Pergamon
00221697(94\8002+A
@
J. Phys.Chem.So/ldsVol. 55, No. 7, pp. 589-593,1994
Copyright O 1994ElsevierScienc€Ltd
Printedin Great Britain. All rights reseiled
0022-3691
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SOLID STATE NMR STUDY SUPPORTING THE LITHIUM
VACANCY DEFECT MODEL IN CONGRUENT LITHIUM
NIOBATE
J. Bruunr,,t E. BonN{ and Ts. Mrrzcnn}
fAnorganisch-chemisches Institut der Technischen Universitdt Mrinchen, LichtenbergstraBe 4, 85747
Garching, Germany
{Lehrstuhl frir Angewandte Mineralogie und Geochemie der Technischen Universitdt Mtinchen,
Lichtenbergstra&e 4, 85'747 Garching, Germany
(Receh:ed 12 Nouember 1993; accepted I March 1994)
Abstract-e3Nb and 7Li wideline- as well as MAS-NMR measurements were performed on powdered and
single crystal lithium niobate of defective congruent composition (48.4%LirO;51.6% NbrOr) using a
magnetic field strength of 7.05 Tesla with the aim to distinguish between a lithium vacancy defect model
(l% of the Nb atoms in a different site) and a niobium vacancy defect model (5% of the Nb atoms in
a different site). Although the line width of the 7Li signal of powdered lithium niobate could be reduced
to 0.6kHz by using MAS-NMR with a rotational lrequency of 4000Hz, thsre was no second 7Li signal
apparent, indicating different chemical surroundings for 7Li nuclei caused by the defect structure. The
wideline spectra of e3Nb NMR measurements on lithium niobate single crystals in various orientations
do not exhibit a second resonanc€ and therefore contradict several earlier e3Nb NMR studies from which
a niobium defect model was substantiated. Additionally, earlier NMR measurements could be explained
on the basis of the complex e3Nb MAS spectra of powdered material.
Keywords'. D. crystal structure, D. defects, D. nuclear magnetic resonance (NMR).
INTRODUCTION
The technical application of congruently grown
lithium niobate (LN) single crystals includes the
production of surface acoustic wave (SAW) band
filters to be integrated in each new TV set; about
250,000 filters are produced daily in one plant. In
spite of the high commercial interest in this material,
there is still a discussion about the defect model in the
congruent non-stoichiometric
LN.
The ideal
structure of LN at room temperature can be
described as distorted hexagonal close packing of
oxygen atoms forming three different octahedral sites
[]. The smallest octahedral site is occupied by Nb
atoms, the octahedron of intermediate size is
occupied by Li atoms and the greatest remains unoccupied [2]. Thus, two-thirds of all octahedra are
occupied by cations. Along the axis c6"* th€
occupancy of octahedra can be written as
{Li o Nb Li o Nb}, where "o" denotes an empty
grown
octahedron.
Czochralsky
crystals
of
congruently melting composition consist of 48.4oh
LirO and 51.6% Nb205 [3], which differs from the l:1
ratio of cation concentrations described by the formula LiNbO3 and must therefore have a defect
structure. Until 1993, a model was favoured [4] that
includes Li substitution by Nb atoms with accompanied Nb vacanciesat Nb sites for charge
neutrality. This model was initiated by an NMR
study[5]. RecentX-ray and neutronscatteringstudies
16,71, however, revealed a fully occupied Nb
sublatticecontradictingthe Nb vacancydefectmodel
proposed in Ref. [4]. Thus, the necessityarisesto
improveseveralNMR measurements
[5,8-14],some
of them supporting the Nb vacancy defect model.
PRESENTSTATEOF NON-STOICHIOMETRY
DEFECTINTERPRETATION
IN LN
The excessof Nb in congruentLN is only expected
on Li sites f4,6,7,151. At the tetrahedralsite it
cannot be expected[6]. The required local charge
neutrality can theoreticallybe guaranteedeither by
oxygendeficiency[7], by Li vacanciesat Li sitesor
by Nb vacanciesat Nb sites.Thesedefectscan be
written in the three notations listed in Table l.
