Crystal-field analysis, upconversion, and excited-state dynamics
for „ U
4 ¿
,U
3 ¿
… :Ba
2
YCl
7
single crystals
Miroslaw Karbowiak, Agnieszka Mech, and Janusz Droz
˙
dz
˙
yn
´
ski
University of Wroclaw, Department of Chemistry, ul. F. Joliot-Curie 14, 50-383 Wroclaw, Poland
Norman M. Edelstein
Chemical Sciences Division, MS 70A - 1150, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received 17 August 2002; revised 7 November 2002
Single crystals of (U
4+
,U
3+
):Ba
2
YCl
7
were grown by the Bridgman-Stockbarger technique. The U
4+
and
U
3+
ions substitute for Y
3+
and have C
1
site symmetry in Ba
2
YCl
7
. Sixty energy levels of U
4+
were assigned
to 40 000 cm
-1
from absorption, excitation, and luminescence spectra, which encompasses all but the
1
S
0
multiplet. A crystal-field analysis was performed by fitting 8 atomic and 19 crystal-field parameters to the
experimental Stark levels. The values obtained from the fit had a standard deviation of 36.6 cm
-1
and agreed
with those calculated from the angular overlap model. Efficient luminescence was observed at 7 K from
the
4
G
7/2
and
4
F
9/2
levels of the U
3+
ions and from the
1
D
2
and
1
I
6
of the U
4+
ions with decay times of 0.5,
0.55, 9.2, and 4.3 s, respectively. For the U
4+
ions, a very strong anti-Stokes emission was observed due to
energy-transfer processes. The emission of the U
3+
ions is strongly quenched by temperature in contrast to that
of the U
4+
ions for which emission was observed even at room temperature. Energy transfer between the U
3+
and U
4+
ions has been shown to occur in the (U
4+
,U
3+
) doped Ba
2
YCl
7
.
DOI: 10.1103/PhysRevB.67.1951XX PACS numbers: 71.70.Ch, 78.40.Kc, 31.70.Hq
I. INTRODUCTION
Crystal-field calculations of actinide ions embedded in
crystalline hosts have been performed primarily for the 3+
oxidation state. The most comprehensive analysis reported
by Carnall
1
encompasses systematic studies of a series of
simple trivalent actinide chlorides with the metal ion at a
D
3 h
site symmetry. In the actinide ions only U
3 +
and Cm
3 +
ions have been investigated to a relatively large extent. The
optical properties and crystal-field analyses of a number of
U
3 +
doped single crystals as well as some polycrystalline
samples have been reported by Karbowiak et al. in recent
years
2–8
and those of Cm
3 +
have been investigated for the
doped LaCl
3
,
9
Cs
2
NaYCl
6
,
10
LuPO
4
,
11
Ref. 12 and ThO
2
single crystals. The parametric approach developed for the
Ln
3 +
ions has been found adequate also for the An
3 +
ions so
the energy levels and calculated crystal-field CF parameters
of these ions could be directly compared with those of their
lanthanide analogues.
The number of papers devoted to An
4 +
ions is relatively
low and most of them deal with U
4 +
systems. More compre-
hensive studies of optical properties of the An
4 +
ions and
especially of U
4 +
started with utilization of some new single
crystals such as ThBr
4
, ThCl
4
, as well as the already known
ThSiO
4
as host materials. All the host crystals possess the
tetragonal structure ( I 4
1
/ amd ) and are isostructural with
UCl
4
. Thus the interpretation of the energy level structure of
U
4 +
ions in this isomorphic series has led to consistent sets
of free-ion and crystal-field parameters
13–16
with least-square
deviations rms between 36 and 71 cm
-1
. In these host
crystals the U
4 +
ions experience a rather weak crystal field
CF, which justifies the application of the same theoretical
approach as for the Ln
3 +
ions in contrast with higher sym-
metry sites such as found in U(BD
4
)
4
/Hf(BD
4
)
4
site sym-
metry T
d
),
17
or Cs
2
UX
6
( X =Cl,Br) (O
h
),
18
where the CF
affecting U
4 +
ions is far stronger and the rms deviation is
about 150 cm
-1
. This indicates that for high symmetry cases
the J mixing is so strong that the assumption of a predomi-
nant SLJ character nominal quantum numbers for an atomic
state for a group of levels is a rather poor approximation.
Recently, a different fit including specifically the configura-
tion interaction with the higher-lying 5 f
1
7 p
1
configuration
has improved the rms deviation considerably for U
4 +
in
Cs
2
UBr
6
.
19
Gajek et al. have presented absorption spectral analyses
of U
4 +
ions doped in UGeO
4
Ref. 20 and -ThGeO
4
Ref.
21 single crystals. Analyses of the CF spectra of the actinide
tetrafluorides—AnF
4
(An=U
4 +
, Np
4 +
, Pu
4 +
, Cm
4 +
,
Am
4 +
) and CeF
4
:An
4 +
(An=Cm,Bk)—have been reported
by Carnall et al.
22
and Liu et al.
23
Fluorescence spectra are
available for U
4 +
-doped -ThBr
4
,
24
-ThCl
4
,
15
and
Cs
2
ZrBr
6
Ref. 25 single crystals. Numerous strong emis-
sion lines in the visible and infrared region have been ob-
served also in the spectrum of -ThBr
4
:U
4 +
.
26
There are,
however, only a few reports concerning upconversion of ac-
tinide ions and ThBr
4
is the only host for which U
4 +
upcon-
version was observed.
27
From among trivalent actinides up-
conversion was observed for U
3 +
:LaCl
3
Refs. 28 and 29
and curium trihalides.
30
In our previous paper
31
the spectroscopic investigations of
U
3 +
:Ba
2
YCl
7
single crystals were presented and discussed
including a crystal-field analysis based on both the angular
overlap model AOM and the conventional parametric
Hamiltonian. In this paper absorption, emission, and upcon-
version spectra of U
4 +
ions in (U
4 +
,U
3 +
):Ba
2
YCl
7
single
crystals are presented. The host crystal possesses monoclinic
symmetry with the space group P 21/c No. 14.
32
The Y
3 +
ions are seven-coordinated and the characteristic feature of
the crystal structure is YCl
7
polyhedra, in the form of a
PHYSICAL REVIEW B 67, 1951XX 2003
0163-1829/2003/6719/1951XX17/$20.00 ©2003 The American Physical Society 67 1951XX-1