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Endor study of Cr5+ in the antiferroelectric NH4H2AsO4

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HAL Id: jpa-00208838

https://hal.archives-ouvertes.fr/jpa-00208838

Submitted on 1 Jan 1978

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Endor study of Cr5+ in the antiferroelectric NH4H2AsO4

J. Gaillard, P. Gloux

To cite this version:

J. Gaillard, P. Gloux. Endor study of Cr5+ in the antiferroelectric NH4H2AsO4. Journal de Physique,

1978, 39 (9), pp.961-964. �10.1051/jphys:01978003909096100�. �jpa-00208838�

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ENDOR STUDY OF Cr5+ IN THE ANTIFERROELECTRIC NH4H2AsO4

J. GAILLARD and P. GLOUX (*)

Section de Résonance Magnétique, Département de Recherche Fondamentale, C.E.N. de Grenoble, 85 X 38041 Grenoble Cedex, France

(Reçu le 28 mars 1978, accepté le 22 mai 1978)

Résumé.

2014

Nous avons vérifié que les configurations de Slater des protons associées au centre Cr5+

étaient de type latéral dans l’antiferroélectrique ADA : une telle situation favorise le couplage du

centre avec son environnement. Les résultats sont comparés à ceux obtenus précédemment pour le centre Cr5+ dans le ferroélectrique KDA.

Abstract.

2014

We have verified that the proton Slater configurations associated with the Cr5+ centre in antiferroelectric ADA were of the lateral type : such a situation enhances the coupling of the centre

With its environment. The results are compared with those obtained previously for the Cr5+ centre

in ferroelectric KDA.

Classification Physics Abstracts 76.70D - 77.80

1. Introduction.

-

According to the recent theory by Halperin and Varma [1] ] certain defects in inter- action with the soft mode may account for the central

peak observed by neutron scattering in crystals undergoing a structural phase transition. Very recent

neutron experiments show a systematic enhancement of the central peak intensity with increasing defect

concentration [2]. Müller and Berlinger made an ESR study of the dynamics at a Cr5 + centre in ferroelectric

chromium-doped compounds of the KH2po4 (KDP) family and identified this centre as being a Halperin-

Varma type defect [3]. By an ENDOR investigation of

the structure of the Cr5 + centre in the ferroelectric

phase of KH2As04 (KDA), Gaillard et al. [4] have

shown that the Cr5 + is associated with lateral Slater proton configurations in disagreement with the fact

that the ferroelectric order is made up only of the polar proton configurations. Hence, the dynamics

observed in the high temperature phase bring only

the above-mentioned lateral configurations into play.

It was natural to surmise that the coupling of the Cr5 +

with its surroundings would be favoured in an anti- ferroelectric crystal such as NH4H2As04 (ADA)

where the order is contructed from the lateral confi-

gurations [5]. For this reason we have undertaken with Müller and Berlinger a study of Cr5 + in

(*) Chercheur CNRS.

chromium-doped ADA. Müller and Berlinger have

examined the dynamics around the defect by EPR [b].

In our experiments we have used ENDOR to verify

that the Cr5 + was, as predicted, associated with lateral

configurations. In this paper, we report our results and relate them to the previous ones obtained for Cr5 + in KDA.

2. Experimental results.

-

The chromium-doped single crystals of ADA were obtained from Müller and Berlinger who had grown them under the condi- tions described in [6]. We studied these crystals with

the EPR-ENDOR spectrometer used previously [4].

After partial saturation of the EPR dispersion signal

at 1.5 K, ENDOR lines appear for frequencies of the

rf source in the range 1 to 40 MHz. We intend to determine the position of the protons in the four

O-H...O hydrogen bonds attached to a Cr 4

tetrahedron for the antiferroelectric phase. We recall

that one expects to find two protons close to the tetrahedron and two protons far from it in agreement with the NH4H2CrO4 formula.

The ENDOR spectrum of the protons for Cr5 + /

ADA when the magnetic field H is orientated parallel

to the c axis is shown in figure 1 ; the c axis is the fourfold axis of the paraelectric phase. This spectrum is analogous to the one recorded in Cr5 + /K DA for the

ENDOR lines l-, 2- and 2+ associated with the close protons 1 and 2 [4]. The + and - signs distinguish

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01978003909096100

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962

between the ENDOR lines related to the + 1/2

and - 1/2 values of the electronic spin. As in Cr5 + /

KDA, the 1 + line was undetectable.

