Effects of oxygen deficiency on the optical spectra of YBa2Cu3O7−x

0038-1098/88 $3.00 + .OO Pergamon Press plc

Solid State Ccmmunications,Vo1.66,No.12, pp.1231-1235, 1988. Printed in Great Britain

.u

EFFECTS

OF OXYGEN

DEFICIENCY

M. Garriga,

ON THE OPTICAL

J. Humlicek,’

M. Cardona,

SPECTRA

OF YBa&usOr+

E. Schijnherr

Max-Planck-lnstitut fiir Festkijrperforschung, Heisenbergstrasse 1, D-7000 Stuttgart 80, Federal Republic of Germany (Received April 18, 1988 by M. Cardona)

We report on the variation

of ellipsometric

pounds with oxygen content compound

two new, rather

in the semiconducting oxygen content,

spectra of the YBa2Cu30r_x

0 < z 5 1. Beside the structures intense peaks are observed

compound

(z > 0.5).

For the

Cu(3d)

understanding

YBa2Cu307_x tronic

materials

structure

deficiency

z.

Several minor features become significant

conducting

T,

The transport

decreasing

tion of one-electron the material

netic

with

fairly

high

we report

Due to the expected should oxygen

ceramic

tematic

tric function. in ceramic,

thin film

are essentially

dependence

ramics

have already

room temperature

in the spectra

but

spectra

been presented

some sys-

[7].

observed

and annealed

and furnace removal metric

with those

J.E.Purkyne

KotlaFsk6 2, 611 37 Brno, Czechoslovakia

of Solid State University,

to room

sive or diamond

of

atmosphere

temperature.

by heating

The oxygen

the pellets

was determined [9].

pro-

was pressed into

at 950 “C in oxygen

was performed

in argon.

by the thermogravi-

For the optical

measurements

polished

with

alumina

paste dissolved in methanol.

the abra-

For the final

step 0.25 pm abrasive was used. The samples were meaature stat.

after

measurements

polishing.

For the low temper-

the samples were placed in a cryo-

After baking out the cryostat

2 x lo-’

Torr was obtained,

a vacuum

which

better than

is necessary to avoid

condensation. The optical ing analyzer

measurements

~1 + icz

were taken

spectroellipsometer

quantity

light

which,

like the YBa2Cu30r_, plex and differences

reflectance isotropic

the relation

are mainly function;

E=

materials more comthin film

orientations

of

161. Fortunately,

in the absolute the positions

tures remain almost unaffected.

function

is much

axes are observed

pseudodielectric

ratio of s-

of sintered,

due to different

the differences

a rotat-

homogenous

polycrystalline

in the spectra

samples

the crystallographic

for

with

In this system the

related to the dielectric

[ll].For anisotropic

and crystalline

[lo].

is the complex

samples, is directly

several weak features

Department

cooled

method

appropriate

and CuO to 950 “C in pure

The powder obtained

The stoichiometry

measured

we have measured the tem-

of Science,

at 1 atm.

pellets

of high T, ce-

function

BaC03

oxygen

and p-polarized

Our results at

of the pseudodielectric

of Y2O3,

results on the

are enhanced.

Faculty

from Ba(5d)

The samples were made by heating portions

sured immediately

mea-

of YBa2Cu307

Preliminary

* on leave of absence from: Physics,

structures

and single crystals

. At low temperatures

YBa2Cu3Oe

transitions

The 1.75 eV peak is possibly due to Cu(3d)

values of the dielec-

agree in the main features

dependence

of optical

the optical

introduces

the spectral

Further,

upon cooling the samples

involving

samples were mechanically

for z N 1 [S].

on single crystals,

of the optical

of Kelly et al. [8].

Fur-

are most easy to obtain

This fact

the same [6].

oxygen

perature

anisotropy

in the absolute

However,

For

to be antiferromag-

the variation

be performed

samples.

[l].

to the predic-

UV with oxygen concentration.

strong

stoichiometries

inaccuracies

of

is super-

see, e.g., [2,3,4].

