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Raman spectra of niobium oxides
Spectrochimica Acts,vol.32A,
pp. 1067 ta 1076.
PergamonPnws, 1976.
PrIntedin Northern Ireland
I%arxanspectra of Robin
oxides
A. ANN MCCONNELL Departmentof Pure and AppliedChemistry,Universityof Strathclyde,GlasgowGl 1XL 3. 5. ANDERSON and C. N. R. RAO* InorganicC~ernist~ Laboratory,Universityof Oxford, OxfordOXI 3&R (Received 30 Jfay 1975) Abstract---Ram&nspectra of various types of niobium oxides have been studied along with theiri.r. spectraand the spectrainterpretedin the light of the structuresof the oxides Besides discussing LO-TO splittingin theseoxides,bandscharacteristic of edge-shared and corner-shared NbOI octahedrahave been assigned. Tetrahedr&l modes of NbOd, PO1 and VO, units present in some of the oxides have been rtesigned to distinctbands in the spectraindioatiugnegligible couplingbetweenthe tetraheclmand Nb06 octahedra. The speotraof PNb,0t6 &ndanalogues suggestthat they belongto I4/m spacegroupratherthan I$ group.
Although it h&s been known for sometime that niobium pentoxide, NbsO,, exists in different polymorphic forms, it is rel&tively recently that structursl relationships among these have been properly understood [l]. The structuresof most of these polymorphs &re derived from the NbOSF (or ReO,) structure and c&n be explained on the basis of crystallogrctphicshe&r. Cryst&llogr&phic she&r &llows aocommod&tion of stoichiometric ch&ngesby & structuml &djustmentthat preserves the cation coordination, but virtually elimin&tes point defects [Z, 31. In many of the NbsO, polymorphs (H-Nb,O,, N-NbsO, etc.), two sets of intersecting cryst&llogr&phic she&r planes dissect the structureinto blocks of NbO, octahedra and det&iled structural studies on some of these block structures h&ve been reported in the lit8rature. Thus, in N-NbSO,, 4 x 4 blocks of cornershsred NbO, octahedra are joined together in infinite ribbons at the same level, as well BBbeing linked to adjacent blocks at & different level, by shsring octahedr&l edges (Fig. 1). In the the~od~&~cally most stable form, H-Nb,O,, 3 x 5 blocks of corner-shared oct&hedr&at one level (y = 0) are joined in ribbons by octahedral edge-sharing, to form stepped rows; at another level (y = Q), octahedra form, 3 x 4 blocks which share edges with ths stepped rows; one Nb atom out of 28 occupies & tetrahedral site in the channel formed between the corners of blocks (Fig. 1). * Commonwealth Visiting Professor (1974-76); Permanent Address: Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
Analagues of Nb,O, pol~o~hs with block structures containing other cations like TiNbSO, (related to N-Nb,O,) and PNb,O,,, with P atoms in tetrahedrrtlsites, are also known &mongst the double oxides of niobium. Unlike N- or H-Nb,O, related to the ReOs structure, the B form of Nb,O, has & structure related to PdFe, with rutile ribbons of edge shared octahedra joined by terminal octahedral oorners. In general, the polymorphs cont&in different types of niobium-oxygen polyhedra, predominantly corner-shared and edgeshared NbO, octahedra. Corner-sharedoct&hedr& ase not appreciably distorted, while edge-shared octahedra exhibit large distortions, resulting in significantvariations in Nb-0 distances. We have studied the Rsznan spectra of sever&l niobium oxides, along with their i.r. spectra, to see how the spectr& elucidate or corroborate the strnctuml fe&tures and cation ordering in these extended StrllCtllr8S. The study shows that, despite the complexity of these oxides examined by us, vibration&l spectroscopy provides information of value when interpreted in conjunction with the known structure features. The method of analysis followed by us, although somewhat simplified, indicates the usefulness of the intern&l mode approach in systems where strict factor group analysis is diBcult. RESULTS
Bd
Al?D DrS~~IO~
for spectralasaignmenfs
Vibrational spectroscopy c&n be useful to study different types of ordering in metal oxides 143: (a) preferential ~st~bution of non-equivalent
1067
A. A. MCCONNELL, J.
1068
S. ANDERSON and C. N. R. RAO the metal-oxygen to
the
octahedra
crystal
binding
axe large compared
forces.
The
internal
vibrations of the MO, group in the solid should be quite
close to the free-ion
modes,
the external
modes occurring at considerably lower frequencies than the internal modes.
Thus, the high frequency
bands in the Raman spectra of many oxides can be satisfactorily octahedra.
assigned to the internal modes of the This is valuable
like LaMgTiO,
containing
in complex essentially
systems
octahedral
MO, units [5], as well as in systems like Mg,Nb,O, with lower symmetry structure [4,6].
than the parent corundum
The Raman spectrum ofMg,NbaO,
would be difficult to interpret formal
factor
frequency
group
Raman
correspond the NbO,
on the basis of a
analysis, band
but
would
to the symmetric
the
highest
undoubtedly
stretching
mode of
octahedron which is only Raman active;
the asymmetric stretching and bending frequencies are seen in the vibrational TiO,
[7,
KNbO,
(4
cations on to non-equivalent in space group symmetry and
distribution in space
inverse
sites, without change or unit cell size, as in
spinels,
on to equivalent
group
symmetry
(b)
only,
group
as in
LiNbO,,
analysis.
[ll].
structures.
consequence
of ordering
centre of symmetry; mutual
lower space
exclusion
The group theoretical is the
removal
of the
this results in the loss of the rule
in
the
Raman
and
i.r.
and
the
compatible
with factor
case of KNbO,,
phase
spectra, the cubic
a second-order
spectrum
When more than one type of metal-oxygen
polyhedron
is present in a cell, we would see a
superposition of the Raman spectrawith frequencies similar to those of the free parent polyhedra,
if
there is little or no coupling. Because the niobium oxides studied by us have rather
complex
structures
of
low
symmetry,
containing extended networks of NbO, difficult.
as in some of the corundum-or
In
phase giving essentially
metry as the parent structure, as in trirutiles and also (d) buildingof superstructurewitha
like
SrTiO,
like
are clearly seen in the Raman
(Table
K,NiF,---based
perovskites
of the
systems
transitions from cubic to lower symmetry structures
preferential
sites with decrease
Analysis
simple
[9, lo], basedon the internal mode approach,
(c) building of superstructures with the same sym-
group symmetry,
81 and
spectrum. of even
yield valuable information
Fig. 1. (a) Structure of H-Nb,O, [OIO] projection, showing columns of octahedra, (5 x 3) in section, at one level (heavy outlines) and (4 x 3) columns at second level. On the left hand side, a schematic representation outlines the rectangular columns as defined by the niobium atom positions. (b) Structure of N-Nb,O,. Similar [OIO] projections showing the columns of (4 x 4) octahedra at both levels.
normal
i.r.
spectra
l),
are not In
the
strict factor
octahedra
group analysis would
be
The structures in most of these systems supercells case
of
of
the
simpler
structural
units.
non-centrosymmetric
block
structures, however, we may be able to see LO-TO splittings in the Raman
spectra since the crystals
could be considered to belong to Loudon’s
type I
in one direction and type II in another [18]. systems like PNbsO,,
Raman
may also provide some information on the ordering
spectroscopy
is particularly
sensitive for
detection of ordering in rock-salt and other types
of cations
of structures
Although
Raman
since ordering gives rise to strong
bands (e.g. in the region corresponding to
the stretching while
none
structures. In metal titanium
mode of metal-oxygen
are
seen
in
completely
or VNb,O,s,
In
spectra, or in the relaxation of the selection rules.
and hence on the crystal
oxide
systems
containing
the binding
in these systems,
we can fruitfully
octahedra)
Raman
disordered
in terms of discrete metal-oxygen
niobium,
forces within
symmetry.
strict factor group analysis is difficult interpret the
spectra with the internal mode approach
The types of polyhedra
or tungsten,
Raman spectra
polyhedra.
present in the different
niobium oxides are shown in Table 1. We see that the oxides predominantly
contain NbO,
octahedra
B-Nb,O,(M) czjc
I4/m (or) I3 ~O~.SNb~O~ I4ImP
VNW’,,
PNb~O*~(T) 14/m (or) Iii
TiNb,O,(M) 82/m
N-Nb,O,(M) cz/na
H-Nb~O~(M) P.2
StNCturt,
Structure can be understood in terms of PNb,O,, (see discussion section of the paper for details). Different altogether from othem. The structure is related to PdFs with rntile ribbons of ES octahedra joined by octahedral comers (very high density).
Derived from NbO,F structure. Has 3 x 6 blocks of cornerlinked (CL) NbOe octahedra at one level joined by octahedral edge sharing to form stepped rows. At another level, 3 x 4 blocks are present and these share edges with stepped rows. One out of 28 Nb atoms in tetrahedral sites. Derived from NbO,F structure. Has 4 x 4 blocks of CL octahedra. These blocks are joined together at the same as well as different levels by sharing of octahedral edges. Related to N-Nb,O, except that block size is 3 x 3. There is substantial ordering of Ti ions in ES octahedra (up to 64%). Related to N-Nb,O, except that block sise is 3 x 4. Ti ions occupy ES octahedra up to 40 %. Derived from NbO,F; contains 3 x 3 blocks of CL octahedra joined to similar blocks at different levels by edge sharing. Tetrahedral sites occupied by P. &pace group either of the two 8hQW7t. Similar to PNb,O,,. VO, tetrahedra present in place of PO, m PNb80r6. GO, tetrahedra may be present. An unusual ES octahedron present in addition to CL and the usual ES octahedra. ES octahedra
Available distances are in 130-2~2 A range.