For protons with a weaker hyperfine coupling giving ENDOR lines in a range of a few MHz around the free proton NMR frequency (which is about

13.8 MHz, dependent on the EPR line position) the spectra are much more intricate than for Cr5 + /KDA

due to the presence of the NH1 groups. Several protons of the NH1 ions nearest to the defect have hyperfine couplings of the same order as those of the far protons in the O-H...O bonds and this complicates seriously

the identification of the ENDOR lines for these far protons. This was particularly the case for H in the c

direction where we have not succeeded in identifying

the ENDOR lines of the far protons because each line is mixed with a packet of closely spaced NH1 proton lines. These groupings of NH4+ lines, corresponding to

sets of nearly equal couplings, are not visible in

figure 1 but appear very clearly in expanded sweeps.

We do not show such sweeps here in so far as we are

not interested in studying the NH+ protons.

FIG. 1.

-

ENDOR spectrum of the Cr5 + centre for H // c. The

two small lines marked by an asterisk are unrelated to this centre.

On figure 2 we reproduce the angular dependences

of the ENDOR lines of the close protons when H is rotated in the ca and ab crystallographic planes.

Figure 3 shows the angular dependences of the far protons 3 and 4. Due to the presence of the NH’ group

proton lines and to the small separation between the lines of protons 3 and 4 for many orientations, a very careful analysis was needed. Even with such an

analysis we have obtained only part of the curves relative to protons 3 and 4 ; however, this was enough

to determine the corresponding hyperfine coupling

tensors. As an example we give in figure 4 the spectrum recorded for H in the 45° direction in the ab plane,

which was the most appropriate orientation to observe the ENDOR lines of the far protons.

As for KDA, in single crystals of ADA there are eight Cr5 + sites related four by four to the two possible

orientations of AS03- tetrahedron. For a given

orientation of H, the problem was to identify the

FIG. 2.

-

Angular dependences of the ENDOR transitions in the ca

and ab planes for the close protons.

FiG. 3.

-

Angular dependences of the ENDOR transitions in the ca

and ab planes for the far protons.

FIG. 4. - ENDOR spectrum of the Cr5 + centre for H // 45°

in the ab plane.

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TABLE 1

Hyperjine interaction tensors of protons of the Cr5 + centre

in ADA ( from this paper) and KDA ( from [4]).

various ENDOR lines belonging to a given site : this

was resolved in the same manner as in f4]. The field H distinguishes four sites A, B, C, D when it lies in the ca

plane and two sites A’, B’ when it is in the 45° direction of the ab plane : these letters are used to designate the corresponding ENDOR lines in figures 2, 3 and 4.

The four Cr5 + sites corresponding to a given tetrahe-

dron are associated with the four sites A, B, C, D in the ca plane : the A, B sites belong to a type of anti- ferroelectric domain, the C, D sites belong to the

other type. The two sites in the same domain are

related by C2 symmetry around the c axis ; two sites

in different domains are related by an S4 symmetry

operation about the c axis.

The angular dependences exhibit overall a remar-

kable similarity with those obtained for Cr5 + /KDA [4].

The analogy is total in so far as we ascertain that the

curves of the same Cr5+ site for the protons 1, 2, 3, 4

are those which have parallel variations in ADA and KDA. As for Cr5+ /KDA, we found that in the + 45°

direction in the ca, bc and ab planes of a direct a, b, c reference frame the ENDOR lines for a given site

are C, A and B’ respectively [7]. In table I, we collect the calculated values of the isotropic and dipolar hyperfine couplings of the four protons for this site.

The principal directions are defined by their direction

cosines in the a, b, c frame. In order to make further

comparisons, the results previously obtained in Cr5 + /

KDA are also included.

3. Discussion.

-

The considerable resemblence between the angular dependences for Cr5+ /ADA

and Cr5 + /KDA noted above is evidently also reflected

in the hyperfine tensors. Taken two by two, the hyperfine tensors are very comparable in magnitude

as well as in orientation. Indeed, as in KDA, the

Cr5+ /ADA centres are associated with four protons in lateral Slater configurations. The protons of same type (close or far) are situated one in a high 0-H ... 0 hydrogen bond, the other in a low O-H...O hydrogen bond, the vertical axis being the c axis. Their principal

directions associated with the positive dipolar cou- pling, closely related to the chromium-hydrogen directions, are related to each other by an approxi-

mate S4 symmetry operation around the c axis. As for KDA, the absence of symmetry for the Crs+ centre leads to unequal hyperfine couplings for protons of the

same type : this is particularly the case for the isotropic couplings of protons 1 and 2.

One might have thought that the change of host crystal which involves atomic substitutions

(K+ - NH4 ) and different type of order (ferro-

electric -> antiferroelectric) would have had more

influence on the hyperfine couplings of the protons.

