlin, Germany d Technological Institute, Moskovskij pr., 26, 198013, St. Petersburg, Russian Federation e Physikalisches Institut, UniversitaÈt Bayreuth, UniversitaÈtsstrasse 30, D-95440 Bayreuth, Germany

b

Received 23 December 1997; received in revised form 1 May 1998

Abstract The structure of phosphate glasses of the nominal composition x á LiF + (1 ) x) á LiPO3 has been studied for 0 < x < 0.35, using 1 H, 7 Li, 19 F, 31 P NMR. By combining data from di€erent nuclei and various solid state NMR techniques structural information on the molecular scale is obtained. The measurements con®rm that the structural units of the glass are polyphosphate chains, and allow determination of their average length. The increase of LiF concentration, x, from 0 to 0.35 leads to a shortening of the chains from 29 to 4 phosphate units by use of Li and F to form terminal groups. Fluorine is partitioned between two structural units: ¯uorophosphate terminal groups and Li‡ Fÿ fragments, in a ratio of about 1:1 at low x and decreasing to 1:4 at high x. Substantial loss of ¯uorine during glass preparation is observed and measured. Ó 1998 Elsevier Science B.V. All rights reserved. PACS: 61.16N; 66.30H; 76.60

1. Introduction In recent years much attention has been given to the study of glassy lithium conducting materials [1±6], since they are promising for a variety of technical applications, particularly as materials for batteries with higher voltages and energy densities. It is a common feature, that electroconductivity of glassy materials is higher and their activation en-

* Corresponding author. Tel.: +49-30 838 5062; fax: +49-30 838 6081; e-mail: [email protected].

ergy lower as compared to the crystalline state of the same compound [7]. In particular, for Li conducting compounds such an e€ect was con®rmed by Huggins [8]. A likely reason for the increase of conductivity in the glassy state is seen in its structural disorder and, induced by this disorder, also in its motional disorder which in crystalline superionic materials is less [9]. Details of the mechanism of ionic conductivity, in particular the relationship between glass structure and ionic mobility are still under discussion. A main problem is still the lack of knowledge about the microscopic structure of ionic glasses.

0022-3093/98/$ ± see front matter Ó 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 8 ) 0 0 7 0 2 - 9

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Phosphate glasses containing Li‡ as conducting ions are considered to be solid electrolytes [10]. At room temperature the dc conductivity is in the range 10ÿ8 ±10ÿ6 Xÿ1 cmÿ1 and it increases by as much as a factor of 105 at temperatures near the glass transition. The structure of phosphate glasses has the ability to accept a wide range of ionic substitution without altering structure, often accompanied by an increase of electroconductivity [3,11,12]. For example, the introduction of alkali metal ¯uoride into the phosphate leads to an increase of dc conductivity by about 1 order of magnitude (see Refs. [13,3] and references therein). In particular this increase was found for x á LiF + (1 ) x) á LiPO3 glasses [3]. A number of other properties can also be modi®ed in a way useful for a variety of applications. For instance, ¯uorine substitution decreases the crystallization process in phosphate glasses, leads to useful optical properties such as a relatively broad spectral transparency, a wide variability and small temperature coecient of the refractive index and allows larger numerical apertures in optical ®bers [14,15]. Our current investigation is aimed at a better understanding of the e€ects of ionic substitution on the microscopic structure of ¯uorophosphate glasses and, in particular, its relationship with the changes in electric conductivity. 1.1. Basic structure of phosphate glasses From X-ray analysis and neutron scattering it is known that in phosphate glasses the phosphorus is covalently bonded to four oxygens which are located at the corners of a tetrahedron [16]. Neighboring tetrahedra are joined by oxygen bridges and form a network structure. The metal cations are less strongly bonded to the non-bridging oxygens of the tetrahedra. As the concentration of metal cations is increased, more non-bridging oxygens are formed around the phosphorus. A way to characterize the phosphate structure is the classi®cation by Q…n† groups [16,17]. The Q…n† are basic structural units where n denotes the number of bridging oxygens per [PO4 ] tetrahedron. For example, from Q…2† units in®nite polyphosphate chains or ring-like structures can be formed, and

they are also the mid-units in ®nite polyphosphatic chains terminated by Q…1† units. In general, di€erent Q…n† groups participate in the polyphosphate structure to an extent which depends on the particular material composition. Accordingly to Van Wazer [17] the glass at the metaphosphate composition (e.g., 0.5Li2 O + 0.5P2 O5 ) ideally consists of PO4 tetrahedra, each unit with two bridging oxygens (Q…2† ), forming a polymer-like chain. Chromatographic analysis of lithium metaphosphate LiPO3 has shown that indeed the structure of such a glass consists mainly of linear chains of phosphate tetrahedra, linked by a bridging oxygen and having the structure [17]

