An EPR study of different Gd3+ centers in LiNbO3

Solid State Communications,

Pergamon Press.

Vol. 12, pp. 737—740, 1973.

Printed in Great Britain

AN EPR STUDY OF DIFFERENT Gd3~CENTERS IN UNbO3 B. Dischler, J. R. Herrington, A. Räuber, and J. Schneider Institut für Angewandte Festkorperphysik dci Fraunhofer-Gesellschaft, D.78 Freiburg, Germany (BRD) and W. Urban

*

Institut für Angewandte Physik der Umversitat, D.53 Bonn, Germany (BRD) (Received 15 January 1973 by E. Moliwo)

Electron Paramagnetic Resonance (EPR) and Zero-(magnetic)-Field (para. magnetic) Resonance (ZFR) spectra have been observed for LiNbO3 :Gd. EPR spectra measured at 9 and 35 GHz consisted of at least seven indepen3~. dent two The patterns; most intense all of predominantly patterns have axial been symmetry analyzed inand detail, all ascribed and while to one Gd was found to be axially symmetric the other, though predominantly axial, had a small rhombic crystal field term. The ZFR spectrum, measured between 1 and 18 GHz, consisted of at least two patterns of zero fIeld transitions. Both EPR and ZFR spectra were found to be insensitive to variations in LiNbO 3 stoichiometry, c- or a-axis crystal growth direction, and various sample treatments.

LiNbO3 is one of the more interesting and intriguing hosts for transition metal and rare earth imperfections. From a practical standpoint, this host is interesting because of its non.centrosymmetric crystal structure, i.e. single crystals are piezoelectric, pyroelectric, and optically nonlinear permitting such applications as phase-matched second harmonic generation or para-

We report here Electron Paramagnetic Resonance (EPR) and Zero-(magnetic)-Field (paramagnetic) Re. sonance (ZFR) spectra which clearly show the existence of two physically inequivalent Gd~centers to~ gether with a number of magnetically inequivalent 4’5 Gd~centers LiNbO3. Of the several rareinearth” and transitioninmetal3~’defects reported LiNbO 3, 1 configuration to be reported. Gd~ Gd’~ is the first f and the previously reported5 Nd3 are the only defects found to form physically inequivalent centers in LiNbO 3 with comparable concentrations.

metric frequency conversion coherent emission from rare earth defects withinofthe same host,1 photo. refractive storage of volume phase holograms,2 and optical rectification.3 From a basic point of view, liNbO3 is intriguing because of the existence of three different six-fold coordinated axial sites for the incorporation of formal defects cation and thevalences relatively largecould difference between which

Single crystals of LiNbO3 were grown using 2 from:Gd both congruent and the enriched Czochralski method’ Nb melts containing Gd 2 03 in concentrations ranging from 0.01 to 1.0% by weight. X-ray fluores. cence measurements confirmed that the Gd concentrations in these crystals were close to the nominal doping level. Crystals were grown in air from both cand a-axis seeds and slowly cooled in an auxiliary furnace to improve crystal quality. EPR spectra were

permit several charge states of the same dopant to exist simultaneously. *

Previously: Physikalisches Institut der Universitat Karisruhe. 737

AN EPR STUDY OF DIFFERENT Gd3~CENTERS IN UNbO3

738

recorded with standard 9 and 35 GHz spectrometers. ZFR spectra were recorded usingbetween the spectrometer 3 operating I and 18 described by Urban’ GHz.

——

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___________________________ ZFR300Gd3 LINbO3 K

LiNbO 3’ ~ 1.300K 3 Gd ~‘.35GI.$z

U!

