STUDY OF U2 CENTRES IN BaClF AND SrClF CRYSTALS

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STUDY OF U2 CENTRES IN BaClF AND SrClF CRYSTALS

M. Yuste, S. Lefrant, L. Taurel, J. Spaeth

To cite this version:

M. Yuste, S. Lefrant, L. Taurel, J. Spaeth. STUDY OF U2 CENTRES IN BaClF AND SrClF CRYS- TALS. Journal de Physique Colloques, 1976, 37 (C7), pp.C7-227-C7-229. �10.1051/jphyscol:1976754�.

�jpa-00216913�

JOURNAL DE PHYSIQUE Colloque C7, supplkment au no 12, Tome 37, Ddcembre 1976, page C7-227

STUDY OF U2 CENTRES IN BaClF AND SrClF CRYSTALS

M. YUSTE, S. LEFRANT, L. TAUREL

Laboratoire de Physique Cristalline (*) UniversitC de Paris-Sud, Bgtiment 490, 91405 Orsay Cedex, France

and J. M. SPAETH

Experimentalphysik, Gesamthochschule, Paderborn, 479 Paderborn, Pohlweg 55, Postfach 454, F R G

R6sum6. - Nous avons calcult les constantes paramagnetiques du centre U2 dans I'approxima- tion de Hartree-Fock et en admettant I'existence d'une liaison covalente entre I'atome d'hydroghe et les anions situes en plus proches voisins. Nous avons compare les resultats experimentaux et theoriques des centres U2 dans SrClF et BaClF a ceux des centres U2 dans RbCl, KC1 et NaCl et nous sommes arrivQ a la conclusion que la distance entre l'hydrogkne et le chlore joue un rble prepondtrant.

Abstract. ^- Calculation of paramagnetic constants of U2 centre was done on the basis of a Hartree-Fock approximation, including a covalent bonding between the hydrogen atom and its nearest anions. The experimental and theoretical results obtained in BaClF and SrClF are compared to those of U 2 centre in RbCI, KC1 and NaCl and we have come to the conclusion that the distance between the hydrogen atom and the nearest chlorine ions plays the most important role.

1. Introduction. - The physical properties of the U, centre have been extensively studied in the past.

The accepted model for this centre is a hydrogen atom located at a n interstitial site of the lattice. The ESR and ENDOR experiments [I, 2, 31 confirm this model and the most striking experimental result is the large anisotropy of the hyperfine interaction between the hydrogen electron and its nearest neigh- bour nuclei.

In order to explain this interaction, it has been assumed a charge transfer from the anion to the hydro- gen atom 141, which increases when the electronic affinity of anion decreases. Our purpose is to study the influence of the distance between the hydrogen atom and the anion on the charge transfer. In the covalent model, the charge transfer is expected to be increased when the Ho-anion distance decreases.

We have performed the measurement of a(Cl), b(C1) and B(proton) constants of U, centres in BaClF and SrClF [5]. In these crystals, the hydrogen atom is located at an interstitial site as it is shown in figure 1.

The distance HO-C1 in BaClF is similar to that of NaCl.

In both crystals, the C1- nucleus is located at the corner of a tetrahedrom, regular in NaCl and slightly distorted in BaClF.

FIG. 1. -Unit cell. Surrounding of the U2 centre (M++ : (*) Equipe de Recherche associke au ^{C .} N. R. S. no 13. Sr++ or Ba++)

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976754

C7-228 M. YUSTE, S. LEFRANT, L. TAUREL AND J. M. SPAETH 2. Wavefunction of U, Centres. - In the wave

function calculation, we have used a method previous described [4].

We have taken as free-ion orbitals ^Qi those pro- vided by Clementi [6] for C1- and F- ions ; for Ba2+

and Sr2+, approximate orbitals have been calculated by Harker [7] from atomic ones, where only the 5s, 5p and 4d outer orbitals of Ba2+ (or 4s, 4p and 3d of Sr2+) have been used.

In the covalent model, the single electron belongs to an antibonding state q;, given by the following expression [4] :

The parameters A, and A, can be fitted from the experimental values of a(C1) and b(C1).

3. Dependance of paramagnetic constants with HO-CI- distance. - The experimental and theoretical values of a(Cl), b(C1) and B(proton) are indicated in tables I and I1 in the case of RbCI, KC1 and NaCI [4], BaClF and SrClF 151. The values of I, and A, para- meters are also indicated (table I).

The main results are the following :

1) the experimental value of b(C1) increases when the HO-C1 distance decreases (Table I). The HO-C1- distance in NaCl and BaClF is very similar and in these crystals the b(C1) experimental values are very close. In order to calculate the b(C1) constant in the charge transfer model two factors are important : the

<

1 s(HO) ( q3,,(C1-)

>

overlap and the A, charge transfer parameter. The constant b(C1) increases when

<

1 s(HO) ] q3,,(C1-)

>

or 2, increases. Obviously, the b(C1) experimental change from one crystal to another is explained qualitatively by the variation of overlap with HO-C1- distance. The variation of 1, with

HO-CI- distance can be explained by the following arguments :

a) The charge transfer between the q,,, and 1 s(HO) states increases when their energetic distance decreases.

The energy level of the 1 s(HO) state is slightly below the top of the valence band which consists of the outer halogen p-orbitals (figure 2) and a reduction in lattice parameters leads to a reduction in the distance bet- ween the 1 s(HO) and q,,,(Cl-) levels.