The probability of forming ring-like structures is low. In practice the length of the phosphate chains is limited by the presence of glass modi®ers. For example, water reduces the chain length by hydrolyzing ±P±O±P± bonds to produce two chain terminating ±P±OH groups. Here, the hydrogen ion acts like a metal cation. In the case of metaphosphate the average chain length typically does not exceed 40 units when water is incorporated into the glass [18,19]. 1.2. The structure of ¯uorophosphate glasses As metal cations are added to the glass, the average chain length is reduced by forming further non-bridging oxygen ions which compensate the additional charge of the metal cations. This leads to the transformation of Q…2† into Q…1† units. In case of ¯uorine addition the mono¯uorophosphate ±PO3 F (and di¯uorophosphate ±PO2 F2 at high ¯uorine concentration) terminal groups are formed [12,20±26]. Thus, the adding of LiF causes depolymerization of the phosphatic chain and the creation of new fragments in the structure in accordance with the reaction

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2. Experimental

In reference [26] a qualitative analysis of the chain length of LiF doped LiPO3 glasses was made using aqueous solutions of the glass. It was found that the average chain length nav in LiPO3 glass is around 22, while in 0.2LiF + 0.8LiPO3 glass nav decreases to 6.6. The question of relationship between the structural units present in solid glasses and in their aqueous solution has been addressed already in early works [27,28] and also in more recent studies [18,19]. It has been argued that the structural units in the glass matrix and in a freshly made aqueous solution are similar, since the process of hydrolysis needs a longer time than the analysis. From the analysis of aqueous solutions of x á LiF + (1 ) x) á LiPO3 [26] it was also found that ¯uorine only partly participates in the phosphate network while a signi®cant percentage stays in the form of Li‡ Fÿ . However, in this approach it remains dicult to take fully into account the hydrolysis process, to estimate the role of residual water for the glass structure, and to get reliable quantitative information. More recently solid glass of the same kind was studied by X-ray photoelectron spectroscopy [3]. It was found that the partition between the two Fÿ positions varies with the ¯uorine content. The relative number of Licoordinated ¯uorine increases with the increase of LiF concentration. Similar e€ects were observed in ¯uorinated aluminophosphate and sodium aluminophosphate glasses, where the presence of both, F±Al and F±P species was found with a relative concentration that depends on ¯uorine content and preparation procedure [12,25]. In the present paper we report a comprehensive structural analysis of solid x á LiF + (1 ) x) á LiPO3 glass with systematic variation of the LiF content x (0 < x < 0.35), by combining various methods of solid state NMR spectroscopy (1 H, 7 Li, 19 F and 31 P NMR, with and without MAS) known to be sensitive to the local structure on the atomic scale.

Synthesis and characterization. The synthesis of glasses of composition x á LiF + (1 ) x) á LiPO3 was carried out in a carbon crucible at 850±880°C in an atmosphere of dry argon and was taking about 30 min. As starting materials commercially available LiPO3 and LiF speci®ed as purissimum and as analytical reagent, respectively, were used. The melt was transferred to preheated metallic forms and annealed for 45 min at a temperature of about 10°C below the glass transition temperature Tg . The quality of the annealed glass was optically controlled under a polarization microscope. Samples with ten di€erent nominal LiF concentrations were prepared (x ˆ 0.00, 0.01, 0.03, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35). It was found, that for concentrations x > 0.35 partial crystallization of the sample material cannot be avoided. Di€erential scanning calorimetry (DSC) measurements. DSC measurements were carried out in the temperature range between 100°C and 380°C at a heating rate of 10°C/min, using a Netzsch DSC-200 calorimeter. Samples were encapsulated in aluminum pans and an empty pan was used as reference. The glass transition temperatures were determined with an accuracy of ‹5°C by the intercept method. Data are compiled in Table 1. Conductivity measurements. DC conductivity r was measured in the temperature range 20±275°C using active Li-amalgam electrodes. Protective conducting rings were used to eliminate surface currents. Room temperature conductivity and activation energy Er are listed in Table 1. Nuclear magnetic resonance (NMR) spectroscopy. The isotopes 7 Li, 19 F, 31 P and 1 H were chosen for our NMR measurements. All these nuclei are well suited for NMR investigations because of their large gyromagnetic ratio and their high natural isotopic abundance. 1 H and 19 F NMR measurements were performed at an external ®eld B0 ˆ 7.05 T corresponding to resonance frequencies of 300 and 282 MHz, respectively (Bruker CXP-300 spectrometer). 1 H and 19 F NMR spectra were obtained by Fourier transform of the free induction decays (FIDs). The length of the 90° radio frequency excitation pulse was 1 ls and thus suciently short

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Table 1 Composition, glass transition temperature Tg , DC conductivity r ˆ r0 x á LiF + (1 ) x) á LiPO3 x (nominal mol%) 1 2 3 4 5 6 7 8 9 10 a b

