THE GOLDANSKII-KARYAGIN EFFECT IN Gd2M2O7 AND Eu2M2O7 COMPOUNDS

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THE GOLDANSKII-KARYAGIN EFFECT IN

Gd2M2O7 AND Eu2M2O7 COMPOUNDS

E. Bauminger, A. Diamant, I. Felner, I. Nowik, A. Mustachi, S. Ofer

To cite this version:

THE GOLDANSKII-KARYAGIN EFFECT

IN Gd

2

M

2

0

7

AND Eu

2

M

2

0

7

COMPOUNDS (*)

E. R. BAUMINGER, A. DIAMANT, I. FELNER, I. NOWIK, A. MUSTACHI and S. OFER

The Racah Institute of Physics, The Hebrew University, Jerusalem, Israel

Résumé. — Un effet Goldanskii-Karyagin (G. K.) important a été mis en évidence dans les

spectres Môssbauer d'un monocristal de Eu2TÎ207 obtenus à l'aide du rayonnement y de 103 keV de I53Eu. La grandeur de l'effet est compatible avec les résultats antérieurs relatifs à des poudres. L'effet G. K. est également présent dans les spectres de GdîT^CH Gd2Sn207, Gd2CrSb07, Gd2SnTi07, Gd2Ru207 et Gd2lr207, obtenus à l'aide du rayonnement y de 105 keV de 155Gd. Dans ces composés, les paramètres de l'interaction quadrupolaire varient de manière considérable, linéairement avec la taille de la maille élémentaire. L'effet G. K. se manifeste aussi dans les spectres de Gd2Ti2Û7, lorsque l'on observe le rayonnement y de 123 keV de 154Gd. Nous examinons l'influ-ence de divers paramètres sur la grandeur de l'effet G. K. observé.

Abstract. — A large Goldanskii-Karayagin (G. K.) effect has been observed in the recoilless absorption measurements of the 103 eV y ray of 153Eu in a single crystal of Eu2Ti2C>7. The size of the effect is consistent with results obtained previously in a Eu2Ti207 powder sample. G. K. effects were observed in the MQssbauer effect of the 105 keV y ray transition of 155Gd in Gd2Ti207, Gd2Sn207, Gd2CrSb07, Gd2SnTi07, Gd2Ru207 and Gd2lr207. The quadrupole interaction parameters in these compounds change drastically and linearly with the size of the unit cell. The G. K. effect was also observed in the Mossbauer spectrum of the 123 keV y ray of 154Gd in Gd2TJ207. The dependence of the G. K. effect on various parameters is discussed.

1. Introduction. — A large Goldanskii-Karyagin

(G. K.) effect has been previously observed in the recoilless absorption measurements of the 103.2 keV gamma ray of 1 5 3Eu in Eu2Ti207, of the 89 keV gamma ray of 1 5 6Gd in Gd2Ti207 and of the 21.6 keV gamma ray of 1 5 1Eu in Eu2Ti207 at several temperatures [1-3]. In order to further establish the effect and determine its dependence on energy and on crystal properties, we performed recoilless absorption measurements in several pyrochlore type compounds, using the 103.2 keV gamma ray transition in 1 5 3Eu, the 105.3 keV gamma ray transition in 1 5 5Gd and the 123.1 keV transition in 1 5 4Gd. The recoilless absorp-tion spectra of the 103.2 keV gamma ray of 153Eu in a Eu2Ti207 single crystal, obtained at 4.1 and 36 K, confirm unambiguously the assumption that the observed changes in the relative line intensities in the polycrystalline samples [1] are solely due to lattice vibrational anisotropy. Measurements of the G. K. effect in the Mossbauer spectrum of the 123 keV gamma ray of 1 5 4Gd in Gd2Ti207 show that the recoilies absorption fractions /j| a n d /±, follow appro-ximately the predictions of the Debye model. Using the 105 keV gamma ray transition in 1 5 5Gd, the G. K. effect in several other Gd pyrochlore type (*) This research was supported by a grant from the United States - Israel Binational Science Foundation (BSF) Jerusalem, Israel.

compounds, G d2M207, was observed and corre-lations between the size of the G. K. effect and pro-perties of the M ion were sought for. The quadrupole interaction parameters in these compounds change drastically and linearly with the size of the unit cell.

