SURFACE AND INTERFACE ANALYSIS, VOL. 24, 91-94 (1996)

XPS Studies of Fluorine Bonding in Phosphate Glasses?

R. K. Brow* and Z. A. Osborne Sandia National Laboratories, MS1349, Albuquerque, NM 87185, USA

The F 1s spectra from fluorine-containing phosphate glasses provide quantitative information about the nature of fluorine bonding that can be used to test structure+omposition models. When fluorine is added to an aluminop- hosphate base glass, AI-F bonds are initially preferred until all available Al sites are filled, after which additional F replaces P-0-P bonds to form P-F sites. In tin phosphate base glasses, the P-0-P bonds are consumed during the initial stages of thoridation, followed then by the formation of So-F bonds. The structural insight provided by XPS is complementary to that provided by more conventional 'bulk' spectroscopic probes.

INTRODUCTION

Quantitative information about chemical bonding in multi-component glasses is often difficult to obtain because their lack of long-range order and overlapping radial distribution functions preclude the use of diffrac- tion techniques for all but the most simple composi- tions. A combination of atom-specific spectroscopic probes, however, including solid-state nuclear magnetic resonance (NMR) spectroscopy, x-ray absorption spec- troscopy (XAS) and x-ray photoelectron spectroscopy (XPS), can provide detailed information about the nature of the chemical bonds in a glass.

Although XPS has traditionally been used to charac- terize the nature of glass surfaces, information about the 'bulk' chemical structure can also be obtained.' In this paper, we will review our XPS studies of fluorine bonding in several different phosphate glasses. Fluoro- phosphate glasses have a variety of technological appli- cations, including low-loss laser host materials, fast ion conducting solid electrolytes and low-temperature matrices for organic/inorganic composites, and the structural role played by fluorine affects these applica- tions. These are interesting systems to probe by XPS because different F-metal (and 0-metal) sites can be resolved and their analyses compared with those obtained by other spectroscopic probes.

EXPERIMENTAL PROCEDURE

We have examined three families of F-containing phos- phate glasses: F-modified sodium phosphate glasses; F- modified sodium aluminophosphate glasses ; and

* Author to whom correspondence should be addressed. t This work was performed at Sandia National Laboratories, sup-

ported by the US Department of Energy under contract DE-ACO4- 94AL85000.

CCC 0 142-242 1/96/02OO9 1-04 0 1996 by John Wiley & Sons, Ltd

F-modified tin phosphate glasses. The sodium phos- phate and aluminophosphate glasses were prepared by melting mixtures of the respective base glasses and NH,HF, in vitreous carbon crucibles at 500-650 "C for up to 30 min. Up to 18 at.% F could be incorporated into the glass structure by this method. Details about glass preparation and properties are given in Refs 2 and 3. Tin fluorophosphate glasses were prepared by melting mixtures of SnO, SnF, and NH,H,PO, in vit- reous carbon crucibles at 800 "C. These glasses are described in more detail in Ref. 4.

The photoelectron spectra were collected with a Kratos XSAM 800 spectrometer. The hemispherical analyzer was operated in the fixed retarding ratio mode. Non-monochromatic, 300 W Mg Ka x-rays provided the excitation radiation. High-resolution spectra were obtained with a bandpass energy of - 10 eV. These set- tings produce an Ag 3d,,, full width at half-maximum (FWHM) of 0.9 eV from a silver reference sample. Glass samples of -6 mm in diameter were fractured in the ultrahigh vacuum (UHV) chamber (-2 x l op9 Torr) immediately prior to analyses and photoelectron spectra were collected over the course of -60 min after fracture. The photoelectron spectra showed no indica- tions of any x-ray-induced changes. The reported binding energies have been referenced to a C 1s binding energy of 284.8 eV for the adventitious carbon,' which could be detected on many of the samples. In general, the reported binding energies are reproducible to f 0.2 eV.

Quantitative spectral analyses were done using photoelectron peak areas after subtraction of a linear background. Sensitivity factors were determined from analyses of a set of samples with known compositions, determined independently by ion chromatography and inductively coupled plasma atomic emission spectrorn- etry (ICP-AES). The compositional analyses were reproducible to better than f 10% relative.

Background-corrected F 1s and 0 Is photoelectron spectra have been decomposed into their respective Gaussian components, using a least-squares fitting algo- rithm that varies the position, width and height of each component until a best-fit solution is obtained. These

Received 7 June I995 Accepted 18 August 1995

92 R. K. BROW AND 2. A. OSBORNE

quantitative curve fittings were reproducible to & 5% relative and the r2 values were typically > 0.95. NaPO, Glass

RESULTS AND DISCUSSION

Fluorine containing sodium phosphate and sodium aluminophosphate glasses

Figure 1 shows the F 1s spectra collected from glasses with compositions (atom fractions) a)

and c) Nao,23Alo~03Po~1600~42Fo~17. All three spectra possess peaks centered at - 688.3 eV. The aluminop- hosphate glass spectra possess a second, more intense peak centered at -685.5 eV. Note that the relative intensity of the higher binding energy peak in the F- aluminophosphate spectra increases as the F content increases.

