Short Notes K 73

phys. stat. sol. (a) __ 83, K73 (1984)

Subject classification: 14.4; 22.8 .1

Fachbereich Physik, Universitat Osnabriick 1 )

Li' Ionic Conductivity in L iXO,

BY H. FRANKE

Introduction LiNbO single crystals are widely used in electrooptics and non-

l inear optics. Diffusion processes in this material are important for the pro-

duction of optical waveguides. The dark conductivity of pure crystals at room

temperature is very low (< 1 0 L2 c m ). Investigations of the charge t rans-

port mechanism in LiNb03 single crystals have been performed at higher tem-

peratures (T > 350 O C ) by Jorgensen and Bartlett /2/ and Bergmann /l/.

Jorgensen and Bartlett found the electronic conductivity at 700 OC t o be

electronic at low oxygen partial p re s su res and to be ionic at one atmosphere of

oxygen. They reported that the ionic conductivity could be increased by a factor

of 1 0 by adding Li20. Bergmann also proposed a considerable fraction of ionic

conductivity using stoichiometric L i m o 3 . Lapshin and Rumyanzev /3/ studied

the diffusion of Na, Rb, Cs, and Nb. From these results one should expect an

activation energy for the Li' diffusion smaller than 0.5 eV.

This investigation was performed in o rde r t o look for the Lit ionic con-

ductivity of L i m o 3 in the temperature range 50 t o 300 OC, which is important

for most applications.

3

-18 -1 -1

3 Experimental Undoped single crystals of the dimension 4~5x1 mm have

been used for the conductivity measurements. Usually L i m o 3 single crystals

are grown from the congruent melt containing 48.6 mol% L i z 0 and 51.4 mol%

Nb205. This composition is also found in the crystals /4/ and leads to a cer ta in

amount of Lit vacancies. The electrode arrangement for measuring the dc Li'

ionic conductivity was e

lLil L i m o 3 1 ~g , LiNb03 single crystals were contacted to molten metallic Li. The molten Li had

to be kept under a dry atmosphere o r under vacuum (0.13 Pa). When this contact

arrangement (1) was used Li' ions could diffuse into the L i m o 3 crystal . A

s imi l a r arrangement served for measuring the Li' conductivity of Li3N /5/.

1) Barbarastr . 7, D-4500 Osnabruck, FRG.

A E @loo OC

(ev) (Q-' c m - l )

I F 0.5 1 3 x 1 0 ~ ~ ~ Li+

II f 0.7 3 ~ 1 0 - l ~

O200 O C

( Q-' c m - l )

2x1 0-1 O

2xlO-l'

Additional charge t ransfer due to Li' diffusion was measured in t h e steady

state a t different temperatures.

In order t o find out the electronic contribution t o the charge t ransfer ac-

cording to arrangement (1) measurements with passive electrodes have been also performed,

(3 Ag (Pt( LiNb031Age . (2)

The use of Ag a s anode contact only did not show detectable differences up to

200 OC. A constant current source (1 0 kV, 1 mA) and a Keithley pA meter were

used. The s i lver electrodes were attached by using silver paste. Results and discussion Below 300 OC the d c conductivity of LiNbOQ was

found to be more than two o rde r s of magnitude higher using metallic Li a s the

I passive

anode than the conductivity in the case of an Ag anode (Table 1). In Fig. 1 the

measured conductivities a r e plotted for passive contacts (curve a ) and metallic

Lf a s the anode in vacuum o r argon atmosphere (curve c) . When a i r is allowed

1.25 1x1~-16 ~ ~ 1 0 - l ~

u;/( I, _f

T

Fig. 1. Semilogarithmic presentation of dif- ferent dc conductivities of LiNb03 single crys- tals; ( a ) passive contacts, in vacuum; (b) metal- lic Li a s the anode, in air, ( c ) metallic Li as the anode, in vacuum

Short Notes K 75

Fig. 2. Semilogarithmic presentation of dc Li+

(a) parallel to c-axis, 0 in vacuum, A argon; (b) perpendicular to c-axis

- -- temperature ('0 250 200 150 700 50 ionic conductivity of L i m o 3 single crystals; ' in

to enter the recipient the conductivity drops to values s imilar t o those for passive contacts ".., (curve b). The experimental values for the

3b ' contact arrangement (2) can be described by $Kl)-

Gel = Go exp( -A Eel/kT)

as electronic conductivity (Fig. 1).

When the metallic Li electrode gets contact with wet air, LiOH is formed accompanied by protons. Recently some ionic exchange treatments with Limo3 have turned out to be due to Hi/Li+ exchange /6/. Bollmann and Stohr /7/ have shown that H-doped L i m o 3 shows a large increase of conductivity. The thermal fixing process of volume phase holograms has turned out t o be due to mobile protons / B / . If the elevated conductivity measured with Li electrodes is also hydrogen controlled, the values should increase upon air inlet. The opposite, however, was observed.

F o r metallic Li electrodes (1) the conductivity values can be expressed as mainly ionic conductivity /5L

bLi = OO exp(-AE /kT) . Li

It was measured parallel and perpendicular to the crystallographic c-axis. Activation energies were determined from the above equations. The achieved

values are listed in Table 1.

A transfer experiment was performed qualitatively. A current of 100 nA was applied for 72 h, carrying 0.026 As or 2

istic red colour due t o Li could be detected in the flame probe of the dissolved cathode material. This was not possible after a usual electronic charge t rans- port of the same magnitude.

Li as equivalent. The character-

Single crystals of L i m o 3 grown in air contain a certain amount of hydrogen, which can be detected via the OH- vibration. Measurements of the IR spectrum

K 76

of the crystal after the t ransfer experiment for Li+ showed an unusual well re-

solved spectrum. The OH- spectrum of as-grown crystal is broadened. One reason f o r this may b e statistically distributed Li vacancies in the neighbour-

hood. These Li+ vacancies are probably partly occupied after the Li in-dif-

fusion. The intensity of the OH- spectrum did not rise during the t r ans fe r ex- periment. Thus, there is no evidence f o r additional H+ in-diffusion.

In summary we conclude that the dc ionic conductivity of LiNb03 due to

Li' could be measured using metallic Li electrodes. Activation energies of

0 . 5 eV perpendicular and 0. 7 eV parallel t o the c-axis were determined. IR

spectra of OH centers give hints upon Li in-diffusion.

physica status solidi (a) 83

+ +

I wish to thank E. Kratzig for valuable discussions and A. F o r s t e r for

measuring the IR spectra.

References

/1/ G. BERGMANN, Solid State Commun. - 6, 77 (1968).

/2/ P.J. JORGENSEN and R.W. BARTLETT, J. Phys. Chem. Solids - 30,

/ 3 / W . Y . LAPSHIN and A. P. RUMYANZEV, Izv. Akad. Nauk SSR, Ser . neorg.

/4/A. RAUBER, in: Current Topics in Nat. Sci., Vol. 1, Ed. E. KALDIS,

/5/;U. VAN ALPEN, A. RABENAU, and G.H. TALAT, Appl. Phys. Letters 30,

/6/ J . L . JACKEL and C.E. RICE, Appl. Phys. Letters - 41, 508 (1982).

/7/W. BOLLMANN and H.-J. STOHR, phys. stat . sol , (a) 39, 477 (1977).

/8 /H. VORMANN, G. WEBER, S. KAPPHAN, and E. KRATZIG, Solid State

2639 (1969).

Mater. - 12, 2199 (1976).

North-Holland Publ. Co., 1978 (p. 481).

- 621 (1977).

-

Commun. - 40, 543 (1981).

(Received March 2, 1984)