Analysis of the absorption and emission spectra of U4+ in α-ThBr 4

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Submitted on 1 Jan 1988

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Analysis of the absorption and emission spectra of U4+

in α-ThBr 4

E. Simoni, S. Hubert, M. Genet

To cite this version:

1425

Analysis

of the

_absorption

and

emission

_spectra

of

U4+ in 03B1-ThBr4

E.

Simoni,

S. Hubert and M. Genet

Institut de

_Physique

Nucléaire,

Laboratoire de _Radiochimie, 91406

_Orsay,

France

(Requ

le 3 dgcembre _1987,

_accepté

le 13 avril

₁₉₈₈₎

Résumé. 2014 La forme basse

température 03B1-ThBr4

a une structure de _type scheelite

_I41/a

dans

_laquelle

l’uranium tétravalent se substitue au thorium

_(symétrie

de site

_S4).

En faisant

_{l’hypothèse}

_{que l’état} fondamental est

₀₃₉₃₄

comme dans la forme

_03B2-ThBr4,

les _spectres

_{d’absorption}

_polarisés

à 4,2 K montrent _que

D2d

est une bonne

_{approximation.}

Cette matrice a la

_{particularité}

d’exalter de très nombreuses fluorescences de

U4+,

ce

_qui

_permet d’indexer _quatre niveaux Stark de l’état fondamental

3H4 : 03935

à 110

cm-1,

03931 à

473

_cm-1,

_{03931 à}

623

cm-1

et

_{03935 à}

830

cm-1.

30 niveaux ont été indexés et les

_paramètres

de

_champ

cristallins de

U4+

(5f2)

ont été calculés dans

_{l’approximation D2d:}

_B20

= - 382,

B40

= - 3

262,

B44

=

1 734,

B60

= - 851 et

_B64

= - 1828

cm-1.

Il est intéressant de montrer

_{qu’une légère}

distorsion dans la

structure scheelite

_03B1-ThBr4

_comparée

avec la structure zircone

_03B2-ThBr4

induit un

_changement

_important

sur

les

_paramètres

de

champ

cristallin.

Abstract. 2014 The low _temperature form

_03B1-ThBr4

has a scheelite structure

_I41/a

in which the tetravalent uranium

_occupies

the thorium site which is

_{S4. Assuming}

that the

_ground

state remains

₀₃₉₃₄

as in the

03B2-ThBr4

form, the

_{polarized absorption}

_spectrum at 4.2 K shows that

_D2d

is a

_{good approximation.}

A

peculiarity

of this host is the exaltation of very numerous fluorescences of

U4+

which

_permit

to

_assign

four Stark levels of the

_ground

state

3H4 :

₀₃₉₃₅

at 110

_cm-1,

₀₃₉₃₁

at 473

_cm-1,

₀₃₉₃₁

at 623

cm-1

and

₀₃₉₃₅

at

830

cm-1.

30 levels have been

_assigned

and the

_crystal

field _parameters

of U4+

_(5f2)

have been calculated in the

D2d

approximation :

B20 = -

382,

B40 = -

3 262,

B44 = - 1734,

B60

= - 851 and

_{B64 = - 1828}

cm-1.

It is

interesting

to note that a small distortion in the scheelite structure of the

_03B1-ThBr4

_compared

with the zircon

structure

_03B2-ThBr4

induces

_important

_changes

in the

_crystal

field _parameters.

J.

_Phys.

France 49

₍₁₉₈₈₎

1425-1434 AOÚT 1988,

Classification

Physics

Abstracts 71.70C - 78.60

t

1. Introduction.

Thorium tetrahalides

_(X

=

Cl,

Br)

_possess numerous

properties

which make them attractive candidates

for

chemical,

physical

and

_{spectroscopic}

studies.

Thorium tetrabromide and thorium tetrachloride

have two

_polymorphic

forms

_[1]

with a transform-ation

_temperature

of 426 °C for the former and

405 °C for the latter

_[2-3].

