Absorption and emission spectra of U4 + diluted in the incommensurate structure of ThCl4

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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:

Absorption

and emission

_spectra

of

U4

+ diluted

in the incommensurate

structure

of

_ThCl4

C. Khan

_Malek,

J. C.

_Krupa,

P.

_Delamoye

and M. Genet

Laboratoire de

Radiochimie,

Institut de

_Physique

Nucléaire, Université Paris-Sud, 91406

_Orsay

France

(Requ

le 11 avril _1986,

_accept6

le 9

_juin

₁₉₈₆₎

Résumé. - Les

spectres

d’absorption

et de fluorescence de

U4 +

dilué dans des monocristaux de

_ThCl4

ont été mesurés de 4,2 K

_jusqu’à

la

_température

ambiante. Les cristaux de

_03B2-ThCl4

_présentent

une structure incommensurable en dessous de 70 K avec une _perte de la

_{périodicité}

le

_long

de l’axe c. Ceci a _pour

conséquence

une variation de la distance

_{métal-halogène lorsqu’on}

_{passe d’une maille} à l’autre. La

symétrie

du

site de l’ion actinide est ainsi abaissée. Les raies

_{correspondant}

au site de

_symétries

_S4

et

_D2

ont été identifiées par

spectroscopie.

La

_{symétrie S4}

a été ramenée à celle de

_D2d

et une

_analyse

_{paramétrique}

des niveaux

d’énergie

de

U4 +

en

_symétrie

_D2d

et

_D2

est donnée. Pour 25 niveaux dans le site

_{D2d, l’écart}

_quadratique

moyen 03C3 est de 46

cm-1

et

de 56

cm-

_{1 pour}

les 34 niveaux en

_symétrie

_D2.

Les

_{paramètres qui}

interviennent

dans les _{calculs pour} les deux

_symétries

sont

_légèrement

différents.

Abstract. 2014 The

absorption

and fluorescence _spectra of

U4+

diluted in

single

crystals

of

_ThCl4

have been measured at _temperatures

_ranging

from 4.2 K to room _temperature.

_03B2-ThCl4

exhibits on incommensurate

structure below 70 K with a loss of

_periodicity

_along

the c axis. This results in a variation of the distance between the metal and the

_halogen

from one cell to another. The site _symmetry of the actinide ions is then

reduced. The lines

_{corresponding}

to the sites of the

_resulting

_symmetries

_S4

and

_D2

are identified

spectroscopically.

The

_S4

_symmetry is

_{approximated by}

the

_D2d

one and a

_{parametric analysis}

of the _energy

levels of

U4 +

in the

_D2d

and

_D2

_symmetries

is

_reported.

For 25 levels in the

_D2d

site the root mean _square

deviation 03C3 is 46

cm-1

and

for 34 levels in

_{D2, 03C3}

= 56

cm-1.

The _parameters which occur in both

_symmetries

are

_only

_slightly

_changed.

Classification

Physics

Abstracts 78.40 - 78.50

1. Introduction.

There has

_recently

been some interest shown in the

studies of the tetravalent uranium ion

U4 +

_(5f)

in solid state matrices that started with the new

parame-tric

_analysis

of

U4 +

in

_{O-ThBr4 [1]}

_{(U4 +}

at a site of

D2d

or

_{D2 symmetry)}

followed

_by

a

_{reinterpretation}

of the

_spectra

of

U4+

in the

_borohydrides

_[2]

with the

U4 +

ion at a site of

_Td

_symmetry.

The

_study

of

U4 +

in

_{ThCl4 brings}

a new set of

_{spectroscopic}

parameters

for

U4 +

at a site of

_{relatively high}

symmetry

( D, )

which can be

_compared

to those

obtained in the bromide matrix

_ThBr4.

Both matrices offer a similar dodecahedron of coordination for the

U4 +

ion and like

_{ThBr4 [3], ThCl4}

_undergoes

a

second order

_{displacive phase}

transition at a

tempe-rature

_Tc

= 70 k. Below

Tc,

neutron diffraction

experiments

revealed that the

_crystal

structure of

ThCl4

is incommensurate and modulated

_[4],

due to

transverse

_{displacements}

_{of the chloride ions}

perpen-dicular to the c axis of the

_crystals.

