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Absorption and emission spectra of U4 + diluted in the incommensurate structure of ThCl4
C. Khan Malek, J.C. Krupa, P. Delamoye, M. Genet
To cite this version:
C. Khan Malek, J.C. Krupa, P. Delamoye, M. Genet. Absorption and emission spectra of U4 + diluted in the incommensurate structure of ThCl4. Journal de Physique, 1986, 47 (10), pp.1763-1773.
�10.1051/jphys:0198600470100176300�. �jpa-00210372�
Absorption and emission spectra of U4 + diluted in the incommensurate structure of ThCl4
C. Khan
Malek,
J. C.Krupa,
P.Delamoye
and M. GenetLaboratoire de
Radiochimie,
Institut dePhysique
Nucléaire, Université Paris-Sud, 91406Orsay
France(Requ
le 11 avril 1986,accept6
le 9juin 1986)
Résumé. - Les spectres
d’absorption
et de fluorescence deU4 +
dilué dans des monocristaux deThCl4
ont étémesurés de 4,2 K
jusqu’à
latempérature
ambiante. Les cristaux de03B2-ThCl4 présentent
une structureincommensurable en dessous de 70 K avec une perte de la
périodicité
lelong
de l’axe c. Ceci a pourconséquence
une variation de la distancemétal-halogène lorsqu’on
passe d’une maille à l’autre. Lasymétrie
dusite de l’ion actinide est ainsi abaissée. Les raies
correspondant
au site desymétries S4
etD2
ont été identifiées parspectroscopie.
Lasymétrie S4
a été ramenée à celle deD2d
et uneanalyse paramétrique
des niveauxd’énergie
deU4 +
ensymétrie D2d
etD2
est donnée. Pour 25 niveaux dans le siteD2d, l’écart quadratique
moyen 03C3 est de 46
cm-1 et
de 56cm- 1 pour
les 34 niveaux ensymétrie D2.
Lesparamètres qui
interviennent dans les calculs pour les deuxsymétries
sontlégèrement
différents.Abstract. 2014 The
absorption
and fluorescence spectra ofU4+
diluted insingle crystals
ofThCl4
have beenmeasured at temperatures
ranging
from 4.2 K to room temperature.03B2-ThCl4
exhibits on incommensurate structure below 70 K with a loss ofperiodicity along
the c axis. This results in a variation of the distance between the metal and thehalogen
from one cell to another. The site symmetry of the actinide ions is then reduced. The linescorresponding
to the sites of theresulting symmetries S4
andD2
are identifiedspectroscopically.
TheS4
symmetry isapproximated by
theD2d
one and aparametric analysis
of the energy levels ofU4 +
in theD2d
andD2 symmetries
isreported.
For 25 levels in theD2d
site the root mean square deviation 03C3 is 46cm-1 and
for 34 levels inD2, 03C3
= 56cm-1.
The parameters which occur in bothsymmetries
are
only slightly changed.
Classification
Physics
Abstracts78.40 - 78.50
1. Introduction.
There has
recently
been some interest shown in the studies of the tetravalent uranium ionU4 + (5f)
insolid state matrices that started with the new parame- tric
analysis
ofU4 +
inO-ThBr4 [1] (U4 +
at a site ofD2d
orD2 symmetry)
followedby
areinterpretation
of the
spectra
ofU4+
in theborohydrides [2]
withthe
U4 +
ion at a site ofTd symmetry.
Thestudy
ofU4 +
inThCl4 brings
a new set ofspectroscopic parameters
forU4 +
at a site ofrelatively high symmetry ( D, )
which can becompared
to thoseobtained in the bromide matrix
ThBr4.
Both matrices offer a similar dodecahedron of coordination for theU4 +
ion and likeThBr4 [3], ThCl4 undergoes
asecond order
displacive phase
transition at atempe-
rature
Tc
= 70 k. BelowTc,
neutron diffractionexperiments
revealed that thecrystal
structure ofThCl4
is incommensurate and modulated[4],
due totransverse
displacements
of the chloride ions perpen- dicular to the c axis of thecrystals.
Thesedisplace-
ments are different in each unit cell and
they
can bedescribed as in
ThBr4
in terms of a localphase
’PI’where I is the index which identifies the cell
[5].
Themodulation reduces the site
symmetry
of the actinide ion that isD2d
at roomtemperature
andS4, D2, C2
atlow
temperature, according
the 91 values : for ’PI= 0,
thesymmetry
of the site isS4,
for 9 = ±
ir /2,
theU4 +
sites are describedby
theD2 symmetry,
for 0
1911 ( Ir/2,
the sites are describedby
theC2
symmetry.
