F CENTERS IN THIN FILMS OF AMORPHOUS
AND HEXAGONAL LITHIUM IODIDE
N. Rücker, W. Kleemann
To cite this version:
N. Rücker, W. Kleemann. F CENTERS IN THIN FILMS OF AMORPHOUS AND HEXAGONAL LITHIUM IODIDE. Journal de Physique Colloques, 1973, 34 (C9), pp.C9-489-C9-490.
<10.1051/jphyscol:1973980>. <jpa-00215456>
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Submitted on 1 Jan 1973
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JOURNAL DE PHYSIQUE Colloque
C9, supplkment au no 11-12, Tome 34, Nouembre-DPcembre 1973, page C9-489
F CENTERS IN THIN FILMS OF AMORPHOUS
AND HEXAGONAL LITHIUM IODIDE (*)
N. RUCKER a n d W. KLEEMANN
I. Physikalisches Institut der Universitat Gottingen,
D 34 Gottingen, Bunsenstrasse 9, RFA
RCsumC. - Des centres F en grandes concentrations sont produits dans des couches minces de
LiI. La structure du sel est arnorphe ou hexagonale apr6s condensation a basse temperature. La
bande F est diplacee vers les plus grandes energies et devient asymetrique pendant la transformation amorphe-hexagonale. L'asyrnetrie est due a une superposition des absorptions de deux
sortes de centres F. L'une a une symetrie cubique, I'autre est perturbee par des dCfauts de Frenkel.
Par reactions photochimiques les deux sortes de centres se transfornient l'un dans l'autre. L'irradiation avec de la lumiere polarisee cause un dichroi'sme faible dans la bande des centres F perturbes.
Abstract. - F centers in high concentrations are produced in quenched films of LiI in the amorphous and hexagonal niodifications. The transition amorphous-hexagonal causes a high energy
shift and an asymmetry of the F band. The asymmetry is due to the superposition of the absorption
bands of two types of F centers, being cubic and lion-cubic (disturbed by nearby Frenkel defects),
respectively. Both types of F centers can be transformed into one another photochemicaIly. After
polarized irradiation the disturbed F centers exhibit a slight dichroic behavior.
The structure of bulk Lil is cubic, whereas films
condensed a t low temperatures become hexagonal
o r even amorphous. O u r task was t o study the influence
of the short range order in these hexagonal and
amorphous layers on F centers.
I n 1960 Fischer and Hilsch [I] showed that the
different modifications of LiI are characterized by
their exciton spectra. Condensation below 80 K
generates the amorphous phase. Its first exciton band
is peaking a t about 5.85 eV, measured at 78 K (Fig. 1).
By annealing up t o about 100 K the layer becomes
hexagonal. The first exciton band now peaks at
6.12 eV. The hexagonal structure occurs immediately,
if the layer is condensed a t temperatures between
80 and 300 K. The proof for the existence of these
modifications of LiI was given by Riihl [2] by means
of Debye-Scherrer diagrams. The hexagonal phase
was shown to be of the wurtzite type.
The only method to produce F centers of high
concentration in such quenched films is to evaporate
simultaneously LiI and Li onto a cooled substrate.
Thus one obtains F centers u p to a concentration of
more than 10" c ~ I - ~ .
We found out that tliere exist two types of F centers.
One
of them is cubic. It is supposed that it consists
6.1.2 eV(78K1, hexagonal
1
I
of
an
electron in an anion vacancy surrounded by
8 0 K J~X L ~ O O K
four nearest cations. If one regards only these four
nearest cations. and takes into accout that the ratio c / a
5.85 eV(78 K), amorphous
of the axes in hexagonal Lil is almost ideal, then the
L 8OK
point symmetry of a lattice site in the wurtzite structure - which originally is trigonal - can be approximated by T, symmetry. Thus tliere will be n o
splitting of the excited states. The second type of
F centers is non-cubic. It seems to consist of a normal
F center as just described, disturbed by a cation
Frenkel
defect. As has been shown for a siniilar
I
1
0
4 eV
6
5
center in cubic Nal by means of a semi-continuurn
p h o t o n energy
model [3], this will result in a splitting of the p-orbitals.