These

displace-ments are different in each unit cell and

_they

can be

described as in

_ThBr4

in terms of a local

_phase

_’PI’

where I is the index which identifies the cell

_[5].

The

modulation reduces the site

_symmetry

of the actinide ion that is

_D2d

at room

_temperature

and

_{S4, D2, C2}

at

low

_temperature,

_according

_{the 91 values :} for ’PI

= 0,

the

_symmetry

of the site is

_S4,

for 9 = ±

ir /2,

the

U4 +

sites are described

_by

the

D2

symmetry,

for 0

_{1911 (}

_Ir/2,

the sites are described

_by

the

_C2

symmetry.

There exists a continuous variation of the environ-ment, thus of the

_crystal

field around the

U4 +

ion and

_consequently

_{the energy levels of}

U4 +

are

modulated 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. The

shapes

of these bands

are then characteristic

_{(Fig. 1) -}

_They

are described

by

the model of

_Delamoye

and Currat

_[5]

as a

continuum of lines limited

_by

two

_{sharp edges. They}

correspond

to the sites of

_D2

_symmetry

_{(u band)}

or

S4

and

_D2

_symmetries

for a 1T band. Site selective

excitation of the

U4 +

_absorption

levels

_permitted

us to correlate the

_absorption

and emission lines

_[6]

and in

_particular

to

_identify

transitions of

U4 +

ions at

sites of

_D2

and

_S4

_symmetries.

Therefore we are in a

position

to

_analyse

the

U4 +

_{energy levels in} these

both sites.

Fig. 1.

- U4+

absorption bandshapes

in the incommensu-rate structure of

_ThCl4.

2.

_Experimental

details and

_analysis

of the data.

/3 - ThCl4 single

crystals

were _grown

_by

the

_Bridgman

method

_[7].

The

_crystal

structure is

_tetragonal

_[8]

space group

14,/amd

at room

_temperature

_(/3

form of

_{ThCl4 [9]). ThCl4}

was

_doped

with about o.1 % of

U4 + ,

the

_doping

material

_{being U02}

or

_UCI4.

The

crystals

were cleaved and

_polished

in a

_dry

box

because of their

_{hygroscopicity}

and

_they

were sealed

under a helium

_{atmosphere (300 torrs)}

in silica

tubes. The

_crystals

used for the measurements were

about 15 x 3 x 5

mm3

or smaller.

_They

_presented

a

cleavage

face which was not

_exactly

_{perpendicular}

to

the fourfold axis so

they

can be oriented in order to

take

_polarized

_{spectra a}

and u + ’1r.

Experiments

at

low

_temperature

were

_performed

in an Oxford

Instruments helium gas

circulating

cryostat.

The

_absorption

and emission

_spectra

were

recor-ded at different

_temperatures

between 4.2 K and 300 K in the visible and infrared

_region

(0.3

>m - 2.5

urn)

on a one meter HR 1000 Jobin Yvon

spectrometer.

It is

_equipped

in the

visible

_region

with a 1200 lines/mm

grating

and a

photomultiplier,

and in the infrared with a 600 lines/mm and a PbS detector. The calibration

was made with a _{low pressure mercury}

_lamp

before

and after the

_recording

of the

_spectrum.

Fluores-cence

_spectra

were excited with a

_Sopra

_nitrogen

pumped dye

laser.

The

_surrounding

of the

U4,

ions in the different

symmetries

mentioned before is

non-centrosymme-tric

_{(non-inversion centre),}

so

_strong

zero

_phonon

transitions are

_expected.

All

_spectra

were

_analysed

with a linear

_polarizer

which can be oriented in both

parallel

and

_{perpendicular}

directions relative to the

C4

optical

axis which is

_preserved

below the

_phase

transition

_temperature

in the

_tetragonal

structure of the incommensurate

_phase.

The at and a

_spectra

were checked to be

_identical,

thus

_leaving

us

_only

with electric

_dipole

transition to deal with.

_Though

the

_polarization

is

_expected

to be

_complete

_(7r

or

ofa)) (for

definition see Table

I),

the a + ff were recorded instead _{of the pure} w

_spectra,

which is due to a

_slight

misorientation

_regarding

the

_C4

axis on

the various

_crystals

that were studied.

