Polarizable complexes in barium fluoride doped with trivalent ions

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Polarizable complexes in barium fluoride doped with

trivalent ions

E. Laredo, D. Figueroa, M. Puma

To cite this version:

j o i K \ A L Dt PHYSIQUE Colloque C6, supplément au n° 7, Tome 4 1 , yMz/fe* 1980, jpage C6-451

Résumé. — Des expériences de thermocourants ioniques ont été réalisées sur des cristaux de BaF2 dopés avec des ions trivalents de différentes terres rares et avec des ions d'yttrium. Les impuretés que nous avons étudiées sont La3 + , C e3 +, Pr3 + , Nd3 + , Sm3 + , G d3 +, Ho3 + , T m3 + et Y3 + , tous les échantillons ont une concentration d'impu-retés de 10~3, exprimée en fraction molaire. Trois pics principaux ont été observés dans tous les cristaux. La relaxation de plus haute température a été attribuée, en raison de son comportement après des traitements ther-miques, aux dislocations présentes dans le cristal. Les pics de plus basse température sont dus aux relaxations de dipôles R3 +- F ~ , en premiers ou seconds voisins qui coexistent dans le BaF2 et non fréquemment dans les autres cristaux de structure fluorite. Deux ensembles de valeurs pour les paramètres de relaxation E0 et T0, obtenus pour les différentes impuretés, sont présentés. D'abord on suppose que l'énergie de réorientation des dipôles a une valeur unique. Ensuite nous supposons que cette énergie a une distribution gaussienne de largeur a autour d'une valeur moyenne E0(a). Les valeurs obtenues avec ce modèle pour les paramètres de la relaxation dipolaire E0(a),

T0(CT) et a sont présentées et discutées.

Polarizable complexes in barium fluoride doped with trivalent ions

E. Laredo, D. R. Figueroa and M. Puma

Umversidad Simon Bolivar, Department of Physics, Apartado 80659, Caracas-Venezuela

Abstract. — Ionic thermal current experiments have been carried out on BaF2 crystals doped with different

trivalent rare earth and yttrium ions. The impurities studied here are La3+, Ce3+, Pr3 + , Nd3 + , Sm3+, Gd3+,

Ho3 + , Tm3+ and Y3+ ; all the samples have an impurity concentration of 10"3 expressed in molar fraction.

Three principal relaxation peaks have been observed in all the crystals. The highest temperature relaxation is attributed, due to its behaviour after thermal treatments, to the dislocations present in the crystal. The low tempe-rature peaks are identified as relaxations due to nearest neighbours and next nearest neighbours RE3+-Ff

com-plexes that coexist in BaF2 and not very often in other fluorite structures. Two sets of values for the relaxation

parameters E0 and x0 obtained for the various impurity ions are presented. First a unique value is assumed for

the reorientation energy of dipoles. Second a distribution of the reorientation energy, with a width a around a mean value E0(o) was assumed. The values obtained with this model for E0(a), T0(a) and a are given and discussed.

1. Introduction. — The introduction of trivalent rare earth cations, RE3 + , in fluorite structures is

responsible for the creation of substitutional impurities in the cation sub-lattice and interstitial fluorine ions, Fj~, as charge compensators. Consequently, dipoles are formed in the crystal whose characteristics are easily studied by Ionic Thermal Currents experi-ments [1] (I.T.C.).

In CaF2 crystals, the dipoles present a tetragonal

symmetry [2, 3, 4] and are therefore attributed to RE3 + -Ff in nearest neighbour (n.n.) positions.

In SrF2 crystals, n.n. or next nearest neighbour dipoles

(n.n.n.), or both, are present and the predominant specie depends on the ionic radius of the impurity [5]. In the case of BaF2 crystals the two observed I.T.C.

peaks have been attributed to the two kinds of dipoles, namely n.n. and n.n.n dipoles in the BaF2 : La3 + [6],

and BaF2 : G d3 + [7] systems. E.P.R. experiments on

BaF2 crystals doped with paramagnetic trivalent

ions have shown the existence of lines with trigonal symmetry (Gd, Yb) [3, 4] or tetragonal symmetry (Ce) [8].

