CO3-3 DEFECTS ASSOCIATED WITH LITHIUM IN SYNTHETIC CALCITE

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CO3-3 DEFECTS ASSOCIATED WITH LITHIUM IN

SYNTHETIC CALCITE

G. Bacquet, L. Youdri, J. Dugas

To cite this version:

G. Bacquet, L. Youdri, J. Dugas. CO3-3 DEFECTS ASSOCIATED WITH LITHIUM IN

C7-208 JOURNAL DE PHYSIQUE Colloque C7, supplément au n° 12, Tome 37, Décembre 1976

C O r DEFECTS ASSOCIATED WITH LITHIUM IN SYNTHETIC CALCITE

G. BACQUET, L. YOUDRI

Laboratoire de Physique des Solides (*)

118, route de Narbonne, 31077 Toulouse Cedex France and

J. DUGAS

Département de Physique, Université Mohamed V. Rabat, Maroc

Résumé. — Après irradiation à l'ambiante de monocristaux de calcite synthétique, on observe deux défauts paramagnétiques qui sont des ions CO|~ associés à des impuretés de lithium et qui ont une symétrie axiale suivant l'axe c de la calcite. On a déterminé les constantes des hamiltoniens de spin décrivant les spectres des deux défauts. Pour celui qui n'est associé qu'à un ion Li+, on trouve

une densité de spin au niveau de Li+ égale à 2,96 %. Pour le centre à deux lithiums, la densité de spin sur le Li+ le plus proche est de 11,89 % alors qu'elle n'est que de 0,1 °/00 sur le deuxième.

On émet quelques hypothèses concernant la position des divers ions Li+ par rapport à l'ion carbonate.

Abstract. — In irradiated synthetic single crystal calcite two defects which are C O | ~ ions

asso-ciated with lithium impurities are observed. Both defects are axially symmetric along the calcite c axis. One defect is associated with one Li+ ion, the other with two Li+ ions. The values of the

various constants of the spin hamiltonians describing the spectra of both defects are given. The spin densities in the 2s lithium orbitals are 2.96 % for the one lithium centre and 11.89 % and 0.1 °/00

in the case of the two lithiums centre. Some hypothesis concerning the location of the various Li+ ions are presented.

1. Introduction. — Single crystals of X-, y- and neutron irradiated naturally occurring calcite (CaC03),

display a number of paramagnetic defects. Electron Spin Resonance (E. S. R.) absorption spectroscopy has been used by Marshall et al. at the Argonne National Laboratory (U. S. A.) to identify many of these radiation-induced centres. We shall only refer here to ionized carbonate ions which exhibit either local [1, 2] or remote charge compensation [3] with the location of the charge compensator affecting to some extent the ligand field symmetry.

On the other hand, we reported recently on the existence of a CO|~ molecular ion associated with one Li+ ion [4] created by X-irradiation at R. T. of

specimens of synthetic single crystal calcite grown by means of the Travelling Solvent Melting Zone method where L i2C 03 was used as solvent [5].

In this report, we shall describe the results obtained on a new ionized carbonate ion which is associated with two lithium ions and we shall compare them to those concerning the one lithium centre.

2. Experiments and Results. — Samples of dimen-sions 4 x 3 x 3 mm3 were X-irradiated at R. T. during

about 15 hours and then studied in the X-band using

(*) AssocieauC.N. R.S.

a conventional 100 kHz field modulation spectrometer at the same temperature. Immediately after irradiation, several spectra due to different defects exhibiting varying degrees of thermal stability were simulta-neously recorded and two of them were already described [4, 6]. Among them two were found to be associated with the presence of lithium impurities.

2.1 Two LITHIUMS CENTRE. — The spectra of this new centre (which bleaches out with a half-life of 2 days at 300 K) are characterized by very narrow lines (AH c=; 50 mG) which are easily saturated. When the Zeeman field vector is either parallel or perpendicular to the crystalline c axis, the spectrum, consists of four sets of quadruplets, the maximum spread out of each quadruplet being 0.5 G. No angular dependence was found in a plane parallel to that containing the host ion, thus indicating that this paramagnetic defect is axially symmetric along the c axis.