Table I. -

Electric

_dipole

selection rules in the

_{D2d (a),}

S4 (b)

and

_{D2 (c)}

_symmetries,

notations

_from

Nielson and Koster

_[11].

A J

_{polarization corresponds}

to a spectrum recorded with the electric

field

E

perpendicu-lar to the

_principal

axis

_C4

whereas a 11: _spectrum

cor-responds

to E

_parallel

to this axis.

Some 75 % of the lines can be seen with the or

polarization

and the lines observed with a ir

polariza-tion are

_generally

weaker than those with a

o-polarization

(Fig. 2).

This trend has also been seen on

_{ThSiO, - U4,}

[10]

where the

U, 4

ion is at a

site of

_D2d

symmetry

as well. The lines in

broader than those with the

_polarization

m. This has

been

_explained

_by

the

_Delamoye

and Currat’s

analy-sis

_[5]

of the line

_shapes

recorded in

Ø-ThBr4 -

U4+.

The width of the lines in

ThCl4 -

U4 +

are broader overall than those in

ThSi04 -

U4 +

which offers a

_{regular D2d}

site for

U4 +

at low

_temperature.

Fig.

2. -

Absorption

spectra of

_{ThCl4 -}

U4 + _{in the}

visible

_(a)

and IR

_(b).

Although

in the modulated structure

_{of B-ThCI4}

the

_symmetry

of the actinide site is lowered

S4 -

C2 -

_{D2) ,}

we shall us the

_{D2d designation}

in

the

labelling

of the energy levels. It can be noted that the electric

_dipole

selection rules for the

D2d

and

_{S4 (subgroup}

of

_D2d)

groups

[11]

only

differ for the transitions

_{(Table I) :}

that are forbidden in the

_D2d

_symmetry

and allowed with a 1T

_polarization

_{in the}

_S4

_symmetry

as

_{r 1 -+ T2}

transitions.

The other selection rules are the same.

In the

_S4

_symmetry

the loss of

_symmetry

elements is

_responsible

for the introduction of

_imaginary

terms in the

_crystal

field

hamiltonian,

Im

_{B 4}

and Im

_B4,

whereas all the

_crystal

field

_parameters

are

real in the

_D2d

symmetry

(Table II). Interpreting

the data in the

_D2d

_symmetry

instead of in the

_S4

symmetry

results in

neglecting

these

_imaginary

terms

in the even rank

crystal

field

_components

that are

used _{for the calculations of the energy levels of the}

optically

active ions. Nevertheless the use of the

D2,d

symmetry

in

_place

of the

_S4

_symmetry

should

give

us the main features of the

_spectra.

Esterowitz

et al.

_[12]

had

_already

noticed with

Pr3 +

_doped

in

LiYF4

at a site of

S4

_symmetry

that the use of the

_D2d

selection rules was a

_good

_{approximation}

in

identi-fying

energy levels. Hence the

D2d

symmetry

will be considered in our case. The

_crystal

field

_eigenstates

carry the

r,

through

r5

irreducible

representations

associated with the

_D2d

site

_symmetry.

_Only

the

_F5

representation

is

_doubly

_degenerate,

the

_remaining

ri

to

_T4

_being

non

_degenerate.

The

_lowering

of the

symmetry

from

_D2d

to

_D2

lifts the

_degeneracy

of the

T5

levels of energy. Thus there are

_only

_singlets

carrying

the

_T

₁ to

_F4

_point

_group

_{representations}

associated with the

_D2

_symmetry.

_The correspon-dance between the sets of

_{representations}

are in the

D2d’ S4

and

_D2

_symmetries

shown in table III. The selection rules for the electric

_dipole

transitions for the

_D2d

and

_D2

_symmetries

are

_given

in the table I.

Table II. -

Crystal field

parameters

of

even rank in

the

_{D2d (a), S4 (b)}

and

_{D2 (c)}

_symmetries.

Table III. -

Correspondence

between the sets

_of

repre-sentations in the

_{D2d, S4}

and

_D2

_symmetries

_[11].

