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
SYNTHETIC CALCITE. Journal de Physique Colloques, 1976, 37 (C7), pp.C7-208-C7-211.
<10.1051/jphyscol:1976748>. <jpa-00216907>
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Submitted on 1 Jan 1976
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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 associated 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 (CaC0 3 ),
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 Li 2 C0 3 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 dimensions 4 x 3 x 3 mm 3 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 simultaneously 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 narrowness of the quadruplets, the angular variation given
in figure 1 is limited to that of the four principal
sets.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976748
CO:'
DEFECTS ASSOCIATED WITH LITHIUM IN SYNTHETIC CALCITE
C7-209
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 determine the nuclear Zeeman term value.
FIG. 1 . - TWOLithiurns 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 spinhamiltonian :
with S
=
112, I(,,
= I(,, =
312 and where :
+ 0.1 MHz, AY' = 29.6 + 0.1 MHz
AK) = 0.5 + 0.1 MHz, AY' = - 0.2 T 0.1 MHz.
A(/'
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.
xAMI=O;OAMI=~;~AMI=~;AAMI=~
= 46.5
It is worthwhile to underline that the exact sign
of the various A(') components is not known. Nevertheless 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-7Lipairs 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
--
In the case of 7Li for which all experimental data
are available the observed spectra may be described
by the spin-hamiltonian :
Xs=pBH.g.S+S.A.I-pNgNH.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.
From 7Ai,, = 8.52 MHz we found that the unpaired
spin density in the lithium 2s orbital is 2.96 %. This
defect was consequently described as being a
G . BACQUET, L. YOUDRI AND J. DUGAS
C7-210
CO:--L~+ centre where the Li+ ion lies on the c
axis in an interstitial site, either above or below a
carbonate, approximatively in a plane containing
Ca2+ ions,
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 Molecular Orbital belonging to A1.
Usually the 0-C-0 bond angle 8 is estimated using
the equations :
cos 6
=
1
3
- c3 c0s2 43
- 11 = - 1/i2
(5)
where q is the angle subtented by the c, axis of the
molecule with the C-0 bond direction, and
r
2"
I 12/ 1a lZ
is the p to s hybridization ratio. In fact, the value
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
and < ri: >C, and
solutions for 1 2 s ( 0 )1
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 following. 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 presented here is not sufficient to ascertain it, we rather
think, by comparison with the works of Marshall
et al. dealing with Y?+ stabilized COZ[l] and
HCO:- [2] that our defects might be visualized as
paramagnetic CO;- molecular ions interacting with
a neighbouring lithium in interstitial site.
CO~' 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.
[9] ATKINS,P. W. and SYMONS,
M. C. R., The structure of
inorganic radicals (Elsevier pub. Amsterdam) (1 967) 170