Factors Controlling the 17O NMR Chemical Shift in Ionic Mixed Metal Oxides

Article abstract of DOI:10.1021/jp9622694

A wide range of 17O-enriched phases ABO3 and A2BO3 (A = Li, Na, Ca, Sr, Ba, and La; B = Ti, Zr, Sn, Nb, and Al) and related compounds has been synthesized and studied using 17O magic angle spinning (MAS) NMR spectroscopy.In these highly ionic phases, the 17O electric field gradients are small, and as a result highly resolved NMR spectra that reveal subtle structural inequivalences are observed.For titanates and zirconates the 17O chemical shifts fall in distinct, well-defined regions (372-564 and 280-376 ppm, respectively).The ratio of isotropic 17O chemical shifts from isostructural titanates and zirconates with the same A cation is constant, and this ratio is close to the ratio of the polarizing powers of titanium and zirconium.The B cation appears to be the dominant influence in determining the 17O chemical shift in these compounds.Additionally the number of oxygen resonances and the shift difference between them increases as the symmetry of the structure decreases. 119Sn MAS NMR has been applied to a variety of stannates and shows a large shift difference (68.2 ppm) between CaSnO3 phases with the ilmenite and GdFeO3 perovskite type crystal structures. 27Al and 17O MAS NMR have been used to study the conversion of lanthanum and aluminum sol-gel precursors to crystalline LaAlO3 perovskite. 17O NMR proves to be more informative than 27Al NMR and shows that the formation of LaAlO3 proceeds via the reaction of separate lanthanum and aluminum oxides initially formed.

Full text of DOI:10.1021/jp9622694

J. Phys. Chem. 1996, 100, 18539-18545
18539
17
Factors Controlling the O NMR Chemical Shift in Ionic Mixed Metal Oxides
†
‡
,†,‡
Timothy J. Bastow, Peter J. Dirken, and Mark E. Smith*
CSIRO DiVision of Materials Science and Technology, PriVate Bag 33, Rosebank MDC,
Clayton, Victoria 3169, Australia, and Physics Department, UniVersity of Kent,
Canterbury, Kent, CT2 7NR, U.K.
Harold J. Whitfield
Department of Applied Physics, Royal Melbourne Institute of Technology, P.O. Box 2476W,
Melbourne, Victoria 3001, Australia
X
ReceiVed: July 25, 1996
1
7
A wide range of O-enriched phases ABO
3
and A
2
BO
3
(A ) Li, Na, Ca, Sr, Ba, and La; B ) Ti, Zr, Sn, Nb,
1
7
and Al) and related compounds has been synthesized and studied using O magic angle spinning (MAS)
1
7
NMR spectroscopy. In these highly ionic phases, the O electric field gradients are small, and as a result
highly resolved NMR spectra that reveal subtle structural inequivalences are observed. For titanates and
zirconates the 17O chemical shifts fall in distinct, well-defined regions (372-564 and 280-376 ppm,
respectively). The ratio of isotropic O chemical shifts from isostructural titanates and zirconates with the
same A cation is constant, and this ratio is close to the ratio of the polarizing powers of titanium and zirconium.
1
7
1
7
The B cation appears to be the dominant influence in determining the O chemical shift in these compounds.
Additionally the number of oxygen resonances and the shift difference between them increases as the symmetry
119
of the structure decreases.
Sn MAS NMR has been applied to a variety of stannates and shows a large
phases with the ilmenite and GdFeO perovskite type crystal
structures. Al and O MAS NMR have been used to study the conversion of lanthanum and aluminum
shift difference (68.2 ppm) between CaSnO
3
3
27
17
1
7
27
3
sol-gel precursors to crystalline LaAlO perovskite. O NMR proves to be more informative than Al
NMR and shows that the formation of LaAlO
oxides initially formed.
3
proceeds via the reaction of separate lanthanum and aluminum
Introduction
latter structure is not a perovskite structure, but is closely related
2+
to it. For t > 1, BaTiO3 (rBa ) 1.52 Å, t ) 1.02) the symmetry
The perovskite structure ABO3 is one of the most important
in materials science and geophysics occurring widely in
ferroelectrics, piezoelectrics, thin film electronics, and optical
3
is tetragonal. In a cubic system all the oxygens are equivalent,
but as the symmetry is lowered the number of inequivalent
oxygen sites increases to a maximum of three. Deviations of
the perovskite structure from cubic to lower symmetry and the
associated loss of a center of symmetry are critical because they
are directly linked to many of the technologically important
properties of these materials. For example, BaTiO3 is a widely
used ferroelectric whose electric dipole is a result of the
1
devices. An estimated 80 vol % of the Earth’s lower mantle
is thought to be composed of (Mg,Fe)SiO3 perovskite, making
it the most abundant mineral in the Earth’s mantle, so that
interpretation of seismic data relies heavily on knowledge of
the perovskite crystal structure. The ideal perovskite structure
2
(e.g., SrTiO3) has cubic symmetry (Pm3m) with the only
2
-
4+
1
displacement of O and Ti from their ideal cubic positions.
