TRANSPORT IN OXIDESNew investigation of oxygen self-diffusion in Cu2O

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TRANSPORT IN OXIDESNew investigation of oxygen self-diffusion in Cu2O

F. Perinet, S. Barbezat, C. Monty

To cite this version:

F. Perinet, S. Barbezat, C. Monty. TRANSPORT IN OXIDESNew investigation of oxy- gen self-diffusion in Cu2O. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-315-C6-318.

�10.1051/jphyscol:1980680�. �jpa-00220118�

JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 7, Tome 41, Juillet 1980, page C6-315

TRANSPORT IN OXIDES.

New investigation of oxygen self-diffusion in CU2O

F. Perinet, S. Barbezat and C. Monty

Laboratoire de Physique des Materiaux, C.N.R.S. Bellevue, 1, place A.-Briand, 92190 Meudon, France

Résumé. — On a effectué de nouvelles mesures des coefficients d'auto-diffusion de l'oxygène dans C u20 mono- cristallin. Le domaine de stabilité a été exploré sous 2 pressions partielles d'oxygène : 4,6 x 10"4 atm et 0,26 atm.

L'isotope stable l sO a été utilisé comme traceur. Il a été introduit soit par la méthode du dépôt mince soit par recuit sous atmosphère de teneur en ' sO constante. On a mesuré les profils de diffusion par spectrométrie de masse de l'émission ionique secondaire. Ces profils obéissent bien aux solutions de l'équation de Fick. Nous obtenons comme résultat :

Le défaut responsable de la migration de l'oxygène qui correspond à cette dépendance en pression partielle d'oxy- gène est l'interstitiel d'oxygène chargé une fois O,'.

Abstract. — New measurements of oxygen self-diffusion coefficients in Cu20 have been performed on single crystals under two oxygen partial pressures (4.6 x 10"* atm and 0.26 atm) in the stability domain.

The stable isotope l sO has been used as a tracer. It has been introduced by a thin film method or by annealing under a constant ^{1 8}0 pressure. The diffusion profiles have been measured by secondary ion mass spectrometry.

They obey quite well the solutions of Fick's equation. The results can be represented by :

The defect responsible for the oxygen migration corresponding to the above oxygen partial pressure dependence is the singly charged oxygen interstitial OJ.

1. Introduction. — Self-diffusion measurements are a good tool for studying point defects. In non-stoi- chiometric oxides, the oxygen partial pressure depen- dence of the diffusion coefficients enables to identify the defects responsible for the migration of the studied species and the temperature dependence provides information about the enthalpies and entropies of formation and migration of these defects [1].

Oxygen diffusion in oxides is poorly known because of various experimental difficulties. However, its measurements can provide us important information about the defects in the oxygen sublattice when these are the minority defects. Measurements of oxygen self-diffusion are also of importance in the interpre- tation of phenomena such as sintering and high- temperature creep in which matter transport occurs.

Recent theory predicts that the kinetics of this matter transport is controlled by the mobility of the less mobile component of the crystal [2], which, in the C u²0 case, is the oxygen.

C u20 is a metal-deficient oxide. The departure from stoichiometry can be higher than 10 "3 [3]. The defects responsible for this departure from stoichiometry are neutral and singly-charged copper vacancies. The

structure of C u²0 is quite unusual : a face-centered sublattice for copper but a body-centered sublattice for oxygen.

In the present work, we have investigated the diffu- sion of oxygen in C u²0 single crystals at two different oxygen partial pressures : 0.26 atm and 4.6 x 10"4 atm in the temperature range of stability. We have mea- sured the temperature dependence at each oxygen partial pressure and the oxygen partial pressure dependence.

2. Experimental. — Samples were cut from large single crystals which were prepared using an arc- image furnace. Their microstructure was investigated in detail [4, 5].

Diffusion treatments were performed at tempera- tures ranging between 812 °C and 1 098 °C.