Following

the

_study

of

U4 +

in

_{(3 - ThBr4}

on which

the

_crystal

field transitions. At 4.2 K the a lines

correspond

to transitions from the

_{( D2d)}

_T4

ground

state to the

_doubly

_{degenerate (Dm)}

_F5

levels of energy while the lines recorded with

the

7T

polarization correspond

to transitions to the

DZd T1

1 non

degenerate

levels of energy.

In the most intense

_peaks

a contribution of

U4 +

ions at a site of

_D2d

_symmetry

is

_expected.

Indeed the

_intensity

of the electric

_dipole

transitions in the same

_{configuration}

and

_regardless

of the site

symmetry

of the

_ion,

is

_proportional

to matrix elements

_involving

the

_crystal

field

_parameters

of

odd

rank

_{( B *k q )2.}

The

_D2

_symmetry

is close to the

_D2d

_symmetry

at

low

_temperature

so the additional

_parameters

B 4 *5" B4*7’

and

_B,*7’

to be added to the odd rank

parameters

for the

_D2d

_symmetry

_{(B *3 B *5}

and

B2’)

can be

_supposed

to be weak. Therefore the

intensity

of the lines

_{corresponding}

to the

U4,

ions in the

_D2d

and

_D2

_symmetries

will follow the same

trend.

There are more lines than are

_{theoretically}

expec-ted from a

_F4

_ground

_state, even if some additional

lines are assumed to come from

U4,

ions at sites of

D2

and

_C2

_symmetries

alone

_according

to the selec-tion rules. Some lines are vibronic in

_origin

and

_they

are found on the blue side of the electronic lines at

4.2 K. When excited

_{they give}

the same fluorescence

spectra

as the electronic lines

_they

are associated

with. Some other lines are

_probably

due to an

impurity

or to

U3 + .

_{In /3 -ThBr4 - U4 ’,}

M. C. D.

experiments

[13]

showed an

_{anomalously high 9}

value for certain lines that are attributed to

impuri-ties.

At

_higher

_temperature

transitions from a

low-lying

level at 55

cm-1

1

are

_readily

_apparent.

This level has also been seen in fluorescence

_experiments

[6].

It can be

_interpreted

as a

_D2

level

_arising

from the

_splitting

_{of the first excited Stark level} represen-tated

_by

_T5

in the

_D2d

_symmetry.

The

_shape

of the bands in

_{J3 - ThCl4 -}

U4 +

is modified when the

temperature

is increased

_{(Fig. 3).}

In

_particular

the

sharp edges

corresponding

to transitions of the

W ’

ions at sites of

_D2

_symmetry

decrease in

intensity.

Those

_{edge singularities finally disappear}

and the

_D2d

line is left alone at 70 K. With

_increasing

concentration of

U4 +

a decrease in the

_{sharp edges}

is also observed.

The fluorescence

_spectrum

of

U4 +

in the solid

state is

_reported

here for the ’third time. It had

already

been exhibited in

_{ThBr4 -}

U4 +

_[14]

and in

Cs2ZrBr6 -

U4,

[15].

At 4.2 K with the

_halogen

lamp

as excitation source two broad features at

5131

A

and 6870

A

are observed.

_{They correspond}

to the radiative transitions between the levels :

li 6-+

3H4

and

_{lD2 _+ 3H4}

_{respectively.}

Those emission

Fig.

3. -

Absorption

spectra of

_{ThCl4 -}

U4 + above and below the

_phase

transition _temperature.

lines are

_{strongly dependent}

on

_temperature

and the

energy of the

exciting

radiation. The results of some

site selective excitation

_experiments

at 4.2 K are

given

in table

IV,

with their

_assignment.

Dye

laser excitation

_experiments

were

_performed

in the

_absorption

bands in the visible

_region

in order

to

_identify

the lines

_{corresponding respectively}

to

the

_D2d

and

_D2

_symmetries.

_Indeed,

the induced green and red fluorescence

permitted

us to correlate the observed transitions and attribute a definite

symmetry

to the

U4 +

from which

_they

_originate

_[6].

In the I.R.

_region

where no fluorescence was

observed,

the middle of the

_absorption

band was

taken as

_{corresponding}

to the

_D2d

both in the Q and

1T

_spectra.

