Surface and intrinsic low frequency conductivity of KTiOPO4 (KTP) in the temperature range 290 – 1100 K

Pergamon

Solid State Communications, Vol. 97, No. 11, pp. 913-917, 1996 Copyright 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-1098/96 $12.00+.00 0038-1098(95)00801-2

SURFACE AND INTRINSIC LOW FREQUENCY CONDUCTIVITY

OF KTiOPO, (KTP)

IN THE TEMPERATURE RANGE 290 - 1100 K. A. Pimenov* and C. H. Riischer, Inslitutjlir

Mineralogie

der Universitdt

Hannover, Werjngarten I, D-30167 Hannover

V. A. Maslov, Institute of

General Physics, Russian Acaa! Sci., Viilow

38, h4oscow

(Received 4 October 1995; accepted 24 November 1995 by H. v Liihneysen)

The ac conductivity and dielectric function of KTiOPO, were measured in the frequency range 6om 10 Hz to 10 MHz and in the temperature range from 290 to 1100 K, using Au evaporated contacts. Strong low frequency relaxation after field annealing experiments up to 600-I 1OOKis obseaved, which can be attributed to surface segregation. Conclusively, by removal ofthe surface layers, intrinsic low frequency behavior is retained. The conductivity spectra indicate a power law behavior U(U)--0’ with a quite small exponent s-0.17. This dependence is possibly caused by the one dimensional character of AC conductivity. Keywords: A. ferroelectrics, A. surfaces and interfaces, D. dielectric response

1. Introduction.

possesses an open skeleton which is formed by chains of

KTiOPG, (KTP) continues to attract attention as a promising

interconnected by slightly distorted PO, tetrahedra in the x

nonlinear material for di&rent applications, particularly for

and y direction, respectively (for details see Ref. 2). The K’

distorted TiO, octahedra along the z axis. The octaedra are

second harmonic generation of 1.06 urn laser.’ KTP is

ions are located in the empty spaces of the structure,

known to be a ferroelectric ’ with space group Pn2,a. A

occupying two positions relative to the screw axis 2,. They

second order phase transition is observed 3 at 1207 K. The

are surrounded by eight and nine oxygen ions respectively, in

dielectric properties of KTP have been the subject of various investigations.*’

These

investigations

reveal

such a way to provide channels for ions to move through

large

parallel to the z axis. It should be noted that the system does

discrepancies in the low frequency dependence of the

not show ferroelectricity in the conventional sense, since the

conductivity (below about 1 GHz) on the order of few

reversing of polarization is strongly hindered by the high

magnitudes. Morris et al9 have shown that the properties

conductivity.3

can vary depending on the difFerem techniques and conditions used for crystal growth. In addition it is known

From our point of view there are still questions wnceming

that doping of &purities mainly decreases the (ionic) specific

the behavior of the low frequency conductivity, e.g. it has to

conductivity of KTP.” It is suggested %*” that the low

be differentiated between electrode effects and bulk

frequency dynamics is given by the mobiity of the R ion

conductivity.

along the crystallographic z axis. The crystal lattice of KTP

investigate the low frequency properties of KTP using impedance spectroscopy, particularly the E II z direction.

+

present address: Ins&t furFestk&perhy&

Technische

Hochschule Darmstadt, 64289 Darmsta&. 913

It is the purpose of the present paper to

SURFACE AND INTR~SIC

914

2.

LOW FREQUENCY CONDUCTIVITY OF KTiOPO,

Vol. 97, No. 11

ExperimentaI.

6 We have used a flux grown high quality single crystal of KTP. The crystal was cut with orientation to achieve a shape ofabout

x~y~z=5~5~0.8mm~,

zbeingthepolar

25 0 T-

direction. The crystal was polished to optical quality (0.3 pm

22

diamond paste). Reflectivity measurements (650-5000 cm-‘) confirm fairly well the spectra for E//G E/lb, and El Ic, reported by Kugel et al.i2T~~ion

m~r~ents

on this

crystal also show no evidence of OH-defects which have been reported by Morris et al9 as a possible defect type for KTP. The resuits given below are fairly well reprodu~

by

using additional singIe crystal of KTP. Au electrodes were

-6

evaporated to the (001) planes. Pt wires were glued by silver paste to them in order to hold the sample and to connect the measurement circuit. Accordingly the electrical field was chosen to be E/lc in our experiments. The low frequency

6

spectra (5 Nz -13 MHz) were measured using a HP4192.k automatically driven impedance analyzer. The probe was given into the Iiunace and heated. The temperature measurem~ts

were

done

using

thermocouple.