I

‘U

L

~

~ ~ ~e.n.25 ~ 5kG

~

Gon. 1

MoQrIetIC

15kG F,iId

I

I 6SGHi

~

-~

10kG

I

I

70Hz

7.56Hz

v—

Gen • 25 20kG

K

FIG. 1. EPR spectrum observed at 35GHz for a UNbO 3~single crystal at room temperature with H3II :Gd c. The first derivative of absorption is shown vs. magnetic field. The two prominent fine structure patterns are labelled I and II. Note that the gain setting was changed as indicated. The EPR spectrum observed at 35 GHz from a typical LiNbO 3 :Gd single crystal is shown in Fig. i for the applied magnetic field, H, parallel to the crystallographic c-axis. For the same nominal Gd concentrations, this spectrum was found independent of melt stoichiometry, axis of growth, or sample treatments such as ‘poling’ or annealing. The H It c 3~ spectrum is composed of at seven differentaxial Gd fine structure patterns,’4 all least of predominantly symmetry. As the magnetic field was rotated away from c, the patterns labelled I and H in Fig. 1 broadened somewhat but a normal angular dependence was observed. The lines of the other fine structure patterns split and broadened with appreciable overlapping and it was not possible to trace their angular dependence. The angular dependences of patterns I and II were fitted within experimental error using results of a computer diagonalisation of a matrix representation of the following spin Hamiltonian.’5 = g~H~S~ +~ S~+ ~ 5~ + (1/3)b~O~ + (l/60)b2O~+ (lIl260)b~O~ + (1/3)b~O~ + (l/36)b~O~ + (I/l260)b~Ot +b~O~ +(l/6)b~O~ +(l/24)b~O~

130 GHjI

1L.0 0Hz

I GHz U.S

I 0Hz 15,0 _____

Fr.qu.ncy

FIG. 2. Relevant portions of the ZFR spectrum observed for a LiNbO 3~single crystal in the range 3 :Gd 1—18 GHz. The assignments were made using the parameters of Table 1. Due to experimental limitations the frequency scale is nonlinear in the lower trace. The ZFRthe signals are centered at the minima in of thethe traces and halfwidth is a qualitative measure statistical scattering of the zero field splittings.

Here, the parameters g~ g~,and g~are the g factors for H assumed parallel to the a, b, and c crystallographic axes, respectively. The O~operators are the conventional operator equivalents for an effective spin of 7/2 which are used to represent the crystal field interaction. The b~parameters are nonzero only for orthorhombic symmetry defects. ,

The results of our analysis of the fine structure patterns labelled I and II in Fig. 1 are given in Table 1. For comparison the parameters reported’6 for ~2 03 :Gd3~are also included. As shown in Table I, the angularbydependence of patternterms I wasinaccurately described the axial symmetry the spin Hamiltonian of equation 1 but the angular dependence of pattern II required the inclusion of a small rhombic

Vol. 12, No.7

Parameter*

=

g1

=g~=g~

b~ b°~ Ib~I b~

Ibti b~ b~ *

AN EPR STUDY OF DIFFERENT Gd3~CENTERS IN LiNbO3 Table 1. Spin Hwniltonian parameters for LLNbO 3 :Gd~ 34 IJ.Nb0 3~ LiNbO3 : Gd 3 : Gd Pattern I Pattern II

739

Al 203 :Gd3t

L9916(5) 1.9916(10)

1.9916(5) 1.9916(10)

1.9912(5) 1.9912(5)

+ 1185 (13) + 8( 3) + 1(1) 33(17)

+ 1260(20) + 8( 4) + 1 ( 2) 33(17)

+

—

1033 (2) 26(1) 1(0.5) 18(1)

—

—

—

0( 5)

+ +

+

40(10)

—

—

—

—

5(0.5) — — —

Crystal field parameters are given in units of iO~cm~.

t Included for comparison, see reference 16. crystal field term, i.e. b~* 0. Those parameters for which no values given inaccuracy. Table 1 have been sign determined withare sufficient The not absolute for the bI parameter was determined by observing the intensity changes in the EPR spectrum at 4.2 K induced by thermal depopulation. Relative signs within each set of parameters follow from data analysis (note that the sign of b~was not determined).

Relevant portions of the ZFR spectrum for2.a The 3~sample are shown in Fig. typical LiNbO3 :Gd peculiar lineshapes are produced by a small amplitude square wave magnetic field modulation used in phase sensitive detection of the signals. Between 1 and 18 GHz four ZFR signals were observed and assigned as indicated in Fig. 2, i.e. the two signals near 7 GHz are assigned to the AM~= ±1 transitions between the ±1/2 and ±3/2 Krarners doublets of the two Gd3’ centers labelled I and ii in the EPR spectrum of Fig. I and the two signals near 14.5 GHz are assigned to the = ±1 transitions between the ±3/2 and ±5/2 Kramers doublets. The frequencies observed in the ZFR spectrum agree well with the zero field splittings predicted from a computer diagonalisation of a matrix representation of the spin Hamiltonian in equation 1 using parameters obtained from the EPR analysis and listed in Table 1. The most probable sites for the Gd3~ions in LiNbO 5~sites, and since both lie 3 are the Lt and Nb

