Local Structure of Cu2+ in the (C2H5NH3)2MCl4:Cu2+ (M : Cd, Mn) Layer Perovskites. Influence of Hydrostatic Pressure in the 0-60 kbar Range*

Zeitschrift für_{Physikalische}Chemie,Bd. 201, S. 151-158 (1997) ©by R. _{Oldenbourg Verlag,} München 1997

Local Structure of

Cu2+

in the

_{(C2H5NH3)2MCl4:Cu2+ (M}

=

Cd,

Mn)

Layer

Perovskites.

Influence

of

_Hydrostatic

Pressure in the

0-60 kbar

_Range*

By

B. A.

_Moral',

F.

_{Rodriguez'**,}

R.

_Valiente',

M. Moreno1 and H.U. Güdel2

1 _{DCITIMAC, Facultadde}

Ciencias,Universidad de Cantabria, E-39005 Santander,Spain

2 _{Institut füranorganischeundphysikalischeChemie,}

UniversitätBern,

Freiestrasse3,CH-3000 Bern 9,Switzerland

(Received_August 24, 1996)

Charge-transfer

spectroscopy

I

_Hydrostatic

pressureI

CuClt

_complex

I

(C2HJSIH3)2CdCh

I

(C2HsNH3)2MnCl4

This paper deals with the effects of pressure on the charge-transfer _spectra ofCuCl¿

complexesformed in Cu2'

doped

(C2H5NH,)2MC14(M = Cd,

Mn). A pressure-induced

redshift is observed forthe first_{charge-transfer}band in bothcrystals. While the shift is continuous for the Mn crystal at a rate of —40cm '/kbar in the 0

—

60 kbar range, it

experiences an abrupt jumpof

—

1400cm ' around 26kbar for the Cd crystal. Such a

discontinuous behaviour is interpreted in terms of a structural change in the CuCl2r

coordination geometry from an axially elongated _octahedron to a more compressed

geometry. The present results are compared with those reported for the pure

(C2H5NH,),CuCl4crystal.

Introduction

The aim of this work isto

_investigate

the local structureof

CuCl4,-

complex-es formed in the

(C2H5NH3)2MC14 (M

=

Cd, Mn)

crystals

doped

withCu2+

and its

_dependence

onthe

hydrostatic

_pressure

through

the

Charge-Transfer

(CT)

spectra.

Attention is

_paid

on whether the formation of

compressed

*

Presented at the 13th International

_Symposium

on Electrons and Vibrations in Solidsand Finite Systems(Jahn-TellerEffect) Berlin 1996.

152 B. A.Moral, F._Rodríguez, R. Valiente,M. MorenoandH. U. Güdel

CuCl4,-

_complexes,

unusual for chlorides

[1, 2],

is

_possible

or not in these

layer perovskites,

and whether an

elongated complex

can be transformed

into a

compressed

one

by applying hydrostatic

pressures. The selected crys-tals are suitable systems for such a

study

since

they provide compressed

octahedron sites for

_{accommodating}

substitutional

_impurities.

In

_particular,

the

_equatorial

and axial M—Cl distances of the

_compressed

MCI4,"

octa-hedra are

/?eq

= 2.67

Â

and

flax

= 2.52

Á

for

(C2H5NH3)2CdCl4 [3]

while

for

(C2H,NH3)2MnCl4

the distances are

Rcq

= 2.59

Á

and

flax

= 2.47

Á

[4].

In contrast, _pure

_{(CnH2n+1NH3)2CuCl4}

_compounds

_display

an

in-plane

antiferrodistortive structure of

axially elongated

CuCl4-

[5].

In

fact,

the

elongated

octahedron is the usual coordination structure for

CuCl4,"

either in pure

_cupric

chlorides or in

doped

compounds.

Exceptions

to this be-haviour are found in

(enH2)MnCl4:Cu2+

[6,

7]

and,

in

general,

in

layer

perovskites

(CnH2n+1NH3)2MnCl4:Cu2+ [8]

where a

_compressed

D4h

co-ordination geometrywas

proposed

for

explaining

the intense CT

absorption

band at 21000cm"'.