CL Octahedra have Nb-0 bonds I.92 + 0.06 A. ES octahedrs have Nb-0 distances in the range 1~762.3 A with l/6 of Nb-0 distances <13 A. P-O distance is 1.66 -& 0.08 A.
CL octahedra l/Sth of total. In addition to ES and CL NbO, octahedra, PO, tetr~e~ present (l/Sth of total).
[I71
Cl61
Cl1
El31
1131
1131
CL Octahedra are slightly distorad. ES octahedra have Nb-0 distances between 1.66 and 2.38 A with l/3 of them
Since Ti preferentially occupy ES octahedra, there would be more CL NbOG octahedra (~6/26).
,,
WI
CL octahedra are only slightly distorted. In ES octahedra, 9 out of 48 Nb-0 distances are < 1.9 A and 22 are > 2.0 A.
r141
Ref.
In addition to CL octahedra, there sm ES octahedra similar to H-NbJ,O,. 1/4th of octahedra are CL.
Bond Distances CL octahedra with lQ2(2) and 1.9’7 A Nb-0 bonds. ES octahedra have Nb-0 distances between 1.73 and 2.26 A, 17 out of 84 bonds are1 <13 A and 36 > 2.0 A. Tetrahedral Nb-0 bonds are short (1.66 A).
In addition to corner-shared (CL) octahedra, there are edgeshared (ES) octahedra (sharing 1, 2 or 3 edges). NbO, tetrahedra are also present. 5/2&h of octahedra are CL.
Types of Polyhedra
Table 1. Nature of metal oxygen polyhedra in niobium oxides
1070
A. A. MCCONNELL,
distorted
to
different
extents,
whether
they are corner-shared
appears
that
NbO,
Nb5f
octahedra
distortion oxygens
electrostatic
quite
octahedra
some elements
and, as a first groups.
In
Table
different NbO,
octahedra,
and
PNb,O,,
tetrahedra the
and
the
(Table
it should
various
highest
perovskite
Where
of
Raman
Since
symmetry
all
other
give
Raman NbO, do
the
bands
band
Raman
of v,(Eo)
5A,[l
= (v,, -
-
Q4/v1[l
along
and
respectively. when
-
A
high
to
I(vi)/l(vs)
u~,/ul approaches
unity,
Instead
of andysing
corner-linked for
In this
in
in
such
0, symmetry
all
to bond bond
ratio
is
as in the
however,
or increase
modes.
approaches
outlined
a basis for assigning
also examined compounds, NbO,F
the
spectra
NbO,F
on the
selections
in Table from
2.
ideal
in unit cell size will give
niobium
of
The given
departures
the
spectra
Our
discussion
oxides
follows
above.
In order
the bands, of two
of the to
we have
simple
model
and NbOPO,.
and NbOPO,
NbOsF
has a cubic structure
per
cell
and
with
Raman
NbO,F
random
spectrum
of
650 cm-l.
Based
for isolated
octahedra,
bands
than
mode
due
to
arising
to
the
be fully
slight
A band
of
around
approach that the
may
around
not be
300 cm-i
Based
on the assign
C,,
the
and
B, + E(T,,)
symmetry. region
departure by
mode
mode.
and
band
we can, in principle,
Such assignments, justified
KNbO,
Raman
in this system
3A + 3E(32’,,) from
Second order
we would conclude
bands in the 600-725 cm-l symmetry.
2). cubic
with a shoulder
the vs( Z’s,)
model,
shows
of a second-order
(Fig. by
of [25],
compound
on the internal
725 cm-i.
array
modes
this
highest
occurs at 725 cm-l
arrays
substitution
bands typical
The
one formula
infinite
anion
of a cubic crystal
Raman
with
contains
broad low intensity
NbO,F.
in
array of octahedra,
are
the
due
the
both
modes
directions.
octahedrs
could arise from
and
bands
modes,
we could also analyse
active
The
should
active active
the internal
rise to Raman
vr(Ar,)
c(~ are the
being
octahedra,
analysis,
infinite
the
modes
also
metsl-
oxides
vs(E)
them on the basis of an infinite
higher
with respect
perpendicular
v4(T2)
and
should
similar
Raman
vl(A1)
[ 11, 261.
( --hv,/K!‘)
a1 and
to
showing
are sufficiently
in the
two
bands are
on the distortion
and
present
rise
bands
mode
PO,
KTaO,
(2a,/aJ12 (c+# exp
ve(Tzu)
are also exhibited
intensity
the
depending
spectra
the
The vs, v4( T,,)
spectra,
spectrum
significantly
frequency;
bond polarizabilities
stretching axis
-t
13A,[l
to
would
of perfect
Raman
give
expect
with respect to ~~(4~~) is given by [23],
A,
observed
would
mode
and
of
however,
the
basis of isolated
unit
containing
and i.r.
octahedra
studies.
since Nb6+ with
participates
ve is the exciting derived
spectra
intensity
We
[21, 221. The relative
IhI) -= I(%) where
of high mode.
to be weak
configuration
n-bonding
low
non-centrosymmetric,
due to the Y,(E,)
octahedra
are
have
for structural
and
different
structures.
octahedron
least
vs is some
could,
NbO,,
corresponding
oxides
of in
distortion
tetrahedra at
type
zero
If the octahedra
active.
oxygen
and
Extensive
the inactive
become
in extensive
intensity
61.
i.r.
of the octahedra.
provide
and
[5,
or splitting,
distorted,
show
[24]
vs bands.
the
asymmetry
of
arise from
to ordering
oxides
in
or
bands due to y1 should be
niobium
+,(Tsp)
The
corundum
of MO,
first-order
Rsmen
arise from
systems.
in disordered
and are generally
would
vibra-
band in the spectra
r1 should be most diagnostic The
stretching
the
seen
rules
octahedra
[5, 61 does indeed
the
sub-units
mode, corresponding
The band is sensitive
Raman
or corundum
Raman
to analyse
isolated
with
or no intensity
perovskite
vs(Ts)
in principle, into
in oxide
more than one type
seen.
the
in such an analysis.
systems
present, multiple
they
polyhedra
Raman
structures
has little
in
m&al-oxygen
could,
metal-oxygen
frequenoy
this mode.
the
Besides
be possible
diagnostic
oxide
for
octahedra
shown.
modes
2), the ~i(Ai,)
many
octahedral
etc. [8], but we do not see
advantage
is most
they
examine
rules
NbO,
One
modes.
to the symmetric tion,
of these
discrete
the various
any obvious
NbO,
(e.g. Cd, or CsV)
we could
selection
are
Nb-0
the
Oh symmetry,
of isolated
like MsO, MsO, (MsO,), Of
the
contain
NbCl,
higher intensity
the
the
in
[20]
and
ions involved
negligible
octahedra
the
some of the oxides like H-Nb,O,
tetrahedral
also divide
Such
since
metal
Accordingly,
NbO,
of symmetry
symmetries
s-bonding.
regular
[19].
Although
2, the
case of transition
It
do not obey Pauling’s
in terms
modes
on
octahedra
approximation,
spectra
vibrational
system
expected
rule
have
to form
and C. N. R. RAO
NbO,
be
covalent.
do not
the Raman
small
octahedra
valence
are
retain
also
depending
or edge-shared.
in any oxide
would in NbO,
bonds
is too
J. S. ANDERSON
The
would
from
the
however,
nature
weak
then be ideal may
of spectrum
0, not of
1071
Raman spectra of niobium oxides
I
I
(b)
(b)
II
PiJ(__hL ,(a),
u 400
600
800
1000
. ‘-, .crn. Fig. 2. Raman spectra of (a) NbO,F and (b) NbOPO*. The Raman bands
aa in
spectrum
of NhOPO,
first-order
spectra
shows sharp
(Fig.
2).
The
spectrum is simple and can be readily assigned on the basis of the published structure [27].
NhOPO,
consists of chains of corner-shared NbO,
octahedra
along the c-axis, with PO, chains.
The NhO,
coplanar bonds
Nb-0
tetrahedra
octahedron
bands
(1.97 A) and two unequal
(1.78 A and 2.32 .& Thus,
plane.
symmetry
the
axis.
13 distinct
has
a
to the four-fold
is symmet-
distance of 1.53 A as in normal
orthophosphates. bands
perpendicular
octahedron
The PO, tetrahedron
rical, with a P-O Reman
linking the
has four identical
Factor group analysis yields 15 (5A,
bands
+ 4B,
+ 6E,)
in the
and
spectrum;
two bands are not seen probably
we
the
see
other
because of the
small splittings of the degenerate modes. In the spectrum of NbOPO, to the PO,
1016 cm-i
follows: (strong),
(Fig. 2), bands due
unit [28] can be readily (weak),
~~(-4~); 445 cm-l
and 375 (medium),
980 cm-l
(weak doublet),
vs(E).