2. Crystal structure. — The pyrochlore type, R2M207 compounds have a cubic structure and a space group symmetry Fd3m. The R (rare earth) atoms occupy the 16c positions with a 3 m point symmetry. All rare earth ions in the compound are crystallographically equivalent and the electric field gradient (EFG) at the rare earth nuclei is axially symmetric (tj = 0). The local symmetry axis is in the [111] direction. The M ions occupy the 16d posi-tions. The nearest M neighbours of an R ion lie in a plane perpendicular to the [111] direction and contain-ing the R ion. This special arrangement may be the main cause for the huge G. K. effect found in these compounds.

3. Experimental results. — In all recoilless absorp-tion spectra reported here, least squares computer fits to the experimental spectra, assuming a pure axial quadrupole interaction and relative intensities of the lines as given by the appropriate Clebsch-Gordan (C. G.) coefficients, do not reproduce well the observed line intensities. In all measurements, very thin absorbers

4

C6-50 E. R. BAUMINGER, A. DIAMANT, I. FELNER, I. NOWIK, A. MUSTACHI AND S. OFER were used, all saturation effects were avoided and were

therefore not taken into account.

3.1 RECOILLESS ABSORPTION OF THE 103 keV GAMMA RAY OF 1 5 3 E ~ IN A SINGLE CRYSTAL OF Eu2Ti207.

-

In all the G. K. effect experiments performed so far on the pyrochlore type structure compounds, polycrys- talline absorbers have been used [I-31. In order to eliminate completely the possibility that the observed changes in the relative line intensities, as predicted by the appropriate C . G. coefficients, are due to preferred orientations in the polycrystalline absorber samples (the texture effect) [4], recoilless absorption measure- ments were performed on the 103.2 keV gamma ray of 1 5 3in a single crystal of Eu2Ti207, at 4.1 and 36 ~ ~ K. The single crystal was grown from Eu203 and TiO, in a PbF, flux which was maintained at 1 220 OC in a platinum crucible until the flux completely eva- porated [5]. A 153SmPd3 source at 4.1 K was used in the present measurements.

In the cubic crystal there are four equivalent [I, 1, 11 directions. The cosine of the angle between them is 113 The single crystal absorber was oriented in such a way that the incident gamma ray was parallel to a [I, 1, 11 direction. With such an orientation there are two inequivalent Eu sites : one site with an angle 0 = 00 between the gamma ray and the symmetry axis, and three equivalent sites with cos 8' = 1/3. In such an arrangement, two different angles are involved in a single measurement, andJillf, can be derived from it in the following way : the angular distribution functions. of the gamma ray for a dipole transition are given by :

F;(O) = sin2 0 for Am = 0 and

Fi(0) = )(1

+

cos2 0) for Am =

+

1

,

which yields for the site with 0 = 0, F~(o) = 0 and P:(o) = 1, and for the 3 sites with cos 0' = 113, F;(O') = 819 and F:(o') = 519. The intensities of the Am = 0 transitions, as given by their respective C . G. coefficients, have therefore to be multiplied by a = 3(8/9) f (0') and those of the Am = 1 transitions by b = 3(5/9) f (0')

+

lfil. The relative intensities of the Am = 0 transitions, as given by the appropriate C . G. coefficients, have therefore to be multiplied by a factor

For the case of axial symmetry of the Debye-Waller factor, the recoil free fraction as function of 0 is given by 161 :

f (6) = exp[- k2

<

X:

>

-

N cos2 01 =

where N = k2[< X ;

>

-

<

X:

>]

=

-

In (fillfi) and k is the wave number of the incoming gamma ray.

It follows therefore that : Jillf(0') = (Jillfi) exp(N cos2 6') =

= exp(- N -t N cos2 6') = exp(- 8 N/9) (3) and

BS,,. = 8/[5

+

3 exp(- 8 N/9)]

.

