PROTON-DOMINATED DEFECT STRUCTURES OF OXIDES AND EFFECTS ON DEFECT-DEPENDENT PROPERTIES

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PROTON-DOMINATED DEFECT STRUCTURES OF OXIDES AND EFFECTS ON DEFECT-DEPENDENT

PROPERTIES

A. Norby, P. Kofstad

To cite this version:

A. Norby, P. Kofstad. PROTON-DOMINATED DEFECT STRUCTURES OF OXIDES AND EF-

FECTS ON DEFECT-DEPENDENT PROPERTIES. Journal de Physique Colloques, 1986, 47 (C1),

pp.C1-849-C1-853. �10.1051/jphyscol:19861130�. �jpa-00225528�

JOURNAL DE PHYSIQUE

Collogue CI, supplement au n°2, Tome 47, fevrier 1986 page ci-849

PROTON-DOMINATED DEFECT STRUCTURES OF OXIDES AND EFFECTS ON DEFECT-DEPENDENT PROPERTIES

T. NORBY and P. KOFSTAD

Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo 3, Norway

Résumé - L'hydrogène peut être dissous dans les oxydes solides et former des défauts ponctuels. On le trouve principalement à l'état de protonsliés aux ions oxygène et peut ainsi être qualifié de proton interstitiel: ti\ ou de façon équivalente d'ion hydroxyde substitué: OH». Dans les oxydes stoechiomé- triques, les protons peuvent constituer le défaut dominant, même à très fai- ble activité d'hydrogène (sous pression de vapeur d'eau) et haute température.

Ils sont compensés électriquement par des électrons ou des défauts ponctuels chargés négativement. Des cas limites pour les conditions d1électroneutralité, impliquant des protons comme défauts majoritaires, sont examinés en tenant compte de la pression de vapeur d'eau. Quand le3 protons dominent l'équilibre des défauts, les propriétés liées aux défauts (i.d. conductivitë,protection anti-corrosion, frittage, fluage) deviennent dépendant de la pression de va- peur d'eau. Ces effets ainsi que leur ordre de grandeur sont démontrés dans quelques exemples. Une méthode pour déterminer la conductivitë protonique est décrite brièvement.

Abstract - Hydrogen may dissolve in solid oxides and form point defects. It is mainly present as protons bonded to oxygen ions, and may as such be termed interstitial protons, Ht , or, equivalently, substitutional hydroxide ions, OH; . In stoichiometric oxides protons may be the dominating positive defects even at very low hydrogen activities (water vapor pressures) and high temperatures. They will be counterbalanced by electrons or point defects with negative effective charges. Limiting cases of electroneutrality condi- tions involving protons as majority defects are examined, with special attention to water vapor dependences. When protons dominate the defect situation, the defect-related properties (e.g. conductivity, corrosion protectivity, sintering, creep) become dependent on the water vapor pressure.

The effects and their magnitude are demonstrated by a few examples. A method for determining proton conductivity is briefly mentioned.

I - GENERAL INTRODUCTION

In recent years it has become increasingly evident that hydrogen defects, intro- duced by the presence of hydrogen-containing atmospheres, may significantly in- fluence the defect structure and defect-dependent properties of many metal oxides at elevated temperatures. Such effects have, for instance, been reported for ZnO /l/, Y203 / 2 / , A 1203 III, Si0² / 4 / , Ti02 /5,6/, Mo03 / 7 / , BaTiOg / 8 / . Yb-doped SrCe03

19/, and Y, La and Sm-doped Th0² /10/. On the other hand, hydrogen has not been found to have significant effects on CoO, NiO, and Cu?0 /ll/, Y-doped ZrO, /12/; and CeO- (Norby, unpublished). From the availible data it may appear that hyarogen defects are only important in oxides with low inherent defect concentrations, i.e.

in oxides with relatively small deviations from stoichiometry.

Water vapor is under many conditions a common source of hydrogen in oxides, and in this paper a few simple relations between hydrogen defects and water vapor pressures are presented.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861130

JOURNAL DE PHYSIQUE

I 1

-

DEFECT EQUILIBRIA INVOLVING PROTONS Hydrogen Defects

T h e o r e t i c a l l v hvdroqen ma.v be ~ r e s e n t i n metal oxides as various defects. such as protons, neutral hydrogen sGcies;and negative hydride ions. A t high temperatures and near-atmospheric oxygen pressures i t has i n the l i t e r a t u r e always been concluded t h a t hydrogen i s predominantly present as protons. Furthermore, they are bonded t o oxygen ions on normal l a t t i c e s i t e s , and as such the hydrogen defects may be termed s u b s t i t u t i o n a l hydroxide ions, OH6

.

A1 t e r n a t i v e l y

,

these hydrogen defects may be considered as i n t e r s t i t i a l protons,

H; .

We have chosen the l a t t e r terminology and n o t a t i o n as t h i s s i m p l i f i e s the w r i t i n g and understanding o f defect equations.