The

_high

_temperature

form

_{J3 - ThBr4 isomorphous}

to

_{J3 - ThCl4}

is

charac-terized

_by

the

_tetragonal

zircon structure

_(space

group

_14,/amd)

[4]

and

undergoes

a

_phase

transition at 90 K

_[5]

with an incommensurate modulated

struc-ture below this

_temperature

_[6].

The low

tempera-ture form

_a-ThBr4

_isomorphous

to

_a-ThCl4

has the

scheelite structure

_(space

_group

_141/a)

_[1].

From

_previous

_{investigations}

of the solid state chemical

_properties

of structural

_type

_[7],

thorium

has a

_D2d

site

_symmetry

in the

/3-form

of zircon

structure, and

_S4

site

_symmetry

in the a-form of

scheelite structure

_type.

_Large

_single

_crystals

of _pure

and

_{doped Ï3-ThBr4}

_{may be grown}

_{easily by}

the

Bridgman

method

_[8],

while the a-form is

_relatively

difficult to _prepare as a

_single

_crystal

since the final form obtained

_by

_{this way is the}

_¡3-form.

Additional studies of _pure

_{8 -TbBr4}

have included _{X ray}

_[9],

neutron diffraction

_[6],

Raman

_{investigations [5]}

and

complete analysis

of

_crystal

field

_parameters

on

U4 +,

Pa 4+,

pr3 +

diluted in the incommensurate

modulated

_phase

_[10,

_11,

_12]

of

_J3-ThBr4.

On the other hand

_only

structural studies have been

_reported

on the a-form of thorium tetrahalides

[1,

2, 3,

7].

Recently

nuclear

_quadrupole

resonance

investigations

of Br-

_[13]

have shown that

a-ThBr4 keeps

the same structure from room

tempera-ture down to 10 K and Raman

_scattering

_[14]

has

confirmed this result.

Some

_{spectroscopic properties}

of trivalent rare

earth ions in

_crystals

with a scheelite structure have been

_{already reported}

and the main interest in

crystals

like

_LiYF4,

_PbMo04

and

_{CaW04 [15, 16]}

lies in their

_properties

for laser materials. However these host materials are not the best candidates to

study

the

_{spectroscopic properties}

of tetravalent

actinides in the scheelite structure.

_Only

tetravalent

neptunium

has been studied in

_PbMo04,

and for the first time fluorescence has been observed

_[17,

_18].

Like scheelite structure

_materials,

_{these pure}

_crystals

exhibit

_good

fluorescence when excited

_by

U.V.

radiation in the blue for

_13-ThBr4

_{(zircon structure)}

[19]

and red for

_{a -ThBr4}

_{(scheelite structure) [20].}

Thus

_{a -ThBr4}

was chosen as a host material for

spectroscopic

properties

of tetravalent uranium be-cause it has the same structure and similar chemical and

_optical

behaviour as the better known

scheelite-structure

_tungstates

and

_molybdates.

On the other hand it is

_interesting

to _{compare the}

_{spectroscopic}

properties

of

U4 I

between

_8-ThBr4

which has an

incommensurate structure below 90 K with multisite

continuum

_going

from

_D2d

to

_{D2 [10]}

and

_a-ThBr4

which has

_only

one

_S4

site.

In the

_present

_{study, optical}

_absorption

and

emis-sion,

Zeeman effect studies have been

_performed.

Some fluorescence of

U4 +

has

_already

been

_reported

in 8 -ThBr4

[21], f3-ThCI4 [22]

and

_{Cs2ZrBr6 [23],}

however in

_{a -ThBr4,}

numerous _{and very}

_strong

fluorescence lines from

U4 +

in the

visible,

as well as in the near infrared and infrared have been observed for the first time. The data have allowed reasonable energy level

assignments.