Neel temperature

spectra in the visible-near

with

properties

as x increases

band theory,

elec-

on the oxygen

in contrast

has been found

In this Communication

states.

in the

of their

For 0 < z < 0.5 the compound

ther,

different

knowledge

show a strong dependence

with

surements

states to O(2p)

of superconductivity

2: 2 0.5, it is semiconducting

depends on the

states.

good

is needed.

these materials

strongly

whereas the energy shows a weak dependence both on oxygen content

and temperature.

and O(2p)

at N 1.75 eV and N 4.1 eV

Their intensity

to 7 K. We associate the 4.1 eV peak to excitations and/or

high Z’, com-

observed in the z = 0

values of the

of spectral

struc-

Spectra between 1.6 and

5.3 eV were taken on a large number of samples of different oxygen deficiency

1231

x, both with Y and Tm as trivalent

THE OPTICAL SPECTRA OF YBa2Cu307-x

1232 ions. In the semiconducting

compound

eral rare earth ions were substituted spectral

structures

main features

in our ellipsometric

with other optical

transmission

and reflection

tron

loss spectroscopy

energy

metric

measurements

features,

YBa2Cu30s

observed

measurements, [14],

structure

including

[7,12,13],

A similar

structure

at -2.7

[13,15].

to transitions

eV has been reported

Therefore,

within

ellipso-

dominate

we found

weaker

weak features appear

at low temperatures.

the spectra

we attribute

the Cu-0

are present in both materials. Two strong absorption bands

elec-

and recent

In addition

mainly

[7].

for La2_,SrxCuO4

results agree in the

spectroscopy

[S].

sev-

for Y. The strongest

Vol. 66, No. 12

planes,

at 1.75 and 4.08 eV

; further,

of YBa2Cu30rj

and three low energy satellites

as oxygen

content

this which

several

(they

dis-

increases in the same way as

the main peak) of the 4.08 eV band, can be resolved in The pseudodielectric exhibits

function

substantially

YBa2Cu307

more

structure

the superconducting

behavior. material

increasing

range, but centered oxygen deficiency

eV for z =

the temperature appreciable There

spectral

of

spectrum

or its first derivative

Fig. 1).

positions

of all the observed

The energy

tures for different

semiconducting

pendence

on the oxygen

dependence trivalent

eV in YBa2Cu307

ion.

are shown

end of the

strong

With

deficiency.

Spectra

for different

in Fig. 2.

The

results

and their

When

is however

a red shift with

with

is observed increasing

ion M in the MBa2Cu307

(Fig.

concerning

at 4.08 eV is the strongest

spectra of the z = 1 materials.

ionic radius

Gd or Sm no significant

compounds

can therefore

change is observed

b .: : _....

:

. . ..

. ....

.. . .../

4

2

1

2

3

Energy

4

5

6

I’

1”’

1

2

(eV1

electric

I”’ 4

3

Energy

Fig. 1. Left: Real and imaginary function

< ~1 >

samples of YBazCusOrj and 300K derivative

(dotted of < q

to the energy, ted line).

< ~2 > of ceramic

measured at 7 K (solid line)

line).

Right:

> of YBa2Cu306

at 7 K (solid

numerical with

first

respect

line) and 300 K (dot-

The arrows indicate

resolved structures.

(eV I

parts of the pseudodi+i

for Tm,

(Fig. 3). We

discard the rare earth ion as being responsi-

.:’

,“5

one in the

When replacingy

Y Ba2cU3ofj

(El)

the two

et al. 181. We

of these two structures

10

;

this

z in TmBa2Cq07_,

7K . . . . . . . 300 K

A

in de-

possible origin.

The structure

no 1).

r Y 6a2cu306

8

a strong

decreasing

to 7 K only a slight sharpening of this structure

are listed

We investigated

bands are similar to those of Kelly

shall discuss the characteristics

of Q down

(see struc-

in two series of samples, with Y and Tm as

of the maximum

1 (see Fig. 2).

samples

Table I. The 1.75 and 4.08 eV peaks exhibit

of

of the two

at lower energies.

the low temperature

z this band shows a progres-

shift

of the trivalent

feature

at the semiconducting

sive decrease in the position to -2.6

that

has been discussed in detail

is the broad band at -2.8

is also present

composition

than

The spectral structure

in Ref. [6]. The only common materials

(Fig. 1)

which shows few broad bands and a low en-

ergy onset of metallic

which

of YBa2Cu306

the position

of the

I 5

6

THE OPTICAL SPECTRA OF YBa2C~30~_~

Vol. 66, No. 12

1233

Table I. Energy positions (in eV) of the maxima in the spectra of ~2 in MBa2Cu& ent trivalent

materials, for differ-

ions M.