According t o a l l a v a i l i b l e l i t e r a t u r e , such hydrogen defects d i f f u s e i n the oxides a t elevated temperatures by protons jumping from one oxygen i o n t o another ( f r e e proton migration). As t o d e s c r i p t i o n o f other p o i n t defects and e l e c t r o n i c defects the Krsger-Vink n o t a t i o n i s used i n t h i s paper.

With water vapor as the source o f hydrogen ( a t near-atmospheric oxygen pressures) the formation o f i n t e r s t i t i a l protons and the corresponding equi 1 i brium expression may be w r i t t e n

where e ' denotes an e l e c t r o n and n t h e i r concentration. I f protons are m i n o r i t y defects, and the e l e c t r o n concentration i s determined v ~ ~ o t h e r defect e q u i l i b r i a , the concentration o f protons w i l l be proportional t o pH a t constant Po

.

2 2

At s u f f i c i e n t l y high protons may eventually become the major p o s i t i v e defects i n the oxide. The L ~ H o O a t which t h i s occurs may vpry widely: Many oxides are unaffected by hydrogen defects a t atmospheric pressures o f H 0 o r Hz, while, f o r instance, Y 0 i s dominated by protons even under the most

free

conditions p r a c t i c a l l y obtgigable /2/ a t h i g h temperatures.

Dominant concentrations o f protons w i l l be counterbalanced by electrons o r various p o i n t defects w i t h negative e f f e c t i v e charges depending upon the oxide system, the presence o f dopants, and experimental conditions. I n the following, various l i m i t i n g e l e c t r o n e u t r a l i t y conditions i n v o l v i n g protons as m a j o r i t y defects w i l l be examined.

Oxides w i t h oxygen deficiency

I n such oxides the major n a t i v e i o n i c defects are metal i n t e r s t i t i a l s o r oxygen vacancies, both w i t h p o s i t i v e e f f e c t i v e charges. These are counterbalanced by equivalent concentrations o f electrons. If, however, the concentration o f

i n t e r s t i t i a l protons exceeds t h a t o f the n a t i v e p o i n t defects, the e l e c t r o n e u t r a l i t y c o n d i t i o n becomes (from Eq. 1 )

This s i t u a t i o n has been reported f o r ZnO / I / and Ti02 /5/ e q u i l i b r a t e d i n hydrogen- r i c h atmospheres.

Stoichiometric and m e t a l - d e f i c i e n t oxides

I n such oxides protons w i l l be balanced by the major negative i o n i c defect, which may be metal vacancies ( i n an oxide w i t h Frenkel- o r Schottky-type defects) o r oxygen i n t e r s t i t i a l s ( i n an oxide w i t h anti-Frenkel-type defects).

I n the oxide MO (where a i s the o x i d a t i o n number o f the c a t i o n ) the formation o f metal vacanciesa&?th an e f f e c t i v e charge a '

,

V;

,

and the corresponding equi 1 i brium expression may be w r i t t e n

Electron holes are described by h' and t h e i r concentration by p.

When protons are counterbalanced by these metal vacancies, the e l e c t r o n e u t r a l i t y c o n d i t i o n w i l l be (from Eqs.1 and 3 )

a/ (2a+2) [Hjl

=

a[V;-1

=

K4 PHZO

where K is an expression containing a, K , Kg, and Kin

(=

np). By combining Eqs.3 and 4, 4the concentrations of the electrojic minority defects become

Protons compensated by metal vacancies are mentioned as possible defect situations in BaTi03

/ 8 /

and A1203 /3/.

When the protons are compensated by oxygen interstitials it may be shown that the electroneutrality condition is given by

1 /3

[Hjl

=

2[0;'1

=

K6 PHZ0

_{( 6 )}

Under this condition the electronic minority defects vary as

The situation given by Eq.6 is believed to dominate in pure Y203 at high temperatures and water vapor pressures /2/,(Norby, to be published).

-dominated oxides are doped with substitutionally dissolved htgher- valent foreign cations(which form positive defects), the concentration of protons will be suppressed. Protons become minority defects and the defect structure becomes independent of the water vapor pressure. This has been observed for Y203 doped with

si /2/.

When low gzg !ent cations Hlb+ are dissob~ed substitutionally, they form negative defects, Mli f , and if the amount of HI present is fully soluble, or

eventually "frozen in", the dopant concentration is constant. If these defects are compensated by protons the electroneutral i ty condition may be written

The proton content may thus be changed by doping. By combining Eqs.8 and 1 , ^one

obtains

This situation has been reported for Y O3 ("pure" and Ca-doped) /Z/,(Norby, to be published), Ti02 15/, and Yb-doped S ~ C ~ O /9/.

Fig.1 illustrates the de endences on $he water vapor pressure for some defects in an oxide M

0

doped with Hlq+ cations. The situation corresponding to Eq.8 is shown in the mid612 section of the figure. The dominating native defects are assumed to be anti-Frenkel defects, and the figure illustrates how oxygen interstitials become dominating at high water vapor pressures (Eq.6). At low water vapor pressures protons become minority defects and oxygen vacancies then become the dominating positive defects.