— At a P⁰² — 0.26 atm the tracer (stable isotope

l sO ) was used in gas phase. A group of zeolitic pumps was used to obtain vacuum in the chamber and release the stocked 1 80 when the required condi- tions (T, P0l) of the experiment were reached. Dif- fusion takes place by isotopic exchange. The initial

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980680

C6-3 16 F. PERINET. S. BARBEZAT AND C. MONTY

concentration in 1 8 0 (90 % in the first runs) remains nearly constant during the experiments (chamber volume

-

- At a Po, = 4.6 x atm, the 1 8 0 quantity available in the chamber would not be sufficient. We used another method to introduce the tracer : a C Uthin film was deposited on the surface of the ~ ~ ~ ~ sample by a R.F. sputtering technique (C. Sella, J. C. Martin, to be published). The surface sample was covered by another pure C u 2 0 crystal to prevent evaporation during diffusion annealing. The annealing was done in a flowing mixture 460 ppm O,/Ar.

The concentration profiles were established by secondary ion emission analysis (CAMECA micro- analyser) [6].

3. Results. - 3.1 CONCENTRATION ^PROFILES. -

Due to the fact that we used two different experimen- tal conditions to introduce the tracer, we have consi- dered two different solutions to the Fick's equation.

In the case of samples annealed under 1 8 0 atmo- sphere, the Fick's solution is :

where x is the penetration, C, is the natural isotopic concentration, D the self-diffusion coefficient, t the annealing time. Our results well fit this equation (Fig. 1).

Penetration Ipm)

Fig. 1 . - Diffusion profile at Po, = 0.26 atm and T = 1 055 "C obeying the solution (1). The c, value in t h ~ s experiment wasfound equal to 60 %. The slope of this curve is equal to 112 J D ~ .

When an original thin-film geometry is used, the solution to the Fick's equation is :

where A is a constant. Our results obtained a t low oxygen pressure well fit that solution (Fig. 2).

Fig. 2. - Diffusion profile at Po, : 4.6 x atrn and T = 1 020 OC obeying the thin-film solution (2). The slope of the line is - 114 Dt.

3 . 2 TEMPERATURE AND PRESSURE DEPENDENCE OF

DIFruslvln. - The Arrhenius diagram has been established separately for Po,=4.6 x atrn (Fig. 3) and Po, = 0.26 atrn (Fig. 4). The least-square ana- lysis gives :

- at Po, = 4.6 x atrn :

- at Po, = 0.26 atrn

x 2

C - C , = A e x p - - (2) ^{Fig. 3. --} Arrhen~us diagram for oxygen selfdiKusion In CuzO

4 Dt at Po, = 4.6 x atm.

NEW INVESTIGATION O F OXYGEN SELF-DIFFUSION IN C u 2 0 C6-3 17

Fig. 4. - Arrhenius diagram for oxygen self-diffusion in CutO at Po, = 0.26 atm. Vertical dotted lines show the limits of stability o f Cu,O.

The difference between the two values of activation energy is smaller than the experimental error. We have calculated the pressure dependence of diffusivity using an average activation energy of 1.55 eV and obtained :

D x (Po,)" with n = 0.40 T 0.05 .

4 . 2 POINT DEFECTS CHARACTERIZATION. - The oxygen selfdiffusion coefficient is proportional to the concentration of the defect responsible for the oxygen migration and to the mobility of that defect.

Using the Kroger-Vink's formalism and the mass action law it is possible to determine the expected dependence on oxygen partial pressure of simple defects in CutO [I].

The formation of a defect can be considered as the result of an oxygen exchange process between the oxide and the gas phase.

For a defect V;,, which is the most important charged defect, it can be written

The equilibrium constant is given by :

For a minority defect of the oxygen sublattice, for example O(, we have :

The combined pressure and temperature depen-

f

+

with a new reaction constant Kgi given by :

D (cm2. s - ' ) = 3 x 10- 3(~o,)0.4 exp - 1.55 (eV) k T (5)

where Po, is in arm. To obtain explicitly the concentration of a given defect, one uses the neutrality equation :

4. Discussion. - 4.1 COMPARLSON WITH OTHER

DATA. vious data of Ebisuzaki - We have compared our results with pre- et al. [7]. There are slight [

vAul -

discrepancies on the activation energy and oxygen (v&u and holes are the most important charged partial pressure dependence. The authors found species). For the majority defect v:u, it gives Q = 1.7 T 0.3 eV and n = 0.50 T 0.09; the latter

[V;.,] = [K:=,]"2 pAi8

value was obtained from a very narrow range of (12)

pressures at 1 050 O C . -

and for the minority defect 01 Bretheau [3] measured the influence of temperature

and Po, on steady-state creep rates of c u 2 0 - a t high [o;] ⁼ K&(K[~,)- p3I8 0 2 .

temperatures. He found : (13)

= ~ ~ 5p o . 4 ~ o . 1 . 2 1.8 T 0.3 (eV) It is important to point out that the temperature

0 2 exp - k T (6) dependence of the defect concentration given by the

where i: is the creep strain rate, A is a constant and 0

is the stress.

