E.S.R. of CO3-3-Li+ centre in irradiated synthetic single
crystal calcite
G. Bacquet, J. Dugas, C. Escribe, L. Youdri, C. Belin
To cite this version:
G. Bacquet, J. Dugas, C. Escribe, L. Youdri, C. Belin. E.S.R. of CO3-3-Li+ centre in irradiated synthetic single crystal calcite. Journal de Physique, 1975, 36 (5), pp.427-429.
<10.1051/jphys:01975003605042700>. <jpa-00208268>
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Submitted on 1 Jan 1975
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LE JOURNAL DE
PHYSIQUE
TOME
36,
MAI
1975,
427
Classification
Physics Abstracts
8.632
E.S.R. OF
G.
CO3-3-Li+
CENTRE IN IRRADIATED SYNTHETIC
SINGLE CRYSTAL CALCITE
BACQUET,
Laboratoire de
J.
DUGAS, C. ESCRIBE, L. YOUDRI (*)
Physique des Solides (**), Université Paul-Sabatier,
31077 Toulouse
Cedex, France
and
C. BELIN
L.E.P., 3,
avenue
Descartes, 94450 Limeil-Brévannes, France
(Reçu le 2 décembre 1974, accepté le 9 janvier 1975)
Résumé.
Dans des monocristaux de calcite synthétique irradiés aux rayons X à la température
observe le spectre de résonance d’un électron célibataire piégé sur un ion carbonate et
ambiante,
couplé à un noyau de lithium. Ce défaut qui a été identifié comme étant un ion moléculaire CO3-3
stabilisé par un ion Li+ en position interstitielle, présente une symétrie axiale suivant l’axe c. Il est
très stable à la température ambiante.
2014
on
An E.S.R. spectrum of effective spin S = 1/2 exhibiting a hyperfine structure quadruAbstract.
plet has been observed in synthetic single crystal calcite X-irradiated at 20 °C. From the g values
it is deduced that this spectrum is due to a CO3-3 molecular ion which is charge stabilized by an
interstitial Li+ ion. This defect which is axially symmetric along the crystalline c axis is very stable
2014
at room
temperature.
naturally occurring single crystal calcite several
paramagnetic species created by irradiation were
identified by means of the E.S.R. technique. Some
have been shown to be molecular ions, originating
from the ionization or degradation of impurities
substituting for CO23 -, by Marshall et al. at the
Argonne National Laboratory. Two others are paramagnetic carbonate ions defect centres (C03 and
CO33 -) which are usually produced by y or X-irradiation at 77 K [1]. Both exhibit poor degrees of stability
upon warming. CO33 -, which is the more stable,
In
bleaches out with a half-life of 10 hours at 300 K.
This latter molecular ion was also found to be stabilized by Y3 + in an interstitial position with equal
probability of being slightly displaced either above or
below the plane of the normal divalent carb’onate
ions [2]. On the other hand, in a recent paper, Cass
et al. [3] reported E.S.R. and E.N.D.O.R. spectra
of a magnetic centre stable at room temperature which
was created by y irradiation of natural calcite. They
proposed that the defect is the HCO23- molecular
ion arising from the ionization of bicarbonate ion
impurities.
(*) Détaché de l’Université
(**) Laboratoire associé au
Mohamed-V de Rabat, Maroc.
C.N.R.S.
’
The results presented here were obtained with
specimens of synthetic single crystal calcite grown
at the L.E.P. of Limeil-Brévannes (France) by means
of the Travelling Solvent Zone Melting method
described by Belin et al. [4], where Li2CO3 was used
as a solvent.
Samples of dimensions 0.4 x 0.3 x 0.3 cm’ were
X-irradiated (20 kV, 10 mA) at R.T. during about
15 hours and then studied in the X-band using a
conventional 100 kHz field modulation spectrometer,
at the same temperature. Immediately after the irradiation, several spectra due to different defects were
simultaneously recorded. These last exhibit varying
degrees of thermal stability and this report is devoted
to the most stable of them, the E.S.R. patterns of
which are characterized by very narrow lines
(AH 100 mG) which are easily saturated.
When the Zeeman field vector lies in any plane
perpendicular to the crystalline c axis (such a plane
is parallel to the planes containing the host carbonate
ions) the spectrum consists of three sets of hyperfine
quadruplets where A 2.55 G. The central set
(Fig. la) being about two hundred times as intense
as the two outer ones which are at a distance of
126 G, this permits the observation of a much less
intense hyperfine triplet (where A
1 G) between
=
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01975003605042700
=
428
FIG. 2.
Experimental and theoretical (full lines) angular dependences of various lines when the magnetic field is rotated about an
axis perpendicular to c.
-
x AMI = 0 ; e AMI = 1 ; c:J AMI = 2; : AMI = 3 .
FIG. 1. - Schematic representations of the ’Li+ stabilized CO33molecular ion central spectrum for various orientations of the crystal
in the static field. Allowed lines are designated by a.
