HYPERFINE INTERACTIONS IN AN ORBITAL DOUBLET : CaF2 : Fe2+

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HYPERFINE INTERACTIONS IN AN ORBITAL

DOUBLET : CaF2 : Fe2+

J. Regnard, J. Chappert

To cite this version:

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 12, Tome 37, Ddcembre 1976, page C6-611

HYPERFINE INTERACTIONS

IN

AN ORBITAL DOUBLET

:

CaF,

:

Fe2+

J. R. REGNARD and J. CHAPPERT

Centre #Etudes Nucliaires de Grenoble, DRF/Groupe Interactions Hyperfines, 85X, 38041 Grenoble Cedex, France

Resume.

-

Le systbme CaFz : Fez+ a kt6 etudie par spectroscopie Mossbauer h des temperatures variables entre 300 K et 1,6 K et en presence de champs magnktiques jusqu'8 55 kOe ?I 4,2 K. Sous I'action combinee du champ cubique et du couplage spin-orbite, I'etat Bectronique fondamental est un singulet &park d'une distance 2 D du premier ktat excite r4. En champ nu1 on

met en kvidence I'apparition d'un doublet quadrupolaire AEp = 3,6 mm/s vers 9 K, superposb h la raie principale du Fez+ et on d6duit 2 D = 16 & 2 cm-1. Ceci est en accord avec les prkdictions de Ham. En presence d'un champ magnktique des interactions quadrupolaire et magnktique sont induites par suite du mklange du singulet fondamental TI avec r4.

Abstract. - The system CaF2 : Fez+ has been studied by Mossbauer spectroscopy at various temperatures between 300 K and 1.6 K and with applied magnetic fields up to 55 kOe at 4.2 K. Under the combined action of the cubic field and of the spin-orbit coupling, the electronic ground state is a singlet TI separated from the first excited triplet r4 by a distance 2 D. In zero field one

observes a quadrupole doublet AE2 = 3.6 mm/s around 9 K, superimposed on the main Fez+

single line and one derives 2 D = 16

+

2 cm-1. This is in agreement with predictions by Ham. When a magnetic field is applied, quadrupole and magnetic interactions are induced because of the mixing of the fundamental T I singlet with r4.

1. Introduction.

-

The behaviour of the Fe2+ ion embedded in a cubic diamagnetic matrix has recently been systematically studied by Mossbauer spectroscopy. In particular, the consequences for the Moss- bauer spectrum of coupling between asymmetric vibrational modes and an electronic state having orbital degeneracy were examined [I]. Up to now most of the experimental results concerned the Fez+ ibn in octahedral symmetry [2, 31. We report here 57Fe Mossbauer

results for CaF, : Fe2+ which has cubic- symmetry (eightfold coordination) and where the cubic field splitting is inverted, i. e. the orbital ground state is a

doublet. As discussed by Ham [I] spin-orbit coupling

leaves a singlet

r,

lowest separated by 2 D

-

20 cm-' from the next excited triplet

r4

(Fig. 1). The CaF, :

free cubic spin

ion field orbit

FIG. 1. - Energy level scheme of the Fez+ ion in cubic eightfold

coordination.

Fe2+ system is of interest because (i) it is strongly ionic, hence covalency corrections are probably negligible (ii) it provides information on the importance and on the nature of the Jahn-Teller effect in an orbital doublet (iii) magnetic and quadrupole interactions can be induced in the singlet spin-orbit level because of the close vicinity of the first excited state.

2. Results.

-

Single crystals of CaF, doped with 57Fe were grown by the Bridgman method. A chemical analysis by atomic absorption spectrophotometry indicated that the concentration of 57Fe was 0.04 at.

%.

Mossbauer spectra of 0.5 mm slices were recorded at various temperatures between room temperature and 1.6 K and in various applied magnetic fields up to 55 kOe at 4.2 K.

Typical zero field spectra are shown in figures 2 and

3. At room temperature the spectrum consists of a single line of width 2

r,

= 0.27 mm/s and of isomer shift 6 , = 1.53 mm/s relative to sodium ferrocyanide. The value of 6 , is typical of a strongly ionic Fe2+ charge state. At very low temperatures, 4.2 K and 1.6K, the line is only slightly broadened (2

r,

= 0.37 mm/s) but no additional contribution appears, indicating that pair effects similar to those observed in ZnS : Fe2+ by Gtrard et al. [4] are not

present. This is probably due to the small iron concentration of our sample, about one order of magnitude less than that of Gtrard et al. However some peculiar

features occur at intermediate temperatures. First around 9 K a doublet with an isomer shift 6 , close to

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J. R. REGNARD AND J. CHAPPERT 1-1

i 'i

i

a/

VELOCITY (mmls) # I I I I I , - 2 - 1 0 1 2 3 4 VELOCITY ( m m h )

FIG. 2. - Mossbauer spectra of CaF2 : @ez+ at various tempe- FIG. 3.

-

Mossbauer spectra of CaFz : Fez+ at low temperatures ratures and zero applied magnetic field. Contribution from the and zero applied magnetic field.

beryllium windows of the dewar is apparent around zero velocity.