Ye
The spectrum
is similar to that of IO, in NaIO,. of NbOPO,
assigned as
ve(Ts);
of PO,
The i.r. spectrum
shows strong bands due to us and Y*,
as expected.
The remaining bands in the Raman
spectrum
NbOPO,
of
Raman-allowed 800 cm-l v&C,)
can
assigned
612 cm-l
shoulder
The position
be
of the NbOs
(strong), pi,
and the
Y~(!Z’sJ.
bands
on the
to
the
octahedron: (very weak),
375 cm-i
of the v1 Raman
band
band in
A. A. MCCONNELL,J. S. ANDERSON
1072
and C. N. R. RAO
some other oxides containing corner-shared NbO, octahedra appears modes
is
also
that either
between
amongst
NbO,
The 290 cm-l
NbO,
Raman
mode
suggests
octahedron Spectra
NbO,
band
61.
active
is
distorted.
C,,
symmetry
400
200
It or
in NbOPO,.
of NbOPO,
becomes
600
800
of the
octahedra
polyhedra
may be to the
when
The
the
Raman
for the NbO,
(Table 2).
of niobium oxides
All the oxides give first-order with
[5,
coupling
mode or, more likely,
which
octahedron
spectrum
800 cm-l
the
and PO,
due to an external va(Tzu)
around
there is negligible
loo0
sharp
bands.
Typical
Raman
spectra
spectra
are shown
B-Nb205
,
in Fig. 3 to illustrate the general features and the data are summarized in the form of a correlation diagram in Fig. 4.
with Nb-0
cm-i
region
stretching.
which
can
be
associated
Certain features in the low
frequency region also appear to be common. 1000 I&
II
I..
.
600 I
I
600 I,
400 I
I
III
I.. 800
000
The characteristic feature in all
the spectra is the appearance of sharp bands in the 700-1000
I
I. 1000
cm
I
I
I,Li//l:
I.. 400
,
, ., 200
-I
Fig. 4. Correlation diagram showing Raman bands of Niobium oxides. Heights of vertical lines give relative intensities. PO, bands of PNbsOzs and VO1 bands of VNb,O*, are not shown here. “d” stands for doublet.
200 I
All the oxides discussed here, with the exception
I
of B-Nb,O,,
are block
centrosymmetric crystallographic octahedra
structures
(Table
1).
and
Along
are not
the
unique
direction in which corner-sharing
form infinite strings, long range forces
are likely to predominate
over local anisotropy;
normal to this direction (i.e. in the plane shown in projection in Fig. l), anisotropy will be dominant. Such a situation,
essentially
similar to Loudon’s
Type I in one direction and Type II in the plane perpendicular to it, should give rise to large LO-TO splitting
in the Raman
spectrum.
It, therefore,
appears that the two bands in the 800-1000
cm-l
region in these block structures (Fig. 4) are due to A,
and E type
with
the
longitudinal
corresponding
optic modes
transverse
(TO) appearing in the 620-680 650 cm-1
Raman
bands
optic
cm-l
region.
are intense
and
appear as doublets due to anisotropy. LO-TO range
splitting order
LO-TO
probably
In
cm-l
is split due to anisotropy [ll]. LO-TO
The often
The large
arises from the long-
in one direction,
splitting is ~250
(LO), modes
KNbO,,
the
and the TO mode This assignment of
modes in niobium oxide block structures
tinds support from the absence of such splitting in NbO,F I
1000
I
so0
600
400
200
cm-’
Fig. 3. Typical Raman spectra of Niobium oxides. Partial spectra are shown in some cases to avoid overcrowding.
N-Nb,O,
Polarized Raman
or NbOPO,. crystals
also suggest
around 1000 and 890 cm-i the
LO
LO-TO
modes.
In
that
spectra of the
bands
are most likely due to
centrosymmetric
B-NbzO,,
splitting would be expected due to ordering
in the direction
of rutile ribbons.
Accordingly,
1073
Raman spectra of niobium oxides the
i.r.
around
spectrum
shows
850 cm-l
and
spectrum
also
splitting,
possibly
shows
broad
bands
some
arising
evidence from
Unfortunately,
centered
The
600 cm-l.
for
small
spectrum
Raman LO-TO
The
bands in
particular mode
stretching
According based
these
one
band
and
The
more
than one band is found
Raman
presence
of
NbO,
spectra
both
is
of
Nb-0
show more
are
show
distorted
the
with
as well.
( >2.0 A with
Nb-0
bonds;
slightly
distorted
in the range correct to
some bonds
of short
corner-shared
the
stretching
are only
have Nb-0
1.9-2.0 A (Table
1).
bonds
It would
not be
vi bands in the Raman
vibrations
of
vs
specific
possible
Raman
are likely
the 550 cm-l in the
band
in
to the
region.
The
block
edge-sharing
overlapping
bands
with
in the region
to be due to the vs(Tz,J
band
being
characteristically
structures.
NbOPO,;
characteristic
of
We
this
also
band
corner-linked
see a
oxides
mode.
exhibit
structure
strong
bands
The strong Raman
regions
modes.
of the octahedra.
bands shown by all the oxides
around 260 cm-l probably
arise from the T,,
with only corner-shared
oxides
exhibit
more
mode
structures. octahedra
than
one band
in this region.
The Tzu mode in rhombohedral
bonds and it is likely that there would be consider-
orthorhombic
KNbO,
able
[Ill.
bands
coupling
of modes
amongst
In the model compound a single sharp Raman
NbOPO,
the
octahedra.
discussed earlier,
band is seen (Fig. 2) although
there is one short and one long Nb-0 octahedron. vi Raman while
It is likely
the
lower
corner-shared to
distinguish
octahedra.
frequency
octahedra.
bond in the
different
This
types
assignment
by the spectra of NbOPO, systems
[5,
v1 bands
as
of
due
In Table
to
difficult
edge-shared
seems to be justified
(vr, 800 cm-i)
61 containing
octahedra;
are
noted,
NbO,
in
vi
band
in
to all the oxides.
3, we have
assignments
listed
some elements
of symmetry
If we employ assignments
would
isolated
octahedra.
760-1000 cm-l
Raman
would
lower
modes
while
band
would
be due to TO(T,,)
different seem
structure,
to
(1.80-2.2 also
that
have A).
gives
the octahedra
relatively The
similar
(850-1000
B-Nb,O, long
published long
Nb-0
cm-i).
has
Apart
basically
a
in this polymorph Nb-0 structure
distances of
distances
It
appears
oxides retain
and are at least C,, or
NbO, [29].
octahedral modes
bands
Bands
is at a considerably
structures
oxides.
[8].
Raman
array model
(Table
2), the
in
the
region
be due to LO(T,,)
than the corresponding
fact
mode
be quite similar to those based
(760 cm-l)
block
This band could
in niobium
of internal
of B-Nb,O,
the
some
140 cm-l
the important
the infinite
spectrum in the
show
vibration
of niobium
octahedra
frequency from
are due to
also
3) with a band around
to a metal-metal
in the analysis
NbO,F,
the
200 cm-1
bands
that the NbO,
on
high-intensity
below
and
290 cm-l
C,, groups.
vi < 750 cm-l. The
band
and other
corner-linked
already
being common
phases is around
These
(Fig.
be assigned
octahedra
It is, however,
modes.
systematics
that the higher frequency
bands are due to edge-shared
Raman
external
all the
which
va, v4(T,,)
which becomes active in these non-cubic Unlike NbOPO,,
in
broad and show multiplet
due to distortion
the niobium
Nb-0
region could be
300-500 cm-l
These bands are generally
be The
In the i.r. spectra,
broad
and
would
octahedra.
band in the 290-260 cm-l
600-700 cm-l
as 2.3A)
octahedra
and generally
to assign multiple
as long
450-550 cm-l
same
earlier that
of some of the bands in the
and The
in the
the
corresponding
can be assigned to the i.r. active
octahedra
a number
bands.
the
( <1.9 A with some bonds as short as 1.7 A) and long
suggested
800-1000 cm-i
due to the v4(T1,)
corner-shared
tetrahedra
vs( T,,)
Raman
bands
have
be due to extensive
octahedra
common
niobium-oxygen
the edge-shared
the
could
the
band around 360 cm-l in all the oxides, close to the
in all the structures
NbO,
earlier,
We
high intensity
present
octahedra. with
in
of the
mode,
generally
in this region
and
are present
are appreciably
vs region
consistent
types
in H-Nb,O,)
As mentioned
octahedra from
NbO,
edge-shared
octahedra (e.g.
ordered
different
polyhedra:
relatively
(see Figs. 2 and 3), indicating
from
modes
internal
intensities
bands
arise
the
and
their
variations,
as the vi modes.
which
these bands were TO modes
of
record
show two
region
LO
the
arise
oxides
(vs)
(Y& are
isolated
could
they
That of
to
not
of niobium
Some of the oxides
sharp and symmetric that
on
bands
modes.
interesting
spectra
760-1000 cm-l
interest.
2),
than
Raman
region
approach
(Table
origin
in the
the
could
due to its dark colour.
All the niobium 600-680 cm-i
departures
from ideal symmetry. oxides
we
of NbO,
in
the
derived
620-680&cm-1 modes.
region
Bands
in the
300-550 cm-i can also be assigned to A, or
E modes
originating
from the
T,,
from
mode would Having
spectra, individual
T,,.