Parameters

describing

spin

orbit

_coupling

and

_crystal

field interactions were

adjusted using

a least _{squares minimization}

pro-cedure in the

_{D2d point}

_group

_{approximation.}

The

calculations were made

_using

a full

_{diagonalization}

of the Hamiltonian matrix in the

_(LSJJz)

represen-tation. Good

_agreement

was obtained between the

observed and the calculated

_energies

in

_spite

of the

D2d

approximation,

as has

_already

been

_reported

in several papers

concerning

the scheelite structure materials

_[15,

24,

25].

2.

_{Experimental procedure.}

The a

phase

of

doped

ThBr4

was

_synthetized

_by

two

techniques.

Polycrystalline

materials could be

_easily

obtained

_by

_heating

U4+

_doped

_{f3-ThBr4 single}

crystals

or

_powder

at 400 °C

_during

one _{week. In any} cases the final form was

_{polycrystalline.}

On the other

hand,

only

one _{green colored}

_single

_crystal

of

a -ThBr4 doped

with about 25 ppm of

U4 +

has been

obtained

_by

the

_Bridgman

method with the same

conditions as for the

_/3-form

[8].

This

_{single crystal}

was

_{large enough}

to obtain linear

_polarization

of the

absorption

spectra

in the directions

_{perpendicular}

and

_parallel

to the

_crystal

axes.

All the

_samples

used for

_{spectroscopic}

studies were

analysed using

X _ray

_powder

diffraction. In all the cases the X _ray

_patterns

showed a _pure «

phase.

Samples

were

_hygroscopic

and less stable than the

phase

and were handled in an _argon

_atmosphere

glove

box and enclosed in a silica tube under helium pressure.

The

_absorption

and emission

_spectra

in the visible and infrared were measured with the

_crystal

excited

by

the full

_light

emission

_{produced by}

a 100 W iodine

lamp

and

_using

a

_high

resolution JOBIN YVON HR 1000

_spectrometer

_(with

a

_{reciprocal dispersion}

of about 8

_A/mm)

at different

_temperatures

_ranging

from 4.2 K _up to 300 K. For

_detection,

a Hamamatsu R374

_{photomultiplier}

was used in the visible

_region

and a PbS

_photodiode

was used in the infrared

region.

In the latter case the

_optical

source was

chopped

and the detector

_signal

measured

_by

a PAR Model 186 A lock in

_amplifier.

The

_wavelength

readouts of the

_spectrometer

were calibrated

_using

various lines of the first and second order of

_Hg

emission lines.

Samples

were introduced in a variable

_temperature

optical

Dewar

_{(Oxford Instrument) operating}

be-tween 4.2 and 295 K. A

_temperature

controller was used to set the

_temperature.

Laser excited fluorescence measurements were

performed using

a

_{pulsed Sopra}

tunable

_dye

laser

pumped

with a

_{Sopra nitrogen}

laser.

Using

a

_{superconducting}

_magnet,

Zeeman

split-tings

were recorded in the visible at 4.2 K with the

crystal

in a

_magnetic

field of 6 T.

3.

_Theory

and

_symmetry

considerations.

The

Th4 +

ions in

_«-ThBr4

which are substituted for

by U4 +

have

_{S4 point}

symmetry

as in the well known scheelite structure.

An

_analysis

of the

_crystal

field

_parameters

for the rare earth ions in scheelite calculated in

_D2d

and’

S4

symmetry

[15, 16]

shows that

_{taking S4}

_symmetry

as

_D2d

is a reasonable

_{approximation.}

For this new host matrix we will show

_by

selection rules in both

symmetries

that

_D2d

is a

_good

_{approximation.}

In the

D2d

symmetry

all the

_crystal

field

_parameters

are

real,

whereas in the

_S4

symmetry,

the loss of

symmetry

elements results in the introduction of

imaginary

terms in the Hamiltonian :

_imB1

and

i mB4.

Assuming D2d

for

_S4

_symmetry

amounts to

_neglect

of both the

_{imaginary crystal}

field

_parameters.