The levels to be taken into account in the I. R. for the

_D2

_symmetry

are the

_limiting

lines of the bands in the cr

_spectrum

and the middle of the bands

in the 1T

_spectrum.

The error committed

_{by doing}

so should not _{be very}

_large,

at least in the

_spectra

where the 1T lines are much narrower than those observed in the Q

_polarization

[5].

3. Calculations and discussion.

The levels were fitted

_by

simultaneous

diagonaliza-tion of the free

_ion,fiio,

and

_crystal

field Hamiltonians

Ílee

+

Íl’ ee

_{[16] :}

where

_Ro

is characterized

_by

the

_parameters

of :

-

interelectronic

_repulsion

_{Fk, k}

=

2, 4,

6

-

spin-orbit coupling t

-

configuration

interaction

_{(x, P}

_and

plus

the

_{pk (k=2,4,6)}

and

_{Mk (k = 0, 2, 4)}

parameters

describing respectively

the

Table IV. - Emission

spectrum

of ThCl4 -

U4+

at 4.2 K.

a)

000, 00,

0 : _{strong lines} in order

_of

_decreasing

inten-sity.

b)

***, **, * : _very weak lines in order

_of

_decreasing

intensity.

Ílcc

corresponds

to the

_crystal

field Hamiltonian

that includes the

_parameters

of the same rank k and

multiplicity q

for the

_D2d

and

_{D2 symmetries.}

The

supplementary

parameters

to be added in the case of

the

_D2

_symmetry

are taken into account in

_kcc,

which is considered to be a

perturbation

of

H, ,c,

due to the low

_magnitude

of the

_crystal

field modulation.

B

Bci, B1, Bg

and

_{B ,}

_parametrize

the action of

Ílcc

in the

_D2d

_symmetry,

and

_{Bo‘, Bo’, B4’, B’O"’}

and

_B4,

parametrize

the main effect of

_{ft, cc}

in the

_D2

symme-try,

with the

_perturbation

to the

_lowering

_of

symme-try

being

taken into account

_by

the additional

D2

parameters

B2‘, B2’, B6’ 6’

on which is built

kcc.

4.

_Fitting

in the

_D2d

and

_{D2 symmetries.}

4.1.

_D2d

SYMMETRY CALCULATIONS. - The

_starting

values for the various

_parameters

were taken from

U4 +

in

ThBr4 [1]

and the

_{Bq were}

varied till a

reasonable fit was obtained. Then both the free ion

parameters Fk

and

_t,

and the

_crystal

field

_parameters

Bk

were varied. In a last

_step

the

configuration

interaction

_parameters

a

and P

were allowed to _vary

but the

_parameter

y was fixed at the value found in

ThBr4 -

U4+

_[1],

because the

_position

of the

_’So

which

_fixes,

with the

_3Po,

_{the y}

value is not known. The

pk

and

Mk

were left as well at the values in

ThBr4 -

U4+.

An r.m.s. deviation Q of 46

cm- ’was

obtained for

25

_experimental

levels. The

_{spectroscopic}

parame-ters

_{corresponding}

to this last fit are listed in table V

and the observed and calculated energy levels

along

with the

U4 +

_eigenvectors

in the

_D2d

symmetry

are

given

in table VI. This table shows the

_fluorescing

level of the

_singlet

_1I6

_{above the energy}

_{gap 116 - 3p 1}

However the

_agreement

between the

_experimental

and calculated levels is overall

_{satisfactory.}

4.2.

_D2

SYMMETRY CALCULATIONS. -

Since the

effects of the incommensurate structure which lowers the

_symmetry

from

_D2d

to

_D2

are

_presumed

to be

small,

we

_adopted

the same

_procedure

as was used

for

_{ThBr4 -}

U4 +

_[1]

in order to

determine

the

_D2

crystal

field

_parameters.