Model I can be ruled out by precisemeasurements
of densityaccordingto Refs[8, 19].Models2 and3
describepoint defectsin an averagedstructure.The
excessof Nb atoms replacesLi atoms at Li sites.
Models 2 and 3 differ in the way of achievingcharge
neutrality. In model 2 the occupation of the Nb
590
J. Bwrwt et al
sublatticeremains 100% and every four vacant Li
sites(chargeof -4) are statisticallyaccompaniedby
one Li site occupiedby a Nb atom (differencecharge
chargeof
of +4 at this site).In model 3 the excessive
five Nb atoms on Li sites(charge +20) is compensatedby four vacantNb sites(charge- 20), as shown
in Fig. l. Thus, in model 2 charge neutrality is
achievedby Li vacancies,whereasin model 3 the
charge neutrality is supposedto be realizedby Nb
vacancies.One can seefrom the formula in Table I
that in model 2 about I % of Nb atoms are located
on "wrong" sitesand model 3 requiresabout 5% of
"wrong" positionedNb atoms.NMR measurements
of e3Nb[5, 14] indicate different chemicalsurroundings (coordination)of about 5% of all Nb atoms in
congruent LN, supporting defect model 3. This
model has also been supported by a later X-ray
structureanalysis[4]. CombinedRietveld diffraction
analysis with both X-rays and neutrons (time of
flight) recentlyshoweda fully occupiedNb sublattice
[6,7], strongly supportingdefectmodel 2. In Fig. I,
the cationic sequences
along c5"*for models2 and 3,
as well as for the idealizedLN structureand llmenitetype structure,which can also be realisedin LN [20],
are shown. The interpretation of models2 and 3 on
an atomic scaleshowsthat Nb atoms at Li sitesin
models 2 and 3 necessarilygive a cationic sequence
parallelto the axiscn"*of two Nb atomsin neighbouring octahedra(seeFig. 1). Model 3 was proposedby
Donnerberg et al. Bll to consist of two different
(a) and (b), wherethe sequence(a) can be
sequences
interpreted as a disorderedIlmenite-type structure.
Model 2 also consists of two different cationic
(a) and (b). It waspredictedbyLerneret al.
sequences
and
confirmed
by Donnerberget al. [21] on the
[5]
basis of computer calcul4tion of defect energy
minima.
Thus, the Li vacancydefectmodel is supportedby
comput€r simulations[21] and, as mentionedabove,
by diffraction studies[6,7], contradictingthe results
of NMR [5, 14] and a structurereport [4]. To clarify
this contradictiona new NMR studv was oerformed.
EXPERIMENTAL
All the measurementswere made on a Fourier
Transform Bruker MSL 300 Solid State NMR
spectrometer.Thus, the Larmor frequenciesfor 7Li
and e3Nbwere116.6and 73.5MHz, respectively.
LN
was measuredeither as a finely ground powder or as
single crystals with defined direction of the c-axis.
For the MAS spectra,the powdered sampleswere
packedin 7 mm ZrO2rotors and spunwith rotational
frequencies up to 4 kHz. Single crystals were
measured in the form of manufactured plates
(dimensions:
5 mm x 0.5mm x l0 mm) in 5 mm (7Li)
and 10mm (e3Nb)solenoidalcoils.The piezoelectric
response l22l of the single crystals could be
suppressed efficiently by wrapping them with
adhesive tape. Therefore, a simple single pulse
sequence
with a short 90'pulse (2 ps) and a deadtime
delayof 8 p s could be used.Further detailsare given
with the respectivespectra.