During the rotation about any axis perpendicular to the calcite < 111 > axis, inside of each of the four sets, the quadruplet collapses in a unique line 50° away from the c axis. Due to the extreme narrow-ness of the quadruplets, the angular variation given in figure 1 is limited to that of the four principal sets.

CO:' DEFECTS ASSOCIATED WITH LITHIUM IN SYNTHETIC CALCITE C7-209

FIG. 1 .

-

TWO Lithiurns centre. Experimental and theoretical (full lines) angular dependence of the centre of each quadruplet when the magnetic field is rotated about an axis perpendicular

to the calcite c axis.

The spectra were fitted to the following spin- hamiltonian :

with S = 112, I(,, = I(,, = 312 and where :

A(/' = 46.5

+

0.1 MHz,

AY'

= 29.6

+

0.1 MHz

AK)

= 0.5

+

0.1 MHz,

AY'

=

-

0.2 T 0.1 MHz. It is worthwhile to underline that the exact sign of the various A(') components is not known. Never- theless we known that AfI" and A:') have the same sign ; this is not the case for Af' and AY).

The obtained g values being identical to those quoted by Serway and Marshall [3] for the ~ 0 : - molecular ion we think we observe the resonance of such a defect in interaction with two I = 312 nuclei located on the calcite

<

111

>

axis. According to the mode of preparation of samples it is reasonable to suppose that these nuclei are 7Li+ ions, the three quadruplets of much lower intensity originating from 6Li-7Li pairs being hidden by the CO:--L~+ spectrum.

The two hyperfine tensors can be written

where each Tci) is a traceless tensor. This leads to = 34.3 MHz and

--

0.03 MHz. For an

unpaired electron fully localized in lithium atom 2s orbital the Fermi contact term K calculated from wave functions given by Clementi [7] equals 288.3 MHz. Comparing these three values we find that the unpaired electron spin density in the two lithium 2s orbitals is 11.89

%

and about 0.1 a/,, respectively.

2 . 2 CO;--L~' CENTRE.

-

This defect has a great thermal stability. We shall recall here that it is also axially symmetric along the calcite c axis. When the crystal is rotated about any axis perpendicular to the c axis we observe the angular dependence displayed in figure 2. The existence of so-called forbidden hyperfine lines (Am, = 1,2,3) permitted us to deter- mine the nuclear Zeeman term value.

FIG. 2.

-

~ 0 : - - ~ i + centre. Experimental and theoretical (full lines) angular dependence of various hyperfins lines when the static field is rotated about any axis perpendicular to c.

x A M I = O ; O A M I = ~ ; ~ A M I = ~ ; A A M I = ~ .

In the case of 7Li for which all experimental data are available the observed spectra may be described by the spin-hamiltonian :

X s = p B H . g . S + S . A . I - p N g N H . I (2) where the nuclear Zeeman interaction is taken to be isotropic, with S = 112 and I = 312 and where

and

IpNgNHI = 5.64 f 0.03MHz.

The A components have the same sign which is unknown. In the spin-hamiltonian (2) the coupling with 13C nucleus was not taken into account as we only measured 13A, = 353 MHz.

C7-210 G . BACQUET, L. YOUDRI AND J. DUGAS

CO:--L~+ centre where the Li+ ion lies on the c where q is the angle subtented by the c, axis of the axis in an interstitial site, either above or below a molecule with the C-0 bond direction, and

carbonate, approximatively in a plane containing

Ca2+ ions, 2"

I

r

12/

1a

lZ

is the p to s hybridization ratio. In fact, the value 3. Discussion. - In both above quoted defects,

~ 0 : - molecular ions were found to be the basic constituent as the gll and g , values are identical to those quoted by Serway and Marshall [2]. This radical has 25 valence electrons and, according to the Walsh's prediction 181, must be pyramidal with an 0 - C - 0 bond angle of about 1100 [9]. With such a structure this radical belongs to the point group C,, and the unpaired electron is on a A, antibonding molecular orbital with a proper mixture of 2s and 2p Carbon and Oxygen atomic orbitals :

+

r.