These results obtained on the same single crystals are in good agreement with our diffusion results;

that seems to prove that the matter transport in the previous creep experiments was controlled by the diffusion of oxygen which is the less mobile species.

Such a correlation has not often been demonstrated in oxides.

Table 1.

reaction constants is generally a combination of various constants which do not only characterize the considered defect but also depend on the majority defect population through the neutrality equation.

The oxygen partial pressure dependence of a given defect is also correlated to the majority defects.

Table I [l] gives the value of the exponent associated with the concentration of oxygen-sublattice defects and of V i and Vh for an intrinsic M 2 0 oxide obeying a neutrality equation of the general type : [Vh] p.

C6-3 18 F. PERINET. S. BARBEZAT A N D C. MONTY

We did not consider- the extrinsic case because the impurity content of' the oxide is certainly much smaller than the departure from stoichiometry in the experimental range of T and Po,.

It appears clearly in table I that in our case, oxygen vacancies are unambiguously excluded and that the defect giving the oxygen partial pressure dependence closest to 0.4 f 0.05 is 0:. It is noted that from their results of the Po, dependence, Ebisuzaki er ul. [7]

proposed that the responsible defect is the neutral interstitial oxygen 0:. Oxygen interstitial may not be unexpected in such a structure where the oxygen sublattice is not compact (B.C.).

Defect complexes are highly improbable consi- dering the low level of concentration of these minority defects. The possibility for oxygen to migrate by an exchange mechanism bet ween two oxygen atoms when a copper vacancy is in the ncighbourhood of the tracer has been emphasized [7, 81. Such a mecha- nism should yield a dependence of the selfdiffusion coefficient on Po, determined by the copper vacancies with the exponents n given in the last two columns of table I. The large difference between these values and the value of n determined in the present work cannot account for such a mechanism.

The temperature dependence of the oxygen self- diffusion coefficient gives the sum of the activation energy of the concentration and of the migration energy of the defect responsible for the diffusion. It is possible to deduce the sum of enthalpy AH:;, associated with the formation process described by the cquation (9), and of the migration energy AHE.

It is clear through equation (13) that our experiments give the sum :

where is associated with the equation (12) and with the concentration of holes through (11).

From recent results of Maluenda [9] for the elec- tronic conductivity a in Cu,O we can deduce, if we assume that ^o is independent of the temperature, a value of 0.7 eV for A H f c U / 2 . Thus we can write :

It is interesting to compare this result to the same quantity for V;., (the value of AHp=u has been taken from [9] and [lo]) :

Experiments which enable us to separate formation and migration enthalpy terms for minority defects would be of great interest. However, it is clear from these results that oxygen interstitials can be created easily in C u 2 0 , relative to copper vacancies.

5. Conclusion. - Cu,O is the first case of non- stoichiometric oxide in which the oxygen interstitial has been identified by a phenomenological approach : it appears singly charged. The original structure of this covalent oxide where the oxygens are not in a compact arrangement is perhaps the main reason for that.

Acknowledgments. -- We want to express our thanks to M. Doumanis N., who, during a University post-graduate formation, has had a contribution to the experimental part of the present work. We are indebted to M. Lam N. for assistance in language difficulties.

DISCUSSION

Comment. ^- Z. MORLIN. Replj. - F. PERINET.

Perhaps it would be of interest to measure the lat- Such results would be vcry interesting. Indeed In tice parameter by X-ray diffraction : one may expect our knowledge, there was not any publication of parameter differences depending upon deviation from accurate values of lattice parameter versus oxygen

stoichiometry. partial pressure and temperature.

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