SOLID STATENMR OF LITHIUM NIOBATE
LN offersthreedifferentnucleithat can in principle
be measured by NMR spectroscopy.However,
quadrupolar t?O with its low natural abundanceis
still beyond reach for solid state NMR of samples
that are not enriched.1Li and e3Nb,on the other
hand, have high natural abundances(92.6%oand
100%), but are also quadrupolar nuclei, with /: !
and l, respectively.Therefore,the NMR signalsare
often very broad and evenasymmetricdueto quadrupolar interactions with the electric field gradient.
Severalstrategieshave been developedrecently in
order to decreasethe line width of quadrupolar
nuclei: magic angle spinning (MAS) [23], dynamic
angle spinning (DAS) 1241, double orientation
rotation (DOR) [25] and using as high a permanent
magnetic field as possible.Additionally, the information contentcan be increasedby two-dimensional
nutational NMR [26]. WhereasDAS and DOR are
still technicallydemanding,MAS and high magnetic
fields are readily availabletoday. It is this approach
that has been chosenhere, using a magnetic field
strengthof 7.05Tesla.
7Li NMR SPECTROSCOPY
Early 7Li studies have already been done by
Halstead[8], who investigatedatomic motions in LN
singlecrystals.At the very low magneticfield strength
availablethen(0.75Tesla,correspondingto a Larmor
frequencyof 8 MHz), the minimum valuefor the line
width at room temperaturewas about 9 kHz. A later
Table 1.
Model no.
J
Parametricformula
Krciger-Vink notation
Lir _2,Nbo3_,
[Lir_5,Nb,]Nbo3
[Lir _ 5*Nbs,]Nbr_4,03
2 VLiVo
NbLi4VLi
5NbLi4VNb
Chemicalformula (48.4o/oLi2O)
Lio.e37rNbo2.e686
[Lio.e47Nbooro6]Nbo3
[Li0.e47Nb0.053]Nbo.rrruO,
Authors
Bollmann [7]
Lerner [5]
Abrahams[4]
Solid stateNMR of lithium niobate
ldeol structure
_____________o__
aa
a
a_____-___-_.o
a
aa
o
Defect Model 2
o
a
I
^
a
a
..E
a__
n{
. Nb qtorns
O
o ljotonr
Q_______________
a
O-
o
OT
a
a
l"-rE,92Ar{
(llo) -
a
o
a
o
a
a
o
o
a
a
o
o
a
a_
seqrerpeb)
onellnl€
DefectModel 3
a
oa
a_
a
o)
Sequence
fourflrn$
llmenitetype structure
a_
o
o
o
'--:{-
a
o_
v
a_
o
o
o
o
o
a
rt
ao
a
a
o-l
a
a_____________9 _
o
a
o
o__
g
o
591
o
a
ioi
iia
i'+4
i
a
Q__
o--+---+i6 l .
o*+
o
O
iai
---+---.i-j
ari
'
o
a
o
r"r
a
L
a
a
a
:--:!4-
ia
iai
---l---Lo
aii
--i3-:ia
o _i::_
a
a
a._
o
o
o
a
o
o
a
m'ffro) mmo)
Fig. 1. Cation arrangements of ideal LN, idealized llmenite-type LN, defect model 2 and defect model
3 are drawn in a cut parallel to the plane (1120). The formal charges of the "wrong" occupied oxygen
octahedra in delect model 2 and defect rnodel 3 are declared. The relationship between the cation
sequences in the llmenite-type structure and delect model 3 is marked by the dashed rectangle.
study of LN powder samples[9] did not make a very
reliable impression, since the spectra were grossly
distorted,maybedue to clippedFIDs or piezoelectric
nnging 1221.
If one considersonly the oxygencoordination of
the Li atoms, there is no reasonfor a secondNMR
resonancecaused by the defect structure given,
becauseall Li atoms occupytheir regular lattice site.