The experimental spectrum obtained at 36 K is shown in figure 1. The dotted line is the expected

I I I I 1 I I

-8 -6 -4 -2 0 2 4 6 8

VELOCITY (mm/s)

FIG. 1. -Recoilless absorption spectrum of the 103.2 keV gamma rays of 153Eu in an Eu2Ti207 single crystal at 36 K. The solid line is the theoretical spectrum obtained from a least squares fit to tht: experimental spectrum, taking into account an aniso- tropic absorption fraction. The parameters of the theoretical spectrum are given in the text. The dashed line is the theoretical spectrum obtained, assuming an isotropic absorption fraction.

spectrum when an isotropic absorption fraction is assumed. Least squares computer fits to the experi- mental spectra obtained at 4.1 K and 36 K, yielded the values of BS,,, as given in table I. The results obtained

Parameters derived from the analysis of the Moss- bauer spectra of the 103.2 keV y ray of ' 5 3 E ~ in a polycrystalline absorber and in a single crystal of

Eu,Ti207.

k c . N N

T(K) (single crystal) (single crystal) (polycrystal)

from the measurements in the single crystal are compar- ed in the table with the results obtained in measure- ments carried out on a polycrystalline absorber [I]. The results are seen to be in very good agreement, proving unambiguously the existence of the large G. K. effect in this compound.

source used in the present work was 5 4 E ~ in 5zSmPd3 at 4.1 K. It was produced by irradiation of SmPd, enriched in 15'Sm. The 1 5 4 E ~ source (TI,, = 16 y) is produced through the following chain of reactions :

Least squares computer fits to the experimental Mossbauer spectrum yield the value 0.9 f 0.1 for Bo, the coefficient which has to multiply the relative intensity of the Am = 0 transition as determined by the C. G. coefficient and the value 0.61

+

0.03 for B,, the coefficient which has to multiply the relative intensities of the Am = f 2 transitions. The fits yield the value of (928

+

8) MHz for the quadrupole interaction parameter eq 154Q (123 key), resulting in 154Q(123)/155~(0) =

-

1.164 f 0.018 [7] and thus 154Q(123) =

-

1.85

+

0.20 b.

Using the formulae given in ref. [2] for B, and B,, a value of

-

2.3 f 0.3 is deduced for N. The value of N derived from the analysis of the Mossbauer spectrum pf the 103 keV transition in 1 5 3 E ~ 2 T i z 0 7 was

-

1.95 f 0.15 [I] and the value derived from the analysis of the spectrum of the 89 keV transition of 156Gd was N =

-

1.55

+

0.15 [2]. Assuming a Debye model for the dependence of

J;I

and

f,

on energy, these two values would yield N =

-

2.8 f 0.3 for the 123 keV transition. Within the limits of errors the experimental value for N of the 123 keV transition and the extrapolated value do not disagree.

3.3 RECOILLESS ABSORPTION OF THE 105 keV GAMMA

RAY OF 155Gd IN GdzMz07.-All Gd2M207

compounds which served as absorbers were prepared by heating stoichiometric amounts of Gdz03 and M0, (M = Ti, Sn, Ru, Ir, Sno.,Ti0.,, Sbo,5Cro,5) at temperatures between 1 300 OC and 1 500 OC for 2 t o 3 days in air. X ray powder diffraction analyses were performed and the structure was confirmed. The unit cell dimensions are given in table 11.

Lattice parameter and parameters derived from the analysis of the Mossbauer spectra of the 105 keV y ray of 155Gd in GdzM,07 compounds

-10 -5 0 5 10

VFLOCITY (mmls )

FIG. 2. - Recoilless absorption spectra of the 105 keV gamma rays of 155Gd in GdzSnz07 at 4.1 K. The solid line is the theore- tical spectrum obtained from a least squares fit to the experi- mental spectrum, taking into account an anisotropic absorption fraction. The parameters of the theoretical spectrum are given in the text. The dashed line is the theoretical spectrum obtained

assuming an isotropic absorption fraction.