The energy levels will be fitted

by

simultaneous

diagonali-zation of the free ion and

_crystal

field Hamiltonian

HFI’ H CF

as described below.

1427

The

Fk

and

_’Sf

_represent

_respectively

the radial

parts

of the electrostatic and

_spin

orbit interaction

between 5f

_electrons,

a,

{3,

y are the

_parameters

associated with the

_two-body

effective

_operators

of

configuration

interaction. The

Mk

and

P k

_parameters

represent

the

_spin-spin

and

_spin

other orbit interac-tions.

The classification of the

_experimental

_energy levels was made

_{by analysing}

the

_polarisation

data.

According

to the transformation

_properties

of the electric

_dipole

_operator,

identical axial and a

polarized

spectra

require

the use of electric

_dipole

selection rules while identical axial and 7T

_polarized

spectra

require

the use of

_{magnetic dipole}

selection

rules. Since axial

_spectra

could not be

_recorded,

we assumed that electric

_dipole

transitions

_predominate

as for

U4 +

in

_/3-ThBr4

_{[21], 13-ThC14 [22]}

and

ThSi04 [27].

In

_examining

the

spectra,

we will consider selec-tion rules for

_{D2d point}

_symmetry

_(S4

is a

_subgroup

of

D2a)

as it has been made in the scheelite structure for the rare earth. If

U4 +

ion is in a

_D2d

_symmetry

_site,

the

_crystal

field states are either

_singlet

_(T1,

r2, F3, F4)

or doublets

_(T5)

and certain transitions would be forbidden as shown in table I. The

corre-sponding

identifications of the

_D2d

levels in

_S4

notation are listed in table II.

Table I. - Electric

dipole

selection rules in

_S4

and

D2d.

Table II.

_{- Identi fication of}

the irreductible

repre-sentations

_of

the

_{D2d point}

_{group in}

_S4

notation.

When the local site

_symmetry

is

_{perturbed by}

a lower

_symmetry

environment such as

_S4,

some of the

forbiddenness should be lifted like

_{F4 ->}

_{T 2}

tran-sition in the

_D2d

_{representation}

_{(t2 -> ri}

transition in

_S4

_{representation).}

4.

_Experimental

results and

_analysis

of the

spectra.

Figures

1 show the

_absorption

and emission

_spectra

of tetravalent uranium in the

_visible,

near infrared and infrared _range at 4.2 K. The u and 7T

polari-zations were obtained for the

_strongest

lines since it was recorded with

_{single crystal doped}

with

_only

25 ppm of

U4 + .

_{Unfortunately}

we did not observe

polarization

on the emission lines.

This is the first time that so _many intense

fluor-escence lines are observed.. We note that in the

a-form,

the lines are

_sharp

_(5-15

cm-1),

this was not

the case in the incommensurate

_phase

of

_f3-ThBr4

(40-80

cm-1

for a

_lines)

_[10].

Nevertheless the

_absorption

_spectra

of

U4 +

in

a -ThBr4

have

_roughly

the same trend as in

f3-ThBr4-

Moreover some

_absorption

lines

_present

a

phonon

sideband structure on the

_high

_{energy side} below 200

cm- 1

which indicates

_strong

_coupling

between the electronic and vibronic states as in the d elements.

If we

_interpret

the

_spectra

under the

_assumption

that

_D2d

is a

_good

_{approximation}

for

_S4

_symmetry

and that the

_3H4

_ground

state is

_F4

as for

U4 +

in

f3-ThBr4

[10], ThCl4 [22], ThSi04 [27],

we

_expect

13 7T zero

_phonon

lines and 19 a zero

_phonon

lines in

the _{range 4 000-25} 000

cm-1

_deriving

from

_r4

T1

and

_{F4 -+ F5}

transitions. We observed 14

_strong

7T lines and 9

_{strong a}

lines in this

_region

and some weak lines the

_polarisation

of which could not be observed. The transition

_{F4 -+}

_{r 2}

forbidden in

D2d

should be allowed in the

_S4

_symmetry

_{(r 2}

-+-T 1 transitions),

which should raise the number of w

predicted

lines to 21.