variable the unit-cell parameter a, and consists of corner-linked
octahedra of oxygen anions with B cations at their centers. The
A cations fill the cuboctahedral interstices between the octahedra
1
+
4+
Other related phases are studied here include A 2B O3
compounds with NaCl-like structures: Na2ZrO3, Li2ZrO3,
5-7
with 12 coordination.3
,4
Li2TiO3, and Li2SnO3 . The oxygen ions show distorted cubic
close-packing and the cations occupy the octahedral interstices,
thus also forming a cubic close-packed network. It is the way
Noncubic perovskite structures are
subsequently formed by either cationic displacement (e.g.,
BaTiO3), tilting of the octahedra (e.g., the mineral perovskite,
CaTiO3), or a combination of both (e.g., NaNbO3). To a first
approximation the deviation from cubic symmetry is controlled
by the relative sizes of the ions within the lattice. This is
described by the tolerance factor t ) (RA + RB)/(RB + RO),
with t ) 1 in the case of an ideal framework structure.
Departures of t from unity result in a loss of cubic symmetry
and eventually, if t varies significantly, the adoption of another
1
+
4+
4
in which the A and the B atoms are distributed that
determines the actual structure. Li2SnO3 and Na2ZrO3 are
isostructural with Li2TiO3. Sr2SnO4 and Sr2TiO4 are examples
8
,9
of a K2NiF4-type structure, which is built up of perovskite
3
layers of composition KNiF and layers of composition KF in
3
the ratio 1:1. By stacking perovskite SrTiO3 with a thickness
of more than one layer and SrO layers along the c-axis, a range
4
+
n+1 n
of structures is formed with composition Sr Ti O3n+1, called
structure type altogether. For instance, in the case of B ) Ti ,
8
2+
Ruddlesden-Popper phases.
SrTiO3 (rSr ) 1.37 Å, t ) 0.97) has cubic symmetry, CaTiO3
rCa2 ) 1.16 Å, t ) 0.89) has orthorhombic symmetry, and the
+
Solid state 17O NMR is becoming increasingly established
as a tool for characterizing crystalline and amorphous oxide
materials in materials chemistry and geochemistry. It is
potentially a very attractive nucleus, since it has a wide chemical
shift range, making it very sensitive to subtle changes in the
(
+
4
Li2TiO3 structure (rLi ) 0.68 Å, t ) 0.67) is monoclinic. The
*
Address correspondence to this author.
CSIRO Division of Materials Science and Technology.
University of Kent.
†
‡
X
10
Abstract published in AdVance ACS Abstracts, November 1, 1996.
atomic scale structure of a material. Additionally it has only
S0022-3654(96)02269-1 CCC: $12.00 © 1996 American Chemical Society

1
8540 J. Phys. Chem., Vol. 100, No. 47, 1996
Bastow et al.
a moderate nuclear electric quadrupole moment (eQ). The
quadrupole interaction between eQ and the electric field gradient
a glass ampule and heating at 110 °C for 2 days. Li2ZrO3,
SrZrO3, Li2TiO3, and SrTiO3 were subsequently prepared by
reacting enriched Li2CO3 or SrCO3 with ZrO2 and TiO2 obtained
by hydrolyzing a zirconium or titanium propoxide solution in
(
(
efg, where eq is the maximum component) broadens the central
1/2, -1/2) transition to second-order11 which is only partially
1
7
averaged by magic angle spinning (MAS), contributing to the
isotropic shift and complicating spectral interpretation, but the
2-propanol with O-enriched H2O. Sr2TiO4 was obtained by
1
7
reacting O-enriched SrTiO3 and SrCO3 at 900 °C for 24 h.
