Solid State lonics 18 & 19 (1986)382-387 382 North-Holland, Amsterdam

P205 BASED VITREOUS ELECTROLYTES: STRUCTURAL CHARACTERIZATION BY 31p NMR AND RAMAN SPECTROSCOPIES

Marco VILLA

Dipartimento di Fisica "A. Volta" e Gruppo Nazionale di Struttura del la Materia del C.N.R. Via Bassi 6 - 27100 Pavia, I t a l y

Gaetano CHIODELLI

Centro di Studio per la Termodinamica ed Elettrochimica dei Sistemi Sal in i Fusi e Sol id i del C.N.R., c/o Dipartimento di Chimica Fisica - Viale Taramelli 16 - 27100 Pavia, I t a l y

Mauro SCAGLIOTTI

Laboratoire de Physique des Solides, LA 153, 4, pl . Jussieu, Universit# P. et M. Curie 75231 Paris Cedex 05

Phosphate salts contain tetrahedral PO. units which may d i f f e r in t he i r nominal charge or, i f one prefers, in the number of oxygens that4are shared with other PO a units. In the binary M~O:PpOK systems (M=Li, Ag), the f ract ions of the d i f fe ren t units are deduced from ~ip NMR spectrg acquired with the Magic Angle Spinning (MAS) technique. 31p NMR and Raman spectroscopies are used to characterize the structural changes occurring in the Li20.2P205 glass when P205 is progressively substituted by B203 and a "dopant sa l t " (LiCl) is added.

I . INTRODUCTION

The chemistry of inorganic phosphates has

been established some t h i r t y years ago by Van

Wazer l , Westman 2, and others. They postulated

that condensed phosphates consist of PO 4 te t ra -

hedra in d i f f e ren t states of oxydation, mostly

arranged in chain of various lenghts. Support-

ing evidence for the i r model ch ie f l y comes from

thin paper chromatography. This technique re l ies

upon the s o l u b i l i t y of condensed phosphates in

water and assumes that the structure of the con-

s t i tuent fragments is maintained in solut ion.

So far , local order of phosphate glasses has been

d i rec t l y probed by comparing infrared and Raman

spectra of c rys ta l l i ne and vitreous phases 3,and,

more recent ly, by a sophisticated appl icat ion of 4 X-ray photoelectron spectroscopy . F ina l l y ,

insert ion of traces of lead or copper oxide

allows the "opt ical bas ic i ty" of a phosphate to

be measured 5. This parameter can be related to

the average covalent character of the bond be-

tween the metal of the modif ier oxide (usual ly

M20 or MO) and the oxygens of the phosphate

network. This technique is somewhat complemen-

tary to the X-ray photoelectron spectroscopy,

which evaluates the re la t i ve density of d i f f e r -

ent ly bonded oxygens.

I t is general ly accepted that condensed

phosphates consist of units as follows.

a) Phosphorus oxide is made of "branching"

units; i . e . , PO 4 units in which three of the

oxygens are bridging and the fourth is double

bonded. Since the double bond cannot resonate,

the branching unit has a rather high energy and

reacts vigorously with water.

b) When we begin adding a "modi f ier" ; i . e . , a

metal oxide such as M20, "middle" units are

formed with two bridging oxygens and one nega-

t ive charge. The metal is bonded to the two

non-bridging oxygens (NBO's, or oxygens that

belongs to a single glass forming unit) and the

double bond resonates between them. The process

of formation of middle units ends at the meta-

phosphate composition, MP03, where the middle

units are arranged c y c l i c a l l y , or, more often,

in very long chains.