0 1 3 5 10 15 20 25 30 35

Fluorine content (mol%)

a

0.8 ‹ 0.2 2.6 ‹ 0.5 1.7 ‹ 0.5 4.4 ‹ 0.5 8.3 ‹ 0.5 12.0 ‹ 1.0 13.4 ‹ 1.5 21.7 ‹ 2.0

exp{)Er /(2kT)} and its activation energy Er of glasses

Tg (°C ‹5°C)

r (Xÿ1 cmÿ1 (‹2%)

b

´10ÿ8 )

325 325 320 315 310 305 295 285 275 270

0.35

1.36

0.63 0.79 1.48 1.86 2.29 4.00 5.01

1.33 1.29 1.24 1.21 1.17 1.13 1.13

Er (eV ‹0.02)

Measured. At 25°C.

for exciting the necessary spectral range. Spectrometer dead time in our experiments was around 2.5 ls. Fluorine spectra were recorded in the temperature range between )50°C and +270°C. No decomposition of samples due to heating was found. The temperature was set with an accuracy of 1°C and stabilized to ‹0.1°C. The chemical shift reference for 19 F spectra was CFCl3 . Room temperature 31 P and 7 Li NMR measurements were performed at a magnetic ®eld B0 ˆ 9.3 T at resonance frequencies of 162 MHz and 155 MHz, respectively (Bruker DSX-400 spectrometer). Typical length of the 90° RF pulse was 2 ls. For recording the static (i.e. no MAS) 31 P spectra a Hahn echo sequence was used with appropriate phase cycling [29] and 10 ls delay between pulses. The chemical shift reference for 31 P spectra was 85% H3 PO4 in H2 O. The 7 Li NMR spectra were obtained by Fourier transform of the solid echo after a 90°x -s-64°y pulse sequence [30]. 31 P and 7 Li measurements under magic angle spinning conditions (MAS spectra, obtained from the FID after a 90° RF pulse) were performed using a Bruker MAS probe with spinning rates up to 12 kHz. MAS spectra recorded at two di€erent spinning rates (typically 10 and 6.5 kHz) were compared in order to separate central transitions from spinning side bands. It shall be mentioned that for the same phosphate glasses aging e€ects have been found [31]. As a precaution we used specimens that were at

least two years old; we did not observe any aging e€ect in our glassy samples, in particular the analysis of NMR spectra measured before and after a waiting time of a full year gave the same results. The macroscopic homogeneity of the samples was checked by analysis of the polarization recovery of their 7 Li and 31 P spins after a saturating RF pulse sequence. A mono-exponential behavior (up to at least 99% of the equilibrium value) was found as is characteristic for a homogeneous sample. 3. Results 7

Li NMR. Lithium ions are usually assumed to be responsible for the charge transport in lithium phosphate glasses. For this reason information from 7 Li NMR is of considerable importance. The static 7 Li (I ˆ 3/2) spectra at room temperature show a rather featureless powder pattern, characteristic for ®rst order quadrupole interaction, consisting of a narrow central line on top of a broad component from satellite transitions (Fig. 1(a)). The approximately Gaussian shape of the central line is indicative of dipolar broadening of the central …‡1=2 $ ÿ1=2† transition. As the temperature increases the spectra are narrowing, re¯ecting the activation of the Li‡ motion. The single narrow line of Lorentzian-like shape observed at high temperature indicates that the Li

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Fig. 1. (a) Temperature dependence of the 7 Li NMR line shape in LiPO3 . (b) 7 Li MAS spectra at spinning rate fr ˆ 10 kHz in 0.3LiF + 0.7LiPO3 .

ions at all structural positions in the glass are highly mobile. All samples show 7 Li MAS spectra of similar shape. As a representative example the room temperature 7 Li MAS spectrum of the compound with x ˆ 0.30 LiF concentration is shown in Fig. 1(b). It is known that the range of 7 Li chemical shifts is only a few ppm wide and as a consequence, it is dicult to resolve individual lines from structurally inequivalent sites in the solid state. Even at high resolution under MAS conditions the spectra of our compounds show only a single symmetrical central line along with spinning side bands (Fig. 1(b)). Thus, as far as the microscopic structure is concerned one cannot gain relevant information from 7 Li NMR. However, for characterizing the glass structure on the macroscopic scale 7 Li relaxation data are useful. For all analyzed samples including that with the highest LiF concentration, x ˆ 0.35, the longitudinal relaxation of 7 Li magnetization is mono-exponential for at least two decades in amplitude, in contrast to the results reported in reference [32]. We see this as evidence