Li2SnO3, SrSnO3, and Sr2SnO4 were prepared by reaction of
stoichiometric amounts of enriched Li2CO3 or SrCO3 and SnO2
at 900-950 °C for 24 h. Heat treatments were performed in
platinum crucibles or sintered alumina boats in a dry nitrogen
atmosphere. The reaction temperatures of the Sr phases were
lowered to 900-950 °C by adding 1 mol of SrCl2 per mol of
strontium perovskite as a flux. The zirconates were heated to
600 °C. CaZrO3, BaZrO3, CaTiO3, BaTiO3, and CaSnO3 were
obtained by double decomposition of Li2ZrO3 or Li2TiO3 (Na2-
ZrO3 in case of CaZrO3) with a BaCl2/NaCl or CaCl2/NaCl
eutectic mixture at temperatures of 600-850 °C. An X-ray
powder pattern of the CaSnO3 sample prepared at 600 °C
confirmed it was the low temperature hexagonal form with the
ilmenite structure. Subsequent heating to 950 °C converted it
into the GdFeO3 structure. A mixture of Nb2O5 and Li2CO3
1
2
efg can itself provide important structural infomation. This
interaction is described by the quadrupole coupling constant CQ
2
17
)
e qQ/h. It has been pointed out previously that CQ for
O
1
3
increases as the ionicity of the M-O bond decreases. Oxide
perovskites are ionic compounds in which the electrostatic bond
strengths (in Pauling’s sense) are all smaller than 1; i.e., they
3
are isodesmic. In such highly ionic compounds eq at the
oxygen site will be small so that the second-order quadrupolar
broadening will be very small, spectral resolution will be high,
and correction of the peak position for isotropic quadrupole
shifts is unnecessary. The MAS line width provides an upper
1/2
limit on CQ which will be less than (20νL(7.5∆) /3), where νL
is the Larmor frequency and ∆ is the root mean square line
14
width in ppm. However CQ cannot then be accurately deduced
from the central transition; nevertheless with small interactions
the first-order broadened noncentral transitions will produce
observable spinning sidebands whose intensity envelope can be
used to deduce CQ.15 The only drawback of O NMR is its
low natural abundance (0.037 atom %). Although some natural
abundance spectra have been obtained from highly ionic
1
7
was heated with O-enriched H2O at 105 °C in a sealed glass
tube for 72 h and subsequent heat treatment at 850 and 950 °C
each for 24 h produced LiNbO3. The preparation of Na2ZrO3
17
2
7
has been described elsewhere. The identity of all phases was
confirmed by X-ray diffraction with a Siemens diffractometer
using Ni-filtered Cu KR radiation.
1
4
compounds, most work requires isotopic enrichment. It has
1
7
been shown, that O NMR spectra can be readily obtained at
Isotopically enriched LaAlO was prepared by reaction of
3
an enrichment of only 2.5%.1
6,17
17
O NMR has subsequently
17
La O with O-enriched H O in a sealed tube at 130 °C for 72
2
3
2
proved to be very useful in providing insight into the synthesis
h, which formed lanthanum hydroxide La(OH) . The lanthanum
3
18
of many materials including transitional aluminas and sol-
gel produced ZrO216 and TiO2 , and was able to detect
nanoscale phase separation in SiO2-TiO2 gels.19 O NMR of
perovskites, other than the high temperature superconductors,
is somewhat sparse but has been used to examine the local
structure and oxide ion motion in defective cubic perovskites
such as Ba(In0.67Zr0.33)y.20
hydroxide was then stirred well with an equimolar amount of
17
aluminum propoxide, Al(OPr) , dissolved in a mixed toluene/
3
17
17
2-propanol solvent. Additional O-enriched water was added
to the stirred suspension. A precipitate formed and the mixture
was allowed to stand for 2 weeks while the solvent slowly
evaporated in a stream of dry nitrogen at room temperature.
The product was heated at successively higher temperatures in
dry nitrogen for 2 h periods and examined by X-ray powder
diffraction. The mixture of crystalline La O and LaAlO
This paper will present an extensive set of magic angle
1
7
spinning (MAS) O NMR spectra from ABO3 and A2BO3
compounds with A ) Li, Na, Ca, Sr, Ba, and La and B ) Ti,
Zr, Sn, Nb, and Al. As all these structures have an oxygen
2
3
3
formed at 950 °C was treated at room temperature with 1 M
acetic acid to selectively dissolve lanthanum oxide. After
drying, the residue gave an X-ray powder pattern of LaAlO3
with a small admixture of Al O .
17
framework, O is the most attractive nucleus to study structural
phenomena related to substitution of the various A and B ions
within the lattice. The aim is to determine which structural
2
3
The NMR experiments were carried out on a Bruker MSL
17
parameters control the O NMR chemical shift in perovskites,
and also examine if 17O NMR is a good probe of structural
distortion. It is shown that the number of resonances in the
oxygen spectra and their difference in isotropic chemical shift
are very sensitive to deviations of a structure away from cubic
symmetry. In addition some multinuclear studies have been
carried out on some stannates (1 Sn) and comparison is made
4
1
00 spectrometer (B0 ) 9.4 T) operating at 54.2, 104.2, and
17
27
119
49.1 MHz for O, Al, and
Sn, respectively. For the
oxygen MAS spectra, a 7 mm double-bearing (DB) MAS probe
was used with 5-6 kHz spinning speeds, 5 s recycle delays
and 1.5 µs pulses (flip angle ∼ π/4). Water was used as an
27
external reference (δ ) 0 ppm).