I1 I - DEFECT-DEPENDENT PROPERTIES Electronic conductivit

The variation in n- orYp-conductivity as a function of the water vapor pressure at constant oxygen activity is generally an easy way of determining possible effects of protons, as the dependence on the water vapor pressure is almost unique for each proton-dominated situation: From Eqs. 2, 5, 7, and 9 slopes of 1/4, 1/(2a+2), 1/6, and 1/2, respectively, are expected for plots of the logarithm of the n-conductivity vs log PHen

(,

and correspondingly the negative slopes for p-conductivity). A slope of zero ifiiicates that protons are minority defects.

In practical uses and laboratory studies the water vapor pressure may typically

C1-852 JOURNAL DE PHYSIQUE

Fig.1 - Schematic defect structure of an oxide M203 as a function of the

3 water v a ~ o r Dressure. The oxide is

dominated by' anti-Frenkel defects and protons, and doped with substitutionally dissolved MI"

cations, which concentration is assumed to be constant (broad, horizontal

1

ine) . Three defect structure situations, corresponding to three limiting electroneutrality conditions , are shown in the figure.

Metal vacancies and electron holes are shown as examples of minority defects. The numbers in the figure represent the slopes of the lines for the various defects.

cover 4 orders of magnitude (e.g. to 10" atm.). For a p-conductor corresponding to the limiting condition of Eqs.8 and 9 such a change in the water vapor pressure would lower the conductivity by a factor of 100 in going from "dry" to "wet atmospheres. For the limiting condition of Eqs.

6

and 7 the conductivity would correspondingly decrease by a %actor of

4.6. Such

effects have been demonstrated in practice; thus for Y 0 at

800

C, for instance, the p-conductivity has been observed to decrease by a face08 of 30 (Norby, to be published).

Ionic conductivities

When protons are major defects in an oxide, the protons will also in most cases dominate the ionic conductivity, as protons are expected to be more mobile than other charged point defects. In such cases sol id electrolytes may correspondingly be proton-conductors.

The transport number for protons, t

+

, can be determined by the use of

conventional electrochemical cells. ~ h u emf over an oxide specimen may, for instance, be expressed as /9/ ,(Norby, to be pub1 ished)

This equation is valid for specimens exposed to small gradients in Po and PHZ0. The proton transport number (t

+)

can be found by measuring the emf with a known

2

gradient in PHQO (and equa! oxygen pressures), while the total ionic transport number (tion) can be Lfound with a known gradient in

Pop

(and equal water vapor pressures).

From these data the native ion transport number bean be found as tMS0

=

tion - ^tH+ .

Diffusion-control l e d properties

f f the defect s i t u a t i o n o f Eq.8 i-s again considered (middle region o f Fig.1).

where D t h e o r e t i c a l l v decreases 100 times from "drv" t o "wet",conditions. i t can be shown t h a t [O"] and-[Vh"] increase by f a c t o r s of-10' and

lob,

respecti;ely, o4er the same r a n g i o f water vapor pressure, w h i l e [ V i ' ] decreases by a f a c t o r o f 10

.

For the s i t u a t i o n o f Eq.6 ( r i g h t hand region o f Fig.1) the corresponding increases i n [O!'] and [ V i " ] are about 20 and 100 times, respectively. The r a t e s o f d i f f u s i o n - c o n t r a l l e d processes such as s i n t e r i n g , creep, and corrosion are generally

proportional t o the concentration o f one o f the n a t i v e i o n defects. These r a t e s may therefore be expected t o vary w i t h the water vapor pressure i f the oxide i s dominated by protons. The d i r e c t i o n o f the change w i l l depend on the charge o f the r a t e - l i m i t i n g defect species. As shown, the e f f e c t s may t h e o r e t i c a l l y be large. I n r e a l cases the e f f e c t s may be influenced by competing e q u i l i b r i a , other d i f f u s i n g species, short c i r c u i t d i f f u s i o n paths, etc., which, f o r instance, may decrease the dependences on the water vapor pressure from the t h e o r e t i c a l ones. S t i l l , the importance o f proton-dominated defect s i t u a t i o n s should be considered f o r many systems. Investigations without c o n t r o l o f the water vapor pressure o r i t s e f f e c t s can correspondingly lead t o unrel i a b l e o r erroneous r e s u l t s and conclusions and unexpected m a t e r i a l s behavior. Knowledge o f proton-dominated defect structures and c o n t r o l o f the water vapor pressure may, on the other hand, provide a useful t o o l i n research, manufacture, and use o f oxides a t high temperatures.

The treatments i n t h i s paper may be applied t o other ceramic m a t e r i a l s than oxides. I n the case o f sulphides, f o r instance, the equations may be used d i r e c t l y by replacing 0 w i t h S.

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