In the

_region

22 000-19 000

cm- 1

the 7r line at

21 601

cm - 1

and the 2 a lines at 20 034

cm - 1

and 19 529

cm-1

were attributed to

3P2

and

_lI6

compo-nents.

_By

_comparing

the

_spectra

of

U4 +

in

a-ThBr4

with those of

U4 +

in

f3

-ThBr4,

3P1

is the

_only

multiplet

well isolated at about 17 000

cm - 1.

In the

or

_spectrum,

we observed the

3P1

(r s )

level at

17 083

cm-1

while in the 7T

_spectrum

the

3P1

(r2 )

is

_missing.

This

_missing

7T transition as others

(r 4 -+- T2)

can be

_{explained by considering}

the

U4 +

to be in

_S4

_symmetry

site which is

_nearly

D2d.

The observed emission line at 16 907

cm- 1

has been attributed to the

_3P1

_{(F2) _}

_3H4

_{(r s )}

transition since when the

_temperature

is raised

_{(80 K),}

an

absorption

line appears at the same _{energy. At the}

same

_time,

some additional _{lines appear and}

particu-larly

another one near the

_3P1

_{(T5 )}

state at

16 973

cm- 1.

This hot line at 16 973

cm- 1

was

as-signed

to the

_3H4

_{(F5) _}

_3P1

_{(r s)}

transition. As a

matter of

_fact,

the observation of a

_large

number of hot lines in the visible and infrared from 40 K indicates that the first excited

_ground

state sublevel

at 110

cm- 1

could be

_{T5 like}

as

U4 +

_{in f3-ThBr4}

_[10],

Fig.

1.

_{- Absorption}

and emission spectra of

_{a -ThBr4 : U4+}

in the visible and infrared

_region

at 4.2 K.

The Zeeman

_spectra

_proved

to be

_particularly

useful in

_identifying

the

_{F5 lines. Using}

the

_magnetic

field of 6 T at 4.2

_K,

Zeeman

_experiments

indicated that the

_absorption

line at 17 083

cm- 1

is

_split

which confirms the

_assignment

of

_3p,

_(F5)-

Moreover

_by

exciting

the

_crystal

with U.V. radiation from a

nitrogen

pumped

laser,

Zeeman

_splitting

was also

observed at 4.2 K in the emission

_spectrum.

In

particular

the emission line at 16 907

cm- 1

as well as

two others at 19 329 and 14 219

cm-1

were

_clearly

split

into two

_components.

These lines

_correspond,

as we assumed

_above,

to transitions between

_singlet

states and the

_doubly

_degenerate

state

_’H4

_(r5)

at

110 cm-1.

In the 15 000-16 000

_{cm- 1 region,}

3

_strong

7r lines

1429

multiplets.

The

_strong

7T lines at

_{14 601, 14 573,}

14 529 and 14 506

cm - 1

were not

_assigned

for the

fitting.

For

U4 +

in

_I3-ThBr4’

the

_3Po,

_1D2

and

_1G4

are close

_together.

_By

_comparison,

the

_position

of the

3po

(T 1 )

was established in the 7T

_spectrum

at

14 389

_cm-1,

while the

_{strong o,-}

line at 14 427

cm-1

was attributed to

_ID2

_{(Ts ).}

A

_peculiarity

of the

_absorption

and emission

spectra

of

U4 +

in this new host material is that most

of the

_absorption

lines like

_lI6

_(Ts)

at 19 529

_cm-1,

the

_3p,

_{(rs )}

at 17 083

_cm-1,

the

_ID2

_{(T1 )}

at

14 389

_{cm- ,}

the

_H6

_(F5)

at 10 647

_{cm ,}

the

3F3

(r5)

at 8 558

cm-1

and the

_3Hs

_{(T 1 )}

at 6 076

cm-1

are followed on the low _{energy side}

_by

a

set of

_2,

3 or 4

_strong

fluorescent lines. When the

crystal

is excited with the

_{Sopra nitrogen}

_laser,

the

lI6

(T 5 ),

the

_3p,

_{(T 5 )}

and the

_3Po

_{(T 1 )}

fluorescence

as

_well,

but with a weak

_intensity.

