PARAMAGNETIC POINT DEFECTS IN ZIRCONIA

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PARAMAGNETIC POINT DEFECTS IN ZIRCONIA

G. Bacquet, J. Dugas, C. Escribe, F. Fabre

To cite this version:

G. Bacquet, J. Dugas, C. Escribe, F. Fabre. PARAMAGNETIC POINT DEFECTS IN ZIRCONIA.

Journal de Physique Colloques, 1973, 34 (C9), pp.C9-137-C9-139. �10.1051/jphyscol:1973926�. �jpa- 00215401�

JOURNAL DE PHYSIQUE Colloque C9, .supplktnc.nt au no 11-12, Totne 34, Nouernbre-DPcemb1.e 1973, page C9-137

PARAMAGNETIC POINT DEFECTS IN ZIRCONIA

G. BACQUET, J. DUGAS, C. ESCRIBE a n d F. F A B R E Laboratoire de Physique des Solides AssociC a u C N R S Universite Paul-Sabatier 31077 Toulouse Cedex, France

RCsumk. - Nous avons etudie par R P E des ions d'impuretes trivalentes dans des monocristaux de zircone monoclinique et de zircone stabilisee i la chaux. Nos resultats montrent queces impuretes n'ont jamais de lacune d'oxygene associee en position de plus proches voisins. Nous avons aussi confirme, par des experiences de recuit, I'existence d'une transition desordre-ordre pour le reseau anionique de la zircone stabiliske a la chaux. Une telle transition n'a pu Ctre mise en evidence dans la zircone yttriee.

Abstract. ^- By means of the ESR technique we studied trivalent impurity ions in single crystals of monoclinic zirconia as well as of calcia stabilized zirconia. Our results show that these impurities never have an associated 0 2 vacancy in a nearest neighbour position. We also confirmed the existence of a disorder-order transformation of the anion lattice of calcia stabilized zirconia under annealing. Such a transformation cannot be observed in Yttria stabilized zirconia.

I. Introduction. - When trivalent impurities are introduced in fluorite type oxides MO, ( M = metal), only one 0,- vacancy is needed t o compensate two M 3 + ions, and this must be kept in mind.

Tlie ESR results concerning T h o , and CeO, showed tliat these impurity ions, which are located in M 4 + sites, are always present i n two different environ- ments [I-41. The first one, for which the cubic symmetry of the host lattice is conserved, gives rise to the most intense spectrum. In the second one, a 0'- vacancy is associated with the paramagnetic ion in a nearest neiglibour position, and spectra corres- ponding t o

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axial symmetry are recorded.

It seemed interesting to us to look at the behaviour of such impurities i n Zirconia (ZrO,) which has a different structure. Tliere exists three well-defined polymorplis of pure ZrO,, namely, the monoclinic, tetragonal and cubic structures. The monoclinic phase, the accurate str~lcture of which was first reported by McCullough and TI-ueblood [5], is stable up to about l 000 (1C and the11 tr;~risfc)rms over a I00 "C tempera- ture rkinge to the tetragonal phase. Both structures are related to the fluorite structure. Finally tlie compound could take the cubic C a F 2 structure above 2 300 ''C.

On the other hand. at high temperatures, tlie addition of variable amounts oi' some MO o r M,O, oxides, enables one to obtain stable cubic solid solutions tliat can be conserved at room temperature by u more or less rapid quenching. C a O and Y 2 0 , are currently used and the corresponding solid solutions are respec- tively called Calcia Stabilized Zirconia (CSZ) and Yttria Stabilized Zirconia (YSZ).

2. Results concerning monoclinic zirconia. - Single crystals of monoclinic zirconia respectively doped with Cr3+. F e 3 + and Gd"' were obtained by tlie flux _rrowtli technique.

There are four cationic sites in the unit cell (Fig. 1).

Due to the existence of the two inversion centres I , and I,, and of two symmetry axes (Ob and Oc), they are magnetically equivalent when the magnetic field is respectively parallel o r perpendicular t o the binary b axis.

FIG. I . - Projection of the ZrO? unit cell on (001) after McCul- lough and Trueblood [S]. The crystallographic a axis is in the

(car) plane at 99" from c.