Al NMR spectra were
19
collected using a 4 mm DB MAS probe with spinning speeds
21
to a previous study of Clayden et al. Finally the application
of 12 kHz, 0.75 µs pulses, and 1 s recycle delays. The
1
7
27
of O and Al NMR to following structural development in
sol-gel formed LaAlO3 is presented. LaAlO3 is an important
material as a possible substrate for high-Tc superconducting
119
corresponding conditions for Sn were 10.5 kHz spinning rates,
.5 µs pulses and 20 s recycle delays. The 27Al and Sn
119
2
spectra were referenced using the AlO6 resonance of yttrium
aluminum garnet (YAG) at 0.7 ppm and SnO2 at 0 ppm,
respectively (which equals -604.3 ppm relative to the primary
2
2
films. Above 447 °C LaAlO3 has cubic symmetry, but below
47 °C it adopts a rhombohedrally distorted perovskite structure
space group R3c) with the distortion gradually increasing with
4
(
21
tin shift reference tetramethyl tin) as secondary standards.
23,24
decreasing temperature.
with Al and La MAS NMR, confirming the presence of
LaAlO3 has previously been studied
2
7
139
Results and Discussion
single Al and La sites with δiso ) 11.7 ppm, CQ ) 0.12 MHz,
2
5,26
Crystallographic data and the 17O NMR chemical shift and
line-width data of the compounds studied are given in Table 1.
and δiso ) 375 ( 5 ppm, CQ ) 6 MHz, respectively.
Experimental Methods
17
Typical O MAS NMR spectra of a selected group of titanium
17
17
Li2CO3, Sr2CO3, and SnO2 were O-enriched by sealing the
unenriched compounds with 10 atom % 17O-enriched water in
and zirconium perovskites are displayed in Figure 1. All
O
NMR spectra essentially consist of one or two sharp lines with

17O NMR Chemical Shift in Ionic Mixed Metal Oxides
J. Phys. Chem., Vol. 100, No. 47, 1996 18541
O ) and Crystallographic Data for ABO and Related Materials
TABLE 1: 17O Isotropic Chemical Shifts (δiso, Relative to H
2
3
δ
iso
∆νMAS(2)
(Hz)
C
Q
max a
structure
type
space
group
no. of O
sites
ref for
structure
compound
Li TiO
CaTiO
SrTiO
Sr TiO
BaTiO
Li ZrO
Na ZrO
CaZrO
(ppm)
(MHz)
2
3
405.8/372.3
448.0/443.4
465
310/310
190/n.d.
240
n.d.
n.d.
350/n.d.
55/55
n.d.
180/n.d
270
2000
n.d.
n.d.
1.5/1.5
1.2/n.d.
1.4
n.d.
n.d.
1.6/n.d.
0.65
n.d.
1.2/n.d.
1.4
3.9
3.4
distorted NaCl
C2/c
3
2
1
2
2
2
3
2
2
1
3
2
2
1
1
6
3
GdFeO
perovskite
NiF
BaTiO
3
Pbnm
Pm3m
I4/mmm
P4mm
C2/c
28, 29
3
8
3, 30
5
6
28, 29
31
3
5
32
8
3
2
4
426/407
564/523
K
2
4
b
3
3
2
3
298.4/280
308.9/286.0c
336/329
343.7/340.2
376.0
85
423
213/198
504
170.2
distorted NaCl
distorted NaCl
2
3
C2/c
3
GdFeO
GdFeO
3
Pbnm
Pbnm
Pm3m
C2/c
Pbnm
I4/mmm
R3c
SrZrO
BaZrO
3
3
3
perovskite
distorted NaCl
Li
SrSnO
Sr SnO
LiNbO
2
SnO
3
3
GdFeO
NiF
LiNbO
3
2
4
n.d.
3.4
1.6
K
2
4
3
1500
340
3
33
23
LaAlO
3
R3c
a
max
are upper estimates for C
Q
based on the centerband line width. b Data from refs 34 and 35. c Data from ref 27.
C
Q
Figure 1. 17O MAS NMR spectra of a selection of titanates (top) and zirconates (bottom) with the same A cation. (a) SrTiO
CaTiO and CaZrO , and (c) Li TiO and Li ZrO . Insets show expansions of the centerbands of the spectra.
3
3
and SrZrO , (b)
3
3
2
3
2
3
their accompanying sidebands. Upper estimates for CQ derived
from the line width are also given in Table 1. As expected
from the ionic character of the bonds in these compounds, all
values are small; this means that changes in the observed peak
position can be largely attributed to chemical changes as there
is no significant quadrupole induced contribution.
it is the B ions building up the octahedral network that determine
17
the O chemical shift range and not the A ions. The difference
in the oxygen NMR data is also reflected in the difference in
the Ti-O and Zr-O bond lengths (1.995, and 2.093 Å on
average in the compounds in Figure 1) which also reflects the
polarizing power.