Bands
be in the region
assigned the major we shall now examine oxides.
originating
200-300 cm-i.
bands in the Raman specific
features
of
A. A. MCCONNELL, J. 8. ANDERSON
1074
i
and C. N. R. RAO
H- and N-Nb,O,. These two block structures show remarkably similar spectra (Fig. 4 and Table 3). H-Nb,O,, however, has one out of 28 Nb atoms in tetrahedral sites and the Nb-0 bonds in the NbO, tetrahedra are short (~1.66 A). Samples of H-Nb,O, annealed at 1200% for several days show clean splittings of bands throughout besidesa weak Raman band at 860 cm-l not seen in the We feel that this band spectrum of N-Nb,O,. may be due to the ri(Ai) Raman active mode of the NbO, tetrahedron, the ~a, rs and rq bands being too weak to be seen. Nbsf ions seldom occur in tetrahedral coordination. YNbO, (with Nb-0 bonds of 1.9 A), one of the few compounds consisting of NbO, tetrahedra, shows the vi Raman band at 830 cm-l [30] which is close to that assigned by us for the tetrahedra in H-Nb,O,. It is indeed remarkablethat Raman spectra can pick out these tetrahedra present in such low concentration. Annealing of the oxide seems to be essential to ensure ordering of the tetrahedral ions into one of two equivalent sets of tertahedral sites. The relative intensities of the two vi bands of NbO, units in H- and N-Nb,O, are interesting. The higher frequency bands assigned to edge-shared octahedra have much higher intensity (Fig. 3 and 4) since these octahedra predominate over corner-sharedoctahedra (Table 1). TiNbaO, and T&Nb,cO,,. These oxides have the same basic structure sa N-Nb,O,, but the Ti4+ ions seem to preferentially occupy edge-shared octahedra [13]. Accordingly, the relative intensity of the vi band due to corner-sharedNbO, is higher in these compounds than in N-Nb,O,. The vi(&) and v5(Tc,) modes of TiO, are probably responsible for the large intensities of the 650 cm-l and 540 cm-1 bands (Fig. 4). TiO, octahedrain perovskites and K,NiF, type compounds also show vi and vr, bands in these regions [5, 61. PNb,0a5 and VNlz1,0,~. These two compounds are isomorphous[I] and PNb,O,,, whose structure has been studied in detail [15], belongs to space group I4/m or I& The unusual feature in the Raman spectra of these compounds is the low intensity of the vi bands (LO) (Figs. 3 and 4). The low intensity is indicative of disorder. Disorder in PNb,0a6 can be caused by a statistical distribution of cations over the tetrahedral sites giving a centre of symmetry to the crystal, and this would be consistent with the Id/m space group. H-NbsO,, in which the tetrahedral sites are ordered (and occupied by Nb’+), gives sharp intense vi bands. Further, the 14 structure should give 76 Raman-active bands while the
Raman spectra of niobium oxides 14/m
structure
Although bands,
should
it is dii&ult
give
42
Raman
bands.
to observe or assign all the
the observed
Raman
spectrum
seems to
appears
Since the P atoms in PNbsO,,
The
are in tetrahedral
spectrum
to see bands charaoteristic
of PO,
should
spectrum.
see the
bands
discussed
earlier) :
Here
945 cm-l;
vi(&), va(T2), 446 cm-l
1045 cm-l; again,
there
between the PO,
and v,(E),
appears
modes.
Bands
of VO,%
(e.g. Dy VO,, NasVO,) 860,359-390
1281.
coupling
In VNbsO,,,
vs modes respectively. seen at 895,860(?),
due to vi, ye, vq and
Such “vanadate”
trum of VNb,Oe,.
Once we eliminate
to the
the
tetrahedra,
VNb,O,,
spectra
found
by
of PNb,O,,
oxide. This
niobium
X-ray
diffraction
isostructural with PNbsO,,, the composition
to
and
(vs) with a-GeOe
NbO,
mode.
and
analogy
shows bands around
with
960 and
niobium
oxide,
but
We find no bands assignable to NbO,
tetrahedra.
The
tetrahedral
concentration
groups
of
(approximately
GeO,
one cation in
10) is relatively low, and their Raman bands could be
too
weak
(900-1000 possible
for measurement.
cm-l) that
distorted
at 940 cm-l
the
other
edge-shared
apparently
recent structural study [lS]. B-Nb,O,.
The
vr region
clearly shows three bands.
the band
octahedron, octahedra,
It is
could
distinct
identified
arise from
in
our
As mentioned earlier, this polymorph
with rutile ribbons shows a much lower frequency
the
crystal
for vr. The ve modes also appear at considerably
oxygen
lower
frequencies.
bands
are shown
incompatible
with
of interstitial
determination
of
the
site.
crystal
number
occupied-i.e.
the
fabric
of
as in PNb,O,s,
is perfect-but
excess of cations in this inherently structure,
: oxygen that
over
ratio, provide
is
the
centred that the
nonstoichio-
formal
accommodated
tetrahedral
lo:25 in
sites.
the
Within
The
assignments
in Table
of bands
rather low (C,,).
channels
cm-l
(vJ
since it is in the same region as the bands due to an
symmetry
metal
ve > v1 by
of germanium
are
metric
cm-l
unless there are either vacant detailed
octahedra,
tetrahedra
660-680
assignment of the v1 band is difficult
structure [16] it is now clear that the oxygen sites fully
regions
There are bands in these regions in
the spectrum unequivocal
tetrahedral
GeO,
(ve), 450-520
from
This composition
sites or cations in some type a
be
in the
cm-l
[31].
occupied that
compound,
wss originally assigned
Ge0,.9NbeO,.
however,
structure From
bands due
are not unlike the spectrum of H-Nb,O,.
Germanium
is,
bands are
420 and 310(?) cm-l in the spec-
920 cm-l
germanium
expect
bands
709-830
SiO,
are generally found
cm-l.
give
%(T2). 330 cm-l.
in the regions 889-900,829-
and 250-310
would
cm-l
we should similarly see bands characteristic of VO, tetrahedra.
(vr),
of
preferentially
We
~300
to be little
and NbO,
atoms
NbOPO,
(just ss in the model compound
random
oxide
sites.
“phosphate”
the
giving rise to
if the
Ge
do indeed
with
niobium
of the polyhedra in the structure, we would expect We
consistent
should show bands due to GeO, tetrahedre
sites and the tetrahedra constitute $th the number in the Raman
be
of sites of each sub-set,
higher symmetry.
favour the 14/m structure.
qualitatively
to
occupation
1075
in the
3.
different
of the octahedra
‘polyrutile.’
rutile are multiplied, and
coupling [4].
Raman
would
the
appear to be as a
and BsP bands of
although there is a spread of
absence
of some modes
In the Ruddelsdon-Popper
(related to SrzTiO,),
the
regions,
We could consider B-Nb,O,
In trirutile, the A,,
frequencies
of
Considering
the Ai,
due to
structures
mode is multiplied,
these channels, cations also occupy highly distorted
but the number of observed bands does not account
octahedral sites. These are only partially occupied;
for all the degrees of freedom, due to ordering and
the relative number of cations in tetrahedral octahedral metry.
channel sites determines
Within
the stoichio-
any one channel, tetrahedral
octahedral cations may approximate
and and
to an ordered
the spectra bear no simple relation to the parent structure expect
[a].
simple
B-Nb,O,
By the same token, we would not multiplication
of rutile
since it has a much
bands
in
lower symmetry.
array, but there is little or no correlation between
BEATTIE and GILSON [8] have assigned the Raman
the
bands of B-Nb,O,
ordering in neighbouring
tetrahedral
octahedral
cations,
ordering, the structure
If only
the space group
would be 14/m; for ‘stuffed’
symmetry with
channels.
sites were occupied,
depending
channels, upon
may be CPn or S,.
the Since
there is no superlattice corresponding to correlation between
channels,
symmetry, Raman 11
a statistical
as for PNb,Oer,,
spectrum
model
with 14/m
is appropriate.
of germanium
niobium
The oxide
(OeNb& advantage However,
in terms of ONb,,
types of groupings. in this type
ONb,
and
We do not see any
of group decomposition.
