STUDY OF U AND U2 CENTRES IN BaCIF

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STUDY OF U AND U2 CENTRES IN BaCIF

M. Yuste, L. Taurel, D. Jumeau, J. Badoz

To cite this version:

M. Yuste, L. Taurel, D. Jumeau, J. Badoz. STUDY OF U AND U2 CENTRES IN BaCIF. Journal

de Physique Colloques, 1973, 34 (C9), pp.C9-277-C9-280. �10.1051/jphyscol:1973948�. �jpa-00215423�

J O U R N A L DE PHYSIQUE

Coiloque C9, .supp/kmef71 au no 11-12, Tome 34, Nooetnbre-Dhcernbre 1973, page C9-277

STUDY OF U AND U, CENTRES IN BaCW

M. YUSTE, L. TAUREL, D. JUMEAU Laboratoire de Physique Cristalline (")

UniversitC de Paris-Sud, Centre d70rsay, 91405 Orsay (France) and J. BADOZ

Laboratoire d'optique Physique de I'EPCI

10, rue Vauquelin, 75231 Paris Cedex 5e (France)

RCsume. - I1 est possible de former des centres U2(H?) par irradiation aux rayons X a 78 K d'un cristal de B~CIF(D:,,) contenant soit des centres U, soit des ions OH-.

L'etude IR du mode localis6 du centre U montre que I'ion H- se substitue a un anion F-.

Le spectre RPE de BaCIF irradie aux rayons X est en accord avec I'existence de centres (HP)

situes en position interstitielle au centre de la maille.

Les parametres de I'hamiltonien sont compares a ceux des centres U2 dans NaCI.

Abstract.

u ~ ( H ? ) centres can be formed by X-irradiation, at 78 K, of B~CIF(D:~) containing either OH- ions or U centres.

The infrared study of the U centre local mode is in agreement with the model of H- ion in substitution of an F- anion.

The ESR spectrum of the X-irradiated BaClF crystal is typical of HO atoms in interstitial site at the centre of the unit cell.

The hamiltonian parameters are compared td those observed in the case of U2 centres in NaCl.

The physical properties of the U and U, centres have been extensively studied in cubic crystals of NaCl o r CaF, structure. I t is interesting to extend the study of these centres in a non cubic lattice, in this case BaCIF.

BaClF crystallizes in the tetragonal system. It belongs t o the space group D:,, [I]. The unit cell contains two BaCIF niolecules. The ions lie in planes perpendicular to the quaternary axis c. The projection of the unit cell in the plane (a, c) is represented in figure I. The syrnmetry of the F- ions is D,, while the symmetry of the CI- ions is C,,.

Infrared absorption of U centres.

After heating the crystal a t 900 OC with strontium vapour and hydro- gen (or deuterium) an infrayed absorption spectrum is observed and this can be attributed to the presence of U centres [2].

We envisage three possibilities : l o The H - ion replaces a C1- ion.

20 The H - ion replaces a n F- ion.

3O The H - ion is located in an interstitial site. The most probable site is the centre I of the cell.

(")

Equipe de ~~cclie~~clics

nu 13.

(9 Equipe de rechcrclics

n u

FIG.

Projection of the unit cell on the (a,

plane

n = 4.38 .k c = 7.22

A

quaternary

Symnictry of

sites

Site

site F-

site I

D 2 d

In the first case the symmetry of the H - ion is C4v. In the other two cases the symmetry of the H - ion is D,,.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973948

C9-178

M . YUSTE,

T A U R E L , D. J U M E A U

J. BADOZ The properties of the infrared spectra related t o

these two types of symmetry are indicated in table I.

For these two types of symmetry the fundamental spectrum consists of two bands v, and v, polarized respectively parallel or perpendicular t o the four fold axis.

The second order spectrum consists of three bands in the case of C,, symmetry and two bands in the case of D,, symmetry.

The experimental infrared spectra are shown in figure 2.

Fundamental m o d e s 2nd Harmonics

FIG.

Infrared absorption spectrum of U centres in BaClF (T

K) E

Polarization vector

E /

and

1 = E l c )

v1

822 cm-1

851.5 cm-1

- - - _E /

The second order spectrum consists of two bands and this last result shows that the U centres are of D,, symmetry.

The observed frequencies are very close to the lone frequency of the U centres in BaF, which is 806 cm- '

at 20 K [3]. In this last case the H - ion replaces the F- ion.

The surroundings of an F- ion in BaClF are very close t o that of an F- ion in BaF,. This fact leads us

to assume that in BaClF the H - ion is located in an F- ion site.

A very rough model which accounts for the observed frequencies can be constructed. We assume that the H- ion vibrates relative to the fixed BaClF lattice, and we assume that the vibrational potential energy is mainly due to the interaction between an H- ion and the neighbouring Ba ions (Fig. 3).

FIG.