Nevertheless,

recent

investigations performed

on

isomorphous

Cd

_crystals

_{[8, 9]}

reveal that

CuCl4,"

is

_elongated

rather than

compressed,

even

though

this is the actual

_symmetry

of the

replaced

Cd2+

site. In order to

_clarify

the different behaviour exhibited

_by

the Cd and Mn

crystals,

we have studied the effect ofthe

hydrostatic

_pressure on the CT spectra

corresponding

to these Cu2+

_{doped layered perovskites. Apart}

from the selective character of the CT

bands,

their

_high

oscillator

_strengths

(f~

10 '

—10~2) [8]

makeit suitable

_probes

for

_detecting

structural

_changes

around Cu2+ in diluted materials

_(<1%)

_subjected

to

hydrostatic

_pressure.

Interestingly,

the results obtained on these diluted

_systems

_{compare well}

with those

_recently

obtained in the pure

(C2H5NH3)2CuCl4

crystal

[10],

in which theoccurrence ofa

pressure-induced phase

transition around 40 kbar was

_interpreted

_{in terms of}

_{disappearance}

of the

in-plane

lattice distortion

of CuCl4"

•

Experimental

The

_{single crystals}

of

(C2H,NH3)2MnCl4

and

(C2H5NH3)2CdCl4

doped

with Cu2+

_(about

0.2 mol

%)

arethesame ones

employed

_{in Ref.}

[8].

Hydrostatic

pressure

experiments

have been

performed

on

microcrystals

of about

100 X100X20

_{pm3 using}

a

sapphire

anvil cell attached to a

specially

designed

double beam

_{spectrometer.}

Details of the

_experimental

_setup

are

given

elsewhere

[11].

Paraffin oil was used as _{pressure transmitter and the}

pressure was calibrated

through

the shifts of the

ruby

R-lines.

Ruby

luminescence was excited

by

a 568nm Coherent I-302-K

Krypton

laser.

Results and discussion

Fig.

1 shows the

_{optical absorption}

(OA)

spectra

of thetwotitle

_compounds

at

atmospheric

_pressure. An

analysis

of the

corresponding

_spectra

is

given

Local Structure ofCu2+ in the_{(C2H5NH,)2MCl4:Cu2f Layer}Perovskites 153 6000 5000 4000 3000 2000 Í 1000 3 U h 1500 a 1000 500 40000 35000 30000 25000 20000 WAVENUMBER(cm')

Fig.1.Optical absorptionspectraof(C2H5NH3)2MnCl4:Cu2+and(C2H,NH,)2CdCl4:Cu2'

at_{atmosphericpressure}androom_temperature. The_spectrawereobtained withpolarized light propagating alongthecdirection (perpendiculartothe layer).

in Refs.

[6-9].

In both

_crystals,

the

_absorption

bands have been

_assigned

toCL—»Cu2+ CT transitions of theformed

CuCl4,"

complex.

The

spectrum

of the Cd

_crystal

is similar to those found in

CdCl2:Cu2+, LiCl:Cu2+,

(C2H5NH3)2CuCl4

where

CuCl4,"

complexes display

an

elongated

octa-hedron structure

_[12

—

15].

In

_particular

_the two intense bands at 25100 and 35700 cm"' observed in

_{(C2HsNH3)2CdCl4:Cu2+}

are

assigned

within an

_elongated

_complex

_of

_D4„

_symmetry to CT transitions

from the

_{bonding mainly}

Cl"

eu(7r

+

_<7)

and

_eu(<7

+

_jr)

molecular orbitals

(MO)

to the

antibonding

mainly

Cu2+

blg

(x2-y2)

MO,

respectively.

In

(C2H,NH3)2MnCl4:Cu2+

however the intense band at 21000 cm"1 is

asso-ciated with a CT transition from the

equatorial

Cl"

eu(7t

+

f7)

MO to the

Cu2+

_alg(3

z2

—

A)

MO within a

compressed

CuCl4,"

complex

with the short

axial bond

_{perpendicular}

to the

_layer

_[6—8].

_{Nevertheless,}

this model is unableto

explain

_{the presence of the} narrowerbandat 25000 cm"1. Recent

investigations

carried outon the mixed

(CH3NH3)2MnlCd1_vCl4:Cu2+

crys-35700cm' 25100cm1

154 B. A. Moral.F. _Rodríguez, R.Valiente,M. Morenoand H. U. Güdel

(C2H5NH3)2CdCl4:

Cu

WAVENUMBER

(cm1)

Fig. 2. Influenceofthe hydrostaticpressure uponthe firstCl" —» _{Cu2+ chargetransfer}

band in (C,H5NH,)2CdCl4:Cu2+.

tal series

_point

out the relevance of the Mn2+ not

_only

for

_explaining

the enhancement of

_{absorption intensity}

on

passing

from x = 0 _{to x} = 1 _as

well as its thermal

dependence,

but also for

explaining

the presence of the

25000cm"'

band

_although

in such a case an additional

D2h

orthorhombic

distortion for the CuCl4" was assumed

[9].