83

that none of the samples has macroscopic heterogeneity like coexistence of glassy and crystalline phases with their strongly di€erent spin lattice relaxation. 1 H NMR. Water is one of the most common impurities in phosphorus oxide based glasses. Its concentration can reach an amount of a few mole percent in glassy systems prepared without special precaution [33,34]. The residual water is incorporated into the glass polyphosphatic chain mainly in the form of ±P±OH groups. To estimate the hydrogen content we performed 1 H NMR measurements and determined the spectral intensities using a CHCl3 sample as reference. The estimated proton concentration in our glasses is in the range of 1±3 mol%; no systematic correlation with the LiF concentration was evident. 19 F NMR. 19 F NMR spectra are shown in Fig. 2. All spectra consist of two components separated by about 140 ppm. The approximately symmetric, Gaussian-like shape of the spectral components at room temperature (Fig. 2(a)) indicates that their line width is determined by homonuclear and heteronuclear (¯uorine±phosphorus or ¯uorine± lithium) dipolar interaction and also by the dis-

Fig. 2. 19 F NMR spectra of x á LiF + (1 ) x) á LiPO3 glasses. (a) x ˆ 0.15, 0.20, 0.25, 0.30, 0.35, room temperature (the integral intensities are normalized with respect to the weight of the sample). (b) 0.35LiF + 0.65LiPO3 , at di€erent temperatures.

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tribution of chemical shift tensors. From the line positions we conclude that the more intense up®eld component centered around r ˆ )200 ppm with respect to CFCl3 is due to ¯uorine in LiF fragments [35], while the less intensive down®eld component centered around )65 ppm is assigned to ¯uorine bonded to phosphorus in the phosphate chains. The latter r is close to the isotropic chemical shift of about )72 ppm as found in 19 F MAS spectra for ¯uorine bonded to phosphorus in ¯uorinated sodium and aluminum phosphate glasses [25]. A qualitatively similar 19 F spectrum of 0.35LiF + 0.65LiPO3 glass is found in reference [32], but the line positions and relative intensities of the components di€er from the corresponding ones in our compound. Two distinct ¯uorine positions in x á LiF + (1 ) x) á LiPO3 glass identi®ed as F±P and F±Li sites were also observed by X-ray photoelectron spectroscopy [3]. As the temperature increases, the width of the up®eld spectral line decreases and at temperatures above 500 K the shape of this line becomes closer to a Lorentzian function (see Fig. 2(b)) due to motional averaging because of ionic exchange. Also the width of the down®eld component becomes smaller at higher temperatures. The intensity ratios of the two components are listed in Table 2 for samples with di€erent concentration x. At room temperature the relative line intensities are estimated from a simulation assuming the superposition of two components of Gaussian shape. At high temperatures the two lines are well separated (Fig. 2(b)) and their overlap is so small that the relative intensities can be obtained with good accuracy by simple integration. For additional

Table 2 Percentage of ¯uorine atoms in LiF fragments wLiF (wLiF + wP±F ˆ 1) and average chain length nav estimated from 19 F NMR x

wLiF (‹0.03)

nav

0.10 0.15 0.20 0.25 0.30 0.35

0.56 0.61 0.64 0.67 0.71 0.76

12.1 ‹ 1 9.2 7.0 5.7 4.7 4.5 ‹ 0.5

analysis spectra were recorded for one sample (x ˆ 0.35) at high and at room temperature. The relative line intensities obtained for the two temperatures are the same. As can be seen from Table 2 even at smaller LiF concentrations a part of the ¯uorine stays in the form of LiF. With increase of LiF content, x, the percentage of F atoms in the LiF form increases. As is well known ¯uorine is highly volatile and its content in the ®nal product can, therefore, di€er from the nominal composition based on the input products and may depend on the preparation procedure and preparation time (see, e.g., Refs. [3,23±25]). Fluorine is lost during the glass synthesis mainly in form of HF in accordance with the reaction 2LiF + H2 O ® Li2 O + 2HF­, where the hydrogen comes from residual water present in the glass volume, but loss in the form of PF3 , PF5 , or LiF is also possible. For an estimate of the ¯uorine concentrations in the samples after the preparation process we used 19 F NMR spectra of carefully weighted glass samples recorded simultaneously with a liquid CFCl3 sample of known weight as reference. From the ratio of the integral spectral intensities of the liquid CFCl3 and the glassy samples the ¯uorine content was calculated (see Table 1). It was found that the actual ¯uorine concentration in the glassy matrix is smaller than the nominal value. 31 P MAS and static NMR. An important advantage of 31 P NMR is the fact that in polyphosphates the di€erent Q…n† groups usually di€er by di€erent 31 P chemical shift tensors, as is well established from numerous studies of phosphate compounds of known structure [36±40]. As a consequence, it is in such cases comparatively easy to measure the Q…n† composition by 31 P NMR. In a ®rst step we observed the presence of the di€erent Q…n† by using the MAS technique. The MAS NMR spectrum of the pure LiPO3 glass shows only a single central line positioned near )23 ppm along with side bands separated by the spinning frequency, fr , (Fig. 3(a)). The position of the central line is in the range expected for Q…2† phosphorus groups [36,37,40] and in good agreement with rs reported in the literature for crystalline [41] and glassy LiPO3 [33,41±44]. The