Y

Y

Tm

Gd

Sm

6

7 K

300 K

300 K

300 K

300 K

1.78

1.75

1.76

1.71

1.74

2.12

2.12

2.12

2.06

2.08

2.59

2.61

2.58

2.51

2.58

3.16

-

-

-

-

3.73

3.74

3.75

3.74

3.72

3.86

3.90

3.90

3.90

3.92

3.95

-

-

-

-

4.08

4.08

4.08

4.08

4.08

4.59

4.60

4.58

4.58

4.60

5.2

5.3

5.3

5.3

5.3

A\\

-

089

1

ble for that feature; see also Ref. [8]. On the other hand the intensity of the satellites in the low energy side of the structure depends on the trivalent

ion substituted for Y:

whereas for Gd the satellites can be clearly resolved even at room temperature

(see real part of < E >,Fig.

3)

for

Y they become clear only upon cooling to 7 K, or in the differentiated

room temperature

spectrum (Fig. 1). The

4.08 eV band shows a rather small temperature

I I.

depen-

I

I

I

2

1

dence. No significant shift in energy or increase in intenWhen decreasing the oxygen

I

4

Energy

sity is observed between 300 K and 7 K, the broadening decreasing by about 20%.

I

1

3

I

I

5

6

(eV)

deficiency 2: the structure exhibits a rapid decrease in intensity and a considerable shift to higher energies (right side of Fig. 4)

remain unchanged, In the complete

Fig. 2. Imaginary part < ~2 > of the pseudodielectric function of TmBa2Cu#_x

whereas the broadening and lineshape in agreement

with Kelly et al.

as a function of oxy-

gen deficiency 2.

[8].

range of z a shift of 160 meV and a

linear decrease in intensity is observed.

In some of the

intensity fall with increasing oxygen content would be re-

one-electron

[3,4] there is a

lated to the diminution

band structure calculations

group of rather flat conduction

bands at -4

eV above

the Fermi level that could be a good candidate for the fi-

In the Ba-0

of the number of 0-Cu-0

units.

case, the oxygen content dependence of the

structure could be understood as due to changes in the

nal states of the observed peak. Those bands are mainly

Ba-0

of Ba(5d) character.

of excitonic interaction by the free carriers. The best way

Therefore one tentative assignment

of the 4.1 eV structure is that corresponds to Ba-0 transitions. ported

This interpretation

would be further

by the fact that the optical spectrum

like sup-

of BaO,

which crystallizes in the sodium chloride structure with a lattice constant of 5.52

A,exhibits

two strong excitonic

peaks at 4.06 and 4.30 eV, accompanied features at 3.88 and 3.95 eV [16]. of YBa2Cus07_,

by two small

In the Ba-0

planes

, the atomic arrangement is similar to

that of the sodium chloride structure with a distance of 2.753

A

between Ba and 0 atoms which corresponds to a

lattice constant of 5.45

A.On

the other hand Kelly et al.

[8] concluded that Ba does not participate because substitution

in the peak

of Sr for Ba does not change sub-

stantially the observed structure.

They attribute

the 4.1

distance, accompanied

to decide between both possibilities would be a measurement of the different components of the dielectric tensor in the 4 eV region.

within the Ba-0 that should be

observed preferentially with the electric field of light perpendicular to the c axis. to 0-Cu-0

Similarly,

if the band is_due

units, it should appear mainly for the E I] c

configuration. The absorption peak at -1.75 onset of strong optical ing compounds.

eV corresponds to the

absorption in the semiconduct-

It lies far above the effective gap of

0.2 eV given by temperature

dependent

resistivity mea-

surements [18]. The strong dependence of this structure on oxygen deficiency z is shown in Figs. 2 and 4. pronounced maximum

isolated linear 0-Cu-0

seen up to 2 N 0.5.