6, and a splitting AE, = 3.6 mm/s is superimposed on linewidth somewhere between 18 K and 42 K the main single line. The relative intensity 12/(11

+

I,) (2 TI = 0.83 mm/s at 18 K) (Fig. 1).

of this doublet varies with the temperature as indicated Upon application at 4.2 K of an external magnetic in table I. With a further increase of temperature the field H parallel to the y-ray propagation direction, a doublet disappears simultaneously with a considerable magnetic hyperfine structure develops (Fig. 4). The broadening of the single line which reaches a maximum spectra for H = 10,20 and 30 kOe can be fitted with a

Results of least-square fits of the spectra ofJgures 2 and 3.

The index 1 refers to the main line and the index 2 to the superimposed quadrupole doublet,

if

any.

12/(11

+

I,) represents the relative intensity of the doublet

Temperature 61 (*) (K>

_-

(mm/s)

-

300 1.53 77 1.70 42 1.71 18 1.73 12 1.72 10 1.72 9 1.72 8 1.72 4.2 1.72 1.6 1.75

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HYPERFINE INTERACMONS IN AN ORBITAL DOUBLET : CaF2 : Fez+ C6-613

VELOCITY (rnmts)

FIG. 4.

-

Mossbauer spectra of CaF2 : Fez+ at 4.2K with different magnitudes of the external field H. The solid lines are least-square fits of the data assuming a single 4-line hyperline pattern (Am = 0 lines vanish in the longitudinal configuration).

single four-line pattern (intensity ratios 3 : 1 : 1 : 3). The effective field at the nucleus

HN

and the quadrupole interaction AEH =

3

e2 qQ are given in table 11.

Values at 4.2 K of the efSectiveJieId

I

HN

I

and of the quadrupole interaction L\E, induced by an external magneticJield H

3. Discussion.

-

According to the energy level scheme of figure 1, the system is in the lowest spin- orbit level (r,) at low temperature. Since this level is a singlet and since the symmetry is cubic, no electric

field gradient at the 57Fe nucleus is expected in zero field. This is exactly what is observed at the lowest temperatures where no quadrupole interaction is present (Fig. 3). The small line broadening observed at these temperatures (Table I) is probably of instru- mental origin. On the other hand at temperatures above

-

77 K it is likely that all spin-orbit levels are populated and, due to relaxation averaging, the quadrupole interaction should also vanish, as it is observed. However Ham [ l ] suggested that at a temperature low enough to avoid relaxation averaging but where the first excited spin-orbit level

(r,)

of figure 1 could be populated, a quadrupole splitting

A E = 6 q [ C E l (1) should appear, where q is a reduction factor represen- ting the strength of the Jahn-Teller coupling and C, is given by

3

With I =

-,

eq. (1) becomes

2

which is, except for the reduction factor q and the missing Sternheimer factor (1

-

R), the quadrupole interaction of a ferrous ion in a singlet orbital state [5].

One knows that in that case, whatever the symmetry of the Fe2+ ion, one can expect AE

-

3-4 mm/s [6]. One can therefore interpret the Mossbauer spectra around 9 K (Fig. 3) as the superposition of two contri- butions : the main single line comes from the

r,

state while the weak doublet AE, u 3.6 mm/s is due to the

population of the

r,

state. When the temperature is further increased, the doublet disappears and the single line broadens because of relaxation averaging. These observations are the first experimental proof of Ham's predictions (1). Since the value of

AE,

-

3.6 mm/s is of the order of the values usually found in other ferrous compounds, it is likely that the reduction factor q is not much less than unity, indi- cating that the Jahn-Teller coupling for Fez' in cubic eightfold coordination is weak. Taking q = 1 and Q = 0.2 barn, one can even use eq. (3) to derive an effective value

<

r b 3 >eff = 3.1 a. U. for the 3d electrons.

An estimate of the distance 2 D between T I and

I', (Fig. 1) can be obtained from the intensity ratio 12/(11

+

I,) (Table I). Assuming a Boltzmann population distribution one gets 2 D = 16

+

2 cm-I. Far infra-red measurements are presently under way in order to measure directly this distance.

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C6-614 J. R. REGNARD AND J. CHAPPERT

mixing of the singlet ground state with the next excited levels through the electronic Zeeman interaction. Indeed such induced hyperfine interactions have already been observed in the case of Fez+ and Fel+ in CaO and KMgF, (octahedral symmetry) where the distance between spin-orbit levels is strongly reduced by the dynamic Jahn-Teller effect (3). Here we observe that HN increases linearly with H, at least up to H = 30 kOe, while AEH varies as Hz. The high field

behaviour (H = 40 and 55 kOe) is not yet completely explained. It is likely that the complexity of the Mdssbauer spectra reflects either the increasing mixing of the ground state with the excited states

r,, r,,

etc

...,

or the possible contribution of the low lying Zeeman excited level Jz =

-

1 of

r4.

Analysis of these data using a perturbation treatment is now being perfor- med. It should also provide an estimate of the distance 2 D.

References

[ I ] HAM, F. S., J. Physique Colbq. 35 (1974) C 6-12 and refe- 141 GERARD, A., IMBERT, P., PRANGE, H., VARRET, F. and

rences therein. WINTENBERGER, M., J. Phys. Chem. Solids 32 (1971)

[2] CHAPPERT, J., FRANKEL, R. B.; MISETICH, A. and 2091.

BLUM, N . A., Phys. Rev. 179 (1969) 518. [5] INGALLS, R., Phys. Rev. 133A (1964) 787. [31 Ray, T. and REGNARD, J. R., Phys. Rev. 14 (1976) 1796 ; 161 GANIEL, U., Chem. Phys. Lett. 4 (1969) 87.