In fact, only some of the data are affected and the modifications are not drastic. This is the case for :

(i) The isotropic coupling of proton 2 which varies

from 15.89 MHz in KDA to 19.63 MHz in ADA.

Such a positive coupling, originating in an overlapping

of orbitals, may be very sensitive to a small modifi- cation in the position of the proton with respect to the electronic orbital. But finally the inequivalence bet-

ween the isotropic couplings of close protons remains

of the same order in both crystals : one might have

thought that it would be more dependent on the

environment, the more so as one of the close protons

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964

is necessarily in conflict with the ferroelectric proton arrangement, a situation which does not exist in the antiferroelectric case.

(ii) The dipolar part for proton 3 which diminishes noticeably on going from KDA to ADA.

(iii) The principal directions associated with the

negative dipolar values for the protons 1 and 3 : their orientations move about 20° on going from KDA to

ADA. Curiously enough, the principal directions of

proton 1 in one crystal are, with a good approximation,

related to those of proton 3 in the other crystal by a C2

rotation around the c axis.

The disposition of protons 1, 2, 3 and 4 around a

Cr034- ‘ tetrahedron is of course identical with the

disposition in KDA : for the site defined by table 1

see the corresponding scheme in figure 3 of [4].

We add that Müller and Berlinger have found that the

deuterated compounds (DKDA and DADA) have

similar g-tensors to each other r3, 6]. The principal g-tensor values are identical and the principal direc-

tions X, Y, Z are very close to each other. The axis X which is in the ab plane makes an angle of 27.5° with a

in DADA instead of 25° in DKDA and the axis Y makes an angle of 30° with the ab plane instead of 35°.

Except for this, the X, Y, Z axes have the same position as in [4] for the site considered. In the same

manner as for Crs + /KDA, the wave function of the

unpaired electron must have a predominant dx2- y 2

character with its lobes pointing preferentially towards

the positive charges of the close protons as discussed in our previous paper [4].

4. Conclusion.

-

The proton configuration at the Cr5+/ADA centre is in complete agreement with the antiferroelectric ordering in the crystal. Moreover this

was the only expected result in so far as we knew that

the Cr5+ centre was already stable with protons in lateral configurations in the unfavourable environ- ment of the ferroelectric phase of KDA. In addition only small modifications have been ascertained bet-

ween KDA and ADA for the Cr5 + centre in the low temperature phase : the electron-proton system under consideration is so little influenced by the kind of

environment that, finally, the coupling values by

themselves are characteristic of the relative position occupied by the corresponding proton in the lateral

configuration.

The arrangement in lateral configurations pre-

disposes the Cr5+ centres to be more sensitive, that is

more coupled to the dynamics of the paraelectric phase in ADA than in KDA. When crossing over the

transition from below, the evidence of dynamic

effects at Cr5+ in KDA was detected only several

tens of degrees above the Curie point [8]. In contrast,

Müller and Berlinger [6] have just observed that the dynamics in ADA appeared abruptly from the Curie point since above this temperature they see only the averaged line characteristic of a rapid exchange.

The sum of the EPR-ENDOR results for the Crs +

centre would justify the hypothesis [9] that either the polar configurations in the ferroelectric KDA or the lateral configurations in the antiferroelectric ADA dominate in the dynamics near the Curie point : this

would be the precursor sign of the ordering of the low temperature phase.

Acknowledgments.

-

We thank Pr. K. A. Müller

and Mr. W. Berlinger for providing us with the chro-

mium-doped single crystals of ADA, and Dr. R. Cox for reading the manuscript.

References

[1] HALPERIN, B. I. and VARMA, C. M., Phys. Rev. B 14 (1976) 4030.

[2] HASTINGS, J. B., SHAPIRO, S. M. and FRAZER, B. C., Phys. Rev.

Lett. 40 (1978) 237, and references therein.

[3] MÜLLER, K. A. and BERLINGER, W., Phys. Rev. Lett. 37 (1976)

916.

[4] GAILLARD, J., GLOUX, P. and MULLER, K. A., Phys. Rev.

Lett. 38 (1977) 1216.

[5] HEWATT, A. W., Nature 246 (1973) 90.

[6] MÜLLER, K. A. and BERLINGER, W., Z. Phys. B, to be published.

[7] There were two typing errors in the previous paper [4] as noted in

an erratum (Phys. Rev. Lett. 40 (1978) 593) : on page 1217.

column 1, m the sentence beginning at line 20, read bc for ba and B’ for A’.

[8] MÜLLER, K. A., DALAL, N. S. and BERLINGER, W., Phys. Rev.

Lett. 36 (1976) 1504.

[9] ADRIAENSSENS, G. J., Ferroelectrics 12 (1976) 269.

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