on the threefold axis, the site symmetry substi3’ ions should be trigonal. Theofdominance tutional Gd of the axial crystal field parameters in Table 1 suggests that the Gd3’ ions indeed do lie on the threefold axis, i.e. substitute for Li’ or Nb5’ or occupy the structural vacancy, while the small orthorhombic perturbation in pattern II could be caused by a nearby charge cornpensator. The EPR and ZFR results give evidence, that Gd3~in LiNbO 3 forms at least two physically different and several different centers. However, no decisionmagnetically on whether Gd34 substitutes for If or Nb5~’or for both can be made using the present results. Two tentative explanations for the observed patterns I and II are the following: (i) Gd3~is found on two of the three different axial sites, which are sixfold oxygen-coordinated. (ii) Gd3~’is found only on one of the three sites, while the differences in the patterns are produced by perturbations due to neighboring charge compensators. In both cases the orthorhombic term of pattern II must be attributed to a charge compensator which lies off-axis, while for pattern I the charge compensator may either be absent or must lie on the same threefold axis as the Gd34. In this situation ENDOR may be capable of identifying the ions surrounding the Gd34, including possible charge compensators.

AN EPR STUDY OF DIFFERENT Gd34 CENTERS IN LiNbO3

740

Acknowledgements We wish to thank Dr. G. Brandt for his X-ray fluorescence measurements which determined the Gd concentration. We also would like to thank Prof. K. A. MUller and Mr. Berlinger at IBM Research Laboratories in Zurich-Rüschiikon for their —

Vol. 12, No.7

hospitality and their assistance with the 4.2 K measurements. The technical assistance of Mr. F. Donner and Mr. B. Matthes in the growth and preparation of the LiNbO3 samples is gratefully acknowledged.

REFERENCES I.

JOHNSON L. F. and BALLMAN A. A.,J. app!. Thys. 40,297(1969).

2.

CHEN F. S., LAMACCHIA J. T. and FRASER D. B.,AppL Thys. Lett. 13, 223 (1968).

3.

AUSTIN D. H., GLASS A. M. and BALLMAN A. A.,Phys. Rev. Lett. 28,897(1972).

4.

BURNS G., O’K.ANE D. F. and TITLE R. S.,Thys. Rev. 167,314 (1968).

5. 6.

EVLANOVA N. F., KORNIENKO L. S., RASHKOVICH L. N. and RYBALTOVSKII A. 0., Zh. Eksp. Teor. Fiz. 53, 1920 (1967). [English translation: Soviet Phys. Solid State 26, 1090 (1968).] ARSENEV P. A. and BARANOV B. A., Phys. Status Solidi (a) 9, 673 (1972).

7. 8.

REXFORDD.G.,KIM Y. M. and STORY H. S.,J. Chem Phys. 52, 860 (1970). GLASS A. M.,J. chem. Phys. 50, 1501 (1969).

9.

HERRINGTON J. R., DISCHLER B. and SCHNEIDER J., Solid State Commun. 10, 509 (1972).

10.

TAKEDAT.,WATANABE A. and SUGIHARA K.,Thys. Len. 27A, 114 (1968).

12

BALLMAN A. A.,J. Am. Ceram. Soc. 48,112(1965).

13 14

URBAN W.,Thys. Status Solidi (b) 47, 543 (1971). Note, from a careful examination of the observed intervals one may exclude hyperfine interactions as being responsible for the additional structure on each of the fine strucutre transitions. The conventional numerical factors have been used, see e.g. ABRAGAM A. and BLEANEY B.,Electron Paramagnetic Resonance of Transition Ions, Oxford (1970). GESCHWIND S. and REMEIKA 3. P.,J. Phys. Rev. 122, 757 (1961).

15 16

Spektren von LiNbO3 :Gd wurden mit EPR and ZFR (Nullfeld-Resonanz) beobachtet. Die EPR Spektren bei 9 und 35 bestehen aus mindestens sieben 3’ zugeordnet. Die beiden Linienmuster wurden im und em. unabhangigen Linienmustern; alleintensivsten mit uberwiegend axialer Symmetric alle Gd analysiert, wobei sich fur das eine axiale Symmetric ergab, während zelnen das andere zwar ttberwiegend axial war, aber einen kleinen orthorhombischen Kristallfeldterm hatte. Das ZFR Spektrum zwischen 1 und 18 GHz bestand aus rnindestens zwei Linienmustern von Nul1feldUbergan~en. Sowohl EPR als auch ZFR Spektren waren unempfIndlich gegen Anderungen in der LiNbO 3 Stochiometrie, c- oder a-Achsen Ziehrichtung des Kristalls und verschiedene Nachbehandlung der Proben.