The situation changesif the effect of vacanciesand
Nb atomspositionedon Li sitescausedby the defect
structureis takeninto account.In this case,in model
2 as well as in model 3 the perfectsequenceof the Li
atomsalong(22.1)is disturbedby vacancies
and Nb
(seeFig. 1), which may give rise
atoms,respectively
to a second 1Li resonance.While a single crystal
measurementof 7Li could not diminish the line width
found earlier[8], rotating a powder sampleabout the
magic angle with a rotational frequencyof 4000Hz
gavea residuallinewidth of only 0.6kHz (seeFig. 2).
This meansan improvementof more than an order
of magnitudeascomparedto earlierresults.Thus, the
quadrupolarinteractionscould efficientlybe reduced
by MAS alonein this case.The overallappearance
of
the single signal is completely symmetric, all
rotational sidebands[23] with the samedistanceto
the centralline haveequalheight;the isotropicline
does not show any shoulder,which additionally
592
J.Brvunr et al.
makesthe assumptionthat another signalshould be
hidden underneathunlikely.
So, the 7Li NMR spectrumsupportsneitherdefect
model 2 nor defectmodel 3, although the impact of
the surroundingsof the 7Li nucleuson its chemical
shift is not known and the overall chemical shift
rangel27l is about three times the observedresidual
line width.
9Nb NMR SPECTROSCOPY
As compared to 7Li, e3Nb has a much greater
chemicalshift rangeofseveralthousandppm [27]and
thereforebecamethe object of further investigation.
esNbNMR spectroscopy
alwaysusesthe most intense
central-ll2+l/2 transition.
Early NMR studiesof e3Nbin LN have already
beenperformedin 1967by Petersonand Bridenbaugh
[0], who determined the quadrupolar coupling
constant (22.02MH2). Later, he used e3NbNMR
spectroscopy
of powdersamplesas a probe of stoichiometry in LN [1j. Finally, an attempt was made by
Petersonand Carnevale[5] to distinguishbetweenthe
defect models 2 and 3 (Fig. l) for Li-deficient maPPH
terials by single crystal e3NbNMR measurements.
In principle, due to the large chemicalshift range Fig. 3.73.5MHz e3NbMASspectra
of powdered
LN at the
indicated.
Singlepulseexcitation
with
of e3Nb(seebetow and ref [27]), different signalsfor rotationallrequencies
a 90' pulsewidthof 2 ps anda deadtime
delayof 5 ps.pulse
Nb nuclei in different chemical surroundings are
repetitionrate500ms.
expected.Consideringonly the oxygensurroundings,
both defectmodelscould theoreticallygivemore than
one e3Nb resonance.Besidesthe prominent e3Nb
The situation will becomemore complicatedif the
NMR signalof the regularlattice {o Nb Li o}, model
effect of Nb atoms and vacanciespositioned on Li
2 requires one more signal with about 1% the
sitesis examinedon Nb atomsin regularlattice sites.
intensity that stems from Nb atoms at Li sites
Model 3 was strongly favoured by Peterson and
{o Nb Nb o}. According to model 3, one more signal Carnevale[5], but later a careful measurementcomwith a relative intensity of about 5% could in prinbined with elaboratesimulation studiesshowedthat
ciple be expectedfrom Nb atbmsoccupyinglarger Li
the interpretation of the e3NbNMR spectrais less
octahedra{o o Nb o} and {o Nb Nb o} (seeFig. 1).
straightforward than thought previously [12]. The
conclusionsdrawn recently by Hu et al. ll4l are
dubious, becausethey are based mainly on e3Nb
NMR powder spectraof very bad signal-to-noise
ratio and the dubious secondsignal might just be a
part of the asymmetricwideline signal.
In contrastto the 7Li NMR measurements,
MAS
did not improvethe e3NbNMR spectrasubstantially.