in K2CuCl3

If the transitions

planes are responsible for the structure,

eV band to localized excitations which take place in the units, arguing that

by an increasing screening

The

in the < Q > spectra is clearly For larger x values the band tends

where the Cu is bonded in a similar linear complex [17],

to disappear in the background of metallic-like

also a strong absorption peak is observed at 4.4 eV. We

tion, but it is still observed as a weak shoulder in the

feel that both interpretations, them, are possible.

or even a combination

In the case of Cu-0

of

excitations the

derivative spectra (Fig. 4). temperature

absorp-

The peak has the strongest

dependence of all the observed structures:

THE OPTICAL SPECTRA OF YBa2C~30~_~

1234

I

1

2

3

4

Energy

5

1

6

I

I

2

Vol. 66, No. 12

I

I

3

Energy

(eV 1

Fig. 3. Real (left)

and imaginary

pseudodielectric

functions

(right)

parts

of sintered

.

I

4

II

5

I

6

(eV) of the

samples

of

, for M = Tm, Y, Gd, and Sm.

YBa2Cu306

lb

-

0mY

_

ooTm

3

d 0

0

2

/:

O0

Q1

0

x Y

JE

al

00 l

1.85

_

s -? sl 1.80 -

a

.=

l

a5

a

8

l

iii c 1.75 -

*

0.2

0.4

0.6

8

l

I

0.8

0

1.0

0.2

0.4

0.6 X

X

Fig. 4. Left: Energy position and intensity eV structure

as a function

in YBa$u307_x (circles). the -4.1 deficiency

Right:

of oxygen

(squares) Energy

eV structure

(circles).

deficiency

z

and TmBa$u307_,

position

as a function

z in YBa2Cu307_,

TmBa2Cu307_x

of the -1.75

and intensity of oxygen

(squares) and

of

4.20

-415

l

8

W

0.0

-

m

l

l

0 0.8

z

E

G - 4.10 t I 5 1.0

1235

THE OPTICAL SE'ECTRAOF YBa Cu 0 2 3 7-x

Vol. 66, No. 12 linear interpolation

of its position between 300 K and 7

K gives a temperature

coefficient of -8

x

10e5eV K-‘.

structures at 1.75, 2.61, and 4.08 eV and several weaker transitions that can be better resolved at low tempera-

Its intensity increases by a factor of two, whereas the

tures.

spectral width decreases by about 10% in that tempera-

has been measured. The 2.6 eV band is nearly indepen-

ture range. The changes with temperature

dent of the oxygen deficiency and is probably related to

become even

The temperature

larger when the samples are heated up to 700 K [19]. On

transitions

the other hand, the peak shows a very weak pressure de-

have a rather strong dependence on oxygen deficiency

pendence 114. The transition seems to be not consistent Since the proposed in-

z. The 4.08 eV peak progressively decreases as z decreases from 1 to 0. This peak could be due to transi-

[12] should be rather weak, an

tions in the Ba-0 planes although fairly strong arguments

with band structure calculations. traionic

d-d excitations

alternative

explanation

in terms of charge transfer tran-

sitions within Cu(3d)-O(2p)

states is also possible [13,7].

in the Cu-0

dependence of the main bands

[8] support a 0-Cu-0

planes.

origin.

The other two features

The 1.75 eV band is ob-

served as a strong peak for z < 0.5.

For larger z values

A coherent explanation of the structure should give a cor-

only weak shoulders in the metallic-like

rect interpretation

been found. The most plausible explanation for this band

of the temperature

in the present work, together

changes observed

with the pressure depen-

are charge transfer excitations within Cu-0

dence reported in Ref. [13]. Again, determination of the optical anisotropy can contrrbute to the assignment of

experiments,

the structure.

cidate the origin of these bands.

materials on oxy-

gen deficiency. At x N 1, the spectra show three strong

in particular,

determination

planes. New

of polarization

dependence of the last two structures would help to elu-

We thank K. Syassen and U. Venkateswaran

In conclusion, we have measured the dependence of the optical spectra of MBa2Cus07_,

absorption have

ful discussions, the crystall preparation

for help-

group, H. Hirt,

M. Siemers, and P. Wurster for expert technical help.

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