our assignments in Table 3 show general
correspondence to those of Beattie and Gilson. EXPERWNTAL Raman spectra were recorded on a Gary 81 Raman spectrometer employing 6328 A He-Ne laser excitation and on a Spex Ramalog 4 with Coherent Radiation
1076
A. A. MCCONNELL,J. S. ANDERSONand C. N. R. RAO
52G argon ion laser, using the 4580 A and 5145 A lines. Infrared spectra were recorded using a PerkinElmer 457 spectrometer in the 4000-250 cm-’ region, powdered samples being dispersed in TlBr pellets. Infrared spectra in the 400-40 cm-’ region wore measured on a Beckman-RllC FS620 interferometer with FTC100 computer, powdered samples being dispersed in 1” Rigidox discs. Polarized i.r. spectra were recorded on a Perkin-Elmer 557 spectrometer using a rotatable gold wire grid polariser in the common beam and also on the interferometer. Single crystals were mounted on a Wilks specular reflectance attachment with an angle of incidence of 15”. Single crystal studies were possible only with N- and B-Nb,O, and polarized data could be obtained only in the case of N-Nb,O,. Acknowledgement-Our thanks are due to Dr. A. P. Lava of the University of Glasgow for the use of the Spex Ramalog, for assistance in obtaining polarized IR data and for much helpful discussion. REFERENCES [l] A. D. WADSLEY and S. ANDERSSON,Perspectives in Structural Chemistry, (edited by J. D. DUNITZ and J. A. IBERS,) John Wiley, New York, Vol. 3, 1970. [Z] J. S. ANDERSONin Surface and Defect Properties of Solids (Specialist Periodical Report), (edited by M. W. ROBERTSand J. M. TIIOMAS,)Chemical Society, London, 1972. [3] J. S. ANDERSONin Solid State Chemistry, (edited by R. S. ROTE and S. J. SCHNEIDER,JR.) NBS Special Publication 364, National Bureau of Standards, Washington, page 295 1972. [4] W. B. WHITE and V. G. KERMADASin Solid State Chemistry, (edited by R. S. ROTH and S. J. SCHNEIDER,JR.), NBS Special Publication 364, National Bureau of Standards, Washington, page 113 1972. [5] G. BLASSEand A. F. CORS~IIT, J. Solid State Chem.. 8, 513 (1973); ibid., 10,39 (1974). [6] G. BLASSE and G. P. M. VAN DEN HEWEL, 2. Phyeik. Chem. (N.P.), 84, 114 (1973); J. Solid State Chem., 10,206 (1974). [7] F. MATOSSI, J. Chem. Phya., 19, 1543 (1951). [S] I. R. BEATTIE and T. R. GILSON, J. Chem. Sot. A, 2322 (1969).
PI J. T. LAST, Phys. Rev., 105, 1740 (1967). [lOI P. S. NARAYANAN and K. VEDAM, 2. Physik. 163, 168 (1961). [Ill C. H. PERRY and N. E. TORNBERG in Light Scattering Spectra of Solids, (edited by G. B. WRIGHT), Springer, New Tork, 1969 page 467. WI S. AXDERSSON, 2. anorg. allgem. them., 351, 106 (1967). r131 R. B. VON DREELE and A. K. CHEETAM, Proc. Roy. Sot. A (London), 338, 311 (1974). P41 B. M. GATEHOUSEand A. D. WADSLEP, Acta &yet., 17,1546 (1956). WI R. S. ROTH, A. D. WADSLEY and S. ANDERSSON, Acto &?/8t.,18, 643 (1965). [16] J. S. ANDERSON,J. M. BROWXE, A. K. CHEETAM, R. VON DREELE, J. L. HUTCEISON,F. J. LINCOLN and D. J. M. BEVAN, J. STRAEHLE,Nature, 243, 81 (1973); J. S. ANDERSON, D. J. M. BEVAN, A. K. CHEETAM,J. L. HUTSHISON,J. STRAEHLE and R. VON DREELE, Proc. Roy. Sot., In Publication. [17] S. ANDERSSONand J. GALY, J. Solid State Chem., 1, 576 (1970); Also see F. LAYES, W. PETER and H. WULF, Naturwias., 51, 633 (1964). [lS] R. LOUDON, Advances in Physics, 13, 423 (1964). [19] L. E. ORGEL, An Introduction to Transition Metal Chemistry, page 174. John Wiley, New York, 1960. [20] L. PAULING, The Nature of the Chemical Bond, Chapter 48. Oxford University Press, 1952. [Zl] G. BLASSE,J. Inorg. Nucl. Chem., 26, 1191 (1964). [22] J. B. GOODENOUGH and J. A. KAF~LAS, J. Solid State Chem., 6, 493 (1973). [23] L. A. WOODWARDand J. A. CREIOHTON,Spectrochim. Acta, 17, 594 (1961). [24] W. VAN BRONSU’IJCK,R. J. H. CLARK and L. ~MAREscA,Inorg. Chem., 8, 1396 (1969). [25] L. K. FREVEL and H. W. RINN, Acta CT@., 9, 626 (1956). [ZS] _ - C. H. PERRY, J. H. FERTELand T. H. MCNELLT, J. C&m. Phye., 47, 1619 (1967). 1271 J. M. LONGO and P. KIERKEGAA~D, Acta Chem. Stand., 20, 72 (1966). [28] Ii. NAKAMOTO,Infrared Spectra of Inorganic and Coordination. Compounds. John Wilev, New York, 1970. [29] B. MARINDER,A&iv Kemi, 19,436 (1962). [30] G. BLASSE,J. Solid State Chem., 7, 169 (1973). [31] J. F. SCOTT,Phy8. Rev., Bl,3488(1970). 1,
pp. 1067 ta 1076.
PergamonPnws, 1976.
PrIntedin Northern Ireland
I%arxanspectra of Robin
oxides
A. ANN MCCONNELL Departmentof Pure and AppliedChemistry,Universityof Strathclyde,GlasgowGl 1XL 3. 5. ANDERSON and C. N. R. RAO* InorganicC~ernist~ Laboratory,Universityof Oxford, OxfordOXI 3&R (Received 30 Jfay 1975) Abstract---Ram&nspectra of various types of niobium oxides have been studied along with theiri.r. spectraand the spectrainterpretedin the light of the structuresof the oxides Besides discussing LO-TO splittingin theseoxides,bandscharacteristic of edge-shared and corner-shared NbOI octahedrahave been assigned. Tetrahedr&l modes of NbOd, PO1 and VO, units present in some of the oxides have been rtesigned to distinctbands in the spectraindioatiugnegligible couplingbetweenthe tetraheclmand Nb06 octahedra. The speotraof PNb,0t6 &ndanalogues suggestthat they belongto I4/m spacegroupratherthan I$ group.
Although it h&s been known for sometime that niobium pentoxide, NbsO,, exists in different polymorphic forms, it is rel&tively recently that structursl relationships among these have been properly understood [l]. The structuresof most of these polymorphs &re derived from the NbOSF (or ReO,) structure and c&n be explained on the basis of crystallogrctphicshe&r. Cryst&llogr&phic she&r &llows aocommod&tion of stoichiometric ch&ngesby & structuml &djustmentthat preserves the cation coordination, but virtually elimin&tes point defects [Z, 31. In many of the NbsO, polymorphs (H-Nb,O,, N-NbsO, etc.), two sets of intersecting cryst&llogr&phic she&r planes dissect the structureinto blocks of NbO, octahedra and det&iled structural studies on some of these block structures h&ve been reported in the lit8rature. Thus, in N-NbSO,, 4 x 4 blocks of cornershsred NbO, octahedra are joined together in infinite ribbons at the same level, as well BBbeing linked to adjacent blocks at & different level, by shsring octahedr&l edges (Fig. 1). In the the~od~&~cally most stable form, H-Nb,O,, 3 x 5 blocks of corner-shared oct&hedr&at one level (y = 0) are joined in ribbons by octahedral edge-sharing, to form stepped rows; at another level (y = Q), octahedra form, 3 x 4 blocks which share edges with ths stepped rows; one Nb atom out of 28 occupies & tetrahedral site in the channel formed between the corners of blocks (Fig. 1). * Commonwealth Visiting Professor (1974-76); Permanent Address: Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
Analagues of Nb,O, pol~o~hs with block structures containing other cations like TiNbSO, (related to N-Nb,O,) and PNb,O,,, with P atoms in tetrahedrrtlsites, are also known &mongst the double oxides of niobium. Unlike N- or H-Nb,O, related to the ReOs structure, the B form of Nb,O, has & structure related to PdFe, with rutile ribbons of edge shared octahedra joined by terminal octahedral oorners. In general, the polymorphs cont&in different types of niobium-oxygen polyhedra, predominantly corner-shared and edgeshared NbO, octahedra. Corner-sharedoct&hedr& ase not appreciably distorted, while edge-shared octahedra exhibit large distortions, resulting in significantvariations in Nb-0 distances. We have studied the Rsznan spectra of sever&l niobium oxides, along with their i.r. spectra, to see how the spectr& elucidate or corroborate the strnctuml fe&tures and cation ordering in these extended StrllCtllr8S. The study shows that, despite the complexity of these oxides examined by us, vibration&l spectroscopy provides information of value when interpreted in conjunction with the known structure features. The method of analysis followed by us, although somewhat simplified, indicates the usefulness of the intern&l mode approach in systems where strict factor group analysis is diBcult. RESULTS
Bd
Al?D DrS~~IO~
for spectralasaignmenfs
Vibrational spectroscopy c&n be useful to study different types of ordering in metal oxides 143: (a) preferential ~st~bution of non-equivalent
1067
A. A. MCCONNELL, J.
1068
S. ANDERSON and C. N. R. RAO the metal-oxygen to
the
octahedra
crystal
binding
axe large compared
forces.
The
internal
vibrations of the MO, group in the solid should be quite
close to the free-ion
modes,
the external
modes occurring at considerably lower frequencies than the internal modes.