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Fig. 3. (a) 31 P MAS NMR of LiPO3 glass, spinning rate fr ˆ 10 kHz. (b) 31 P MAS NMR of 0.3LiF + 0.7LiPO3 glass, spinning rates fr ˆ 10 kHz (top) and fr ˆ 6.5 kHz (bottom).

central line has approximately a Gaussian shape with a width of about 10 ppm. This rather large line width is due to the distribution of isotropic chemical shifts and thus the structural heterogeneity of the chemical environment of the 31 P nuclei, as is typical for disordered glassy systems. The absence of any contribution from other Q…n† (n ¹ 2) sites con®rms that the glass is close to the ideal metaphosphate composition with long chains. The minor asymmetry of the central line (small shoulder at the left side, near )10 ppm) is attributed to the contribution of end groups of the polyphosphatic chain, terminated by hydroxyl groups, ±P±OH [40,42]. A similar contribution of ± P±OH groups was found in several other phosphates, e.g., in sodium/lithium metaphosphate [42,45,46] and in sodium and potassium dihydrogen diphosphate [40], where, near )8 ppm, a small additional line was observed.

85

In the LiF-doped glasses the MAS spectra show an additional down®eld line centered between )1 and )4 ppm (Fig. 3(b)). Two MAS spectra at two di€erent spinning frequencies are shown in Fig. 3(b) to demonstrate that the central lines (marked by the values of the chemical shift) are easily distinguished from the spinning side bands. The amplitude of the second line increases with the LiF concentration and in the compound with the largest LiF concentration, x ˆ 0.35, this line reaches an integral intensity comparable with the intensity of the up®eld line. The width of this line is slightly less than that of the up®eld line. The position of the down®eld line can be attributed to the end units Q…1† of the polyphosphatic chain or pyrophosphates [37], where the phosphorus atom is bonded to only one bridging oxygen and the chain is terminated by two lithium ions. This line position is in agreement with the isotropic chemical shift of end units as observed in nominal lithium metaphosphate [42]. The intensity ratio of these two center components of the MAS 31 P spectra plus their respective side bands, which are of considerable intensity, gives the relative population of the Q…1† and Q…2† sites (see Table 3). For the quantitative evaluation of tensor parameters, however, side band analysis is not the method of choice because of its limited accuracy. From the MAS spectra we conclude that only two chemical shift tensors with di€erent isotropic shift constants (corresponding to the position of the central lines) are giving a contribution. Hence, in good approximation the static 31 P NMR spectra of x á LiF + (1 ) x) á LiPO3 glasses with LiF concentration x > 0 can be considered to be a superposition of two tensor patterns corresponding to Q…2† and Q…1† phosphorus sites. Along this line we simulated all static 31 P spectra at di€erent LiF concentration by varying the relative content of the two tensors. Some representative static 31 P NMR spectra at di€erent concentration x of lithium ¯uoride are shown in Fig. 4(a). For the composition LiPO3 (x ˆ 0) the line shape is very close to a shape characteristic for a non-axially symmetric (g > 0) chemical shift tensor (spectrum (1)). As expected the tensor parameters are typical for the inner groups of the phosphate chain, Q…2† , where the

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Table 3 Chemical shift tensor parameters of the mid units Q…2† and end units Q…1† of the phosphate chain, relative weight w…1† of the Q…1† tensor and average chain length nav estimated from 31 P NMR x 0.00 0.01 0.03 0.10 0.15 0.20 0.25 0.30 0.35

Mid-units Q…2†

End units Q…1†

w…1† (‹0.04) nav

r, ppm (‹1) g (‹0.03)

d, ppm (‹3)

r, ppm (‹1) g (‹0.1)

d, ppm (‹3)

)23 )23 )22 )21 )20 )20 )21 )20 )21

)185 )185 )185 )181 )179 )179 )179 )174 )174

)1 )1 )1 )2 )3 )3 )4 )4 )4

126 126 130 130 131 131 134 129 129

0.50 0.51 0.54 0.52 0.54 0.54 0.54 0.54 0.53

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

0.07 0.08 0.09 0.17 0.24 0.25 0.35 0.40 0.43

29 ‹ 5 25 22 11.8 8.3 8.0 5.7 4.7 4.0 ‹ 0.4

The reference for r is 85% H3 PO4 in H2 O. Isotropic chemical shift r ˆ (r11 + r22 + r33 )/3, anisotropy parameter d ˆ r33 ) (r11 + r22 )/2, asymmetry parameter g ˆ (r22 ) r11 )/(r33 ) r), |r33 ) r| > |r11 ) r| > |r22 ) r|.