The

accuracy

of

the

~-~:l~~

the

temperature

determination was better than k2 K. Microprobe analysis was carried out on representative crossections which were cut from the sample used for the impedance investigations.

Fig. I. Dielectric spectra of KTP in the temperature range 295 to 522 K. Upper graph: Real part of dielectric fimction. Lower graph: Conductivity. The line corresponds to power law LI- o”.165.

3. Results and Discussion. (E, = 0.36 eV at I kHz) by Kalesinskas et al.’ and has been Dielectric spectra obtained for El/c up to the temperature

attributed to I<” ion rn~~.

522 K are given in Fig. I. In the upper graph the real part of

activation energies varying from 0.26 to 0.36 eV. They also

the dielectric Iunction is given. We observe an increase of

show that specific room temperature values of conductivity

two to three orders of magnitude of c’ at lower Gequencies,

vary over four orders of magnitude depending on sample

while above about 100 kHz E’changes only slightly. Sharp

preparation.

lines occur in the frequency range of I - 10 MHz, which can be attributed to piezore~~~s.5 The ~~~~n~rng

conducts

frequency dependent conductivity spectrum is given in the lower graph of Fig. 1. The lines above about 1 MHZ again are due to the ~~r~o~~s.

At higher temperatures their

magnitude is too low to be resolved

cIw’&

from the

increased conttibution of the conductivity. The temperature dependence of conductivity at 1 k%k follows an Arrhenius law: a =

The

Morris et ak9 reported on

characteristic

room

of our sample (2.610”9 @kni’

temperature at 1 kHz)

belongs to the lower values ofthose reported by Morris et al. For sake of compatison we also measured diekctric spectra for ZZi10.and obtained one to two orders of magnitude lower specific conductivity values compared to the El/c one in accordance with observation by Yanovskii and Voronk~va.~ The low frequency dielectric constant (helow 1 MHz and at T = 293 K: ~‘111 for E/la and c’- 18 for E[Ic) also show rather good agreement to those known from literature.’

(5,.47

exp(-EJr)

0)

with E,= 0.53 eV. A similar vahie (E, = 0.31 eV at 1 MHz) has been reported by Yanovskii and Voronkova3 and earlier

Therefore, our w&es could be considered to show rather good agreement with those reported by others, although the activation energy appears to be 1.5 timeshuger.

Vol. 97, No. 11

SURFACE AND INTRINSIC LOW FREQUENCY CONDUCTIVITY OF KTiOPO,

915

It should be pointed out that using an Arrhenius type plot for conductivity values shown in Fig. 1 (at least below 500 K) is not very favorable because the conductivity shows certain

g-2x106 0

Up to the lowest measured

G

frequency (5 Hz) the conductivity shows no clear saturation.

h

frequency

de~d~ies.

1x106 0

Instead, we observe a weak power law frequency dependence (0: with s-0. I?), which is indicated as a

0

2~10~

straight line in Fig. 1 for T= 349 K data set. For a system

4~10~

6~10~

Z(szcm)

with the hopping charge transport the frequency independent ~~

at low Muncie

is general& observed” and a

power iaw at higher frequencies with exponent s - 0.6-0.9. Such a behavior is reported’ even in KTP for low conducting directions ( &/kr, El lb ) and clearly contradict to our data on 4 lc. We suppose that the observed frequency dependence of AC conductivity is related with the one-diiensionahty KTP. A simiiar frequency dependence of ~ndu~~~

Fig. 2. Cole-Cole plots of the T= 377 K dielectric spectra of KTP demonstrating a deviation from ‘semicircle dependencies”. SemicircIes are expected for ionic conductor, which can be described by equivalent circuits (arc fkom bu& electrode, grain boundaries, etc.).

of with

s = 0.09 in one-dimensional hollandites was analysed by Jonscher”. In such systems the charge carriers are forced to follow a prescribed options

channel. The probability

to change a conduction path in case of blockage is very small due to much smaller conductivity in other directions. Thus,

in Fig. 1 by the steeper decrease of conductivity below 0.1 kHz. A plot for the high temperature dependence of E’at 1 and 0.1 MHz is shown in Fig. 3. It is observed that E'shows a maximum at about 780 K. Yanovskii and Voronkova 3 also

even

reported on a similar observation on the dielectric constant

slowly relaxing high-energy barriers become efhective giving

on their KTP crystal, which they suggested for K’ hopping

in contrast to systems with a higher ~e~io~~,

rise to a frequency dependent conductivity at very low

relaxation. In our study this relaxation turned out to be due to the surface segregation which strongly depends on the

&quencies.

m&g

history of the sample. To show this, the frequency

The possible tool for obtaining the dc part of the

dependence of E’(a) and 0 (b) measured at 600 K is plotted

conductivity for ionic conductors is the Cole-Cole diagram:

in Fig. 4. Shown are the spectra from the first heating run to 600 K (curve l), after first cooling down from 700 K to

for conductivity: a” = f (a?,

a*(o) = 0’ - id’

(2)

for impedance: z” = f (z).