The F 1s peak at 688.3 eV is assigned to F bonded to P (F-P), in agreement with previous studies of alkali fluorophosphate glasses.293s6 The lower binding energy peaks represent F bonded to a less electronegative cation, in the present case either A1 or Na. Raman and solid-state NMR studies of these same glasses indicate that F replaces 0 that bridge neighboring P-tetrahedra and Al-octahedra to form F-A1 and F-P bonds3s7 and so the 685.5 eV peak in the F 1s spectra from these glasses is assigned to F incorporated into those F-A1 bonds.

Figure 2 shows the 0 1s spectra collected from an NaPO, base glass (0 at.% F) and a second, fluorinated metaphosphate glass containing 12 at.% F. There are two peaks in each spectrum. The lower intensity peak, centered near 534 eV, is due to oxygens that bridge neighboring P-tetrahedra (P-0-P) and the peak near 532 eV is due to oxygens bonded to a single P-

Na0.20P0.1900.49F0.12~ b, Na0.23A10.03p0.1700.50F0.07

F-AI

F-P h

695 690 685 680

F 1s Binding Energy (eV) Figure 1. Fluorine 1 s spectra collected from a sodium fluoro- phosphate glass (a) and two sodium fluoroaluminophosphate glasses (b, c). Glass compositions are given in the text.

540 535 530 525

01s Binding Energy (eV) Figure 2. Oxygen I s spectra collected from a sodium meta- phosphate base glass (top) and a metaphosphate glass containing 12 at.% F (bottom).

tetrahedron (P-0 .-), in agreement with previous studies.'

The ratio of the two 0 1s peak areas in the spectrum from the NaPO, base glass ([POP]/[PO-] = 0.49) agrees well with that expected from the metaphosphate stoichiometry (0.50), illustrating the quantitative nature of the information one can obtain about bonding in glass by XPS. The addition of F clearly reduces the rela- tive concentration of bridging oxygens (Fig. 2), indicat- ing that they are preferentially replaced by F-P bonds, according to2,,

Raman spectroscopic studies of these glasses confirm that F depolymerizes the metaphosphate chains through the formation of terminal F-P As a consequence, the glass transition temperature of NaPO, is reduced from - 260 "C to - 180 "C with the addition of 12 at.% F.

The lithium fluorophosphate system is similar in that F-P bonds are preferred when F replaces 0. However, significant concentrations of F-Li bonds also form in these glasses (particularly when the Li/P ratio is increased) and the d.c. conductivity of these glasses is shown to be related to the extent of the 'LiF' network.6

For the F-containing sodium aluminophosphate glasses, Fig. 1 indicates that there is a dependence of the relative F 1s peak intensities on the total F content. This dependence is shown more clearly in Fig. 3, which summarizes the quantitative spectral analyses from a series of glasses with a base composition of 20A1F3.80NaP03. Plotted are the number of F ions bonded to A1 per A1 ion, and the number of F ions bonded to P per P ion. These values were obtained by multiplying the respective relative peak areas from the F 1s spectra by the total F concentration, and then dividing by the respective metal cation concentration.

XPS STUDIES OF FLUORINE BONDING IN PHOSPHATE GLASSES 93

L 1

3.0 1 A A

A

A

A

0 0

0

8 - 0

0.0 0 , ? , , , , ~, , , , , , , , , 0.0 '

0.00 0.05 O Y O 0.15 0.20

atomic fraction F Figure 3. Quantitative fluorine bonding from a series of sodium fluoroaluminophosphate glasses determined from their respective F 1 s spectra.

From Fig. 3, a mechanism for the incorporation of F into sodium aluminophosphate glass can be developed. Initially, F preferentially forms F- A1 bonds until there are three fluorines bonded to each Al. Once the octa- hedral aluminum sites are saturated, further incorpor- ation of F is then accomplished through the formation of F-P bonds. This 'chemical progression' is shown schematically below.

0- 0- I o\/o I

-0-P-0-Al-0-P-0- + F+

0- 0- I T / F I

-0-P-0-Al-0-P-0- + F-b I / \ I 0- F F 0-

The 3 : 1 F/Al stoichiometry indicates that an 'AlF,' structure coexists with a fluorophosphate network in glasses with the highest F contents. The different anionic sites are charge-balanced by alkali ions (not shown). Similar structural moieties have been proposed for more complex fluorophosphate optical glasses for which their highly ionic bonding character leads to low dispersion and low non-linear refractive indices.'