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

M. Villa et al. / P205 based vitreous electrolytes 383

c) The metaphosphate chain is terminated by "end"

uni ts, with one bridging oxygen and two negative

charges. Two end units l inked by an oxygen

form the pyrophosphate group M4P207.

d) Further addi t ion of metal oxide causes for -

mation of "monomeric" uni ts , which correspond + po~ 3 to the orthophosphate composition M 3

For the binary metal oxide:P205 systems, we

now have some understanding of the re la t ionsh ip

between composition and density of the d i f f e ren t

uni ts. However, structural character izat ion of

the phosphate containing glasses as so far been

l imi ted by lack of a technique which quant i ta-

t i v e l y evaluates these densi t ies. By contrast,

the a b i l i t y of ~B NMR to dist inguish among

symmetric BO 3 uni ts , asymmetric BO 3 units,and

BO 4 units has brought about a fundamental under-

standing of the structural chemistry of borate

glasses, as well as borophosphate, boros i l i ca te ,

and boroaluminate glasses 6. This paper shows that h igh- f ie ld ~P NMR,

coupled with the Magic Angle Spinning (MAS)

technique, is able to measure the f ract ions of

the various units in binary phosphates and of fers

new insights on the structure of more complex

systems, such as the borophosphates. In par t i c -

u lar , by making a cor re la t i ve use of 31p NMR and

Raman spectroscopies, we have been able to char-

acter ize the modif icat ions occurring in the

glass network upon doping with LiCI or Agl. lon

conduction and thermal propert ies of these sub- 7 stances are discussed in a separate paper .

2. EXPERIMENTAL DETAILS

Samples belonging to the quaternary systems

LiCI:Li20:B203:P205 and AgI:Ag20:B203:P205 have

been prepared and absence of c rys ta l l i ne phases

has been control led by X-rays. NMR-MAS exper i -

ments have been performed at room temperature

and at a working frequency of 60.76 MHz. Ingots

of ~ 1 cm 3 were crushed in a mortar and part of

the powder was inserted in a Delr in sample

holder which was then t i g h t l y capped. To reduce

water absorption, the time in terval between

removal of the ingot from a sealed container

and the end of NMR measurements was kept to a

minimum (~ I0 ' ) . Traces of aqueous H3PO 3 were

added to few samples to give an internal refer-

ence ("zero frequency"). S t a b i l i t y of the f i e l d

to frequency ra t io was monitored by per iod ica l -

ly running a 3~p MAS spectrum of b is (d iphen i l -

phosphine)-butane.

Raman experiments have been performed on a

double monochromator (Coderg PHI). The spectra

of L i -contain ing samples were excited by the

514.5 nm l ine of an Ar + laser while the 647.1

nm l ine of a Kr + laser was used for the s i l ve r

borophosphate glasses. The power of the laser

beam was ~I00 mW and the spectral s l i t width

was ~2 cm - I . Al l data have been taken with

f reshly polished samples to reduce the effects

of water absorption.

3. THE BINARY M20:P205 SYSTEMS (M=Li, Ag)

A general knowledge of the condensed phos-

phates and Raman analyses of our samples al low

to put into re la t ion the features of the NMR -

MAS spectra of binary phosphates and densi t ies

of the d i f fe ren t phosphate units. Figure 1

i l l u s t r a t e s the steps of a NMR experiment, per-

formed on a sample with nominal composition

Ag20 (0.75)P205. Figure la is the non-spinning

spectrum, which is dominated by the anisotropy

of the chemical sh i f t tensor and by i t s d i s t r i -

bution. Trace b is the MAS spectrum at a spin-

ning rate of 2250 Hz. Notice that the envelope

of the spinning sidebands roughly fol lows the

spectrum (a) and that the two components of the

central peak have d i f f e ren t sidebands patterns,

as i t could have been expected 8. Trace c is

the blown up representation of the central peak,

which was iden t i f i ed by comparing three MAS

spectra at d i f f e ren t spinning frequencies (from

2 to 5 kHz). Figure (d) is the computer as-

sisted deconvolution of spectrum (c) into Gauss-

ian components. The root mean square deviat ion

384 M. Villa et al. / P205 based vitreous electrolytes

i L i I

200 100 0

/_] J

f f

H o ~

, J L

i -100 -200 I

/ E ", // M / I

20 10 0 -10 -20 -3'0

chemical shilt (ppm)

FIGURE 1 Steps of a NMR-MAS experiment (see tex t ) .

between experimental spectrum and sum of the

deconvoluted components (dotted curve in Fig.