In each group of

emission,

the energy difference between the

fluores-cent lines is the same as indicated in table III :

_363,

Table III. - Observed

fluorescences of

U4 +

in

a -ThBr4 at

4.2 K towards

_3H4

_ground

state Stark levels.

(a)

Zeeman

_splitting

observed.

(b)

Lines for which an

_absorption

line rises when the

temperature increases.

513 and 720 cm-

1.

When the

_temperature

is raised

from 4.2 K to 100

_K,

the

_highest

_{energy emission}

lines from each _group decrease faster in

_intensity

than the other ones, and

absorption

lines rise up at

the same _{energy like for}

3P1

_(T2).

All these

fluores-cent lines come from different

_singlet

excited levels near those labelled

_above,

to reach 4

_ground

state

Stark levels

_including

the low

_lying

one

3H4

(r s)

at

110

cm-1.

Then we could deduce three other Stark

levels to be at

_473,

623 and 830

cm-1.

No Zeeman

splitting

was observed for the emission lines

_reaching

these Stark levels. The fluorescences from each group

reaching

the

_3H4

state at 830

cm-1

could not

be tested

_clearly

since

_they

are

_generally

broad and weaker. However a

_slight

_broadening

with

_magnetic

field has been observed on one of them.

Selective excitation

_experiments

with a tunable

dye

laser _{showed that each group} of fluorescence

lines comes from forbidden

_singlet

excited levels very near the allowed lines observed in the

absorp-tion

_spectrum.

From

_D2d

selection

rules,

and the energy

position

of the hot lines observable at 80

K,

we could reach forbidden states and

_assign

_lI6

(r4)

at 19 439

_cm-1,

_3p,

_(F2)

at 17 017

_cm-1,

ID2

(T4)

at 14 355

_cm-1,

_iD2

_(F3)

at 14 329

cm-1,

3H6

(T4 )

at 10 504

_cm-1,

_3F3

_(r4)

at 8 170

cm-1

and

3H5

(T4 )

at 5 765

_{cm-1. Figure}

2

represents

the

v

partial

energy level

diagram

for fluorescent

tran-sitions

_reaching

the

_3H4

levels in

_D2d

_{representation.}

The

_3H4

_ground

state Stark levels consist of 2

_T1

+

T 2

+

_{T 3}

+

_{T 4}

+ 2

_{T 5}

for

_{D2d point}

_group

_symmetry.

In the case where the emission lines are

_coming

from

r4

states

_(group

_{1, 3, 5, 6,}

₇₎

the observation of 3 or 4 emissions indicates that these lines should

corre-spond

to

_{F4 --+}

_r1

₁ and

_{T 4 ->}

_rs

transitions. In the case where the emission lines are

_coming

from

F2

or

_{r 3}

states,

only

two fluorescent lines are observed and

_correspond

to

_{F2 --I’ rs}

or

_{r3 -> F5}

transitions. However in this case, the allowed tran-sitions

_{r 2 ->}

_{r 3}

or

_{r 3 -> T 2}

are

_missing.

From these

considerations we attributed the

_{r s}

_symmetry

to the

3H4

Stark levels determined at 110 and 830

cm-1

and

Table IV. - Observed

energy levels caracteristics

for a -ThBr4 :

u4+.

(*) Asterics indicate energy levels deduce.d from the

fluor-escence lines or from hot lines.

the

_T1

_symmetry

to the

_3H4

Stark levels deduced at

473 and 623

cm- 1

Results of fluorescence and

_absorption

measure-ments are summarized in table IV. The table

pre-sents the line

_frequency

in vacuo, the

polarization,

D2d

and

_S4

_symmetry

_assignment.

