STIMULATED PHOTON- AND ELECTRON
EMISSION FROM LATTICE DEFECTS IN BARIUMAND STRONTIUM SULPHATE
G. Holzapfel, M. Krystek
To cite this version:
G. Holzapfel, M. Krystek. STIMULATED PHOTON- AND ELECTRON EMISSION FROM
LATTICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE. Journal de Physique
Colloques, 1976, 37 (C7), pp.C7-238-C7-240. <10.1051/jphyscol:1976758>. <jpa-00216917>
HAL Id: jpa-00216917
https://hal.archives-ouvertes.fr/jpa-00216917
Submitted on 1 Jan 1976
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C7-238
JOURNAL DE PHYSIQUE
Colloque C7, suppKment aau no 12, Tome 37, Ddcembre 1976
STIMULATED PHOTON- AND ELECTRON EMISSION
FROM LATTICE DEFECTS IN BARIUM- AND STRONTIUM SULPHATE
G. HOLZAPFEL and M. KRYSTEK
Physikalisch-Technische Bundesanstalt, Institut Berlin, 1 Berlin 10,
Abbestr. 2-12, FRG
Rksumk. - L'Btude des pieges d'electrons dans les sulfates alcalino-terreux actives par des terres
rares a kt6 axke principalement sur le BaS04 et le SrS04. Ces derniers, cristallisent dans le m6me
rkseau de barite et sont dopQ en E u ~ +L'kmission
.
simultanee de photons et d'electrons (luminescence et exokmission thermostimul6es) ont pour origine des pieges d'klectrons equivalents, mais il
n'y a pas d'interfkrence entre ces phknomenes. La dependance des niveaux d'energie des pieges par
rapport aux parametres de reseau est aussi discutke ;elle signale des defauts intrinseques.
Abstract. - Investigation of electron trapping sites in rare earth activated alkaline earth sulphates is focussed on Euz+ doped Bas04 and SrS04, both crystallizing in the barite lattice. Simultaneous photon- and electron emission (thermally stimulated luminescence and exoemission)
originate in equivalent electron traps, but the phenomena do not interfere. The dependence of the
trap energy levels on the lattice parameters, which points to intrinsic defects, is discussed.
Sulphate phosphors became interesting on account
of their high response to ionizing radiation 11, 21.
Optimum efficiencies are achieved with earth alkali
sulphates, especially CaSO,, BaSO, and SrSO,.
Activators for thermoluminescence (TL) are conveniently provided by rare earth (RE) admixtures.
The TL emission characteristics are largely determined by the valency of the incorporated RE ions.
RE3+ ions produce individual line-structured spectra
which can be identified with transitions between
known RE3+ levels shielded against the host crystal
lattice field. In contrast the RE2+ ion, which is represented only by Eu2+, exhibits a non-structured, broad
TL emission band, suggesting strong coupling with
the lattice. The RE3+ line emission spectra are entirely independent of the host lattice, whereas the Eu2+
emission band shifts on the wavelength scale when the
lattice structure is varied.
Another feature, which is controlled by the trapping
sites, is the glow peak location on the temperature
scale. Here the TL glow peaks are independent of the
RE3+ species but dependent on the individual sulphate lattice. This glow peak shift is even more pronounced for Eu2+ doping.
These facts suggest trapping sites which are related to the interatomic distances of the sulphate lattices.
Excluding CaSO,, BaSO, and SrSO, crystallize in the
same lattice and would appear to be suitable for investigation of the nature of trapping sites in more detail.
Consideration of the TL emission spectra indicates
that only Eu2+ doping can produce classical recombination phosphors with the trapping sites clearly
separated from the activator sites.
General information on electron traps in ionic
lattices is often available from measurements of exoelectron emission 131. This may take place simultaneously with luminescence from recombination phosphors.
Common to both is the detrapping of electrons by
thermal or optical stimulation (I). The mobile electrons may then recombine with holes trapped in activators, so causing photon emission (TL). Alternatively, the detrapped electrons may escape from the
surface (TSEE). Outside the crystal the exoelectrons
can be sensitively detected by particle counters. As a
result of their small energy the electrons are emitted
only from a very thin surface layer, whereas photon
emission occurs in the bulk of the emitter. If penetration of the surface is not seriously affected by the
charge of the escaping particle, TL and TSEE will
yield comparable results [4].
Since activators are not required for exoemission,
the initial trap spectrum of phosphors can be investigated before and parallel with TL activation. The
results for BaSO, and SrSO, increasingly doped with
Eu2+ are shown in figure 1. The undoped materials
already show strong TSEE which remains unaffected
by the Eu2+ doping up to concentrations of loz0cmd3.