Tlie substitutional site is located in sevenfold coor- dination. and as such, has no symmetry. Nevertheless it is reasonable to think that the corresponding spectra would be very different according as there exists a

0 2 - vacancy in nearest neiglibo~lr positions o r not. In all cases. we will be obliged to fit tlie spectra to a very general spin-Hamiltoniari :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973926

C9-138 G. BACQUET, J. DUGAS, C. ESCRlBE AND F . FABRE where the 0: are the normalized operator equivalents

defined by Michoulier 161, and with n = 2 for Cr3+ ; n = 2 , 4 for Fe3+ and 11 = 2, 4, 6 for G d 3 + .

From the values of the spin-Hamiltonian constants, it is possible to find, using the method proposed by Michoulier and Gaite [7], the principal axes of the second or fourth order tensors of the crystal field.

This last tensor is mainly sensitive to the nearest neighbours, and its symmetry must be at least the same as that of the immediate environment.

On the other hand, using ZrOz point ion lattice it is possible to calculate sums of Tesseral harmonics which are proportional to the crystal field constants.

From them, we can find the theoretical axes corres- ponding to the various tensors describing the crystal field. Comparison between experimental and theore- tical axes will give us informations about tlie imme- diate neighbourhood of the paramagnetic ion.

2.1. Cr3+ ION. - The X-band spectra showed the resonance of two distinguishable centres and could be fitted to a type (1) spin-Haniiltonian the constants of wich were the sa~,ie for both defects. Only the orien- tation of the second-order crystal field tensor axes is slightly different [8]. These facts rule out the possibi- lity to have a defect without vacancy and the other with an associated vacancy. As all the trivalent ions cannot be associated with 0'- vacancy, we can conclude that an anion vacancy is never bounded to C r 3 + .

2.2. S-STATE IONS : Fe3+ A N D G d 3 + . - Previous experiments undertaken on Fe3+ doped ZrOz only gave second order crystal field componant [9]. Our Q-band spectra obtained with Fe3+ as well as Gd3', are due to a unique type of defect. Consequently both ions are not associated to a 0'- vacancy. The theore-

FIG. 2. - Stereographic projection of the experimental (cros- ses) and theoretical (circles) fourfold pseudosynlmetry axes of the fourth order term of the Fe3+ spin Hamiltonian. a, b and c

are the crystallographic axes.

FIG. 3. - Stereographic projection of the experimental (crosses) and theoretical (circles) fourfold pseudosymmetry axes of the fourth order tern1 of the Gd3+ spin Hamiltonian. a, b and c

are the crystallographic axes.

tical and experimental fourfold axes of the fourth order tensor for Fe3+ and G d 3 + are respectively given in figures 2 and 3.

It can be seen that Fe", the ionic radius of which (0.64

A)

is lower than the z r 4 + one (0.79

A)

and which cannot stabilize zirconia, does not modify its imme- diate neighbourhood. Such is not the case with the big Gd3+ ion ^{(1. =} 0.97

A),

the oxide of which is a good zirconia stabilizer, and for which the local symmetry evolved to that of cubic zirconia (a' bc axes).

3. Results concerning CSZ and YSZ.

3.1 YSZ. ^- As grown (ZrOz)o,e6(Y203),,,4 single crystals doped with G d 3 + or Yb3+ only exhibited a very broad line ( - 800 G) without resolved structure.

They were annealed at about 1 000 OC during a week and no change was observed in tlie ESR spectra.

Consequently no information about the possible asso- ciation of a M 3 + ion with a 0 2 - vacancy, was obtained.

Recently, in both as grown and current blackened single crystals of 8 and 12 mole '%, YSZ, Thorp ct al. [lo] observed anisotropic ESR lines characterizing a defect with

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axial symmetry. They also recorded spectra that, according them, correspond to single electrons trapped in vacancies surrounded by a tetrahedron of four zirconium.

3.2 CSZ DOPED WITH Gd3 ⁺ on Y b3

+.