Ic) was t y p i c a l l y 5%. In Fig. Id, peak M is

assigned to middle uni ts , by comparison with

spectra of metaphosphates, and peak E is as-

signed to end uni ts. Based upon the nominal

composition, we would have expected peak M to

account for 67% of the to ta l in tens i ty , rather

than 59%, as observed. Such a discrepancy is

cer ta in ly due to evaporation of phosphorous

oxide during glass preparation. Thus, the

spectrum of Fig. Ic gives Ag20.(O.71)P205 as the

real composition of the glass. We found that ,

in favorable cases, the water content and the

M20/P205 ra t io can be determined within ~1% from

the 3~p NMR-MAS spectra.

A summary of the Raman and NMR data col lected

in the binary phosphates containing Li and Ag is

given in Table I. We have l i s ted the frequency

TABLE I : Structural units, Raman and 31p NMR shi f ts in M20:P205 (M = L i , Ag)

Structural Raman frequency 3~P-NAS sh i f t uni t of symmetric P-~ from aqueous

stretching (cm-) H3PO 3 (ppm)

Branching 1390 -42 ÷ -35

Middle I150 + I180 -28 ÷ -15

End 950 ÷ I050 -12 ÷ +3

Monomer 920 ÷ 960 +25 f + Measured in c rys ta l l i ne Ag3PO 4

ranges where features assigned to d i f fe ren t

phosphate units have been observed. Under the

Raman column, we l i s ted only the frequencies

typical of the symmetric stretching of the P-O

bonds in the various units. The datum for pure

P205 has been taken from the l i t e ra tu re 9.

Within the same un i t , the frequency of the P-O

stretching is believed to be mostly dependent

upon the metal-oxygen force constant and the

O-P-O angle lO. Since l i th ium and s i l ve r d i f fe rs

great ly in p o l a r i z a b i l i t y , ionic radius, and

mass, there is l i t t l e doubt that these ranges

are of the order of the possible cat ionic ef-

fects upon Raman and NMR-MAS spectra of the

phosphates.

4. THE (O.5)Li20.(Y)B203.(l-Y)P205 SYSTEM

We w i l l begin by commenting corresponding

traces of Figs. 2 and 3 which i l l u s t r a t e the

ef fect of progressive subst i tu t ion of boron

with phosphorous upon Raman and NMR spectra.

In the lowest trace of Fig. 2, obtained in

the pure phosphate (y=O), the intense branching -l band at 1325 cm is substant ia l ly shif ted with

respect to the 1390 cm - l band reported for P205

and assigned to P=O stretching 9. The sh i f t is

probably due to a par t ia l de loca l iza t ion of the

double bond I I . Analysis of the NMR-MAS spectrum

reported below supports this assignement. The - l . strong band at If70 cm ms due to the O-P-O

symmetric stretching of the middle units. An-

other major feature of the lowest trace is the

M. Villa et al. / P205 based vitreous electrolytes 385

y=0.8

0.6

0.4

260 4()0 6()0 8()0 10'00 12'00 1400 wave number (cm -1)

FIGURE 2 Raman spectra of (O.5)Li20.(l-y)B203.(Y)P205

peak at 675 cm -I which is due to P-O-P stretching

modes. This band occurs at lower frequency with

respect to the metaphosphate (675 vs 700 cm - I )

but at higher frequency re la t i ve t o P205 (675 vs

640 cm-l). The corresponding NMR spectrum (see

Fig. 3) gives a f ract ion of middle units higher

than expected (60% rather than 50%; peak M at

-24 ppm) and 36% of branching (peak B at -42

ppm). A small shoulder on the middle signal is

deconvoluted in peak W which is t en ta t i ve l y as-

signed to H+-bonded middle groups.

