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 fürPhysikalischeChemie. Bd. 201. S. 151-158(1997)

© by R. Oldenbourg Verlag,München 1997

Local Structure of

Cu2+

in the

_{(C2H5NH3)2MCl4:Cu2+}

_(M

=

Cd,

)

Layer

Perovskites.

Influence of

_Hydrostatic

Pressure in the

0-60

kbar

_Range*

By

B. A.

_Moral1,

F.

_{Rodriguez1**,}

R.

_Valiente',

M. Moreno' and H. U. Güdel2

1

DCITIMAC,Facultad deCiencias,UniversidaddeCantabria,

E-39005 Santander, Spain

2 _{Institut füranorganische undphysikalischeChemie, Universitat Bern.}

Freiestrasse3, CH-3000 Bern 9,Switzerland

(Received

August

24. 1996)

Charge-transfer

spectroscopy

I

_Hydrostatic

pressureI

CuClf

complex

I

(C2H5NH3)2CdCl41 (C2HsNHJ2MnCh

This paper deals with the effects ofpressure on the charge-transfer _spectra ofCuCl?,"

complexes

formed in Cu2+

_doped

(C2H5NH,)2MCl4 (M = Cd, ). A

pressure-induced

redshift isobserved for the first

_{charge-transfer}

band inboth crystals. While the shift is continuous for the Mn _crystal at a rate _{of —40 cm"'/kbar in the 0—60 kbar range,} it

experiences an abrupt jumpof —1400 cm"' around 26 kbar for the Cd crystal. Such a

discontinuous behaviour is interpreted in terms _of a structural change in the CuCl¿

coordination geometry from an axially elongated octahedron to a more compressed

geometry. The _present results are compared with those reported for the _pure

(C2H,NH,)2CuCl4crystal.

Introduction

The aim of this work isto

_investigate

the localstructureof

_CuCl^"

complex-esformed in the

(C2H3NH3)2MC14

(M

=

Cd,

)

crystals doped

withCu2+

and its

_dependence

on the

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

Solids and Finite

_Systems

(Jahn-TellerEffect)Berlin 1996.

** _{Author for}

152 . . Moral,F. _Rodríguez, R. _Valiente,M. Moreno and H. U. Giidel

CuCi6~

complexes,

unusual for chlorides

[1, 2],

is

_possible

ornot in these

layer perovskites,

and whether an

_{elongated complex}

can be transformed

intoa

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

_MCl^

octa-hedra are = 2.67 A and

Rd%

= 2.52

À

for

(C2H5NH3)2CdCl4 [3]

while

for

_{(C2H5NH3)2MnCl4}

the distances are

fleq

= 2.59

Á

and

/?ax

= 2.47

Á

[4].

In contrast, pure

(CnH2n+1NH3)2CuCl4

compounds display

an

in-plane

antiferrodistortive structure of

axially elongated

CuCl?r

_[5].

In

fact,

the

elongated

octahedron is the usual coordination structure for

CuCljT

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

D4I,

co-ordination

_geometry

was

proposed

for

explaining

the intense CT

absorption

band at 21000 cm1.

Nevertheless,

recent

investigations performed

on

isomorphous

Cd

_crystals

_{[8, 9]}

reveal that

CuCl¿"

is

_elongated

rather than

compressed,

even

though

this is the actual symmetry of the

replaced

Cd2+

site. In orderto

clarify

the different behaviour exhibited

_by

the Cd and Mn

crystals,

we have studied the effect of the

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"1

_{—10"2) [8]}

make it 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}

(C2H5NH,)2CuCl4

_crystal

[10],

in which the occurrenceofa

pressure-induced

phase

transition around 40 kbar was

interpreted

in terms of

_{disappearance}

of the

_in-plane

lattice distortion of

_CuCir.

Experimental

The

_{single crystals}

of

_{(C2H5NH3)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 568 nm 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,)2MCI4:Cu2tLayerPerovskites 153

6000

_I_,_I_,_I_,_ 40000 35000 30000 25000 20000

WAVENUMBER(cm1)

Fig.1.Optical absorptionspectraof(C2H5NH,)2MnCl4:Cu2+ and(C2H5NH,)2CdCl4:Cu2+

at

_atmospheric

_{pressure and}room_temperature.Thespectrawereobtained with

polarized

light

propagating along

thecdirection

_{(perpendicular}

tothe _layer).

in Refs.