FIG. 1. - First exciton bands of amorphous and hexagonal This gives rise to two absorption bands. One of
them, which is due to r i single transition, i \ peaking
films of Lil.
a t the same wavc[ength as the absorptiorl b:tnd of
the cubic F centers. The other one. which is ascribed
(*) Research supported in part by the Sonderfo~.schungsto a double 1ransition. is peaking at shorler \ j a w bereich 126 Gottingen-Cla~~sthal.
A
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1973980
lengths. We found out that the two types of F centers
can be transformed into one another photochemically,
and that the anisotropic F centers can be oriented
optically.
Curve 1 in figure 2 shows the absorption of a
layer of LiI doped with Li, which was condensed at
120 K. The temperature of measurement is 20 K.
The exciton band shows that the hexagonal phase is
present. The F center concentration is 5 x 10'' cmP3.
The sub-band peaking at 425 nm is ascribed to the
double transition of the disturbed center. The other
one peaking at 480 nm belongs to its single transition
and to the transition of the cubic F center as well.
i
I
6.12 eV, hexogonol
I
L25nm
photon
480nm
I
energy
FIG.2. - Optical absorption of a film of LiI : Li, condensed
at 120 K, measured at 20 K (curve I), after illumination at 20 K
with 410 nm-light (curve 2), and 500 nm-light (curve 3). Below :
Dichroic difference spectrum after irradiation with linearly
polarized 420 nm-light.
After illumination with 410 nm-light one obtains
curve 2. Obviously non-cubic F centers are transformed into cubic ones. After illumination with
500 nm-light it is just the contrary (curve 3). If linearly
polarized light is irradiated for instance at 420 nm,
the disturbed F centers are partially oriented and
cause a dichroic difference spectrum with extrema
peaking at 425 and 480 nm, respectively (Fig. 2,
below).
In amorphous films of LiI stable F centers could
be produced, too. Figure 3 presents the absorption
spectrum of a film condensed at 20 K doped with Li.
The exciton band shows that the amorphous phase
is present. The broad band peaking at 480 nm (curve 1)
is ascribed to F-like electron excess centers. It lacks
any structure. The halfwidth is 0.9 eV. After illumination with 430 nm-light (curve 2) or with 500 nmlight (curve 3) the F centers are only bleached.
The existence of stable F centers in the amorphous
phase shows that there must be some sort of short
5 87eV, a m o r p h o u s
480 nm
0.02
I
6
1 -.y
.-,
5.5
photon
0
2 eV
3
energy
FIG.3. - Optical absorption of a film of LiI : Li, condensed
and measured at 20 K (curve I). Curve 2 (3) : after bleaching
with 430 (500) nm-light.
range order. This is believed to be hexagonal, because
by annealing the amorphous phase does not convert
into the cubic but into the hexagonal phase.
Condensation at higher temperatures - for instance
at 68 K - produces still the amorphous phase. This
is shown in figure 4 (curve 1). The F band however
has shifted to shorter wavelengths. Now it is peaking
at 450 nm and is asymmetrical. It now resembles the
F band in hexagonal layers. Obviously the hexagonal
short range order has extended. By annealing up to
110 K the layer becomes completely hexagonal
(curve 2) and the F band shifts to still shorter wavelengths. Actually it now peaks at the same wavelengths
as the absorption band of F centers in hexagonal
layers. Photochemical reactions have shown that
there exist now two types of F centers. These exhibit
the same behavior like those which are generated
in films that are hexagonal from the beginning.
6
4
photon
3
eV
2
energy
FIG. 4. - Optical absorption of a film of LiI : Li, condensed
and measured at 68 K (curve l), annealed up to and measured
at 110 K (curve 2).
We can conclude from our measurements that the
F center absorption is a more sensitive tool to detect
the gradual phase transition of LiI than the exciton
absorption. This may be explained by a relatively
large exciton radius as compared with the extension
of the F electron in its unrelaxed excited state.
References
[ I ] FISCHER,
F., HILSCH,R., Z. PIVS. 158 (1960) 553.
[2] R ~ ~ HW.,
L , Z. Phys. 143 (1956) 591.
[3] ROCKER,N., KLEEMANN,
W., Z. PIIJ's.242 (1971) 220.