8001

z* = i/u*

(3) This method was applied to KTP by Sigaryov’ to extract the

DC conductivity from the fresuency dependent impedance data. However, as can already be seen from Fig. 1 and which is exemplified for the T = 377 K data set in terms of Cole-

r o...

E’

0’

0

400 _

‘.

‘0

.’

6’ .b :, *.,’

Cole plots in Fig. 2, extrapolation of conductivity to zero frequency is not trivial, i.e. in the 2” =I@)

I

‘. *.*

.‘t MHz ,6

plot the data

cannot be resolved into arc contributions to get any clear

o

separation into AC and DC effects.

400

0’4

‘b

y

.,,. 100 kliz _ ,.,.o-.-..O .*__..-.

i ..a’

,

I 800

1200

Probably a better data approximation could be achieved above T m 500 K. However, strong relaxation effects for heating to higher temperatures are observed This behavior can already be observed in the T- 522 K conduction curve

Fig.3.R~~oft~eff~ & ofKTPatlMHzand 100 kH2 as function of temperature.

916

SURFACE AND INTRINSIC LOW FREQUENCY CONDUCTIVITY OF KTiOPO,

Vol. 97, No. 11

600 K (curve Z), and second and third cycle between 600 ~~~~~

ll~~~~v~y.~~ng~e~to

1100 K temperature cycle completed, the low fnequency conductivity shows a fairly stable tiequency dependence, i.e. further heating runs show reproducible results. However, afhx removing surface layer of about 10 pm spectra (1) and the behavior shown in Fig. 1 is retained. Therefore it can be concluded that the irreproducibihty shown in Fig. 4 is due to changesin the surfaceof the KTP crystal. It should be noted

that the strength of AC ehxtrical field was about 15 V/cm and we see no nodal

in the sample response. The

nonlinear e&cts start to be observed at high temperatures and about one order of magnitude higher field strengths. In order to investigate the nature of surface effects, the microprobe analysis was carried out. It is observed that i.) I

no contact material exists near the surface, ii.) the quantity of potassium and titanium are equal near surface and bulk and iii.) the qu~ti~ of p~~~~s

1

t

J

I

0

6

is smaller near the surface

than in the bulk. The spatiai resolution of our microprobe analysis was restricted to 2 5pm. Thus no profile of the phosphorus content can be given. However, the nature of the surface layer can qualitatively be related to phosphorus depletion, being responsible for an additional relaxation in the low frequency spectrum of KTP at high temperatures (above about 500 K). Because the activation energy of the surfirce layer ~~u~~~

is much higher than the bulk

(about 1.05 eV, compared to 0.53 ev), the ~~~~~

Fig. 4. Results of anneahng experiments measured at 600 K: 1 -fusttemperaturenmto6OOK 2-afterheatingrunto700Kandcoolingdownto600K, 3 - temperature cycle 600 to 900 K 4 - temperature cycle 600 to 1100 K. Given are the dielectric constant +s’(above) and conductivity u’ (below).

of

the surface becomes higher than in bulk (m our case for T 2 780 K) and the blocking e&t

of the surface disappears.

This explains the decrease of e’ above 780 K in Fig. 3.

related with the high degree of onedimenaionality of the

It ah&d be pointed out that the surface layer is created not only by heating itself but is it a cooperative effect of high t-e

KTP at temperatures 300 - 500 K. This dependence can be

and AC voltage. Qniy heating without AC

charge transport. At higher temperatures we observe an additional relaxation due to the surface -on.

It turns

out that phosphorus is released from the cry&

~~~~~~,~ttoa~chs~~~. Acknowiedgement - A. Pimenov grate&& acknowiedges In con&&on

a power law behavior u(w) - 0’ with

s n 0.1-0.2 is observed in the low &equency conductivity of

financial

support

from

Deutscher

Academischer

AustaMMenst.

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2.

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p. 288 (I982).

_.-..- ..__ -

Vol. 97, No. 11

SURFACE AND INTRINSIC LOW FREQUENCY CONDUCTIVITY OF KTiOPO.,

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