_[6-9].

In both

_crystals,

the

_absorption

bands have been

_assigned

toCl" — Cu2+ CT transitions of the formed

CuCit

complex.

_The

spectrum

of the Cd

_crystal

is similar to those found in

_{CdCl2:Cu2+, LiCl:Cu2+,}

(C2H5NH,)2CuCl4

where

CuCl£

complexes display

an

elongated

octa-hedron structure

[12-15].

In

_particular

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

_{(C2H3NH,)2CdCl4:Cu2+}

are

assigned

within an

elongated

complex

of

D4/, symmetry

to CT transitions

from the

_{bonding mainly}

_{Cl_eu(7r}

+

_rj)

and

_„(

+

₎

molecular orbitals

(MO)

to the

_{antibonding mainly}

Cu2+

_b,g

_(x2—y2)

_MO,

_{respectively.}

In

(C2H5NH3)2MnCl4:Cu2+

however the intense band at 21000 cm"1 is

asso-ciated with a CT transition from the

equatorial

Cl"

eu(n

+

_a)

MO to the Cu2+

_a,g(3

z2-

r)

MO within a

compressed

CuCl^"

complex

with the short

axial bond

_{perpendicular}

to the

layer

[6—8].

Nevertheless,

this model is unable to

_explain

_{the presence}ofthenarrower bandat 25000 cm"'. Recent

crys-154 . . Moral,F. _Rodríguez, R. Valiente,M. Moreno and H. U. Güdel

(C2H5NH3)2CdCl4:

Cu2+

1 ' ' 1 1 ' ' ' 1 I . I , I . I . I . I . I . I I i I 30000 25000 20000 WAVENUMBER

(cm"1)

Fig. 2. Influence of the

_hydrostatic

_{pressure upon the} firstCl" —» Cu3+ charge _transfer

band in(C2H5NH3)2CdCl4:Cu2+.

tal series

_point

out the relevance of the Mn2+ not

only

for

explaining

the

enhancement of

_absorption

_intensity

on

passing

from = 0 _to = 1 _as

well as its thermal

dependence,

but also for

explaining

the presence of the

25000 cm"1 band

_although

in such a case an additional

D2h

orthorhombic

distortion for the

CuCli!"

was assumed

[9].

The effect of the

_hydrostatic

_pressure on the OA spectra of

(C2H5NH3)2MCl4:Cu2+

(M

=

Cd, )

is illustrated in

Figs.

2 and 3. The

corresponding

variations of the

_peak

energy with pressure are

plotted

in

Figs.

4 and 5.Note thatin 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"1 bands at shift rates, El

_,

of -6.1 and -40cm

"Vkbar,

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 to25

kbar,

the CT band

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

_steep

variation exhibited

by

the first CT band at 26 kbar is

noteworthy.

This reflects structural

changes

of the

_CuCl^"

_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

shortest Cu—Cl bondsare

perpendicular

to the

_layer.

The

_comparison

of the

present

results with those obtained

by

Moritomoetal. in

(C2H5NH3)2CuCl4

using

OA and Raman

_{spectroscopy supports}

this view

[10].

These authors

Local Structure ofCu2+ in the _{(C,H,NH,)2MCl4:Cu2' Layer}Perovskites 155

(C2H5NH3)2MnCl4:

Cu2+ 2 kbar Rubyluminescence kbar

I

4

i _¡I

91

P=_{61 kbar} 690 700 ._(nm) 25000 20000 15000 WAVENUMBER

_(cm"1)

Fig.3. Variationof theoptical

absorption

spectraof(C2H5NH,)2MnCl4:Cu2+ with

hydro-static_pressure at room_temperature in the 0

—

61 _{kbar range.} The correspondingvariation

of the Ruby luminescence used for_pressurecalibration, is shownon the rightside.