5. Discussion.

The levels were fitted

_by

simultaneous

diagonali-zation of the free ion

_Hn

and

_crystal

field

_{H CF}

Hamiltonians

_describing

_{the energy levels of}

U4 +

in

D2d

symmetry.

Fitting

the

_experimental

levels with

the

_starting

_parameters

obtained for

_{f3 - ThBr 4 :}

U4 +

_[10]

_gives

a _very

_large

rms deviation for

a -ThBr4. Using

the

_{f3-ThBr4 :}

U4 +

parameters,

we

diagonalized

the matrices with various values of

B 0 2.

Only

B 0 2

400

cm-1

gave the correct separ-ation between the

_3pl

_F2

and

_F5

levels. With this value for

_Bo,

we tried to fit

_separately

_T1

and

T5

levels. Both fits gave a different set of

_parameters.

While the 11

_{assigned F5}

levels led to an rms

deviation of 62

cm-1,

the

_{11 r 1}

levels were fitted with an rms deviation of 72

_{cm- 1. Finally}

the best fit for the 22 levels

_together

was obtained with the

starting

values for the

_T1

levels. From the calculated energy

levels,

30 transitions could be

assigned

with

good

agreement

between the calculated and measured

_energies.

The

_Bq

were first

varied,

then the

Fk

and £

parameters.

Finally

all the

_parameters

were varied

_{simultaneously.}

For the fit with 30 levels an rms deviation of 77

cm-1

was obtained with

11

_parameters

_varying.

In this fit three levels could

not be well

fitted,

the

_F5

level at 19 529

cm-1

and the two

_ground

state Stark levels

_3H4

at 473 and

623

cm-1

attributed to

_T1

states. Moreover 4

_strong

7T lines in the

_region

of the

3Po

(14 500-14

600

cm-1)

could not be attributed to calculated levels. The calculated _{energy levels and}

_eigen

vectors are

_given

in table V and the final values of the

_parameters

are

given

in table VI.

Compared

with the

_{spectroscopic}

_parameters

of

U4 +

in

_f3-ThBr4

_[10]

and

_{f3-ThCI4 [22],}

the calcu-lated

_parameters

of

U4 +

in

_{a -ThBr4}

are

different,

particularly

Bo

which is smaller and the

_sign

of

Bo

and

_B4

which becomes

_negative.

If the rms

deviation is not as

_good

as in

_f3-ThBr4’

the

_crystal

field

_parameters

are however determined with an

uncertainty

of about 10 %

except

Bo

and

_Bo.

Moreover a decrease in the

Fk parameters

_especially

for

F4

is

observed,

which

_gives

a ratio

F4/F2

smaller

compared

to the values obtained for

_f3-ThBr4.

The Auzel

_parameter

_[28]

_{N v}

can be introduced to

1431

Table V. - Calculated and

experimental

energy levels

of a -ThBr4:

U4 + .

Table V

_(continued).

Table VI. -

Spectroscopic

parameters

for

U4 +

in

_a-ThBr4

in

_comparison

with those

_of

13-ThBr4-(1)

The

Mk

and

P k

values were fixed :

Mo

= 0.99,

M2

= 0.55,

M4

= 0.38,

P 2 = P 4 = p 6

= 500,

y value

_was fixed.

1433

Though

the

_parameters

are _{very different from the}

J3-form,

the

_crystal

field

_strength

has almost the same value for a and

_8-ThBr4

(Tab. VI).

This set of

_parameters

however is not

_completely

satisfactory

because it is

_disturbing

that three intense

transitions

_(the

two

3H4

Stark levels at 473 and 623

cm-1

well determined from several fluorescence

lines and the

_TS

at 19 529

_cm-1)

fit so

_poorly.