There exists a continuous variation of the environ- ment, thus of the
crystal
field around theU4 +
ion andconsequently
the energy levels ofU4 +
aremodulated between two extreme
positions
corres-ponding
to 9 = 0 and ’PI = ±ir /2
for a ’IT band and cp 1= ±Ir /2
for a Q band. Theshapes
of these bandsArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198600470100176300
are then characteristic
(Fig. 1) - They
are describedby
the model ofDelamoye
and Currat[5]
as acontinuum of lines limited
by
twosharp edges. They correspond
to the sites ofD2 symmetry (u band)
orS4
andD2 symmetries
for a 1T band. Site selective excitation of theU4 + absorption
levelspermitted
usto correlate the
absorption
and emission lines[6]
andin
particular
toidentify
transitions ofU4 +
ions at sites ofD2
andS4 symmetries.
Therefore we are in aposition
toanalyse
theU4 +
energy levels in these both sites.Fig. 1.
- U4+absorption bandshapes
in the incommensu- rate structure ofThCl4.
2.
Experimental
details andanalysis
of the data./3 - ThCl4 single crystals
were grownby
theBridgman
method
[7].
Thecrystal
structure istetragonal [8]
space group
14,/amd
at roomtemperature (/3
formof
ThCl4 [9]). ThCl4
wasdoped
with about o.1 % ofU4 + ,
thedoping
materialbeing U02
orUCI4.
Thecrystals
were cleaved andpolished
in adry
boxbecause of their
hygroscopicity
andthey
were sealedunder a helium
atmosphere (300 torrs)
in silicatubes. The
crystals
used for the measurements wereabout 15 x 3 x 5
mm3
or smaller.They presented
acleavage
face which was notexactly perpendicular
tothe fourfold axis so
they
can be oriented in order to takepolarized spectra a
and u + ’1r.Experiments
atlow
temperature
wereperformed
in an OxfordInstruments helium gas
circulating cryostat.
The
absorption
and emissionspectra
were recor-ded at different
temperatures
between 4.2 K and300 K in the visible and infrared
region (0.3
>m - 2.5urn)
on a one meter HR 1000Jobin Yvon
spectrometer.
It isequipped
in thevisible
region
with a 1200 lines/mmgrating
and aphotomultiplier,
and in the infrared with a600 lines/mm and a PbS detector. The calibration
was made with a low pressure mercury
lamp
beforeand after the
recording
of thespectrum.
Fluores-cence
spectra
were excited with aSopra nitrogen pumped dye
laser.The
surrounding
of theU4,
ions in the differentsymmetries
mentioned before isnon-centrosymme-
tric(non-inversion centre),
sostrong
zerophonon
transitions are
expected.
Allspectra
wereanalysed
with a linear
polarizer
which can be oriented in bothparallel
andperpendicular
directions relative to theC4 optical
axis which ispreserved
below thephase
transition
temperature
in thetetragonal
structure ofthe incommensurate
phase.
The at and aspectra
were checked to be
identical,
thusleaving
usonly
with electric
dipole
transition to deal with.Though
the
polarization
isexpected
to becomplete (7r
orofa)) (for
definition see TableI),
the a + ff wererecorded instead of the pure w
spectra,
which is due to aslight
misorientationregarding
theC4
axis onthe various
crystals
that were studied.Table I. - Electric
dipole
selection rules in theD2d (a), S4 (b)
andD2 (c) symmetries,
notationsfrom
Nielsonand Koster
[11].
A Jpolarization corresponds
to aspectrum recorded with the electric
field
Eperpendicu-
lar to the
principal
axisC4
whereas a 11: spectrum cor-responds
to Eparallel
to this axis.Some 75 % of the lines can be seen with the or
polarization
and the lines observed with a irpolariza-
tion are
generally
weaker than those with a o-polarization (Fig. 2).
This trend has also been seen onThSiO, - U4, [10]
where theU, 4
ion is at asite of
D2d symmetry
as well. The lines inf3 - ThCl4 - U4 +
with apolarization a
are ingeneral
broader than those with the
polarization
m. This hasbeen
explained by
theDelamoye
and Currat’sanaly-
sis
[5]
of the lineshapes
recorded inØ-ThBr4 - U4+.
The width of the lines inThCl4 - U4 +
are broader overall than those inThSi04 - U4 +
which offers aregular D2d
site forU4 +
at lowtemperature.
Fig.
2. -Absorption
spectra ofThCl4 -
U4 + in thevisible
(a)
and IR(b).