In the present work, BaF2 crystals doped with La,

Ce, Pr, Nd, Sm, Gd, Y, Ho and Tm are studied using I.T.C. techniques. Experimental results are consistent with the presence of n.n. and n.n.n. dipoles in the crystal, the second kind is found to be the most abundant for all the impurities studied here. The relaxation parameters are calculated for the two kinds of dipoles, assuming that the dipole reorienta-tion energy is single valued and then, a gaussian dis-tribution for this activation energy.

In the first model, the crystal contains N dipoles per unit volume, characterized by a dipolar moment \i and a relaxation time t(T) = z0 exp(E0/kT), where T0

is the reciprocal frequency factor, and E0 is the

activa-tion energy which has, in this model, a unique value for each dipolar specie. The current density J(T), due to the reorientation of each dipolar specie, can be written

30

C6-452 E. LAREDO, D. R. FIGUEROA AND M. PUMA

where Po is given by Po = N p 2 Ep/(3 k T p ) , Ep is the

polarization electric field, and T, the polarization

temperature if we suppose that the polarization time is long enough to allow the polarization to reach its saturation value. From the variation of J as a function of temperature, we can calculate the activa- tion energy, Eo, the reciprocal frequency factor, r,, and the number of dipoles, N, if we assume a value for the dipolar moment.

In the non mono-energetic model it is assumed that the reorientation energy is not single-valued but has a gaussian distribution with a width o and a mean

value Eo(o). The relaxation parameters Eo(a), ~ , ( o )

and o are obtained with a computer model. This model

takes into account the main features of the experimen- tal I.T.C. curve which show the need of an energy distribution to fit the experimental results.

2. Experimental procedure. - The samples are

BaF, asgiven single crystal slabs with an area of about 1 cm2 and a thickness of 1 mm. Most of them were purchased from Optovac, Inc. and all of them have a nominal impurity concentration of expressed in molar fraction. The impurities are rare earth trivalent cations and'yttrium. The crystal faces are not painted or coated with any conductive material. The experi- ment has been already described in detail [6].

3. Results and discussion. - In figure 1 we have drawn the I.T.C. curves obtained for the nine impu- rities studied here. As the crystals have not been thermally treated, the intensities of the second peak have been normalized for all the samples. It can be seen that all the crystals present a similar spectrum, with three main relaxation peaks A, B and C, ordered with increasing temperature. The intensity of peak B is always greater than that of peak A. For La and Ce doped BaFz the two peaks A and B are well separated ;

for Pr, Nd, Sm and Gd doped crystals, peaks A and B are very close and only after polarizing the samples at low temperatures could the two peaks be distin- guished. For Y, Ho, and Tm, the two peaks appear again clearly in figure 1.

In table I, we have reported in the second and third columns the relaxation parameters Eo and r,, for the

TEMPERATURE (K)

Fig. 1. - I.T.C. Spectra of barium fluoride doped with of

nine different impurities.

reorientation of dipoles responsible for the B peak, calculated with the mono-energetic model. In the fourth and fifth columns are listed the values obtained for the relaxation parameters E,(o), r,(o) using the

gaussian distribution whose calculated width is given in the sixth column. The temperature TMB at which the maximum current occurs for the B peak can also be read for the different impurities studied here. The behaviour of peaks A and B in the case of La and Y doped crystals is completely similar after ther- mal treatments and as a function of the doping amount. Quenching from high temperature leads to an increase in the intensity of peaks A and B while

annealed crystals show a decrease in the height of these peaks. For a given impurity, the relative inten-

Table I. - Reluxatior~paranieters for dipoles of type II in BaF, :

RE^

+

POLARIZABLE COMPLEXES IN BARIUM FLUORIDE DOPED WITH TRIVALENT IONS C6-453

process related to the dislocations present in the crystal.

sity of peaks A and B is maintained when the concen- I I I I I I

Finally, we can conclude that in BaFz crystals the

I

'

1

tration of the samples is changed. The energies cal-

culated for the A peaks are always lower than those

most abundant structure is the

RE^'-F;

-in n.n.n.

positions coexisting with n.n. dipoles in lower concen-

,

_1.12 116 1.20 1.24 1.28 1.32

tration. For the concentration studied here _{IONIC RADIUS}

₍₈₁

T~ HOY Gd Sm Nd Pr Ce Lo

the do show any Frg. 2. - Mean actlvatlnn energy r ~ ~ r s o r the impurity mnic i a d m

the samples present a C peak whose behaviour can be _{for the peak of BaF,,} understood if it is associated with the dislocation

0.70

-

i J 3

1

3

4 . 1 . 1 J .

for the B peak. For example E t = 0.39 eV for La and E,A = 0.45 eV for Y . Following the arguments given

in the case of the BaF, : La system [6], peaks A and B

are identified with the relaxation of n.n. and n.n.n. 0.66

-

dipoles, respectively.