The effect of the

_hydrostatic

_pressure on the OA

_spectra

of

(C2H5NH3)2MCl4:Cu2+ (M

=

Cd,

Mn)

is illustrated in

Figs.

2 and 3. The

corresponding

variations of the

_peak

_{energy with pressure} are

plotted

in

Figs.

4 and 5. Notethatin both

_crystals

the CT bands shifttolower

_energies

upon

_increasing

pressure,

although

the

variation,

E(P)

is rather different in each case. While a continuous redshift is observed for the 25000 and

21000 cm"' bands at shift rates,

AEIAP,

of -6.1 and -40

cm'/kbar,

respectively,

for the

_{(C2H5NH3)2MnCl4:Cu2+,}

this variation for the

(C2H5NH3)2CdCl4:Cu2+ crystal undergoes

an

abrupt

shift of -1400cm"1

around 26 kbar. From

_atmospheric

pressure to 25

kbar,

the CT band

experi-ences a small redshift of about -100 cm"1. The

_steep

variation exhibited

by

the first CT band at 26 kbar is

_noteworthy.

This reflects structural

changes

of the

CuCl4,"

complexes

which are

probably

associated with a

shortening

of the

_in-plane

axial bond

_(and

_likely

a

lengthening

of the

in-plane

short

_bonds)

_leading

to a more

compressed

_geometry

where the

shortestCu—Cl bonds are

perpendicular

tothe

_layer.

The

_comparison

ofthe

present

results with those obtained

_by

Moritomoetal. in

_{(C2H5NH3)2CuCl4}

using

OA and Raman

_spectroscopy

_supports

this view

[10].

These authors

report

the existence ofa

pressure-induced

structural

phase

transition around UJ O 5 o ir, <

Local Structure of Cu2' inthe _{(C2H5NH,)2MCl4:Cu2' Layer}Perovskites 155

(C2H5NH3)2MnCl4:

Cu2+ u z < PQ c < Rubyluminescence P=2kbarI 30000 25000 20000 15000 WAVENUMBER

(cm1)

i iil

ill

4

JA

690 700 X_(nm)

Fig.3. Variation ofthe_{optical absorptionspectra}of(C2H,NH,)2MnCl4:Cu2'

with

hydro-static pressure atroomtemperaturein the 0-61 kbar_range. Thecorrespondingvariation

ofthe Rubyluminescence used for pressurecalibration,is shown onthe rightside.

40 kbar that involves the deactivation of the Raman

_peaks

associated with the

_stretching

vibrations of the

_in-plane

axial and

_equatorial

bonds of the

elongated

CuCl4,"

_{complexes (Ä„}

= 2.98

Á

and

R«,

= 2.28

Á)

[5].

The

occurrenceof such a

_phase

transition is

important

since it would

imply

the

disappearance

of the antiferrodistortive structure

displayed by

the

CuCl4,"

complexes

and,

_consequently

a

change

in the

magnetic

behaviour of the

crystal

from

_{ferromagnetic}

to

antiferromagnetic

should be

_expected

above 40 kbar

[10].

The variation of the CT energy upon pressure

reported

in that work resembles those observed for the

_present

diluted

_systems

but in differ-ent _{pressure ranges. The redshift of -3200 cm"1 observed between 15 and} 30 kbar in

(C2H5NH3)2CuCl4

is similar to the variation shown in

_Fig.

3 for

(C2H5NH3)2CdCl4:Cu2^ although

a smaller shiftis observed for the _present case. Above 40 kbar the CT band of the pure

crystal experiences

a

156 B.A. Moral, F._Rodriguez, R. _Valiente,M. Moreno and H. U. Giidel

(C2H5NH3)2CdCl4:

Cu2+ E 25000 W

g

24000

è

a, 23000 "i i i r

/

E=25120- 4.3 P P=26 kbar 10 20 30 PRESSURE

(kbar)

Fig.4. Variation of thepeakenergy of the first CT band in(C2H,NH,)2CdCl4:Cu2+ with the hydrostatic pressure. The straight line corresponds to _{the least square fit in the}

0—25 kbar range.

(C2H5NH3)2MnCl4:

Cu2+ 24000 a >h 22000

I

20000 18000 SecondCT band E=24660-6.1 P First CT band E=21000-40P 10 20 30 40 50 PRESSURE

(kbar)

60 70

Fig.