Nearest neighbours of the H- centre This rough model leads to a ratio v2j1~, equal to X,/Z, where X, and Z, are the Ba' ion coordinates in the OXYZ axes (Fig. 3). The expected ratio is 1.05 at 20 OC, while the experimental result is 1.03 at 20°C and 1.04 at 10 K. Taking into account the limits of the precision of the crystallographic parame- ters, this rough model gives a rather good agreement.

EPR spectrum of U2 centres.

After X-irradiation at 78 K of a crystal containing U centres, a n EPR spectrum due to atomic hydrogen is obtained.

The same result is observed with a crystal which does riot contain U centres but which was grown i n air and this is probably due t o the presence of OH- ion [4].

The following results have been obtained in crystals grown in air.

The EPR spectrum for a direction of tlie magnetic field parallel to tlie fourfold axis c is represented in figure 4. This spectrum can be divided in two parts :

Experimental results at T

10 K and expected spectra in the case of C,, and D2, s~~nin?erries H - substituted for F-

.2d

( ^H- interstitial in the center C,, (H- substituted for C1-) of the unit cell

- -

(A1 1) v ,

1)

\ I 2

( E 1)

( E 1)

2 v , +(A1 1) ^{2 v ,}

( A , ) inactif 2v2 + ( A 1 1) 2

( B , 1)

v, + v, + ( E I)

+ ^v2 + ( E L )

Observed frequencies (cm- ')

H- D-

-

8 2 2 7 ?

851.5 1 606.5 1

STUDY OF U A N D Uz CENTRES I N BaClF C9-279

2 800 3000 3 2CO 3400 3600

H (gauss).

FIG. 4.

First derivative of the X-band EPR absorption spectrum o f X-irradiated BaClF at T

78

with

// c

( s p e c t r ~ ~ m a t T

78 K).

I) A central spectrum due to [CI;] centres which can be optically bleached in the ultraviolet.

2) A spectrum consisting of two groups of lines, I , and I, being the centres of each group. The distance between I , and l 2 is 467 G.

It is known that in the case of the hydrogen atom (H') the distance between the hyperfine lines is 507 G.

Within the accuracy of our measurements, no ani- sotropy of the g factor or of the B proton hyperfine constant was detected. This result is in agreement with the assumption that the perturbation term due to the crystal field is low.

From the A[ and A l experimental values the a and b terms which respectively characterize the contact superhyperfine constant and dipolar superhyperfine tensor(A[= I a + 2 b I ; A l = l a - b 1 ) c a n b e d e t e r - mined. The physically significant a and b values are :

a b

-

36 MHz

= 10 MHz

I1

h

The results can be compared with those obtained in NaCl [S] (Table 11). In both cases, the H0 atom is surrounded by 4 CI- ions located at the same distance d

2.436 A. The only difference is that in NaCI the 4 C1- ions are located at the corners of a regular tetrahedron while in BaClF the tetrahedron is slightly distorted.

E.~peri,nental EPR parameter ualues of HI'' ^{in BaClF}

and NaCl crystals The observed spectrum is in agreement with the hypo-

thesis of H0 in the lattice. H:~,, : g,

2.0023 ; - B0 - - 1420 MHz h

The shf structure due to interactions between the

electron of the H0 centre arid the nearest nuclei is BaClF NaCl [5]

only well resolved for a direction of the magnetic - -

field either parallel to the crystallographic directions a ₌ - g, - ³ -

and c, or parallel to the binary axis a', bisector of the two a axes [4].

The shf structure is accounted for by assuming that the t1° atom is located in the interstitial site I and that the superhyperfine structure is due to the four C1- nearest neighbours ( k ) located at the corners of a slightly deformed tetrahedron.

The spin hamiltonien which is used is :

The experimental values of the parameters are

-

a

h 36 MHz 44.7 MHz

-

b

h 10 MHz 9.8 MHz

It is interesting to remark that the b dipolar super- hyperfine interaction is the same for the two cases.

This could be explained assuming that the b term can be calculated taking into account mainly the ortho- gonalization between the Is wave function of the

atom and the p, orbitals of the neighboured CI- ions and a partial covalency bonding between the H0 atom and the nearest halogens [6].

References

[I]

ASMT Card Irtdex, no 30304. B ~ u o z ,

Solid State Phys. (to be

JUMEAU, D., YOSTE, M., TAUIII:I.. L. and BILLAKDON, M., published) (1973).

PIiys.

Let!.

(1973) 189.

SPAETH,

M . and S T U R ~ I , M . ,

5:rrt.

(1970)

ELLIOT. R .

DC)NES, G. D., MACDONALD, 739.

H . F. and SENNET, C. T.,

R.

SI>AI:I.~I.

M . and Srilolrr., H.,

(h)

(1971)

YUSTE, M . , RAHLIAWI, M., JUMEAU,

TAUIIEL, L. and

M. YUSTE, L. TAUREL, D . JUMEAU AND J. BADOZ

DISCUSSION

J. M. SPAETH. - The observed change of proton indicates that so far the effect of crystalline field on hyperfine interaction is very interesting in compari- the proton hyperfine interaction has not yet been son to NaCI, where the distance of the C1 neighbours considered enough.

is very similar. The low observed value possibly