phosphorus is bonded to two bridging oxygens [36,37,40]. With increasing LiF addition the spectrum changes in a systematic way: a shoulder appears at about )50 ppm and becomes more and more pronounced, in accordance with the occurrence of phosphate groups having a Q…1† -like tensor. The observed chemical shift tensor parameters and their changes with phosphorus chain length are consistent with that reported [36±40,47]. For example, the isotropic chemical shift of Q…2† increases (becomes more positive) with increase of the LiF content and the degree of glass depolymerization (see Table 3). This tendency is typically observed in phosphate glasses [34,45,47,48]. The Q…2† sites close to the terminal group have a more positive chemical shift with respect to the internal Q…2† groups, as is consistent with the change of the average p-component of the P±O bonds [34]. In MAS spectra, because of the distribution of chemical shifts in the glass, this shift shows up as shift of the central line. The slight reduction of the shift anisotropy was attributed to an increase of the bond length of the bridging oxygen and an increase in p-bond component [47,49]. In our simulation the parameters of the Q…2† tensor were taken from the sample without LiF (x ˆ 0), and only slightly varied for the spectra from LiF doped glasses. The range of parameter variation was kept well within the limits known from literature (see, e.g., Refs. [36,39]). The values

for the isotropic part of the chemical shifts were taken from the MAS spectra. In simulating the spectrum of pure LiPO3 the small contribution by Q…1† units (of about 7% of the total intensity) was taken into consideration to improve the agreement with the experimental line shape. For the numerical ®ts line broadening by convolution with a Gaussian function of 16 ppm (2.6 kHz) half width was applied to take into account the e€ects of dipole±dipole interaction and anisotropic J-coupling. The results of such a simulation are collected in Table 3 and some selected simulated spectra are shown in Fig. 4(a). The intensity of the Q…1† tensor pattern with respect to the total spectral intensity is plotted in Fig. 4(b) as function of the LiF concentration x. One can see, that with increase of the LiF concentration the weight of the Q…1† tensor monotonically increases, showing the growing number of end groups in the polyphosphatic chains. The agreement between experimental and simulated spectra is in general good with the di€erence well within the experimental noise, except for a small deviation near 10 ppm for samples with the larger LiF concentration. We attribute this minor but systematic deviation from the ®t that grows with LiF concentration to the appearance of orthophosphate groups Q…0† in the more depolymerized glasses. Since its integral intensity is less than a few percent, the central line expected for this group in the region between +9 and +20 ppm [37,40,45], is not observed in MAS spectra.

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Fig. 4. (a) 31 P Static NMR spectra of x á LiF + (1 ) x) á LiPO3 glasses: (1) x ˆ 0; (2) x ˆ 0.15; (3) x ˆ 0.30. Solid narrow line ± experimental spectrum, bold solid line ± simulated spectrum, dashed and dotted lines ± contribution of Q…2† and Q…1† components, respectively. (b) Relative weight w…1† of the Q…1† component in x á LiF + (1 ) x) á LiPO3 glasses.

4. Discussion 4.1. Polyphosphatic chain length In compounds with phosphorus occupying only Q…1† and Q…2† sites their population number, P…1;2† ,

87

can be used to estimate the average length, nav , of the polyphosphatic chain, nav ˆ 2/w…1† [17], where w…1† ˆ P…1† /(P…1† + P…2† ) is the relative weight of the Q…1† tensor (see Table 2). In the case of the pure LiPO3 glass (x ˆ 0) minor losses (of a few percent) of phosphorus oxide are expected during preparation. This loss is a reason for the decrease of the chain and the creation of Q…1† units [25,34,44]. In ultraphosphate compositions it is known that there are losses of P2 O5 during the melting process [16]. The chain length of nav ˆ 29 as estimated from our 31 P NMR is in the range typically observed for metaphosphates. In the case of LiF-doped compounds, however, an estimation using the relation nav ˆ 2/w…1† can give only an upper limit of the chain length because the phosphate chain can be terminated also by ¯uorine atoms forming mono¯uorophosphate ± PO3 F or di¯uorophosphate ±PO2 F2 terminal groups [3,25,26]. Such end groups have a similar chemical shift tensor as the Q…2† phosphorus sites. In Ref. [50] this was shown for several crystalline ¯uorophosphates of known structure. In Ref. [25] the same result was deduced from comparison of 31 P NMR, Raman and photoelectron spectroscopy data for several ¯uorinated and ¯uorine-free phosphate glasses. This conclusion was supported by the similarity of ¯uorine and bridging oxygen with respect to their electronegativity and covalent radius. Accordingly, an F-terminated chain end contributes to the 31 P NMR spectrum very much like a Q…2† tensor and cannot be identi®ed as terminal group. To estimate correctly the contribution of ¯uorine to the chain depolymerization the fraction of ¯uorine bonded to phosphorus and the losses of F during glass preparation (Table 1) must be taken into account. As was discussed above, only a part of the ¯uorine couples to the phosphate chain, while the rest stays in the form of Li‡ Fÿ (see Table 2) and does not participate in terminating the chain. By combining the 19 F NMR data on the actual ¯uorine concentration (Table 1) and the relative population of ¯uorine sites (Table 2) the number of ¯uorophosphate terminal groups is easily determined and the calculation of the nav values corrected.