Although
in the modulated structureof B-ThCI4
the
symmetry
of the actinide site is loweredS4 - C2 - D2) ,
we shall us theD2d designation
inthe labelling of the energy levels. It can be noted
that the electric dipole
selection rules for the
D2d
andS4 (subgroup
ofD2d)
groups[11] only
differfor the transitions
(Table I) :
that are forbidden in the
D2d symmetry
and allowed with a 1Tpolarization
in theS4 symmetry
asr 1 -+ T2
transitions.
The other selection rules are the same.
In the
S4 symmetry
the loss ofsymmetry
elements isresponsible
for the introduction ofimaginary
terms in the
crystal
fieldhamiltonian,
ImB 4
andIm
B4,
whereas all thecrystal
fieldparameters
are real in theD2d symmetry (Table II). Interpreting
thedata in the
D2d symmetry
instead of in theS4
symmetry
results inneglecting
theseimaginary
termsin the even rank
crystal
fieldcomponents
that are used for the calculations of the energy levels of theoptically
active ions. Nevertheless the use of theD2,d symmetry
inplace
of theS4 symmetry
shouldgive
us the main features of thespectra.
Esterowitz et al.[12]
hadalready
noticed withPr3 + doped
inLiYF4
at a site ofS4 symmetry
that the use of theD2d
selection rules was a
good approximation
in identi-fying
energy levels. Hence theD2d symmetry
will be considered in our case. Thecrystal
fieldeigenstates
carry the
r, through r5
irreduciblerepresentations
associated with the
D2d
sitesymmetry. Only
theF5 representation
isdoubly degenerate,
theremaining ri
toT4 being
nondegenerate.
Thelowering
of thesymmetry
fromD2d
toD2
lifts thedegeneracy
of theT5
levels of energy. Thus there areonly singlets carrying
theT
1 toF4 point
grouprepresentations
associated with the
D2 symmetry.
The correspon- dance between the sets ofrepresentations
are in theD2d’ S4
andD2 symmetries
shown in table III. The selection rules for the electricdipole
transitions for theD2d
andD2 symmetries
aregiven
in the table I.Table II. -
Crystal field
parametersof
even rank inthe
D2d (a), S4 (b)
andD2 (c) symmetries.
Table III. -
Correspondence
between the setsof
repre- sentations in theD2d, S4
andD2 symmetries [11].
Following
thestudy
ofU4 +
in(3 - ThBr4
on whichZeeman and M.C.D. studies were
performed [13],
the
ground
state level is assumed to be a(Dm ) r4
level,
which is consistent with ourinterpretation
ofthe
crystal
field transitions. At 4.2 K the a linescorrespond
to transitions from the( D2d) T4
ground
state to thedoubly degenerate (Dm) F5
levels of energy while the lines recorded with
the
7Tpolarization correspond
to transitions to theDZd T1
1 nondegenerate
levels of energy.In the most intense
peaks
a contribution ofU4 +
ions at a site ofD2d symmetry
isexpected.
Indeed the
intensity
of the electricdipole
transitions in the sameconfiguration
andregardless
of the sitesymmetry
of theion,
isproportional
to matrixelements
involving
thecrystal
fieldparameters
ofodd
rank( B *k q )2.
The
D2 symmetry
is close to theD2d symmetry
at lowtemperature
so the additionalparameters
B 4 *5" B4*7’
andB,*7’
to be added to the odd rankparameters
for theD2d symmetry (B *3 B *5
andB2’)
can besupposed
to be weak. Therefore theintensity
of the linescorresponding
to theU4,
ionsin the
D2d
andD2 symmetries
will follow the sametrend.
There are more lines than are
theoretically
expec- ted from aF4 ground
state, even if some additionallines are assumed to come from
U4,
ions at sites ofD2
andC2 symmetries
aloneaccording
to the selec-tion rules. Some lines are vibronic in
origin
andthey
are found on the blue side of the electronic lines at 4.2 K. When excited
they give
the same fluorescencespectra
as the electronic linesthey
are associatedwith. Some other lines are
probably
due to animpurity
or toU3 + . In /3 -ThBr4 - U4 ’,
M. C. D.experiments [13]
showed ananomalously high 9
value for certain lines that are attributed to
impuri-
ties.
At
higher temperature
transitions from a low-lying
level at 55cm-1
1 arereadily apparent.
This level has also been seen in fluorescenceexperiments [6].
It can beinterpreted
as aD2
levelarising
fromthe
splitting
of the first excited Stark level represen- tatedby T5
in theD2d symmetry.
Theshape
of thebands in
J3 - ThCl4 - U4 +
is modified when thetemperature
is increased(Fig. 3).