40 kbar that involves the deactivation ofthe Raman

_peaks

associated with the

_stretching

vibrations of the

_in-plane

axial and

_equatorial

bonds of the

elongated

CuCli"

complexes (Rm

= 2.98

À

and = 2.28

A)

[5].

The

occurrence of such a

phase

transitionis

important

since it would

_imply

_the

disappearance

of the antiferrodistortive structure

_displayed

_by

the

CuCl¿

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

_{(C2H5NH,)2CuCl4}

is similar to the variation shown in

_Fig.

3 for

(C2H5NH1)2CdCl4:Cu2+

although

a smaller shift is observed for the

_present

case. Above 40 kbar the CT band of the pure

crystal

experiences

a continu-ous redshift at a rate of -16

cm"'/kbar,

_analogous

to that followed

by

the

156 . .Moral, F. _Rodríguez, R. _Valiente,M.Moreno and H. U. Güdel

(C2H5NH3)2CdCl4:

Cu/+

r

0 10 20 30

PRESSURE

_(kbar)

Fig.

4. Variation of the_{peak energy}of the first CT band in_{(C2H,NH1)2CdCl4:}Cu24 with

the hydrostatic pressure. The straight line corresponds to the least square fit in the

0 — 25 kbar range. 24000 22000 20000 18000

(C2H5NH3)2MnCl4:

Cu:+ First CT band E=21000 -40 "T" Second CT band E=24660 -6.1 10 20 30 40 50 60 70 PRESSURE

_(kbar)

Fig.

5. Pressure

_dependence

of CT bands at 21000

(C2H5NH,)2MnCl4:Cu2+. Thestraightlinesare least square fits.

and 25000cm

21000cm"1 band in

_{(C2H5NH3)2MnCl4:Cu2+}

_(Fig.

_5).

Moreover the CT energy of

(C2H3NH3)2CuCl4

at 90

kbar,

E = 21500

cm1,

is _near the CT

Local Structure of Cu-' _{in the}

(C2H5NH,)2MCl4: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 withsimilarCTstates of

_compressed

CuCl4,"

complex-es of either

tetragonal

(D4I,)

ororthorhombic

(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

to reduce

significantly

the axial Cu-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

_{anisotropie 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 be very useful}to

clarify

the coordination

geometry

of

CuCl4,"

in this

_phase.

Finally,

itmustbe noted that the narrow band

appearing

at 25000 cm"1 in the OA

_spectrum

of

(C2H3NH3)2MnCl4:Cu2+,

is not observed in the

_high

pressure

phase

of either

(C2H,NH.,)2CdCl4:Cu2+

or

(C2H,NH.,)2CuCl4.

This

feature is consistent with the

_{interpretation given}

in

[9]

for such a band as

dueto an electronic transition

involving

bothCT~ — 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

₍

=

-6cm"'/kbar)

and for the

4A,4E(G)

peak

in

MnCl2 (

= -5

cm"'/kbar) [16]

supports

that

inter-pretation.

From the pressure shift of the broad band at 21000

_cm1,

we

estimate that the pressure

required

to shift the first CT band from

(C2H,NH,)2CdCl4:Cu2+

to

_{(C2H5NH3)2MnCl4:Cu2+}

is about 100 kbar.

Summary

Hydrostatic

pressure

experiments

carried out in

(C2H5NH,)2CdCl4:Cu2+

show the existence of a 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,NH,)2CuCl4}

indicates that the

_{high-pressure}

structureof the

CuCl4,"

in this

crystal

is

probably

similartothat attained for

(C2H5NH,)2MnCl4:Cu2+

at

_atmospheric

_{pressure. The}

_present

_hydrostatic

pressure

experiments performed

onthese

Cu2+-doped

layer

perovskites

pro-vide direct epro-vidence about the redshift

_{undergone by}

the first CT band of

CuCl4,"

_complexes

upon pressure in dilutedsystems.

158 . . Moral, F._Rodríguez, R. Valiente,M. Moreno and H. U. Güdel

Further work in order to

_investigate

whether the

_steep

redshift around 26 kbaris associatedwith a structural

_phase

transition of the _host

_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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