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The results are given in Table 3. The correction is almost negligible (less than 5%) for x < 0.3 because of the relatively larger ¯uorine losses at low LiF input. For x ˆ 0.30 and x ˆ 0.35, the correction reduces nav from 5.0 to 4.7 and from 4.7 to 4.0, respectively. In principle, an analogous correction has to be performed to take the e€ect of residual water in the matrix into account. As already mentioned it leads to depolymerization of the phosphate network. Such an e€ect is expected to be particularly larger at lower LiF concentrations. In glasses with exactly equal concentration of lithium and phosphorus the presence of residual hydroxyl groups located at chain ends is the main factor limiting the chain length. The MAS NMR technique has been applied to determine the structural modi®cation of phosphate glasses due to such residual water [33,51]. NMR data are interpreted as showing that water is not present as H2 O molecule, but that it reacts preferentially to form terminal ±OH groups, rather than displacing metal ions at middle units [34,51]. As for ¯uorine the termination of the phosphorus chain by H-atoms does not lead to the formation of a Q…1† -like tensor, but instead this Q…1† (H) group exhibits a chemical shift powder pattern which is similar to the Q…2† site [40]. Hterminated chain ends, therefore, do not contribute in 31 P NMR spectra to the intensity of the Q…1† component. However, because the H content in our samples is only 3 mol% or smaller as measured by 1 H NMR this e€ect is negligible. The average chain length of lithium metaphosphate (x ˆ 0), estimated simply from the hydrogen content exceeds 70, and thus is much longer than the experimental data in Table 3. In an independent approach we take the 19 F data by themselves, estimate the number of ¯uorine in terminal groups and determine from that the average chain length. The results are included in Table 2. The discrepancies observed between these numbers and the data based on the 31 P spectral analysis are not greater than the uncertainties in the estimates and well within the experimental error margins. Qualitatively, the analysis of the average chain length for the aqueous solution of a particular glass gives results which are in agreement with the

corresponding NMR data from the solid compound. Quantitatively, however, nav in water is somewhat shorter than in solid matrix (e.g., for the composition with x ˆ 0.2 the average chain length is nav ˆ 6.6 in solution and 8.0 in the solid glass [26]), indicating that the hydrolysis process cannot be totally avoided. In contradiction to the experimental data the calculation of nav on the basis of the nominal glass composition, even under the rather unrealistic assumption that all Li ions are coupled to the phosphate chain, gives a number of Q…1† sites which is signi®cantly too small and, accordingly, a chain length which is too long, particularly at low doping concentrations. For pure LiPO3 glass at the metaphosphate concentration 0.50Li2 O + 0.50P2 O5 the chain length ideally is in®nite, under consideration of residual water in the glass matrix it merely reduces to 70 units. The discrepancy demonstrates that losses during the glass synthesis have a major impact on the matrix structure and emphasize that details of the glass preparation procedure require closer inspection. 4.2. Motional e€ects The aim of the present article is the structural analysis of the ¯uorophosphate matrix, and ionic motion will be dealt with in a subsequent paper. Therefore, only a short and qualitative discussion shall be given here. The change of mobility is re¯ected in the e€ects of temperature on relaxation behavior and on spectral features. Such spectral changes are observed only for 7 Li and 19 F, but not for 31 P. Fig. 2(b) illustrates them for 19 F NMR of the 0.35LiF + 0.65LiPO3 glass. On increasing the temperature a reduction of line width is found for the ¯uorine in the LiF fragment, while the line width of ¯uorine bonded to phosphorus stays nearly the same. The likely explanation for this line width decrease is mostly the change in lithium mobility. As can be concluded from the temperature dependence of the 7 Li spectral line shape (cf. Fig. 1(a)) the lithium ions are highly mobile at temperatures above 470 K (see also Section on 7 Li NMR). Accordingly, the local ®eld due to heteronuclear dipole interaction of ¯uorine with lithium spins, giving a considerable contribution to