Thus, the high frequency
bands in the Raman spectra of many oxides can be satisfactorily octahedra.
assigned to the internal modes of the This is valuable
like LaMgTiO,
containing
in complex essentially
systems
octahedral
MO, units [5], as well as in systems like Mg,Nb,O, with lower symmetry structure [4,6].
than the parent corundum
The Raman spectrum ofMg,NbaO,
would be difficult to interpret formal
factor
frequency
group
Raman
correspond the NbO,
on the basis of a
analysis, band
but
would
to the symmetric
the
highest
undoubtedly
stretching
mode of
octahedron which is only Raman active;
the asymmetric stretching and bending frequencies are seen in the vibrational TiO,
[7,
KNbO,
(4
cations on to non-equivalent in space group symmetry and
distribution in space
inverse
sites, without change or unit cell size, as in
spinels,
on to equivalent
group
symmetry
(b)
only,
group
as in
LiNbO,,
analysis.
[ll].
structures.
consequence
of ordering
centre of symmetry; mutual
lower space
exclusion
The group theoretical is the
removal
of the
this results in the loss of the rule
in
the
Raman
and
i.r.
and
the
compatible
with factor
case of KNbO,,
phase
spectra, the cubic
a second-order
spectrum
When more than one type of metal-oxygen
polyhedron
is present in a cell, we would see a
superposition of the Raman spectrawith frequencies similar to those of the free parent polyhedra,
if
there is little or no coupling. Because the niobium oxides studied by us have rather
complex
structures
of
low
symmetry,
containing extended networks of NbO, difficult.
as in some of the corundum-or
In
phase giving essentially
metry as the parent structure, as in trirutiles and also (d) buildingof superstructurewitha
like
SrTiO,
like
are clearly seen in the Raman
(Table
K,NiF,---based
perovskites
of the
systems
transitions from cubic to lower symmetry structures
preferential
sites with decrease
Analysis
simple
[9, lo], basedon the internal mode approach,
(c) building of superstructures with the same sym-
group symmetry,
81 and
spectrum. of even
yield valuable information
Fig. 1. (a) Structure of H-Nb,O, [OIO] projection, showing columns of octahedra, (5 x 3) in section, at one level (heavy outlines) and (4 x 3) columns at second level. On the left hand side, a schematic representation outlines the rectangular columns as defined by the niobium atom positions. (b) Structure of N-Nb,O,. Similar [OIO] projections showing the columns of (4 x 4) octahedra at both levels.
normal
i.r.
spectra
l),
are not In
the
strict factor
octahedra
group analysis would
be
The structures in most of these systems supercells case
of
of
the
simpler
structural
units.
non-centrosymmetric
block
structures, however, we may be able to see LO-TO splittings in the Raman
spectra since the crystals
could be considered to belong to Loudon’s
type I
in one direction and type II in another [18]. systems like PNbsO,,
Raman
may also provide some information on the ordering
spectroscopy
is particularly
sensitive for
detection of ordering in rock-salt and other types
of cations
of structures
Although
Raman
since ordering gives rise to strong
bands (e.g. in the region corresponding to
the stretching while
none
structures. In metal titanium
mode of metal-oxygen
are
seen
in
completely
or VNb,O,s,
In
spectra, or in the relaxation of the selection rules.
and hence on the crystal
oxide
systems
containing
the binding
in these systems,
we can fruitfully
octahedra)
Raman
disordered
in terms of discrete metal-oxygen
niobium,
forces within
symmetry.
strict factor group analysis is difficult interpret the
spectra with the internal mode approach
The types of polyhedra
or tungsten,
Raman spectra
polyhedra.
present in the different
niobium oxides are shown in Table 1. We see that the oxides predominantly
contain NbO,
octahedra
B-Nb,O,(M) czjc
I4/m (or) I3 ~O~.SNb~O~ I4ImP
VNW’,,
PNb~O*~(T) 14/m (or) Iii
TiNb,O,(M) 82/m
N-Nb,O,(M) cz/na
H-Nb~O~(M) P.2
StNCturt,
Structure can be understood in terms of PNb,O,, (see discussion section of the paper for details). Different altogether from othem. The structure is related to PdFs with rntile ribbons of ES octahedra joined by octahedral comers (very high density).
Derived from NbO,F structure. Has 3 x 6 blocks of cornerlinked (CL) NbOe octahedra at one level joined by octahedral edge sharing to form stepped rows. At another level, 3 x 4 blocks are present and these share edges with stepped rows. One out of 28 Nb atoms in tetrahedral sites. Derived from NbO,F structure. Has 4 x 4 blocks of CL octahedra. These blocks are joined together at the same as well as different levels by sharing of octahedral edges. Related to N-Nb,O, except that block size is 3 x 3. There is substantial ordering of Ti ions in ES octahedra (up to 64%). Related to N-Nb,O, except that block sise is 3 x 4. Ti ions occupy ES octahedra up to 40 %. Derived from NbO,F; contains 3 x 3 blocks of CL octahedra joined to similar blocks at different levels by edge sharing. Tetrahedral sites occupied by P. &pace group either of the two 8hQW7t. Similar to PNb,O,,. VO, tetrahedra present in place of PO, m PNb80r6. GO, tetrahedra may be present. An unusual ES octahedron present in addition to CL and the usual ES octahedra. ES octahedra
Available distances are in 130-2~2 A range.
CL Octahedra have Nb-0 bonds I.92 + 0.06 A. ES octahedrs have Nb-0 distances in the range 1~762.3 A with l/6 of Nb-0 distances <13 A. P-O distance is 1.66 -& 0.08 A.
CL octahedra l/Sth of total. In addition to ES and CL NbO, octahedra, PO, tetr~e~ present (l/Sth of total).
[I71
Cl61
Cl1
El31
1131
1131
CL Octahedra are slightly distorad. ES octahedra have Nb-0 distances between 1.66 and 2.38 A with l/3 of them
Since Ti preferentially occupy ES octahedra, there would be more CL NbOG octahedra (~6/26).
,,
WI
CL octahedra are only slightly distorted. In ES octahedra, 9 out of 48 Nb-0 distances are < 1.9 A and 22 are > 2.0 A.
r141
Ref.
In addition to CL octahedra, there sm ES octahedra similar to H-NbJ,O,. 1/4th of octahedra are CL.
Bond Distances CL octahedra with lQ2(2) and 1.9’7 A Nb-0 bonds. ES octahedra have Nb-0 distances between 1.73 and 2.26 A, 17 out of 84 bonds are1 <13 A and 36 > 2.0 A. Tetrahedral Nb-0 bonds are short (1.66 A).
In addition to corner-shared (CL) octahedra, there are edgeshared (ES) octahedra (sharing 1, 2 or 3 edges). NbO, tetrahedra are also present. 5/2&h of octahedra are CL.
Types of Polyhedra
Table 1. Nature of metal oxygen polyhedra in niobium oxides
1070
A. A. MCCONNELL,
distorted
to
different
extents,
whether
they are corner-shared
appears
that
NbO,
Nb5f
octahedra
distortion oxygens
electrostatic
quite
octahedra
some elements
and, as a first groups.
In
Table
different NbO,
octahedra,
and
PNb,O,,
tetrahedra the
and
the
(Table
it should
various
highest
perovskite
Where
of
Raman
Since
symmetry
all
other
give
Raman NbO, do
the
bands
band
Raman
of v,(Eo)
5A,[l
= (v,, -
-
Q4/v1[l
along
and
respectively. when
-
A
high
to
I(vi)/l(vs)
u~,/ul approaches
unity,
Instead
of andysing
corner-linked for
In this
in
in
such
0, symmetry
all
to bond bond
ratio
is
as in the
however,
or increase
modes.
approaches
outlined
a basis for assigning
also examined compounds, NbO,F
the
spectra
NbO,F
on the
selections
in Table from
2.
ideal
in unit cell size will give
niobium
of
The given
departures
the
spectra
Our
discussion
oxides
follows
above.
In order
the bands, of two
of the to
we have
simple
model
and NbOPO,.
and NbOPO,
NbOsF
has a cubic structure
per
cell
and
with
Raman
NbO,F
random
spectrum
of
650 cm-l.
Based
for isolated
octahedra,
bands
than
mode
due
to
arising
to
the
be fully
slight
A band
of
around
approach that the
may
around
not be
300 cm-i
Based
on the assign
C,,
the
and
B, + E(T,,)
symmetry. region
departure by
mode
mode.
and
band
we can, in principle,
Such assignments, justified
KNbO,
Raman
in this system
3A + 3E(32’,,) from
Second order
we would conclude
bands in the 600-725 cm-l symmetry.
2). cubic
with a shoulder
the vs( Z’s,)
model,
shows
of a second-order
(Fig. by
of [25],
compound
on the internal
725 cm-i.
array
modes
this
highest
occurs at 725 cm-l
arrays
substitution
bands typical
The
one formula
infinite
anion
of a cubic crystal
Raman
with
contains
broad low intensity
NbO,F.
in
array of octahedra,
are
the
due
the
both
modes
directions.
octahedrs
could arise from
and
bands
modes,
we could also analyse
active
The
should
active active
the internal
rise to Raman
vr(Ar,)
c(~ are the
being
octahedra,
analysis,
infinite
the
modes
also
metsl-
oxides
vs(E)
them on the basis of an infinite
higher
with respect
perpendicular
v4(T2)
and
should
similar
Raman
vl(A1)
[ 11, 261.