For y=O.2, the branching band shi f ts to 1310 -I cm and decreases while the middle band shows

-I a peak at 1165 cm and is much broader than in

pure phosphate. This broadening arises from

contr ibut ion due to BPO 4 groups and to middle

units bonded to borates, which w i l l be cal led

"middle-B" units. The P-O-P region now shows

three p a r t i a l l y overlapping peaks at 715, 665 and -I 640 cm , respect ively. We ten ta t i ve l y re late

the 715 cm -I band to P-O-P and the others to

Y=0.8

0.6 . .

4o o -4o -2'o -3'0 -4o -~o

0.0 ~ W " ~

To 6 - ~ -2~0 -3'o -40 -50 -60 c h e m i c a l sh i f t (ppm)

FIGURE 3 31p NMR-MAS spectra of the glasses of Fig.2. When the spectra can be deconvoluted, the i r components are represented by dotted curves.

P-O-B modes. The NMR spectrum has a main peak

covering a30 ppm wide region and a smaller one

in the "end units region". Since the signal of

c rys ta l l i ne BPO 4 occurs at -30 ppm and substi-

tu t ion of phosphorous with boron shi f ts the

middle peak in the paramagnetic d i rec t ion, the

main peak probably contains unresolved contr ibu-

t ions of branching, BP04, middle, and middle-B

units.

For y=O.4, a broad band at 1120 cm - I , with a

shoulder at 1070 cm - I , is seen in the O-P-O

stretching region, while contr ibut ions from the

branching units have almost disappeared. Since

c rys ta l l i ne BPO 4 has a peak at 1116 cm - I , and a

smaller feature at 1066 cm -I 12, i t is argued

that the O-P-O modes at y=O.4 are due to a pre-

dominant contr ibut ion of the BPO4's and to

middle-B units. The main contr ibut ion to the

386 M. Villa et al. / P 2 0 5 based vitreous electrolytes

- l band peaking at 635 cm is l i k e l y to come from

P-O-B modes. The NMR spectrum cons is ts only of

a very broad peak (21 ppm at ha l f he ight) , cen-

tered at -21 ppm, i . e . , in the middle reg ion,

and extending wel l in the middle-B and BPO 4

reg ions. Not ice that the most dramat ic e f f e c t

o f adding boron is the paramagnetic s h i f t o f

the main peak, which moves 8 ppm in going from

y=O.2 to y=O.4.

At y=O.6, there is a f u r t h e r s h i f t o f the

BPO 4 and middle-B s t r e t c h i n g peak to lower f r e -

quency (I045 cm- l ) . Based upon NMR evidence,

t h i s fac t is co r re l a t ed w i th an increase od - I middle-B dens i t y . The peak at 725 cm in -

creases wi th respect to the fea tu re at 640 cm - l ,

which may i nd i ca te r e c o n s t i t u t i o n of P-O-P mo-

t i f s . A shoulder at 775 cm - l may be due to bo-

rate groups 13 wh i le a comparison w i th the NMR - l spectrum ru les out tha t the fea tu re at I045 cm

is due to end un i t s . The main NMR peak is cen-

tered in the middle-B region w i th a shoulder at

the pos i t i on of the middle un i ts in pure phos-

phates. This f i n d i n g , and observat ion of a pure

borate mode, suggests tha t the y=O.6 composi t ion

is p a r t i a l l y phase-separated.

When y=0.8, the O-P-O s t re t ch ing region has

a band at I005 cm - l w i th a shoulder a t l lO0 cm - l

which is i n t e rp re ted in terms of a f u r t h e r in -

crease of middle-B un i ts , even i f these modes

cover a frequency band which is t y p i c a l of end - l un i t s . A small shoulder at 720 cm , near the

borate band at 778 cm - l may be due to P-O-P

NMR sees a large peak in the middle-B reg ion ,

which supports the assignement of the O-P-O

modes.