The most important observation to be made in Figure 1 is
that the peak positions of the centerbands of the zirconates are
at systematically lower chemical shifts than those of the titanates
with the same stoichiometry and A cation. For instance,
compare chemical shift data of BaTiO3 (δiso ) 523 and 564
In addition to the chemical shift differences between the
groups with different B cations, there are more subtle effects
that influence the actual value of the chemical shift within each
group. Most importantly, the number of oxygen resonances
increases as the symmetry of the perovskite decreases. SrTiO3
34
5
17
ppm) with that of BaZrO3 (δiso ) 376.0 ppm). Titanates have
and BaZrO3, both cubic perovskites, have a single sharp
O
1
7
O chemical shifts in the range 372-564 ppm, while zirconates
resonance as is to be expected since all the oxygen sites are
crystallographically equivalent and the sites also have a very
small CQ due to both the site symmetry and ionic nature of the
structure. Compounds with the next highest symmetry, BaTiO3
at room temperature and the GdFeO3 type phases, have two
1
7
have O chemical shifts of 298-376 ppm. This is consistent
16
17
with earlier results on pure ZrO2 and TiO2 systems. In fact,
the ratio between the average 17O chemical shift in isostructural
titanates and zirconates with the same A cation is remarkably
constant: 0.74 ( 0.03. This value is very close to the ratio of
1
7
O signals in the ratio 2:1, also in agreement with their
the polarizing powers (R) of Zr4 and Ti (where R ) r /z, in
+
4+
2
crystallography. In BaTiO3 Ti and Ba are displaced from their
ideal positions and the oxygens become inequivalent. In
4+
which r is the ionic radius (0.64 Å for Ti and 0.77 Å for
4+ 36
Zr ) and z is the valence of the ion (4 for both ions)). Hence
GdFeO3 compounds the BO6 octahedra are tilted, having the
4
+
4+
28,29,31,32
for Ti and Zr R is 0.41 and 0.59, respectively, so that RTi/
RZr equals 0.7. This is an indication that in ABO3 perovskites
same structural effect.
In compounds with an even
lower symmetry, the monoclinic C2/c phases, the number of

1
8542 J. Phys. Chem., Vol. 100, No. 47, 1996
Bastow et al.
oxygen signals does not increase. However the actual 17O
chemical shift difference between the resonances increases
considerably in the monoclinic A1 2B O3 structure. For
GdFeO3-structured compounds CaTiO3, CaZrO3, and SrZrO3
the separations are 4.3, 7, and 3.5 ppm, respectively, while for
monoclinic Li2TiO3, Na2ZrO3, and Li2ZrO3 the corresponding
separations are 33.5, 22.9, and 18.4 ppm, respectively. Dorrian
+
4+
7
and Newnham refined the Li2TiO3 structure and concluded that
1+
4+
in A 2B O3 compounds there are only two possible configu-
rations for the oxygen coordination, each with oxygen bonding
4
+
1+
to two B ions and four A ions. In the Li2TiO3 phases only
one of these sites exists, while in Li2ZrO3 both configurations
are present in a 1:2 ratio. Although in the Li2TiO3-type
structures only one oxygen geometry is present, the orientation
of the OB2A4 group within the lattice is different for O(1) and
O(3) as compared to O(2). The 17O NMR spectrum of
27
Na2ZrO3 has been presented elsewhere and consists of two
lines, with an intensity ratio of 1:2, from O(2) and O(1)/O(3)
respectively. The oxygen spectrum of Li2TiO3 (Figure 1c) is
very similar to the Na2ZrO3 spectrum, again consisting of two
lines in an approximately 2:1 intensity ratio, representing the
two distinct oxygen sites, in agreement with the fact that they
are isostructural. The only difference between the two spectra
is the fact that the two isotropic lines in the Li2TiO3 spectrum
each consist of a doublet (with approximately equal splittings
of 140 Hz). Only two distinct oxygen sites are present in the
structure, and the quadrupole interaction is too small to cause
quadrupolar structure in the resonances. These compounds are
however susceptible to stacking faults, and these effects may
be large enough to cause small differences in the chemical shift
of 140 Hz.
Figure 2. 17O MAS NMR spectra of (a) Sr
Li SnO
2
4 3
SnO , (b) LiNbO and (c)
2
3
.
The 17O NMR spectrum of Li2ZrO3 exhibits resonances at
2
80 and 298.4 ppm in an approximately 1:4 intensity ratio. The
The 17O NMR spectrum of BaZrO3 is shown in Figure 3. It
Li2ZrO3 crystal structure contains two crystallographically
distinct oxygen sites O(1) and O(2) in a 1:2 ratio. This
exhibits a complex spinning sideband envelope that spreads out
over a range of approximately 2300 ppm (125 kHz). It is well-
known that if the spinning speed under MAS conditions is less
than the chemical shift anisotropy, the centerband will be flanked
5
discrepancy in intensity has also been observed for Li2HfO3,
which is isostructural with Li2ZrO3.37 Despite this discrepancy,
on the basis of the higher intensity of the 298.4 ppm peak this
is attributed to the O(2) site and the 280 ppm resonance to the
O(1) site. Li2SnO3 is isostructural with Na2ZrO3 and Li2TiO3,
3
8
by a series of spinning sidebands, and for spin-1/2 nuclei the
intensities of these sidebands can be used to recover the chemical
3
9
shift parameters. These parameters can be very useful in
providing information about the symmetry of the bonding at
1
7
and therefore two resonances would be expected in the
O
1
7
NMR spectrum. However, only one broad (fwhh ) 1.5 kHz)
the nuclear site. For O if the quadrupole interaction is not
oxygen signal is observed (Figure 2c). As has been pointed
too large (C e 3.3 MHz), and by applying fast MAS all the
Q
1+
4+
out above, A 2B O3 phases can have a high degree of stacking
disorder. Although two-dimensional order exists within the
layers of Li and Sn octahedra parallel to the ab plane, the double
cubic close-packed stacking sequence is absent. As a result,
no distinct oxygen resonances are observed, but only one broad
signal, which is probably indicative of stacking disorder.