I ~ P >

>))

(3)

where the various reference axes are shown in figure 3.

FIG. 3.

-

Reference axes used to establish the LCAO Mole- cular Orbital belonging to A1.

Usually the 0-C-0 bond angle 8 is estimated using the equations :

1

cos 6 =

-

₃ c3 c0s2 43

-

11

=

-

1/i2 ( 5 )

of A2 drastically depends on the wave functions chosen to describe the

I

2s

>

and

I

2p

>

carbon orbitals : for instance starting from the ESR data of Serway and Marshall [3] concerning the hyperfine interaction between the unpaired electron and the I3C nucleus (I = 112) we find A2 = 3.076 using Hartree-Foch solutions for

1

2 s ( 0 )

1

and

<

ri: >C, and l2 = 3.891 using Clementi's wave functions. The corresponding 0-C-0 angles are 1090 and 105O respectively.

It would be interesting to know the values of the 0-C-0 angles in the case of the two ~ 0 : -

_-

defects associated with lithium impurities and, if possible, to localize exactly the various Li' ions. Unfortunately this is impossible as we don't have sufficiently data concerning the hyperfine coupling of the unpaired electron with the various nuclei possessing a nuclear spin ("0 and essentially I3C).

Some hypothesis can be proposed nevertheless. According to Marshall et al. [I-31 the unpaired electron is mainly located on the carbon, thus creating a net weak negative charge (- 3 6) at this point and a net positive charge (+ 6) on each oxygen. So, the resulting dipolar moment is directed along the c axis. Consequently it is reasonable to assume that the distorsion of the CO:- molecule then comes off towards the interstitial lithium (attraction between the two electric charges of opposite signs). In the foregoing, we saw, in the two Li+ centre case, that the spin density on the closest lithium (11.89

%)

is greater than that obtained in the case of CO;--L~+ (2.96

%).

A possible explanation could be the foll- owing. The second Lif being located in a Ca2+ substitutional site on the c axis (on the opposite side of the Carbon with respect to the first Li+) creates at this point a net negative charge. This would increase the distorsion of the ~ 0 : - molecule towards the interstitial Lif, the Carbon going still closer to this last lithium.

Now, is the interstitial lithium hyperfine structure due to either inter-or intra molecular electron-nucleus interaction ? Although the spectral information pre- sented here is not sufficient to ascertain it, we rather think, by comparison with the works of Marshall

C O ~ ' DEFECTS ASSOCIATED WITH LITHIUM IN SYNTHETIC CALCITE C7-211

References

[ I ] MARSHALL, S. A., MC MILLAN, 5. A. and SERWAY, R. A., J . Chem. Phys. 48 (1968) 5131.

[2] CASS, J., KENT, R. S., MARSHALL, S. A. and ZAGER, S. A., J. Mag. Res. 14 (1974) 170.

[3] SERWAY, R. A. and MARSHALL, S. A., J. Chem. Phys. 46 (1967) 1949.

[4] BACQUET, G., DUGAS, J., ESCRIBE, C., YOUDRI, L. and

BELIN, C., J. Physique 36 (1975) 427.

[5] BELIN, C., BRISSOT, J. J. and JESSE, R . E., J. Cryst. Growth, 13/14 (1972) 597.

[6] BACQUET, G., DUGAS, J. and BELIN, C., Proceedings of the

18th Ampere Congress, Nottingham, 1 (1974) 161. [7] CLEMENTI, E., Tables of atomic functions (1965). [8] WALSH, A. D., J. Chem. Soc. (1953) 2301.