5. Pressure

_dependence

of CT bands at 21000 and 25000 cm"' in

(C2H5NH,)2MnCl4:Cu2t.The

_straight

linesare least square fits.

21000 cm"' band in

_{(C2H,NH3)2MnCl4:Cu2+ (Fig.}

_5).

Moreover the CT energy of

(C2H5NH3)2CuCl4

at 90

kbar,

E = 21500

cm',

is _near the CT

Local Structure of Cu2+ in the(C2H,NH,)2MC14:_Cu2' _LayerPerovskites 157

These two facts suggest that the CT band observed in the

_{high-pressure}

phase

of the pure

_crystal

and in the Mn

_crystal

at

_atmospheric

_{pressure is}

probably

associated with _{similar CT}states of

_compressed

CuCl4,"

complex-esof either

tetragonal

_(D4h)

or_orthorhombic

(D2h)

_symmetry.

_{Consequently,}

the variations

_{experienced by}

the first CT band _{upon pressure in} the three related

_crystals

can be

reasonably explained

in terms of structural

_changes

of

CuCl4,"

taking

into account that the _{effect of pressure} on the

elongated

CuCl4,"

complex

is

_mainly

toreduce

_{significantly}

the axialCu—Cl

_distance,

and

_only

_slightly

(or

even

increase)

the

in-plane

_{short Cu-Cl}

_distance,

leading

to a more

compressed

_geometry

with the shortest Cu—Cl bonds

perpendicular

tothe

layer.

This

anisotropic

_compression

of the

CuCl4,"

com-plex

upon pressure is

propably responsible

for the observed CT band red-shifts. A structural

_study

of the

_{high-pressure phase}

of the pure Cu

_crystal

using

diffraction

_techniques

would beveryusefulto

_clarify

the coordination

geometry

of CuCl4" in this

_phase.

Finally,

it mustbe noted that the narrow band

appearing

at 25000 cm-1 in the OA spectrumof

(C2H,NH3)2MnCl4:Cu2+,

is notobserved in the

_high

pressure

phase

of either

(C2H5NH3)2CdCl4:Cu2+

or

(C2H5NH3)2CuCl4.

This

feature is consistent with the

_{interpretation given}

in

_[9]

for such a_band as

due to an electronic transition

involving

both Cl" — Cu2+ CT _states of the

CuCl4"

_complex

and the

_4A,4E(G)

excited state of the

_{exchange-coupled}

Mn2+

_{neighbours, although}

in such a case, an additional orthorhombic

D2h

distortion was assumed for

CuCl4,".

The similar

pressure-induced

shift

rates measured for this band

(AEIAP

=

—

6

cm"'/kbar)

and for the

4A,4E(G)

peak

in

_MnCl2

(AEIAP

= -5

cm"'/kbar) [16]

_supportsthat

inter-pretation.

From the pressure shift of the broad band at 21000

cm"',

we

estimate _{that the pressure}

_required

to shift the first CT band from

(C2H,NH3),CdCl4:Cu2+

to

_{(C2H,NH3)2MnCl4:Cu2+}

is about 100kbar.

Summary

Hydrostatic

pressure

_experiments

carried out in

(C2H5NH3)2CdCl4:Cu2+

show the existence ofa structural

_change

in _the CuCl4"

_complex

_from an

elongated

octahedron to a

_{nearly compressed}

one at above 25 kbar. This

change

is evidenced

_through

the

_abrupt

shift of -1400 cm"1

_undergone

_by

the first CT band around that pressure. The

comparison

of these results with those available for

(C2H,NH3)2CuCl4

indicates that the

_{high-pressure}

structureof the

CuCl4,"

in this

_crystal

is

_probably

similartothat attained for

(C2H5NH3)2MnCl4:Cu2+

at

_atmospheric

_{pressure. The}

_present

_hydrostatic

pressure

experiments

performed

on these

Cu2+-doped

layer perovskites

pro-vide direct epro-vidence about the redshift

_{undergone by}

the first CT band of

158 B. A.Moral, F._Rodríguez, R. Valiente,M. Moreno and H. U.Gtidel

Further work in order to

_investigate

whether the _steep redshift around 26 kbar is associated witha structural

phase

transition ofthehost

crystal

or

whether is

_just

a _pure local

_{phenomenon, using}

diffraction

techniques

is

under way.

Acknowledgments

Financial

_support

from

_Caja

Cantabria and CICYT

_(Project

No.

_PB92-0505)

is

_{acknowledged.}

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