_-

The highest temperature peak, C, shows for all the >

samples the same features. It is the most intense peak

-

of the spectrum and its height is not clearly related to

5

the impurity concentration ; its position changes with w

the thermal treatment and the amount of doping. 0.58

-

It is believed that the C peak is due to a relaxation

structure of the crystal. In figure 2 we have reported _{tlltional trivalent ions in the Ba2+ sub-lattice is} the variation of E,(a) as a function of the ionic radius _{responsible for the peak broadening observed in all}

of the impurity [9]. It can be seen that there is a _{the spectra.}

maximum for E,(o), located around Sm3+ which

happens to have a radius close to that of the fluorine AcknowIedgments. - This work was partly sup-

anions. This seems to support the idea that the elastic ported by the CONICIT (Venezuela) and we wish lattice deformation due to the introduction of substi- to express our gratitude to them.

DISCUSSION

Question. - H. W. DEN HARTOG.

I agree with you that the broadening of the I.T.C. peaks can be described by assuming a distribution of I.T.C. peaks (see VAN WEPEREN et al., Phys. Rev. B 16 (1977) 2953) ; I also agree with you that electro- static dipole-dipole interaction may not be the only origin for the broadening. Did you analyse your C peaks ; what were the activation energies as compared to that of the motion of free F--interstitials ?

Reply.

-

E. LAREDO.

We analysed our C peaks, but due to its behaviour (its position changed with the thermal treatment or with the concentration as reported in reference [6])

we were not able to find a unique value for E,. For

the as given crystal, E, ranged from 0.85 to 0.65 eV

as c increased, which are around the 0.76 eV found

for the migration of the free F; by ionic conductivity.

Question. - R. W . WHITWORTH.

The high temperature peak attributed to dislo- cations is of interest as there is some evidence for such an effect in the alkali halides. Have you any

information about the dependence of this peak on annealing or plastic deformation ?

Reply.

-

E . LAREDO.

Yes, in reference [6] we have reported the behaviour

of the C peak as a function of thermal and mechanical treatments. After bending the crystal, dipole peaks did not change while the C peak intensity was increased by a factor of three; moreover for the annealed crystal the C peak moves to higher temperatures while for the quenched crystal the temperature of the maximum was reduced again (Fig. 7 , reference [6]).

Comment. - J . J . FONTANELLA.

1) A relaxation associated with the n.n.n. complex has been observed in CaF, for rare earths smaller than Gd. It is our R,, relaxation (0.15 eV).

2) Care must be taken in interpreting the appea- rance of n.n.n. complexes in lanthanum doped SrF, at high concentrations. While it may be due to a concentration effect, it may also be due to an impurity

E. LAREDO, D. R. FIGUEROA AND M. PUMA

References

[I] Buccr, C., FIESCHI, R. and GUIDI, G . , Phys. Rev. 148 (1966) 816. [6] LAREDO, E., FIGUEROA, D. R. and PUMA, M., Phys. Rev. B 19 [2] KIEL, A. and MIMS, W. B., Phys. Rev. B 7 (1973) 2917. (1979) 2224.

[3] CHI-CHUNG YANG, SOOK LEE and BEVOLO, A. J., Phys. Rev. [7] KITTS, E. L. Jr and CRAWFORD, J. H. Jr., Phys. Rev. B 9 (1974) B 13 (1976) 2762. 5264.

[4] RANON, U. and YANIV, A., Phys. Lett. 9 (1964) 17. [8] KIEL, A. and MIMS, W. B., Phys. Rev. B 6 (1972) 34. [5] LENTING, B. P. M., NUMAN, J. A. J., BIJVANK, E. J. and DEN HAR- [9] SHANNON, R. D., Acta Crystallogr. A 32 (1976) 751.