PHYSICAL REVIEW B 85, 144119 (2012)
Electron paramagnetic resonance study of Eu
2+
centers in melt-grown CsBr single crystals
H. Vrielinck,
1,*
D. G. Zverev,
1,2
P. Leblans,
3
J. P. Tahon,
3
P. Matthys,
1
and F. Callens
1
1
Ghent University, Department of Solid State Sciences, Krijgslaan 281-S1, B-9000 Gent, Belgium
2
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany
3
Agfa-HealthCare NV, Septestraat 27, B-2640 Mortsel, Belgium
(Received 18 January 2012; published 27 April 2012)
The structure of Eu
2+
monomer centers in CsBr single crystals is investigated using electron paramagnetic
resonance (EPR) spectroscopy. These centers are produced by heating the melt-grown crystals above 600 K in
vacuum followed by a rapid quench to room temperature (RT) or 77 K. The angular dependence of their EPR
spectrum demonstrates that these centers have cubic symmetry. At RT the EPR spectrum decays by aggregation
of the Eu
2+
ions. This strongly contrasts with the situation for CsBr:Eu needle image plates synthesized by
physical vapor deposition, where the Eu
2+
-related EPR spectrum was observed to exhibit long-term stability at
RT.
DOI: 10.1103/PhysRevB.85.144119 PACS number(s): 78.60.Lc, 76.30.Kg, 87.59.bd
I. INTRODUCTION
It is well known that in alkali halides (AX) many divalent
cations (M
2+
) aggregate at ambient temperature; these include
Pb
2+
,
1,2
the transition metal ions Mn
2+ 3,4
and Fe
2+
,
5
and
the rare-earth ions Sm
2+
, Eu
2+
, and Yb
2+
.
6,7
Very often, these
divalent ions have a cation vacancy (V
A
) in their direct vicinity
which preserves charge compensation. Attraction between the
electric dipoles M
2+
A
–V
A
is considered to be the driving force
for the aggregation, while vacancies assure the mobility of
the impurity ions. In many cases, the initial decay kinetics of
the dipole monomer centers were observed to be of third order,
suggesting the formation of M
2+
A
–V
A
trimers as a first step in
the aggregation process.
3,7
Direct trimer formation seems very
unlikely, in view of the low probability of coincidence for
three M
2+
A
–V
A
dipoles. However, Crawford demonstrated that
a monomer dipole decay proportional to the third power of the
monomer concentration is observed if trimers are formed by
capture of dipole monomers by dimers, in which the monomers
exhibit low binding energy.
8
For many M
2+
impurity–AX
host combinations the location of the V
A
could be determined
from the symmetry of the defect, as established from angular
dependent electron paramagnetic resonance (EPR) studies on
single crystals. Such experiments demonstrated that several
vacancy configurations may coexist and that charge compen-
sation of divalent substitutional impurities in alkali halides
is not necessarily local (see, e.g., also Rh
2+
in NaCl
9,10
and
AgCl
11
).
Among these systems, CsBr:Eu has regained interest in the
past decade. Since the late 1990s research efforts have been
directed toward the development of a CsBr:Eu
2+
based x-ray
storage phosphor for medical image plates (IPs) in computer
radiography that would solve the resolution problems of the
BaFBr:Eu
2+
powder IPs, while maintaining high sensitivity.
It was found that thermal evaporation of CsBr:Eu
2+
on an
Al substrate may result in oriented needle growth [needle
image plate (NIP)], with needles only a few μm in diameter
and up to several 100 μm in length, matching resolution and
sensitivity requirements.
12–14
This in itself is quite remarkable,
as melt-grown Eu
2+
doped single crystals perform rather
poorly as photo-stimulated luminescence (PSL) phosphors.
Our EPR investigation of NIPs demonstrated that they exhibit a
Eu
2+
-related spectrum which can be detected at room tempera-
ture (RT), and whose intensity is correlated with the sensitivity
of the plate.
15,16
While at RT the spectrum can be interpreted as
being due to a single type of Eu
2+
center with axial symmetry
around a 〈100〉 axis, below 20 K the spectrum exhibits two
contributions due to axial and orthorhombic centers, occurring
in a fixed ratio.
17
A detailed electron nuclear double resonance
(ENDOR) study revealed the presence of a H
2
O (or OH
−
)
molecule in the close vicinity of the Eu
2+
ion, and a model
was proposed in which a cesium vacancy (V
Cs
) accounted for
the occurrence of two distinct centers at low temperature in a
fixed ratio (immobile V
Cs
), while only one type of center is
observed at high temperature (V
Cs
bound to the complex, but
free to move around it).
18
Like the PSL properties, also the EPR spectra of NIPs and
melt-grown single crystals of CsBr:Eu differ substantially.
In previous reports, no Eu
2+
-related EPR spectra had been
detected in as-grown single crystals,
19
which was attributed to
the aggregation of Eu
2+
ions in CsBr forming diamagnetic
pairs or larger clusters. Even earlier, Savel’ev et al.
7
had
shown that Eu
2+
ions in CsBr qualitatively exhibit the
same behavior as in the rocksalt-type alkali halides: an
EPR spectrum exhibiting a hyperfine structure typical for a
natural abundant mixture of
151
Eu/
153
Eu is produced after
quenching the crystals from T> 600 K to 77 K, but it
decays at RT following third-order kinetics in the early
stage. Hence the isolated Eu
2+
centers were assumed to be
Eu
2+
-V
Cs
dipoles, although, unlike for the centers with a near-
est neighbor V
A
in the NaCl-type lattices,
6
no angular depen-
dent EPR spectra were presented specifying the position of the
vacancy.
This paper is devoted to a study of the angular dependence
of the EPR spectrum of these quenched-in Eu
2+
centers in
CsBr crystals. It is further organized as follows. In Sec. III A
the EPR spectrum of isolated Eu
2+
centers in CsBr single
crystals is introduced and its limited stability at RT is
examined, while in Sec. III B the symmetry of the center is
determined, resulting in a defect model. The implications for
the aggregation mechanism for the Eu
2+
ions is discussed in
Sec. IV. Sections II and V present experimental details and
conclusions, respectively.
144119-1 1098-0121/2012/85(14)/144119(7) ©2012 American Physical Society