transitions are narrowed to form a set of well-defined spinning
sidebands. The intensity envelope closely mimics the static line
shape and here can be used to accurately determine the
quadrupole interaction parameters CQ and η and the isotropic
chemical shift δiso. Normally the chemical shift anisotropy
cannot be directly deduced from such sidebands because of the
1
7
In Sr2SnO4 and Sr2TiO4, two distinct oxygen sites are present,
one within the SrBO3 layer and the other bridging with the Sr.
The terminal B-O bonds are shorter (1.90 Å) than the bridging
B-O-B (1.94 Å). The oxygen spectra of Sr2SnO4 (Figure 2a)
and Sr2TiO4 (not shown) consist of two lines from the two
oxygens in the lattice. The stannate 17O resonances have
chemical shifts considerably more shielded than the titanate
signals (213/198 ppm and 426/407 ppm, respectively), but the
separation is very similar (15 ppm and 19 ppm, respectively).
Although the difference in the range of the chemical shifts does
not correlate with the difference in the polarizing power, since
presence of the quadrupole interaction. The BaZrO3
O
spectrum represents a special intermediate case of the magnitude
of CQ, in which both the quadrupole interaction and the chemical
shift parameters can be effectively estimated independently from
different transitions. In the spectrum the two singularities of
the ((3/2, (1/2) transitions are visible, from which CQ ) 430
kHz. This value is small and is about one-third of the value of
CQ calculated from the line width (1.4 MHz) emphasizing that
1
7
O residual MAS NMR line widths in many highly ionic
crystalline compounds are dominated by chemical shift disper-
sion, and not by second-order quadrupole effects. Since CQ is
small, it is a very good approximation that the (1/2, -1/2)
transition spinning sidebands are determined largely by chemical
shift anisotropy (csa). The values for csa derived by Herzfeld
the ionic radii are almost identical (rTi ) 0.68 Å, rSn4+ ) 0.71
4
+
3
6
Å), it supports the general trend that the most important
influence on the chemical shift is the type of B ion; then other
smaller effects cause further separation of the resonances.
Sr2SnO4 and Sr2TiO4 are isostructural, and as a result the
separation of the constituent resonances is almost identical.
3
9
and Berger analysis are listed in Table 2. The asymmetry
parameter η, expressing the divergence of the chemical shift
tensor from axial symmetry, is defined as (σ22 - σ11)/(σ33 -

17O NMR Chemical Shift in Ionic Mixed Metal Oxides
J. Phys. Chem., Vol. 100, No. 47, 1996 18543
Figure 3. 17O MAS NMR spectrum of BaZrO
. The inset shows an
3
expansion of the centerband.
TABLE 2: NMR Chemical Shift Parameters of a Selected
Group of Perovskites
Figure 4. 119Sn MAS NMR spectra of (a) Sr
hexagonal CaSnO , (d) orthorhombic CaSnO , and (e) Li
the fine structure at the base of the centerbands in (a), (b), and (d),
2
SnO
4
, (b) SrSnO
3
, (c)
3
3
2
SnO . Note
3
σ
iso
σ
11
σ
22
σ
33
∆σ
1
17
119
which is interpreted as Sn- Sn spin-spin coupling.
compound
BaZrO
SrTiO
(ppm)
(ppm)
(ppm)
(ppm)
η
(ppm)
3
367.2
468.1
340.1
443.9
329.3
444.7
564
508
647
469
599
482
715
422
583
414
521
379
514
171
174
137
211
126
105
0.44
0.22
0.27
0.34
0.51
0.59
-294
-441
-305
-349
-305
-509
-520
-480
site which is rather nonaxial. The spectrum, however, indicates
that the electron distribution around the oxygen is close to axial
(η close to 0, so that σ22 - σ11 , σ33 - σiso). Herzfeld and
Berger analysis is only really applicable to cases where the
chemical shift anisotropy is the dominant broadening mechanism
of the static spectrum, and the much broader individual lines in
this spectrum probably indicate an increase in CQ (maximum
value 3.4 MHz). Compared to the CQ values of the other
compounds studied here (≈400 kHz) LiNbO3 is clearly much
higher, and as a result Herzfeld and Berger analysis is not really
applicable as the sideband intensity is determined by a convolu-
tion of chemical shift and quadrupolar interactions.