- As grown 14 mole %, CSZ gave the same unresolved spectra as the YSZ samples. After annealing ( I 000OC during a week) the crystals became milky and exhibited resolved spectra corresponding to a weak

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axial symmetry [I I]. Such a distorsion is not due to the presence of a 02- vacancy in a nearest neighbour position (< 1 1 1

>

type deformation). We found that

PARAMAGNETIC POINT DEFECTS IN ZIRCONLA C9-139 both as grown and annealed CSZ samples gave tlie ronments (limited to the first shell) of the A, B same perfectly cubic X-ray pattern with a lattice and C sites can be deduced one from the other by constant equal to 5.1305 f 0.0005 A, thus confirming f 2 n/3 rotations about the

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directions, and that the whole crystal is not tetragonal. In such expe- possess a twofold Oz axis directed along a

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riments, however, we only observe the cationic lattice. direction. When the magnetic field is aligned along a If there was an interstitial C a 2 + ion near the para-

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direction, two sites are magnetically equi- magnetic impurity, a much stronger anisotropy in valent because they have their respective Oz axes the y b 3 + g factor (gll = 3.4788 and g , = 3.4032) perpendicular to the static field, the Oz axis of the would beexpected. Finally the observed

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axial third site being parallel to the d-c field. Consequently, symmetry can be explained assuming displacements we must observe a (c perpendicular ⁾⁾ spectrum twice of the 0'- ions from the ideal fluorite anion sites as as intense as tlie (( parallel )) one, this is the case.

suggested by Carter and Rot11 [12]. These authors The D site has a particular configuration consisting already found that CSZ undergoes an ordering trans- of two imbricate tetrahedra having the same C, axis, formation near I 000 "C and proposed an ordered and as such might give a cc cubic ⁾⁾ spectrum which arrangement of 0'- ions, an extension of which up was not observed. We think that the Yb3' and Gd3' to the four ZrO, constituing a unit cell is given i n avoid this site where four 0'- ions move in, because figure 4. The impurity ion can occupy the four direrent their ionic radii are more important than the Zr4+ one.

sites respectively labelled A, B, C and D. The envi- The presence of at least one c a 2 + in the twelve next nearest neighbour positions, and also of its associated 0'- vacancy in more remote anion sites can explain tlie anomalous linewidths (from 20 to 70 G) we observed.

4. Conclusion. - In monoclinic zirconia, as well as in CSZ, we observed that the oxygen vacancies created to preserve lattice neutrality are not in nearest neigh- bour sites with respect to the trivalent impurity ions.

This result conflicts with some of the ESR observa- tions of Thorp et 01. on YSZ single crystals. We have seen that in YSZ. the anionic lattice is always disor- dered. The ordering of this lattice being generally a consequence of an ordering of the cations, it appears possible in YSZ that some 0'- vacancies, essentially if they trap an electron. can be associated with Y3' ions. Contrarily, when these vacancies do not trap FIG. 4. - Extension of the ordered arrangemellt of 0'- pro- electrons, they are free enough to give rise, in stabilized posed by Carter and Roth [12], showing the f o L l r possible substi- zirconia, to a strong ionic conductivity as reported in

tutional sites. the literature.

References

[I] ABRAHAM, M., LEE, E. J. and WEEKS, R. A,, J . P I I ~ ~ s . [7] MICHOULIER, J . and GAITE, J . M., J. CIIPIII. P11)js. 56

Clletrl. Solids, 26 (1965) 1249. (1972) 5205.

[2] ABRAHAM, M., WEEKS, R. A,, CLARK, G. W. and FINCH, [81 BACQUET, G . , DUGAS, J. and ESCRIBE, c., ~ l r y s . slat.

C. B., Plr~.s. Rcv. 137A (1965) 138. Sol. 0, 47 (1971) 177.

[3] ABRAHAM, M., WEEKS, R. A., CLARK, G . W. and FINCH, [9] GERVAIS, F. and CABANNES, F., ^Phys. Stat. Sol. 33 (1969)

C . B., Plzys. Rev. 148 (1966) 350. 453.

[lo] THORP, J. S., AYPAR, A. and Ross, J. S., J. Muter. Sci.

141 ABRAHAM, M., BOATNER, L. A,, FINCH, C. B., LEE, E. J. 7 (1972) 729.

and R. J . Phys. Che112. "lids 28 [I l ] BACQUET, G., DUGAS, J., ESCRIBE, C. and FABRE, F.,

[5] MCCULLOUGH, J. D. and TRUEBLOOD, K. N., Acta Cryst. J . Phys. C. (Solid state Physics) 6 (1973) 1432.

12 (1959) 507. [I21 CARTER, R. E. and ROTH, W. L., General Electric Res.

[61 MICHOULIER, J., Thesis, Univ. of Grenoble, 1970. Rep., 67C (1967) 308.