TABLE 1
13C hyperfine coupling
constant
values for various
C03 - defects
the two inner lines. The relative intensity and position
of lines inside the central pattern indicate that the
unpaired electron is coupled with one lithium nucleus
which has two isotopes having different nuclear
spins : these are 7Li (I = 2, 92.6 % and y 3.256)
and ’Li (I
1, 7.4 % and y 0.822).
When the crystal is rotated about any axis perpendicular to the c axis extra lines appear which can
only be seen in the central set (Fig. lb and lc). According to their positions and intensities, it is clear that
these are so called forbidden hyperfine transitions
=
=
=
1, AMI 1, 2, 3).
With the magnetic field parallel to the c axis, the
central pattern is again composed of four lines,
each of them being the superposition of several
transitions (Fig. Id). Only the central line of the 6Li
spectrum is seen, the two others being hidden by the
(AM,
=
TABLE II
Spin hamiltonian parameters of ’Li stabilized
CO33 -
molecular ion
=
spectrum of another stable defect. On the other hand
the line intensity is too weak (see below) to permit
the observation of the two satellites. The experimental
line positions for ’Li are given in figure 2.
What can be said about the nature of the defect ?
The unpaired electron is weakly coupled with a
lithium nucleus and is essentially located on a carbonate as indicated by the observation, when H 1 c,
of two sets of hyperfine quadruplets at a distance of
126 G due to 13C of I = 1/2 with 1.1 % natural abundance. This value of the 13C splitting is equal to that
measured in the case of y3 1 stabilized CO33 - (see
Table I). On the other hand, our measured gll and
Y.L values (see Table II) are identical to those of both
CO33 - and Y3+ stabilized CO33 -. Consequently we
can assert that we are observing an axially symmetric,
Li+ stabilized, C03- molecular ion.
Aand Al.
In the
are
have the
case
same
sign
which is unknown.
of ’Li for which all
experimental
data
available, the observed spectra may be inter-
preted by
the
Je=
spin-Hamiltonian :
PB H. g. S + S. Ã. 1 - PN ON H. 1
(1)
where the nuclear Zeeman interaction is taken to
be isotropic, and with S = 2 and I
2. The various
constants of (1) are summarized in table II.
It is worth while to underline here the importance
of the nuclear Zeeman term. Its value, which is
equal
A Il’ enables one to explain the position
(full lines in Fig. 2) as welt as the intensity of forbidden
transitions. They can still occur when the static field
is very close to the c axis, but vanish when H and c
are carefully aligned. We think that the insufficient
intensity of the lines in figure 1 d may be explained
=
to 2
429
by a slight ( 10) misalignment of the crystal inside
the cavity. It can be seen in figure 2 that experimental
and theoretical angular dependences are in good
agreement. The experimental uncertainty ( ± 0.15 G)
which seems large is due to the fact that we are obliged
to measure the field values by means of proton resonance outside the spectrum as even at the lowest
level available the 50 Hz field modulation causes
broadening of E.S.R. lines.
The hyperfine tensor can be written Â
Aiso + Î,
where T is a traceless tensor and1 Air,.1 = 8.52 MHz.
For an unpaired electron fully localized in the lithium
atom 2s orbital the Fermi contact terni A;SO, calculated from wave functions given by Clementi [5],
equals 158.5 MHz. Comparing these two values, we
find that the unpaired electron spin density in the
lithium 2s orbital is 5.37 %. In the case of a pure COI
molecular ion only l.1 % of the spin density is localized
on a nearest neighbour Ca2+ [2]. This indicates that
the lithium nucleus is closer to the carbonate than
such a calcium. Since the Li+ stabilized C03 - molecular ion has an axial symmetry about the c axis,
we suppose that the lithium ion lies in an interstitial
site, either above or below a carbonate, approximately
in a plane containing calcium ions as shown in figure 3.
With such a configuration the defect has a net doubly
negative charge, like HCO23- molecular ion, which
explains its great stability. It is necessary to warm
the crystal up to 400°C for half an hour to completely
bleach out this paramagnetic centre.
=
FIG. 3.
Schematic representation of the calcite structure showing
the proposed localization (shading line) of the interstitial lithium
ion (r
0.68 Á) along the c axis above a carbonate ion.
c’’0 17.020 A. White circles : oxygen; dashed circles : calcium;
Black circles : carbon.
-
=
=
’
References
R. A. and MARSHALL, S. A., J. Chem. Phys. 46 (1967)
1949.
MARSHALL, S. A., MC MILLAN, J. A. and SERWAY, R. A.,
J. Chem. Phys. 48 (1968) 5131.
[1] SERWAY,
[2]
[3] CASS, J., KENT, R. S., MARSHALL, S. A. and ZAGER, S. A.,
J. Mag. Res. 14 (1974) 170.
[4] BELIN, C., BIUSSOT, J. J. and JESSE, R. E., J. Cryst. Growth
13-14 (1972) 597.
[5] CLEMENTI, E., Tables of Atomic Functions (1965).