In a first

_step,

the

_D2

levels were treated as

_D2d

_by

_fixing

the

_{F kand t}

_parameters

and

_fitting

those which

ar6 common

to

_D2d

and

_D2

symmetries

to the

_(D2)

_IB

levels and to the centers

of

_gravity

of

_{r2 - r4}

_pairs

_(r5

in

_{D2d .}

Then each

experimental r2 - r4

pair

was

adjuste

to

_reproduce

the calculated centers of

_gravity

so that the variation

of the

_Bq

_parameters

would fit

_only

the

_{r2 - r4}

splitting.

With these

_parameters

as initial

_values,

the Slater

_parameters,

the

_spin-orbit

constant

_t,

and all of the

_{crystal-field}

_parameters

were allowed to _vary.

For 34

_levels,

the r. m. s. deviation was 56 cm- 1.

The

_parameters

_{(Table V)}

common to

_D2d

and

_D2

symmetries

have close values and the additional parameters are all small in both

_symmetries.

The results are

_reported

in tables V and VII.

5. Discussion.

We used the

_crystal

field model to

_successfully

interpret

the energy levels of

U4 +

in the sites of

approximate

D2d

and

_D2

_symmetry

in the

incommen-surately

modulated structure

_of

_ThCl4.

Both the

large spin-orbit

and

_crystal

field interactions result in

Table V. -

Spectroscopic

parameters

of

U41

_(in

- - _the

r.m.s. deviation a is :

cm -1)

- Uv :

_free

ion

- U4+ :

in the

_borohydrides

in

_Td

symmetry

- U4+

in ThX4 in

_D2d

and

_D2

_symmetries

where n is the number

of

observed levels and m the

number

_of

_parameters varied.

Table VI. -

Table VI

_(continued).

(X) Eigenvectots are

given

with the _percentage of each SLJ level.

(*) From fluorescence.

Table VII. -

Calculated and

_experimental

energy levels

of

ThCl4 -

U4+

in

_D2

_symmetry with the _main

Table VII

_(continued).

can be introduced to measure the relative

_strength

of

the

_crystal

field and table VIII shows their values for different

_compounds.

_{ThCl4 -}

U4+ ,

as

ThBr4 -

U4 +

_[1]

is a

_system

with a

_relatively

weak

crystal

field,

in

_comparison

to U

_{BD4 4}

_[2]

or

cs2ucl6 [18],

and this results in a

_fitting

with a

reasonably

small r.m.s.

_despite

of the modulation of

the matrix and the

_approximate

_symmetry

_D2d

used for

U4 +

in

_ThCl4.

In the one

_hand,

this weakness of the

_crystal

field in

_{ThX4 -}

U4 +

was a favourable

element that

_permitted

us to use the

_parameters

of

ThBr4 -

U4 +

[1]

as

_starting

values in view of the

similarity

of the

_spectra

of

U4+,

in the tetrachloride and the tetrabromide. But on the other

_hand,

instead of

_enhancing

the

_possible

variations from a bromide host to a

chloride,

the

_parameters

in both

hosts showed no

_systematic

_trend,

whereas there is a

clear decrease in the values of the free ion

Fk

parameters

for

U4+,

in

_borohydrides

_[2]

_compared

to

those in the thorium

_{tetrahalides,}

which is due to

covalent character of the

_{hydrogen (deuterium)}

-uranium bond

_compared

to the much more ionic halide-uranium one.

Table VIII. -

Values

_of

the

_{Auzel parameter for}

U4+

in

_different

_{compounds (in}

_{cm -1).}

6. Conclusions.

The

_optical

_spectra

_{of ThCl4 - U4 +}

_represent

with fi -

ThBr4 -

U4 +

_[1]

the

_only

cases of an

optically

active ion

U4 +

_{( 5f2 )}

_fully

studied in an

incommensurately

modulated structure. _The

spec-troscopic

identification of lines

_{corresponding}

to

U4+,

in sites of

_S4

_{(approximated}

_by

_D2d)

and

_D2

symmetries

permitted

us to _propose a

_parametric

analysis

of

U4 +

in both

_symmetries.

The

_results,

comparable

to those obtained for

_{ThBr4 -}

U4 +

do

not exhibit a

_systematic

trend in the values of the

parameters

in

_passing

from the bromide to the

chloride. This fact could come from the

_relatively

large

deviations from the

_crystal

field model for the

actinides that would hide this effect when the coordination

_polyhedra

of the actinide are too

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