5. (X )L iC l ( l -X )EL i20 . (O .8 )B203 . ( l . 2 )P205 ]

SYSTEM

The major e f f ec t s of doping w i l l now be d i s -

cussed separa te ly f o r Raman (F ig . 4) and NMR-MAS

spectra (F ig . 5) . The shoulder at I070 cm - l in

the undoped glass becomes more intense at X=O.2

X = 0 7

0.6

_= ~ _ 0.2

............ i

200 400 600 800 10'00 12'00 14'00

w a v e n u m b e r (cm -~)

/ ' 'x /.

/ /

/ X=0.7 J " / " .....

/ /

/ 0.6 . . . . . ~ / - - -- , . • ..........

/ / \

/ \ O0 ~ / "~--~

10 0 -10 -20 -30 -40 -50

chemica l shi f t (ppm)

FIGURE 4 Raman spectra of Li20.(O.8)B203.(l.2 ) P205 doped wi th LiCl

FIGURE 5 31p NMR-MAS spectra of L i20. (O.8)B203. ( l .2 )P205 doped wi th LiCl

M. Villa et al. / P205 based vitreous electrolytes 387

(see Fig. 4). A fur ther increase and a sh i f t to

lower frequencies of this band are observed at

X=O.4,while the main peak remains at 1120 cm -I

and the features in the P-O-P region are essen-

t i a l l y unchanged. When more dopant is added

(X=O.6 and 0.7), the shoulder formerly located

at 1070 cm -I becomes the major feature of the

spectrum and sh i f ts to 1025 cm -I (at X=O.7) while

the 1120 cm -I band decreases. As before, the

1120 cm -I band is interpreted in terms of BPO 4

uni ts, which appear to be present in a l l glasses

of this series, with a density that decreases at

high LiCI content. The continuous increase and

sh i f t to lower frequency of the 1070 cm -I fea-

ture is due to conversion of middle units into

middle-B uni ts , which occurs par t ly at the ex-

penses of the BPO 4 units.

In the NMR spectra (Fig. 5), the increase of

the middle-B units is signaled by the paramag-

netic sh i f t of the broad peak,centered at the

middle region, of the undoped glass. This sh i f t ,

combined with an increase of in tens i t ies in the

diamagnetic side, allows deconvolution of a peak

centered at the BPO 4 posi t ion and having an

apparent in tens i ty of 28% for X=O.2 and 8% for

X=O.7. Due to the large overlap, deconvolution

is not unique and we bel ieve that these numbers

just indicate a trend. In conclusion, middle-B

units in l i th ium borophosphates contr ibute a

band near 1050 cm -I to the Raman spectra and a

broad feature near -20 ppm to the NMR spectra.

6. CONCLUSIONS

Correlat ive use of Raman, 3~p and 11B NMR is

l i k e l y to provide a very accurate descr ipt ion of

the re la t ionship between composition and struc-

ture in the borophosphate glasses. As Figs. 4

and 5 indicate, doping with LiCI causes some in-

crease in the number of middle units bonded to

borons and some decrease in the BPO 4 f rac t ion.

Although deconvolution of spectra such as those

of Fig. 5 can hardly be interpreted quant i ta-

t i v e l y , i t can be stated that the predominant

phosphate uni t in the series of Fig. 5 is a

middle uni t and that i t s weight changes l i t t l e

with composition. This fact suggests that the

connect iv i ty of the glass network and the glass

t rans i t i on temperature do not undergo major

changes upon doping with l i th ium chlor ide. This

agrees with the behavior of the glass t rans i t i on

temperature in l i th ium borophosphates 7.

ACKNOWLEDGEMENTS

Work supported by C.N.R. - Progetto F ina l i z -

zato Energetica. The NMR work has been per-

formed at the Regional NMR Center of the Colo-

rado State Univers i ty, funded by NSF.

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