WCWM

cmwc-noN BAND

-

^Y~PU(CI-) -

-

-

^{WLMCE BAND}

\ s(He)

'f3~(cr)

FIG. 2. - Qualitative diagram of ~ 3 ~ ~ ( C l - ) , p)3s(Cl-) and

ls(H0) energy levels in alkali chlorides. (After Cho et al. 181.) b) On the other hand, since A, parameter describes a covalent effect we may reasonably assume that it increases with the

<

1 s(HO)

1

q3,,(C1-)

>

overlap.

On the basis of these arguments, A, increases when the No-C1- distance decreases and, consequently, the b(C1) variation may be reasonably explained by assuming the charge transfer model.

It is more difficult to find an explanation of the expe- rimental variation of a(C1) (Table I). As in the case of b(C1), the change in the experimental value of a(C1) is qualitatively explained by the

<

^{1 s(HO)}

I

q,,(Cl-)

>

overlap variation, but in the case of A,, the two factors,

U2 centre : experimental and calculated a(CI3 ') and b(C1 ^5, superhyperme constants in BaClF, SrClF and in alkali chlorides : (a(C1) and b(C1) in MHz ; distance in atomic units).

RbCl KC1 NaCl BaClF

- - ^{( I )} - ⁽²⁾ -

Ho-Cl- distance 6.047 5.105 4.575 4.605

(~135) ^\ exp. 17.34 23.74 44.7 36

) Lowdin 9.30 12.40 27 24.6

h ( ~ 1 ~ ~ ) i ^{5.47 6.71 9.8 10}

) Lowdin 2.38 2.75 5.73 5.1

SrClF (2>

-

4.38 44.8 43.3

(*) Calculated recently by A. H. Harker (private communication).

(1) Spaeth and Seidel [4].

(2) Yuste et a{. [5].

STUDY OF Uz CENTRES IN BaClF AND SrClF CRYSTALS C7-229

<

1 s(HO)

I

q3,(C1-)

>

and energetic distance bet- ween 1 s(HO) and pJs(CI-), leads to opposite contri- butions. So, it is impossible to explain the a(C1) change in a qualitative way.

2) The experimental value of the B(proton) hyper- fine constant (Table 11) decreases with the HO-CI- distance and, in all cases, its value is slightly less than the B(proton) value of the free hydrogen atom.

This means that the electronic spin density at the proton decreases when the hydrogen atom is located in the interstitial site of the crystal. In the charge trans- fer model we have taken into account three causes of this spin density variation : first, an increase due to orthogonalization (AB, (Lowdin) in table II), secon- dly, a decrease due to the Van der Waals interaction (AB, (Van der Waals) in Table 11) and, finally, a

decrease due to the C1- ⁴ H0 charge transfer (AB, (covalent) in Table 11).

It is interesting to note that the experimental B shift is close to AB, (covalent). This weans that the total shift due to orthogonalization and Van der Waals effects is small with respect to the covalent one.

4. Conclusion. - In the case of U, centres, the dis- tance between hydrogen atom and the anion plays a very important role on the variation of anisotropic spin density distribution. On the other hand, since spin density calculations in the charge transfer model are in good agreement with experimental results for U , centres of different symmetry (T, in alkali-halides and D2, in BaClF and SrCIF) the crystal field effect can be neglected in the choice of the envelope function.

U2 centre : experimental and calculated B (proton) hyperfine constant in BaClF, SrClF and in alkali chlorides. (B(HO free) = 1420.4 MHz ; AB = B (proton) in crystal- B (HO free) ; B (proton) in MHz ; distance in atomic units).

RbCl KC1 NaCl BaClF SrClF

( 9

- - - ^(2> - (2)

-

HO-C1- distance 6.047 5.105 4.575 4.605 4.38

B (proton) experimental 1 383.8 1 377.6 1 352.6 1 338 1 290

AB exp. - 36.6 - 42.8 - 67.8 - 82 - 130

AB, Lowdin

+

118

+

130

+

270 ^f 269

+

378

AB, Van der Waals - 57 - 158 - 278 - 285 - 383

AB, covalent - 33 - 48 - 100 - 82 - 146

B (proton) theoretical 1 448 1 344 1312 1 322 1 269

(1) Spaeth and Seidel [4].

(2) Yuste et al. [S].

References

[I] KERKHOFF, F., MARTIENSSEN, W. and SANDER, W., 2. Phys. 151 YUSTE, M., LEFRANT, S., SPAETH, J. M. and TAUREL, L., 176 (1963) 305. J. Phys. C : Solid State Phys. 8 (1975) _{. .} 3491.

[2] SPAETH, J. M., 2. Phys. 192 (1966) 107. [6] CLEMENTI, E., ZBM J. Res. Dev. Suppl. 2 (1965) 2.

[3] LEHNERT, G. and SPAETH, J. M., Phys. Status Solidi 31

(1969) 703. [7] HARKER, A. M., D. Phil. Thesis University of Oxford (1973).

[4] SPAETH, J. M. and SEIDEL, H., Phys. Status Solidi (b) 46 181 CHO, K., KAMIMURA, H. and UEMURA, Y., J. Phys. Soc.

(1971) 323. Japan 21 (1966) 2244.

DISCUSSION

R. BAUER. - Have you made any estimates of the M. YUSTE. - We have neglected all polarization spin polarization in the cores of the neighbouring effects. Indeed, the Ho-electronic density is very

ions ? low in the cores of the neighbouring ions.