3
SrZrO
CaTiO
3
3
CaZrO
LiNbO
3
3
a
BaTiO
3
523
a
Data from ref 34.
σiso) and ranges between 0 and 1. The divergence from spherical
symmetry is expressed by the parameter ∆σ which is defined
as σ33 - 1/2(σ22 + σ11).
The most important observation to be made from the values
in Table 2 is that the ∆σ values fall in different regions for
compounds with different B ions. The values for the titanates
are much higher and cover a broader range (-349 to -580 ppm)
than the values for the zirconates (-294 to -520 ppm). This
is again a reflection of the fact that the B ion is the most
important influence on the oxygen chemical shift, being largely
responsible for both the magnitude and symmetry of the
shielding around the oxygen nucleus.
The 119Sn NMR spectra of Sr2SnO4, SrSnO3, CaSnO3, and
1
19
Li2SnO3 are displayed in Figure 4.
Sn shifts for Li2SnO3,
SrSnO3, Sr2SnO4, and the orthorhombic polymorph of CaSnO3
have previously been reported as 160, -36.5, 24.7, and -7.4
ppm, respectively (relative to SnO2). The Li2SnO3 spectrum
(Figure 4e) consists of one sharp line at 159.4 ppm. The Sn
spectrum of SrSnO3 (Figure 4b) consists of one line with a
chemical shift of -36.5 ppm, while the Sr2SnO4 spectrum
(Figure 4a) has a sharp signal at 40 ppm and broader signals
with a low intensity at 0.8 and -37 ppm, representing unreacted
SnO2 and SrSnO3, respectively. Orthorhombic CaSnO3 (Figure
4d) has a shift of -7.4 ppm. The only discrepancy of these
results with the data of Clayden et al. is for Sr2SnO4 with a
The LiNbO3 chemical shift interaction derived from sideband
analysis (Table 2) does not correspond well with the value
anticipated from the shape of the sideband envelope (Figure
2
b). A trial Herzfeld and Berger analysis gives 0.6 for the
asymmetry parameter, representing a symmetry of the oxygen

1
8544 J. Phys. Chem., Vol. 100, No. 47, 1996
Bastow et al.
Figure 5. 27Al (a) and 17O (b) MAS NMR spectra of the LaAlO
mixture at 450 (top), 800, and 950 °C (bottom).
3
difference of 15.3 ppm. The measurements were repeated here
several times, giving consistent results and providing confidence
in the values quoted. Close examination of the data of Clayden
et al. indicates a difference between the value of the shift
presented in their table and the value from their Figure 3, the
latter being close to the value obtained here.
converted into an amorphous aluminum (oxy)hydroxide which
has been dehydroxylated with heat treatment. The fraction of
AlO5 decreases with temperature, in agreement with previous
studies on amorphous aluminas. At 450 and 800 °C there is
no indication in the Al NMR spectra of the form of the
lanthanum within the sample. It is only at 950°C that La can
be shown to have reacted with the Al component, as the broad
AlO4, AlO5, and AlO6 resonances have been converted into one
sharp line with a chemical shift of LaAlO3 (11.3 ppm). The
marked decrease in the line width indicates an increase in the
crystallinity of the sample. The upper limit of the CQ (1.6 MHz)
as determined from the line width (Table 1) corresponds well
with the rest of the perovskite phases determined here.
CaSnO3 has two polymorphs, a low temperature, hexagonal
form and a high temperature, orthorhombic form with a GdFeO3
1
19
perovskite structure. Here we report for the first time the Sn
chemical shift of the hexagonal form of 60.8 ppm (Figure 4c).
The data of both CaSnO3 polymorphs show that the structure
has a very pronounced effect on the Sn chemical shift, causing
a shift difference of 68.2 ppm so that the interesting structure-
shift correlation given by Clayden et al. should be limited to
isostructural compounds. Close examination of the 1 Sn NMR
spectra of Sr2SnO4, SrSnO3, and CaSnO3 (orthorhombic form)
reveals fine structure in the centerbands, which can be attributed
2
7
17
In contrast to the Al NMR spectra, the O NMR spectra
(Figure 5b) contain information on both the La and Al
components of the sample at all stages of LaAlO3 synthesis.
The spectrum at 450 °C consists of three signals, at 546.4, 68.2,
and 44.5 ppm. The two rather broad lines at 68.2 and 44.5
ppm are from oxygens in OAl4 and OAl2H units in aluminum
19
119
117
to Sn- Sn spin-spin (J) coupling. The magnitude of this
interaction varies, but is in the range of 200-300 Hz.