Inparticular
thesharp edges corresponding
to transitions of theW ’
ions at sites ofD2 symmetry
decrease inintensity.
Thoseedge singularities finally disappear
and the
D2d
line is left alone at 70 K. Withincreasing
concentration of
U4 +
a decrease in thesharp edges
is also observed.
The fluorescence
spectrum
ofU4 +
in the solid state isreported
here for the ’third time. It hadalready
been exhibited inThBr4 - U4 + [14]
and inCs2ZrBr6 - U4, [15].
At 4.2 K with thehalogen lamp
as excitation source two broad features at 5131A
and 6870A
are observed.They correspond
to the radiative transitions between the levels :
li 6-+ 3H4
andlD2 _+ 3H4 respectively.
Those emissionFig.
3. -Absorption
spectra ofThCl4 -
U4 + above andbelow the
phase
transition temperature.lines are
strongly dependent
ontemperature
and the energy of theexciting
radiation. The results of somesite selective excitation
experiments
at 4.2 K aregiven
in tableIV,
with theirassignment.
Dye
laser excitationexperiments
wereperformed
in the
absorption
bands in the visibleregion
in orderto
identify
the linescorresponding respectively
tothe
D2d
andD2 symmetries. Indeed,
the induced green and red fluorescencepermitted
us to correlatethe observed transitions and attribute a definite
symmetry
to theU4 +
from whichthey originate [6].
In the I.R.
region
where no fluorescence wasobserved,
the middle of theabsorption
band wastaken as
corresponding
to theD2d
both in the Q and1T
spectra.
The levels to be taken into account in the I. R. for theD2 symmetry
are thelimiting
lines of thebands in the cr
spectrum
and the middle of the bands in the 1Tspectrum.
The error committedby doing
soshould not be very
large,
at least in thespectra
where the 1T lines are much narrower than those observed in the Qpolarization [5].
3. Calculations and discussion.
The levels were fitted
by
simultaneousdiagonaliza-
tion of the free
ion,fiio,
andcrystal
field HamiltoniansÍlee
+Íl’ ee [16] :
where
Ro
is characterizedby
theparameters
of :- interelectronic
repulsion Fk, k
=2, 4,
6-
spin-orbit coupling t
-
configuration
interaction(x, P and
plus
thepk (k=2,4,6)
andMk (k = 0, 2, 4)
parameters describing respectively
the electrostati-cally
correlatedspin-orbit ( p k) , spin-spin
andspin-
other orbit
(Mk)
interactions.Table IV. - Emission spectrum
of ThCl4 - U4+
at4.2 K.
a) 000, 00,
0 : strong lines in orderof decreasing
inten-sity.
b)
***, **, * : very weak lines in orderof decreasing intensity.
Ílcc corresponds
to thecrystal
field Hamiltonian that includes theparameters
of the same rank k andmultiplicity q
for theD2d
andD2 symmetries.
Thesupplementary parameters
to be added in the case ofthe
D2 symmetry
are taken into account inkcc,
which is considered to be a
perturbation
ofH, ,c,
dueto the low
magnitude
of thecrystal
field modulation.B Bci, B1, Bg
andB , parametrize
the action ofÍlcc
in the
D2d symmetry,
andBo‘, Bo’, B4’, B’O"’
andB4,
parametrize
the main effect offt, cc
in theD2
symme-try,
with theperturbation
to thelowering
of symme-try being
taken into accountby
the additionalD2 parameters B2‘, B2’, B6’ 6’
on which is builtkcc.
4.
Fitting
in theD2d
andD2 symmetries.
4.1.
D2d
SYMMETRY CALCULATIONS. - Thestarting
values for the various
parameters
were taken fromU4 + in ThBr4 [1]
and theBq were
varied till areasonable fit was obtained. Then both the free ion
parameters Fk
andt,
and thecrystal
fieldparameters
Bk
were varied. In a laststep
theconfiguration
interaction
parameters
aand P
were allowed to vary but theparameter
y was fixed at the value found inThBr4 - U4+ [1],
because theposition
of the’So
which
fixes,
with the3Po, the y
value is not known.The
pk
andMk
were left as well at the values inThBr4 - U4+.
An r.m.s. deviation Q of 46
cm- ’was
obtained for 25experimental
levels. Thespectroscopic
parame-ters
corresponding
to this last fit are listed in table V and the observed and calculated energy levelsalong
with the
U4 + eigenvectors
in theD2d symmetry
aregiven
in table VI. This table shows thefluorescing
level of the
singlet 1I6
above the energygap 116 - 3p 1
However the
agreement
between theexperimental
and calculated levels is overall
satisfactory.