All spectra obtained could be fitted by multiplying the relative intensities of all the Am = 0 transitionslas determined by the squares of the appropriate C. G, coefficients by a factor B. The solid line in figure 2 is the theoretical computer fit obtained for Gd,Sn,O,, assuming Q(105)/Q(0) = 1.00 [7]. The fits yield the values of B and the quadrupole interaction parameters summarized in table 11. The isomer shift found for all the compounds was (0.57 f 0.02) mm/s relative to the source used.

In figure 3 the dependence of the quadrupole interac- tion parameter on the size of the unit cell for all

a0

Compound eqQ B

MHz

The source used in these measurements was 155SmPd,. In figure 2 the Mossbauer spectrum of the 105 keV gamma ray of 155Gd in Gd2Snz07 is shown.

FIG. 3. - The quadrupole interaction parameter in the various R2M207 compounds with tetravalent M, as function of the size

C6-52 E. R. BAUMINGER, A. DIAMANT, I. FELNER, I. NOWIK, A. MUSTACHI AND S. OFER compounds with tetravalent M is shown. As seen

in the figure the quadrupole interaction parameter decreases linearly with the unit cell's size. A similar

behaviour has been observed by Loebenstein et a].

[%I

for the field gradients acting on the '19Sn nucleus in various R,Sn,O, compounds. The gradients on the Gd and on the Sn nuclei [8], change by about 40

%,

while the size of the unit cells changes by 3

%

only. This change of the gradients is about five times larger than expected if it is assumed that the ratio of the distances between any two ions in R,M,O, compounds are the same as the ratio of the unit cell sizes of the compounds. It is extremely difficult to explain why both the gradients at the Gd and the Sn nuclei change so drastically and linearly with the unit cell. A possible explanation is based on the

assumption that the changes of the gradients are caused by the changes of the parameter x, which determines the positions of the oxygen ions in the unit cell.

Using the B values obtained from the computer fits

to the experimental spectra and the formulae given in ref. [I], the N values given in table I1 were deduced. As seen from the table, there is no simple correlation between the size of the G. K. effect (as expressed by N ) and the quadrupole interaction parameters or size of the unit cell. For the few compounds measured the general trend is such that the size of the G. K. effect decreases with increasing mass of the M ion. This may be due to the change of the character of the external << d )) electrons (3d for Ti, 4d for Sn and Ru

and 5d for Ir). The effect obtained in G ~ , M ~ M ~ O , (when mixing two different M ions in the compounds) does not give the average of the effect obtained in ~ d , ~ f 0, and G~,M;o,. In order to obtain a clearer picture more compounds of this series should be measured, yet there are severe difficulties in obtaining single phase compounds with M ions other than those reported here. For example, Gd2Zrz07 and Gd,Hf20, were prepared, but very broad lines were obtained, probably because of the existence of vacancies which were distributed at random throughout the lattice.

References ARMON, H., BAUMINGER, E. R., DIAMANT, A., NOWIK, I. and

OFER, S., Phys. Lett. 44A (1973) 279. (The true value in this ref. for N at 4.1 K should read

-

1.95 (15).)

ARMON, H., BAUMINGER, E. R., DIAMANT, A,, NOWIK, I. and OFER, S., Solid State Commun. 15 (1974) 543.

[3] BAUMINGER, E. R., DIAMANT, A., FELNER, I., NOWIK, I. and OFER, S., Phys. Lett. 50A (1974) 321.

[4] PFANNES, H. D. and GONSER, U., Appl. Phys. 1 (1973) 93.

151 GARTON, G. and WALKLYN, B. M., J. Mater. Sci. 3 (1968) 395. [6] GOLDANSKII, V. I. and MAKAROV, E. F., in Chemical Appli- cation of Mossbauer Spectroscopy, Eds Goldanskii, V . I. and Herber, R. H. (Academic Press), 1968, p. 103. [7] ARMON, H., BAUMINGER, E. R., DIAMANT, A., NOWIK, I. and

OFER, S., Nucl. Phys. A 233 (1974) 385.

181 LOEBENSTEIN, H. M., ZILBER, R. and ZMORA, H., Phys. Lett.