( --hv,/K!‘)
a1 and
to
showing
are sufficiently
in the
two
bands are
on the distortion
and
present
rise
bands
mode
PO,
KTaO,
(2a,/aJ12 (c+# exp
ve(Tzu)
are also exhibited
intensity
the
depending
spectra
the
The vs, v4( T,,)
spectra,
spectrum
significantly
frequency;
bond polarizabilities
stretching axis
-t
13A,[l
to
would
of perfect
Raman
give
expect
with respect to ~~(4~~) is given by [23],
A,
observed
would
mode
and
of
however,
the
basis of isolated
unit
containing
and i.r.
octahedra
studies.
since Nb6+ with
participates
ve is the exciting derived
spectra
intensity
We
[21, 221. The relative
IhI) -= I(%) where
of high mode.
to be weak
configuration
n-bonding
low
non-centrosymmetric,
due to the Y,(E,)
octahedra
are
have
for structural
and
different
structures.
octahedron
least
vs is some
could,
NbO,,
corresponding
oxides
of in
distortion
tetrahedra at
type
zero
If the octahedra
active.
oxygen
and
Extensive
the inactive
become
in extensive
intensity
61.
i.r.
of the octahedra.
provide
and
[5,
or splitting,
distorted,
show
[24]
vs bands.
the
asymmetry
of
arise from
to ordering
oxides
in
or
bands due to y1 should be
niobium
+,(Tsp)
The
corundum
of MO,
first-order
Rsmen
arise from
systems.
in disordered
and are generally
would
vibra-
band in the spectra
r1 should be most diagnostic The
stretching
the
seen
rules
octahedra
[5, 61 does indeed
the
sub-units
mode, corresponding
The band is sensitive
Raman
or corundum
Raman
to analyse
isolated
with
or no intensity
perovskite
vs(Ts)
in principle, into
in oxide
more than one type
seen.
the
in such an analysis.
systems
present, multiple
they
polyhedra
Raman
structures
has little
in
m&al-oxygen
could,
metal-oxygen
frequenoy
this mode.
the
Besides
be possible
diagnostic
oxide
for
octahedra
shown.
modes
2), the ~i(Ai,)
many
octahedral
etc. [8], but we do not see
advantage
is most
they
examine
rules
NbO,
One
modes.
to the symmetric tion,
of these
discrete
the various
any obvious
NbO,
(e.g. Cd, or CsV)
we could
selection
are
Nb-0
the
Oh symmetry,
of isolated
like MsO, MsO, (MsO,), Of
the
contain
NbCl,
higher intensity
the
the
in
[20]
and
ions involved
negligible
octahedra
the
some of the oxides like H-Nb,O,
tetrahedral
also divide
Such
since
metal
Accordingly,
NbO,
of symmetry
symmetries
s-bonding.
regular
[19].
Although
2, the
case of transition
It
do not obey Pauling’s
in terms
modes
on
octahedra
approximation,
spectra
vibrational
system
expected
rule
have
to form
and C. N. R. RAO
NbO,
be
covalent.
do not
the Raman
small
octahedra
valence
are
retain
also
depending
or edge-shared.
in any oxide
would in NbO,
bonds
is too
J. S. ANDERSON
The
would
from
the
however,
nature
weak
then be ideal may
of spectrum
0, not of
1071
Raman spectra of niobium oxides
I
I
(b)
(b)
II
PiJ(__hL ,(a),
u 400
600
800
1000
. ‘-, .crn. Fig. 2. Raman spectra of (a) NbO,F and (b) NbOPO*. The Raman bands
aa in
spectrum
of NhOPO,
first-order
spectra
shows sharp
(Fig.
2).
The
spectrum is simple and can be readily assigned on the basis of the published structure [27].
NhOPO,
consists of chains of corner-shared NbO,
octahedra
along the c-axis, with PO, chains.
The NhO,
coplanar bonds
Nb-0
tetrahedra
octahedron
bands
(1.97 A) and two unequal
(1.78 A and 2.32 .& Thus,
plane.
symmetry
the
axis.
13 distinct
has
a
to the four-fold
is symmet-
distance of 1.53 A as in normal
orthophosphates. bands
perpendicular
octahedron
The PO, tetrahedron
rical, with a P-O Reman
linking the
has four identical
Factor group analysis yields 15 (5A,
bands
+ 4B,
+ 6E,)
in the
and
spectrum;
two bands are not seen probably
we
the
see
other
because of the
small splittings of the degenerate modes. In the spectrum of NbOPO, to the PO,
1016 cm-i
follows: (strong),
(Fig. 2), bands due
unit [28] can be readily (weak),
~~(-4~); 445 cm-l
and 375 (medium),
980 cm-l
(weak doublet),
vs(E).
Ye
The spectrum
is similar to that of IO, in NaIO,. of NbOPO,
assigned as
ve(Ts);
of PO,
The i.r. spectrum
shows strong bands due to us and Y*,
as expected.
The remaining bands in the Raman
spectrum
NbOPO,
of
Raman-allowed 800 cm-l v&C,)
can
assigned
612 cm-l
shoulder
The position
be
of the NbOs
(strong), pi,
and the
Y~(!Z’sJ.
bands
on the
to
the
octahedron: (very weak),
375 cm-i
of the v1 Raman
band
band in
A. A. MCCONNELL,J. S. ANDERSON
1072
and C. N. R. RAO
some other oxides containing corner-shared NbO, octahedra appears modes
is
also
that either
between
amongst
NbO,
The 290 cm-l
NbO,
Raman
mode
suggests
octahedron Spectra
NbO,
band
61.
active
is
distorted.
C,,
symmetry
400
200
It or
in NbOPO,.
of NbOPO,
becomes
600
800
of the
octahedra
polyhedra
may be to the
when
The
the
Raman
for the NbO,
(Table 2).
of niobium oxides
All the oxides give first-order with
[5,
coupling
mode or, more likely,
which
octahedron
spectrum
800 cm-l
the
and PO,
due to an external va(Tzu)
around
there is negligible
loo0
sharp
bands.
Typical
Raman
spectra
spectra
are shown
B-Nb205
,
in Fig. 3 to illustrate the general features and the data are summarized in the form of a correlation diagram in Fig. 4.
with Nb-0
cm-i
region
stretching.
which
can
be
associated
Certain features in the low
frequency region also appear to be common. 1000 I&
II
I..
.
600 I
I
600 I,
400 I
I
III
I.. 800
000
The characteristic feature in all
the spectra is the appearance of sharp bands in the 700-1000
I
I. 1000
cm
I
I
I,Li//l:
I.. 400
,
, ., 200
-I
Fig. 4. Correlation diagram showing Raman bands of Niobium oxides. Heights of vertical lines give relative intensities. PO, bands of PNbsOzs and VO1 bands of VNb,O*, are not shown here. “d” stands for doublet.
200 I
All the oxides discussed here, with the exception
I
of B-Nb,O,,
are block
centrosymmetric crystallographic octahedra
structures
(Table
1).
and
Along
are not
the
unique
direction in which corner-sharing
form infinite strings, long range forces
are likely to predominate
over local anisotropy;
normal to this direction (i.e. in the plane shown in projection in Fig. l), anisotropy will be dominant. Such a situation,
essentially
similar to Loudon’s
Type I in one direction and Type II in the plane perpendicular to it, should give rise to large LO-TO splitting
in the Raman
spectrum.
It, therefore,
appears that the two bands in the 800-1000
cm-l
region in these block structures (Fig. 4) are due to A,
and E type
with
the
longitudinal
corresponding
optic modes
transverse
(TO) appearing in the 620-680 650 cm-1
Raman
bands
optic
cm-l
region.
are intense
and
appear as doublets due to anisotropy. LO-TO range
splitting order
LO-TO
probably
In
cm-l
is split due to anisotropy [ll]. LO-TO
The often
The large
arises from the long-
in one direction,
splitting is ~250
(LO), modes
KNbO,,
the
and the TO mode This assignment of
modes in niobium oxide block structures
tinds support from the absence of such splitting in NbO,F I
1000
I
so0
600
400
200
cm-’
Fig. 3. Typical Raman spectra of Niobium oxides. Partial spectra are shown in some cases to avoid overcrowding.
N-Nb,O,
Polarized Raman
or NbOPO,. crystals
also suggest
around 1000 and 890 cm-i the
LO
LO-TO
modes.
In
that
spectra of the
bands
are most likely due to
centrosymmetric
B-NbzO,,
splitting would be expected due to ordering
in the direction
of rutile ribbons.
Accordingly,
1073
Raman spectra of niobium oxides the
i.r.
around
spectrum
shows
850 cm-l
and
spectrum
also
splitting,
possibly
shows
broad
bands
some
arising
evidence from
Unfortunately,
centered
The
600 cm-l.
for
small
spectrum
Raman LO-TO
The
bands in
particular mode
stretching
According based
these
one
band
and
The
more
than one band is found
Raman
presence
of
NbO,
spectra
both
is
of
Nb-0
show more
are
show
distorted
the
with
as well.
( >2.0 A with
Nb-0
bonds;
slightly
distorted
in the range correct to
some bonds
of short
corner-shared
the
stretching
are only
have Nb-0
1.9-2.0 A (Table
1).
bonds
It would
not be
vi bands in the Raman
vibrations
of
vs
specific
possible
Raman
are likely
the 550 cm-l in the
band
in
to the
region.