The secondorder quadrupolarinteractionsare too
great to be removed by MAS alone. Even at a
rotational frequencyof 4000Hz the isotropiclines
cannot be resolved(Fig. 3). At the same time the
typical changesof the appearanceof the signalwith
.t.t.t,
-200
200
0
spinningfrequencyare obvious.Sincethe detection
PPl.I
of a resonance
of low intensitywasimpossiblein the
lLi MAS
Fig. 2. ll6.6MHz
spectrum of powdered LN.
complex
MAS
spectra, a single crystal study
Single pulse excitation with a 90' pulse width of 4 ps and a
analogous
to
performedby Peterson
the
one
deadtime delay of 5 p s; 2000 transients were accumulated at
et al. [l2l
a rotational frequencv of 4kHz.
was undertaken.The single crystalswere rotated
593
Solid stateNMR of lithium niobate
two-dimensional nutational NMR spectrum, no
second resonance with a different nutational
frequencycould be detected.
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0
1000
2000
PPI,I
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4848(1972't.
with a 90' pulsewidthof 2 ps anda
Singlepulseexcitation
Zotoy
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in
steps.
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none
of
these
about the axis c5"* 5'
8. HalsteadT. K., "/. Chem.Phys. 53,3427 (1970).
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The
line
a
shoulder.
minimum
width
spectraor even
10. PetersonG. E. and BridenbaughP. M., ,/. Chem.Phys.
(see
Tesla
kHz
magnetic
field
of
7.05
was
5
at our
,18,3402(1968).
Fig. a) and the maximum one 22kHz; the total 11. PetersonG. E. and CarruthersJ. R.,,/. Solid State
C h e m . 1 , 9 8( 1 9 6 9 ) .
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12. DouglassD. C., PetersonG. E. and McBrierty V. J.,
field
resonance
with
a
deviant
electric
a
second
Since
Phys.Reu.840, 10694(1989).
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5%ointensity as comparedto the main signal should
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havebeendetectedwith our procedure,thereare two
W. 5., Mod. Phys.Lett.85, 789 (1991).
15. LernerP., LegrasC. and Dumas1.P., J. Cryst.Growth
explanationsthat can be given.
3,4,231 (1968).
(l) The secondresonancefound earlierwasjust an 16. Nassau K. and Lines M. E., ,/. Appl. Phys. 41, 533
(1970).
artefact due to the low magnetic field strengths
17. BollmannW., Cryst. Res.Technol.18, 9, 1147(1983).
available then. This-together with recent X-ray 18. KovacsL. andPolgarK., Cryst.Res.Technol.2l,K10l
(1986).
results[6, 7]-would strongly favour defectmodel 2,
19.
HolmesR. J. and Minford W. J., Ferroelectrics
15,63
which requiresa secondresonancein the l% intensity
(1987).
regionthat cannotpossiblybe detectedin the spectra 20. Kumada N., Ozawa N., Muto F. and Kinomura N.,
J. Solid State Chem.57,267 (1985).
of strongly quadrupolar nuclei.
(2) The soughtafter secondresonancebehaveslike 21. DonnerbergH. J., TomlinsonS. M. and Catlow C. R.
4., J. Phys. Chem.Solids 52,203 (1991).
the main signaland is alwayshiddenunder the latter. 22. HughesD. G. and PandeyL., J. Magn. Reson.56,428
In this case NMR spectroscopywould not be the
fi 984).
right choice of probe to distinguishbetweendefect 23. Herzfeld J. and BergerA. E., .f. Chem.Phys. 73,6021
fl980).
models 2 and 3.
24. MuellerK. T., WootenE., W. and PinesA., J. Magn.
Reson.92,620 (1991).
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25. SamosonA. and Lippmaa8., J. Magn. Reson.84,410
(1e8e).
spectrumof an LN single crystal with the axis cn"*
A. and Lippmaa E., Phys. Rea. 828, 6567
aligned to the direction of the magnetic field, in 26. Samoson
(1983).
analogyto the one performedby Man el a/. with LN 27. MultinuclearNMR (Editedby J. Mason),PlenumPress,
powder [3], was undertaken.However, even in the
New York (1987).