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the ¯uorine line width for LiF fragment, is averaged to zero at high temperature because of this high lithium mobility [46]. In contrast, for the down®eld line the e€ect of the Li dipole moment is less; instead the coupling with phosphorus, directly bonded to the ¯uorine, is predominant. The Lorentzian shape of the line indicates that ¯uorine ions, too, have considerable mobility at high temperature. The changes in the spectrum with temperature agree with the expectation of a higher mobility for the less restricted ionically bonded ¯uorine in LiF fragments in comparison with the covalently bonded ¯uorines in the polyphosphatic chains. 5. Conclusion Multinuclear solid state NMR provides detailed information on the structure of ¯uorophosphate glass on the atomic scale. For comprehensive and reliable results comparison of data obtained from various spin species is essential. Our data con®rm that the structural elements of the glass matrix are polyphosphate chains. For the case of metaphosphate, LiPO3 , they have an average length of 29 phosphate units. When adding LiF as glass modi®er homogeneous stable phases are formed over a range of LiF concentration, 0 < x < 0.35. No aging or clustering e€ects were detected. The total ¯uorine content increases monotonically with the LiF concentration, x, added to the batch, but the actual ¯uorine percentage in the glass is less than calculated from the nominal composition x á LiF + (1 ) x) á LiPO3 because of its volatility. Fluorine is distributed between two structural fragments: about 55±75% stay in the form of LiF while the rest is incorporated into mono¯uorophosphate chain terminating groups. Likewise, Li is partly incorporated into terminal groups. Therefore the increase of LiF concentration leads to a shortening of the polyphosphatic chain length. The average polyphosphate chain reduces to about 4 units at x ˆ 0.35. Further shortening of the chain length is possibly the reason to the tendency of crystallization for compositions with LiF concentrations x > 0.35. At lower LiF concentrations residual water probably plays an important role in

89

terminating the polyphosphatic chain and it takes also part in the evaporation of ¯uorine during the glass preparation, in the form of volatile HF. Based on the knowledge of the microscopic glass structure the investigation of ionic motion on the atomic scale becomes feasible. Such studies are currently been carried out using 7 Li and 19 F NMR lineshape, relaxation and di€usion measurements. References [1] P.P. Tsai, S.P. Szu, M. Greenblatt, J. Non-Cryst. Solids 135 (1991) 131. [2] F. Borsa, D.R. Torgeson, S.W. Martin, H.K. Patel, Phys. Rev. B 46 (1992) 795. [3] B.V.R. Chowdari, K.F. Mok, J.M. Xie, R. Gopalakrishnan, Solid State Ionics 76 (1995) 189. [4] B. Gee, H. Eckert, J. Phys. Chem. 100 (1996) 3705. [5] K.H. Kim, D.R. Torgeson, F. Borsa, S.W. Martin, Solid State Ionics 90 (1996) 29. [6] R. Winter, K. Siegmund, P. Heitjans, J. Non-Cryst. Solids 212 (1997) 215. [7] M. Watanabe, T. Yamamoto, T. Yamada, Mem. Colleg. Engin Chubu Univ. 20 (1984) 71. [8] R.A. Huggins, Electrochem. Acta 22 (1977) 773. [9] A.F. Privalov, A. Cenian, F. Fujara, H. Gabriel, I.V. Murin, H.M. Vieth, J. Phys.: Condens. Matter 9 (1997) 9275. [10] S.W. Martin, C.A. Angell, J. Non-Cryst. Solids 83 (1986) 185. [11] B.V.R. Chowdari, K.F. Mok, J.M. Xie, R. Gopalakrishnan, J. Non-Cryst. Solids 160 (1993) 73. [12] R.K. Brow, D.R. Tallant, Z.A. Osborn, Y. Yang, D.E. Day, Phys. Chem. Glasses 32 (1991) 188. [13] V.D. Khalilev, Yu.P. Tarlakov, B.V. Petrosyan, A.A. Pronkin, Sov. J. Glass Phys. Chem. 9 (1983) 190. [14] B. Kumar, R. Harris, Phys. Chem. Glasses 25 (1984) 155. [15] X. Zou, K. Itoh, H. Toratani, J. Non-Cryst. Solids 215 (1997) 11. [16] S.W. Martin, Eur. J. Solid State Inorg. Chem. 28 (1991) 163. [17] J.R. Van Wazer, in: Phosphorus and its Compounds, Vols. I and II, Interscience, New York, 1951. [18] B.C. Bunker, G.W. Arnold, J.A. Wilder, J. Non-Cryst. Solids 64 (1984) 291. [19] B.C. Sales, R.S. Ramsey, J.B. Bates, L.A. Boatner, J. NonCryst. Solids 87 (1986) 137. [20] J.-J. Videau, J. Portier, B. Piriou, J. Non-Cryst. Solids 48 (1982) 385. [21] J.P. Fletcher, S.H. Risbud, S. Hayashi, R.J. Kirkpatrick, Di€us. Defect Data 53&54 (1987) 493. [22] N.N. Gurova, V.A. Vopilov, V.M. Buznik, L.N. Urusovskaya, Sov. J. Glass Phys. Chem. 15 (1989) 687.

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