E.S.R. of CO3-3-Li+ centre in irradiated synthetic single crystal calcite

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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�

427

E.S.R. OF CO3-3-Li+ CENTRE IN IRRADIATED SYNTHETIC SINGLE CRYSTAL CALCITE

G.

BACQUET,

^J.

DUGAS,

^C.

ESCRIBE,

^{L. YOUDRI}

(*)

Laboratoire de

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. ²⁰¹⁴ Dans des monocristaux de calcite synthétique ^{irradiés aux rayons X à la} température ambiante, ^on observe le spectre de résonance d’un électron célibataire

piégé

^{sur un} ion carbonate et

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.

Abstract. ²⁰¹⁴ An E.S.R. spectrum of effective spin S

= 1/2

exhibiting ^a hyperfine ^structure quadru- 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 at room temperature.

LE JOURNAL DE PHYSIQUE TOME 36, MAI 1975,

Classification Physics ^Abstracts

8.632

In

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} para-

magnetic

carbonate ions defect centres

(C03

^and

CO33 -)

^{which are}

usually produced by

y ^or X-irradia- tion at 77 K

[1].

^{Both exhibit} poor

degrees

^of

stability

upon

warming. CO33 -,

which is the more

stable,

bleaches out with a half-life of 10 hours at 300 K.

This latter molecular ion was also found to be stabi- lized

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 Mohamed-V de Rabat, ^Maroc.

(**) Laboratoire ^{associé au 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 irra-

diation,

^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. 1. - Schematic representations ^{of the ’Li+} stabilized

CO33-

molecular ion central spectrum for various orientations of the crystal

in the static field. Allowed lines are designated by ^a.

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 _perpen- dicular to the c axis extra lines _appear which can

only

^{be seen in the central set}

(Fig.

^{lb and}

lc).

^Accord-

ing

^{to their}

positions

^and

intensities,

it is clear that these are so called

forbidden hyperfine

transitions

(AM,

⁼

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

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 carbo-}

nate 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 abun- dance. 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.

FIG. 2. ^- Experimental and theoretical (full lines) angular depen- dences 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 .

TABLE 1

13C

hyperfine coupling

^{constant values}

for

^various

C03 - defects

TABLE II

Spin

hamiltonian parameters

of

^{’Li stabilized}

CO33 -

^{molecular ion}

A and

^{Al. have the same sign} which is unknown.

In the case of ’Li for which all

experimental

^data

are

available,

the observed

spectra

may be inter-

preted by

^the

spin-Hamiltonian :

Je=

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 to 2 ^{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

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 ^reso-

nance 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

and 1 Air,. 1

= 8.52 MHz.

For an

unpaired

^electron

fully

localized in the lithium atom 2s orbital the Fermi contact terni

A;SO,

^calcu-

lated 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 -

^mole-

cular 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 [1] ^{SERWAY, R. A. and MARSHALL, S. A., J. Chem.} Phys. ⁴⁶ (1967)

1949.

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

[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).