Sol-gel formation of LaAlO3 was examined by X-ray
18
(oxy)hydroxides. The signal at 546.4 ppm has been observed
before in the synthesis of La2O3 using lanthanum propoxide
2
7
17
diffraction, Al MAS NMR, and O MAS NMR. These
compounds are all essentially X-ray amorphous except at 950
4
1
(La(OPr)3) and can be assigned to oxygen in LaOOH. The
presence of these different units in the oxygen NMR spectrum
indicates that, in the initial gel and in the sample at 450 °C, the
La and Al precursors are not atomically mixed and no reaction
between them has taken place. At 800 °C, the OAl4 and OAl2H
units are still present, but the LaOOH unit has been replaced
by two signals at 582.4 and 466.5 ppm that can be attributed to
°
C where crystalline La2O3 and LaAlO3 have formed so that
NMR is necessary to reveal structural information in the
intermediate stages. The 27Al MAS NMR spectra of the LaAlO3
mixture at 450, 800, and 950 °C are shown in Figure 5a. At
4
50 °C the spectrum consists of three broad lines at ≈6.5, 40,
and 65 ppm from AlO4, AlO5, and AlO6 sites. The line widths
4
1
of the lines at 450 and 800 °C indicate that the material is
oxygens in La2O3. Even at this temperature, the La and Al
2
7
disordered on an atomic scale and the spectra resemble Al
NMR spectra of amorphous aluminas.40 The Al NMR spectra
indicate that the aluminum propoxide has been hydrolyzed and
components have not reacted. At 950 °C a new signal at 170.2
27
17
ppm appears in the O spectrum, while the signals representing
the Al and La units have both decreased considerably in

17O NMR Chemical Shift in Ionic Mixed Metal Oxides
J. Phys. Chem., Vol. 100, No. 47, 1996 18545
intensity. At this temperature X-ray diffraction indicates the
presence of La2O3 and LaAlO3 in a 1:1 ratio, and therefore, the
(8) Ruddlesden, S. N.; Popper, P. Acta Crystallogr. 1957, 10, 538.
(9) Weiss, R.; Fairre, R. C. R. Acad. Sci. Paris 1959, 248, 106.
(
10) Oldfield, E.; Coretsopoulos, C.; Yang, S.; Reven, L.; Lee, H. C.;
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(11) Cohen, M. H.; Reif, F. Solid State Phys. 1957, 5, 321
1
70.2 ppm signal is attributed to LaAlO3. Summarizing, at
27
temperatures <950 °C Al NMR only gives information on
the Al component of the system, and indicates that Al is present
in AlO4, AlO5, and AlO6 coordinations. At 950 °C LaAlO3
has formed. The 1 O NMR gives information on both the La
and Al components and indicates that at 450 °C these elements
are present separately as (oxy)hydroxides, which at higher
temperatures slowly convert (partly) to oxides and eventually
(
(
(
(
12) Smith, M. E. Appl. Magn. Reson. 1993, 4, 1.
13) Schramm, S.; Oldfield, E. J. Am. Chem. Soc. 1984, 106, 2502.
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7
Kosfeld, R. T., Eds.; Springer-Verlag: Berlin, 1994; Vol. 31, p 134.
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, 989.
17) Bastow, T. J.; Moodie, A. F., Smith, M. E.; Whitfield, H. J. J.
(
2
(
(at a temperature between 800 and 950 °C) react to form
Mater. Chem. 1993, 3, 697.
LaAlO3.
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(
19) Dirken, P. J.; Smith, M. E.; Whitfield, H. J. J. Phys. Chem. 1995,
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9
Conclusion
(
17O MAS NMR has been shown to be an effective method
for probing the structure of mixed metal oxides, and in these
highly ionic compounds oxides the line widths are usually small
and therefore well-resolved spectra can be obtained. The results
1
(
(
1
7
show that the O chemical shift interaction is dominated by
the type of cation building up the octahedral framework (B)
and that the cations within the cavities in the structure (A) are
of secondary importance. Furthermore, small deviations from
the ideal cubic symmetry and the presence of stacking disorder
can be readily detected. Application of 17O NMR to the
formation of LaAlO3 from aluminum and lanthanum alkoxides
clearly provides more insight into the reaction than Al NMR
and allows structural evolution to be followed. 17O solid state
NMR should be increasingly used to characterize ceramic oxide
materials in general and is an important component of a
multinuclear approach.
(
23) Geller, S.; Bala, V. B. Acta Crystallogr. 1956, 9, 1019.
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(
1
(
F. Phys. Lett. 1964, 9, 223.
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27
(28) Sasaki, S.; Prewitt, C. T.; Liebermann, R. C. Am. Mineral. 1983,
6
8, 1189.
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(
3
(
Acknowledgment. The authors thank David Hay (CSIRO,
(
Melbourne) for access to the X-ray diffractometer. M.E.S. and
P.J.D. thank the EPSRC for funding 17O NMR characterization
(
of materials through Grant GR/J23938.
(
References and Notes
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(
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(
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(
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JP9622694