This evidently means that electron traps are not introduced into the sulphate lattice b y the activator ;
rather they appear to be formed by natural defects
(or impurities) in the original material. This result
is supported by closer inspection of the corresponding luminescence and exoemission glow curves
(1) Code : TSEE Thermally Stimulated Exoelectron Emission
OSEE Optically Stimulated Exoelectron Emission.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1976758
STIMULATED PHOTON- AND ELECTRON EMISSION FROM Bas04 AND SrS04
C7-239
'//-
1
a) B ~ S O L
U
Sr
c
-u
P
a,
W
0
-u
61
5-
a1 TL
lY
'
stottonory
ernlss~on ,
-
-
o=
b) SrS04
I
to
O
-
,
01
EuO
,,
n
#
,
-
II
I
I
I
I
I
I
-
03 mol% 0.5
concentratton
-
b) TSEE
FIG. 1. - Relative TL- and TSEE efficiencies of Bas04 and
SrS04 dependent on the dopant concentration.
;;
-;z.
0
-2
&QL
n
'
(Fig. 2) which exhibit characteristic single peaks. The
TL-and TSEE glow curves are almost identical without
any change in TSEE between zero and high Eu2+
doping. The specific peak shift from BaSO, to SrSO,
occurs for TSEE as well as for TL.
Analysis of the glow curves by a high resolving,
computer-assisted method [4, 51 revealed fi~storder
kinetics governing both the TL- and the TSEE process. This indicates that the effects do not interfere as
already shown by the independence of the TSEE
efficiency with regard to the growing TL activation
(Fig. 1). The term data (activation energy, frequency
factor) could therefore be uniquely evaluated and definitely attributed to single trapping levels, after separation of minor satellite levels [4] (Table I). The frequency factors are of the order of the DEBYE frequency. Identical activation energies prove that the
same type of trapping site is effective for TL and TSEE
and that these traps are not distorted by the vicinity
of the surface.
The energy shift of the trapping levels when the sulphate is altered may be correlated to lattice parameters [2]. A decrease in the average nearest neighbour
0,2
statlonary
ernlss~on
j
o I
-
500
400
Temperature
K
FIG. 2. - TI- and TSEE glow curves of Bas04 and SrS04 ;
heating rate 0.14 Kis. Dashed curves computed using the term
data of table I.
distance (BaSO, 0.286 nm, SrSO, 0.275 nm) results in
a shift of the activation energy to higher values
(BaSO, 1.08 eV ; SrSO, 1.27 eV). Hence it should be
discussed whether a relation similar to the well known
MOLLWO rule can be found.
An immediate application to our results does not
appear feasible since the MOLLWO-rule relies on
optical absorption measurements. The thermal activation energies obtained from TL- and TSEE- measurements differ from the optical activation energies
by the FRANCK-CONDON ratio [6]. Due to varying
polarisation properties this ratio may change even in
Term data of prin~i~val
electron trap levels in BaSO, and SrSO,
Material
BaSO,
(*) Normalized to q
SrSOa
TSEE
term data
=
1 Kjs.
600
C7-240
G. HOLZAPFEL and M. KRYSTEK
the same lattice structure, i. e. from BaSO, to SrSO,.
Leaving SrSO, for later investigation, only the optical
activation energy of BaSO, is known from optical
measurements (absorptance and respectively reflectance) combined with photothermal exoelectron emission (selective OSEE) [7].
Another aspect refers to the lower symmetry of the
barite lattice, since the MOLLWO-rule.
EOP'= A . d - @
(1)
(Eop, optical activation energy, d lattice constant)
was originally formulated for NaC1-type lattices [8].
The constants A, a, however, change with the lattice
type. In principle, if the lattice structure is non-cubic,
one cannot expect the simple relation (1) to be valid
a priori. Hence the intrinsic nature of the defects
producing the electron traps in the highly sensitive
sulphate phosphors must be substantiated by continued study.
References
[1] YAMASHITA,
T., NADA,N., ONISHI,H., KITAMURA,
S.,
Proc. 2nd Int. Conf. Luminescence Dosimetry, Gatlinburg, C0nf-680920
and
Physics 21
(1971) 295.
[2] DIXON,R. L., EKSTRAND,
K. E., J. Lumin. 8 (1974) 383.
[31 BOHUN,A+, SCHARMANN,
A., K ; ~ x H.,
~ ~proc.,
~ ~4 , th
Int. Symp. Exoelectron emission and dosimetry,
Liblice 1973, Czechosl. Acad. Sc. Ed. A. Bohun
Prague (1974).
[4] HOLZAPFEL,
G., KRYSTEK,
M., Phys. Status Solidi (a) 37
(1976) 303.
[5] HOLZAPFEL,
G., KRYSTEK,
M., WOLBER,
L., Vide 30 (1975)
I n7
A",.
161 NINK,R.2 HOLZAPFEL,
G . Physique Collo4.34 (1973) C 9.
[71 NINKy
R. Optik 41
515.
[8] MARKHAM,
J. J., F-Centers in Alkali Halides (Academic
Press, New York and London) 1966.
5.9