Fluorine-containing tin phosphate glasses

Tin fluorophosphate glasses have an uncommon com- bination of very low glass transition temperature (T, < 150 "C) and good resistance to chemical attack, making them candidates for a variety of technological applica- tions, including inorganic hosts for air-sensitive optically-active organic molecules.' When first devel- oped, these glasses were presumed to have structures

based on tin oxyfluoride and PO, tetrahedra, with no F-P bonds."-13 Subsequent XPS analyses, however, showed that F-P and F-Sn sites are p r e ~ e n t ~ . ' ~ . ' ~ in . the glass and led to the development of a structural model based on tin fluorophosphate crystal structures that is consistent with their unusual proper tie^.^

Figure 4 shows the F 1s spectra collected from a number of the tin fluorophosphate glasses with constant F atomic fraction but varying Sn/P ratios. In general, glasses with lower Sn/P ratios (and lower total F contents) have F 1s spectra with a single peak near 688.3 eV, whereas glasses with higher Sn/P ratios (and greater total F contents) have F 1s spectra with a second peak centered at 685.6 eV.

As for the F 1s spectra from the F-containing alumin- ophosphate glasses, the high binding energy peaks in Fig. 4 represent F bonded to P. The lower binding energy peak is due to F bonded to Sn.47'4,15 (Note that the Pauling electronegativities of A1 (1.61) and Sn(I1) (1.80)16 are similar, and so one expects that F-A1 and F-Sn bonds in fluorophosphate glasses will have similar F 1s binding energies.)

There is an interesting compositional dependence of the nature of fluorine bonding in these glasses. In con- trast to aluminophosphate glasses, F-P bonds are preferentially formed on the initial addition of F to a tin phosphate base composition. There is a concomitant decrease in the relative concentration of bridging oxygen^,^ which indicates that phosphate chains are depolymerized by the formation of F-P bonds in a manner similar to reaction (1).

Tin-rich (i.e. Sn/P ratio > 1) fluorophosphate com- positions, which would have relatively few P-0-P bonds available for such a reaction, have F 1s spectra with relatively larger F-Sn contributions (Fig. 4). Figure 5 summarizes the compositional dependencies of fluorine and oxygen bonding. Much of the scatter in the data results from the variation in F contents for glasses with similar Sn/P ratios. Despite this, there is a clear change in the nature of the glass structure at Sn/P = 1

[F] = 10 atom% F-Sn

695 690 685 680

F 1s Binding Energy (eV) Figure 4. Fluorine 1s spectra collected from a series of tin fluoro- phosphate glasses containing - 10 at.% F.

94 R. K. BROW AND Z. A. OSBORNE

. 1.0 - . ' ' ' ' ' ' ' ' ' ' ' ' I ' ' ' '

0 - 0 0 .

- 0.8 - 0 0

0 rn TI c 0

- 0.6 I 50 Y 8 OO

O . c

0

LL

.Q 0.4 -

- 8 c

2 0

B&",+- 0.2 -

, , , . , I , , , , , , , , , 0.0

I

I 0.4 F. 0 F F

-0-6 - 0 - s n - 0 - p -0- -0-b -0 -Sn- F 2 n - O -

A b (5 b I 0 I I I I

0.3 8 0 c SnlP< 1 SnlP>1 m 4

0.2 9 a 5 2

At Sn/P ratios < 1, the number of bridging oxygens in the 'base' tin phosphate glass is relatively high and these oxygens are replaced by fluorines to form the terminal F-P bonds. When Sn/P > 1, F is incorporated into F-Sn bonds to produce a glass structure based on a combination of tin oxyfluoride and fluorophosphate polyhedra.

.- Y

0.1

0.0

Figure 5. Quantitative fluorine and oxygen bonding from tin flu- orophosphate glasses.

(the pyrophosphate stoichiometry), from structures based on F-P and P-0-P bonds, to ones based on F-Sn and P-0-. As a result, the thermal and physi- cal properties of the glasses show breaks in their com- positional dependencies at or near Sn/P = l.4*12*i3

The compositional dependence of the XPS results can be summarized by the following schematic structures :

X-ray photoelectron spectroscopy can provide a level of quantitative chemical bonding information about glass that cannot be obtained by other spectroscopic tech- niques, and as a result can provide new insight into the compositional dependencies of glass structures. The large chemical shifts associated with the F Is binding energies from F species in phosphate glass makes these materials excellent candidates for such analyses and the ability to quantify the effects of composition on fluorine and oxygen bonding provides a detailed understanding of these technologically interesting materials.

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