4.2.
D2
SYMMETRY CALCULATIONS. - Since the effects of the incommensurate structure which lowers thesymmetry
fromD2d
toD2
arepresumed
to besmall,
weadopted
the sameprocedure
as was usedfor
ThBr4 - U4 + [1]
in order todetermine
theD2 crystal
fieldparameters.
In a firststep,
theD2
levelswere treated as
D2d by fixing
theF kand t parameters
andfitting
those whichar6
common toD2d
andD2 symmetries
to the(D2) IB
levels and to the centersof
gravity
ofr2 - r4 pairs (r5
inD2d .
Then eachexperimental r2 - r4 pair
wasadjuste
toreproduce
the calculated centers of
gravity
so that the variation of theBq parameters
would fitonly
ther2 - r4 splitting.
With theseparameters
as initialvalues,
theSlater
parameters,
thespin-orbit
constantt,
and all of thecrystal-field parameters
were allowed to vary.For 34
levels,
the r. m. s. deviation was56 cm- 1.
The
parameters (Table V)
common toD2d
andD2 symmetries
have close values and the additional parameters are all small in bothsymmetries.
Theresults are
reported
in tables V and VII.5. Discussion.
We used the
crystal
field model tosuccessfully interpret
the energy levels ofU4 +
in the sites ofapproximate D2d
andD2 symmetry
in the incommen-surately
modulated structureof ThCl4.
Both thelarge spin-orbit
andcrystal
field interactions result inan effective
J mixing
in theU4+ eigenvectors.
TheAuzel and Malta’s
parameter [17] :
Table V. -
Spectroscopic
parametersof U41 (in
- - the r.m.s. deviation a is :cm -1)
- Uv :
free
ion- U4+ :
in theborohydrides
inTd
symmetry- U4+
in ThX4 inD2d
andD2 symmetries
where n is the number
of
observed levels and m thenumber
of
parameters varied.in
D2d
symmetry inD2
symmetryTable VI. - Calculated and
experimental
energy levelsof ThCl4 : U4+ in D2d
symmetry with the main compo- nents(in squared %) of
thecorresponding eigenvectors.
Table VI
(continued).
(X) Eigenvectots are
given
with the percentage of each SLJ level.(*) From fluorescence.
Table VII. - Calculated and
experimental
energy levelsof ThCl4 - U4+
inD2
symmetry with the main compo- nents(in squared %) of
thecorresponding eigenvectors.
Table VII
(continued).
Table VII
(continued).
can be introduced to measure the relative
strength
ofthe
crystal
field and table VIII shows their values for differentcompounds. ThCl4 - U4+ ,
asThBr4 - U4 + [1]
is asystem
with arelatively
weakcrystal field,
incomparison
to UBD4 4 [2]
orcs2ucl6 [18],
and this results in afitting
with areasonably
small r.m.s.despite
of the modulation of the matrix and theapproximate symmetry D2d
usedfor
U4 +
inThCl4.
In the onehand,
this weakness of thecrystal
field inThX4 - U4 +
was a favourableelement that
permitted
us to use theparameters
ofThBr4 - U4 + [1]
asstarting
values in view of thesimilarity
of thespectra
ofU4+,
in the tetrachloride and the tetrabromide. But on the otherhand,
instead of
enhancing
thepossible
variations from abromide host to a
chloride,
theparameters
in both hosts showed nosystematic trend,
whereas there is aclear decrease in the values of the free ion
Fk parameters
forU4+,
inborohydrides [2] compared
tothose in the thorium
tetrahalides,
which is due to covalent character of thehydrogen (deuterium)
-uranium bond
compared
to the much more ionichalide-uranium one.
Table VIII. - Values
of
theAuzel parameter for U4+
in
different compounds (in cm -1).
6. Conclusions.
The
optical spectra of ThCl4 - U4 + represent with fi - ThBr4 - U4 + [1]
theonly
cases of anoptically
active ionU4 + ( 5f2 ) fully
studied in anincommensurately
modulated structure. The spec-troscopic
identification of linescorresponding
toU4+,
in sites ofS4 (approximated by D2d)
andD2 symmetries permitted
us to propose aparametric analysis
ofU4 +
in bothsymmetries.
Theresults, comparable
to those obtained forThBr4 - U4 +
donot exhibit a
systematic
trend in the values of theparameters
inpassing
from the bromide to the chloride. This fact could come from therelatively large
deviations from thecrystal
field model for the actinides that would hide this effect when the coordinationpolyhedra
of the actinide are toosimilar.
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