Moreover the w line at 21 601

cm-1

has a

_discrepancy

with the calculated level of 234

cm-1

which is much

more than the average

_{discrepancy (39}

cm-1 ).

In

fact this level fits _very well when all the

_T1

levels are

fitted. From the studies of Naik

_[29],

Auzel et al.

_[30]

and Schoenes

_[31],

we can assume that the

_proximity

of the 6d band to the 5f ones could

_provoke

an

interaction between both

_{configurations,}

which is

not taken into account in the Hamiltonian. It could shift the _{energy of the}

_highest

levels

_rl

(19 529

cm-1)

and

_ri

₍₂₁₆₀₁

_cm-1).

_Many

fits were

tried from various

_starting

parameters

and

particu-larly

the one obtained for

_{J3 - ThBr4,}

but

_they

result in poor

agreement

with all the levels. Another reason

for these

_{discrepancies}

can be due to the fact that the site

_symmetry

for

Th 4,

in this matrix is

_S4

and that we made the

_{approximation}

of

_D2d

_symmetry.

The difference obtained in the

_crystal

field

calcu-lation between both matrices a and

_8-ThBr4

indi-cates

_that,

_although

tetravalent uranium ion is

surrounded

_by

the same

_ligands,

the most

_important

difference in the two forms

_namely

the relative

orientation of the coordination

_polyhedron

within the structure, the distances U-Br and the

_angles

Br-U-Br

_{(Tab. VII)}

_play

an

_important

role in the

_crystal

field

_parameters.

Table VII. -

Principal

bond distances

_(A)

and

angles

in the

_{polymorphic forms of ThBr4}

and

J3 - ThCl4

(Ref. [1])

a,

_{J3 - ThBr 4°}

In the

0-form,

the axes of the

_polyhedron

lie in the

(100)

plane

of the unit cell while in the a-structure,

the

_polyhedron

has been rotated

_by

about

45°,

the c axis and the

polyhedron

axis lie somewhat outside

the

₍₁₁₀₎

_planes.

This rotation allows for a more

efficient

_packing

of the Br- atoms in the a-form.

Moreover,

the metal

_ligand

distances are

approxi-mately

all the same for

_{a -ThBr4}

while there exist

two different Th-Br bond distances for

_{13 -ThBr4.}

For

a -ThBr4

all the bond distances Th-Br can be seen as

intermediate between the

_longest

and the shortest

ones in

_f3-ThBr4.

6. Conclusion.

The

_optical

_spectra

of

U4 +

in

_a-ThBr4

have been

analysed

with 30 transitions

_assigned

in the

_D2d

approximation.

Although

both matrices a and

f3 - ThBr 4

look

_similar,

it _appears that the

spectro-scopic

properties

of the uranium tetravalent ion are

quite

different from one matrix to the other. As a

matter of

_fact,

U4 +

ions

_give

_very

_strong

fluor-escences in the visible and infrared

_region

in the a

form while

_only

2 weak fluorescence lines were

observed in the

_J3-form.

On the other hand the

crystal

field

_parameters

obtained in this

_study

are

very different from those calculated in the

{3-form.

Since the same

_ligands

surround the uranium

ion,

it

seems that the

_change

in the structure

_plays

an

important

role for the

_crystal

field even if the site

symmetry

in the a-form which is

_S4

can be

approxi-mated

_{by D2d.}

It is

_interesting

to show that a small

change

in the structure with the same ions

_produces

an

_{important change}

in the

_crystal

field

_parameters

and that this scheelite structure material induces so

many intense fluorescence lines for

U4 +

as the others scheelites do for rare earth ions and

tetrava-lent

_neptunium

_[17].

The fact that the

_highest

_energy levels of

U4 +

do not fit as well as the others could be due to 5f-6d

_{configuration proximity.}

Acknowledgments.

We wish to thank Dr. G. Goodman for

_helpful

discussions and Dr. N. Edelstein for

_providing

us with the matrix elements and least _{squares program.}

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