The
block
edge-sharing
overlapping
bands
with
in the region
to be due to the vs(Tz,J
band
being
characteristically
structures.
NbOPO,;
characteristic
of
We
this
also
band
corner-linked
see a
oxides
mode.
exhibit
structure
strong
bands
The strong Raman
regions
modes.
of the octahedra.
bands shown by all the oxides
around 260 cm-l probably
arise from the T,,
with only corner-shared
oxides
exhibit
more
mode
structures. octahedra
than
one band
in this region.
The Tzu mode in rhombohedral
bonds and it is likely that there would be consider-
orthorhombic
KNbO,
able
[Ill.
bands
coupling
of modes
amongst
In the model compound a single sharp Raman
NbOPO,
the
octahedra.
discussed earlier,
band is seen (Fig. 2) although
there is one short and one long Nb-0 octahedron. vi Raman while
It is likely
the
lower
corner-shared to
distinguish
octahedra.
frequency
octahedra.
bond in the
different
This
types
assignment
by the spectra of NbOPO, systems
[5,
v1 bands
as
of
due
In Table
to
difficult
edge-shared
seems to be justified
(vr, 800 cm-i)
61 containing
octahedra;
are
noted,
NbO,
in
vi
band
in
to all the oxides.
3, we have
assignments
listed
some elements
of symmetry
If we employ assignments
would
isolated
octahedra.
760-1000 cm-l
Raman
would
lower
modes
while
band
would
be due to TO(T,,)
different seem
structure,
to
(1.80-2.2 also
that
have A).
gives
the octahedra
relatively The
similar
(850-1000
B-Nb,O, long
published long
Nb-0
cm-i).
has
Apart
basically
a
in this polymorph Nb-0 structure
distances of
distances
It
appears
oxides retain
and are at least C,, or
NbO, [29].
octahedral modes
bands
Bands
is at a considerably
structures
oxides.
[8].
Raman
array model
(Table
2), the
in
the
region
be due to LO(T,,)
than the corresponding
fact
mode
be quite similar to those based
(760 cm-l)
block
This band could
in niobium
of internal
of B-Nb,O,
the
some
140 cm-l
the important
the infinite
spectrum in the
show
vibration
of niobium
octahedra
frequency from
are due to
also
3) with a band around
to a metal-metal
in the analysis
NbO,F,
the
200 cm-1
bands
that the NbO,
on
high-intensity
below
and
290 cm-l
C,, groups.
vi < 750 cm-l. The
band
and other
corner-linked
already
being common
phases is around
These
(Fig.
be assigned
octahedra
It is, however,
modes.
systematics
that the higher frequency
bands are due to edge-shared
Raman
external
all the
which
va, v4(T,,)
which becomes active in these non-cubic Unlike NbOPO,,
in
broad and show multiplet
due to distortion
the niobium
Nb-0
region could be
300-500 cm-l
These bands are generally
be The
In the i.r. spectra,
broad
and
would
octahedra.
band in the 290-260 cm-l
600-700 cm-l
as 2.3A)
octahedra
and generally
to assign multiple
as long
450-550 cm-l
same
earlier that
of some of the bands in the
and The
in the
the
corresponding
can be assigned to the i.r. active
octahedra
a number
bands.
the
( <1.9 A with some bonds as short as 1.7 A) and long
suggested
800-1000 cm-i
due to the v4(T1,)
corner-shared
tetrahedra
vs( T,,)
Raman
bands
have
be due to extensive
octahedra
common
niobium-oxygen
the edge-shared
the
could
the
band around 360 cm-l in all the oxides, close to the
in all the structures
NbO,
earlier,
We
high intensity
present
octahedra. with
in
of the
mode,
generally
in this region
and
are present
are appreciably
vs region
consistent
types
in H-Nb,O,)
As mentioned
octahedra from
NbO,
edge-shared
octahedra (e.g.
ordered
different
polyhedra:
relatively
(see Figs. 2 and 3), indicating
from
modes
internal
intensities
bands
arise
the
and
their
variations,
as the vi modes.
which
these bands were TO modes
of
record
show two
region
LO
the
arise
oxides
(vs)
(Y& are
isolated
could
they
That of
to
not
of niobium
Some of the oxides
sharp and symmetric that
on
bands
modes.
interesting
spectra
760-1000 cm-l
interest.
2),
than
Raman
region
approach
(Table
origin
in the
the
could
due to its dark colour.
All the niobium 600-680 cm-i
departures
from ideal symmetry. oxides
we
of NbO,
in
the
derived
620-680&cm-1 modes.
region
Bands
in the
300-550 cm-i can also be assigned to A, or
E modes
originating
from the
T,,
from
mode would Having
spectra, individual
T,,.
Bands
be in the region
assigned the major we shall now examine oxides.
originating
200-300 cm-i.
bands in the Raman specific
features
of
A. A. MCCONNELL, J. 8. ANDERSON
1074
i
and C. N. R. RAO
H- and N-Nb,O,. These two block structures show remarkably similar spectra (Fig. 4 and Table 3). H-Nb,O,, however, has one out of 28 Nb atoms in tetrahedral sites and the Nb-0 bonds in the NbO, tetrahedra are short (~1.66 A). Samples of H-Nb,O, annealed at 1200% for several days show clean splittings of bands throughout besidesa weak Raman band at 860 cm-l not seen in the We feel that this band spectrum of N-Nb,O,. may be due to the ri(Ai) Raman active mode of the NbO, tetrahedron, the ~a, rs and rq bands being too weak to be seen. Nbsf ions seldom occur in tetrahedral coordination. YNbO, (with Nb-0 bonds of 1.9 A), one of the few compounds consisting of NbO, tetrahedra, shows the vi Raman band at 830 cm-l [30] which is close to that assigned by us for the tetrahedra in H-Nb,O,. It is indeed remarkablethat Raman spectra can pick out these tetrahedra present in such low concentration. Annealing of the oxide seems to be essential to ensure ordering of the tetrahedral ions into one of two equivalent sets of tertahedral sites. The relative intensities of the two vi bands of NbO, units in H- and N-Nb,O, are interesting. The higher frequency bands assigned to edge-shared octahedra have much higher intensity (Fig. 3 and 4) since these octahedra predominate over corner-sharedoctahedra (Table 1). TiNbaO, and T&Nb,cO,,. These oxides have the same basic structure sa N-Nb,O,, but the Ti4+ ions seem to preferentially occupy edge-shared octahedra [13]. Accordingly, the relative intensity of the vi band due to corner-sharedNbO, is higher in these compounds than in N-Nb,O,. The vi(&) and v5(Tc,) modes of TiO, are probably responsible for the large intensities of the 650 cm-l and 540 cm-1 bands (Fig. 4). TiO, octahedrain perovskites and K,NiF, type compounds also show vi and vr, bands in these regions [5, 61. PNb,0a5 and VNlz1,0,~. These two compounds are isomorphous[I] and PNb,O,,, whose structure has been studied in detail [15], belongs to space group I4/m or I& The unusual feature in the Raman spectra of these compounds is the low intensity of the vi bands (LO) (Figs. 3 and 4). The low intensity is indicative of disorder. Disorder in PNb,0a6 can be caused by a statistical distribution of cations over the tetrahedral sites giving a centre of symmetry to the crystal, and this would be consistent with the Id/m space group. H-NbsO,, in which the tetrahedral sites are ordered (and occupied by Nb’+), gives sharp intense vi bands. Further, the 14 structure should give 76 Raman-active bands while the
Raman spectra of niobium oxides 14/m
structure
Although bands,
should
it is dii&ult
give
42
Raman
bands.
to observe or assign all the
the observed
Raman
spectrum
seems to
appears
Since the P atoms in PNbsO,,
The
are in tetrahedral
spectrum
to see bands charaoteristic
of PO,
should
spectrum.
see the
bands
discussed
earlier) :
Here
945 cm-l;
vi(&), va(T2), 446 cm-l
1045 cm-l; again,
there
between the PO,
and v,(E),
appears
modes.
Bands
of VO,%
(e.g. Dy VO,, NasVO,) 860,359-390
1281.
coupling
In VNbsO,,,
vs modes respectively. seen at 895,860(?),
due to vi, ye, vq and
Such “vanadate”
trum of VNb,Oe,.
Once we eliminate
to the
the
tetrahedra,
VNb,O,,
spectra
found
by
of PNb,O,,
oxide. This
niobium
X-ray
diffraction
isostructural with PNbsO,,, the composition
to
and
(vs) with a-GeOe
NbO,
mode.
and
analogy
shows bands around
with
960 and
niobium
oxide,
but
We find no bands assignable to NbO,
tetrahedra.
The
tetrahedral
concentration
groups
of
(approximately
GeO,
one cation in
10) is relatively low, and their Raman bands could be
too
weak
(900-1000 possible
for measurement.
cm-l) that
distorted
at 940 cm-l
the
other
edge-shared
apparently
recent structural study [lS]. B-Nb,O,.
The
vr region
clearly shows three bands.
the band
octahedron, octahedra,
It is
could
distinct
identified
arise from
in
our
As mentioned earlier, this polymorph
with rutile ribbons shows a much lower frequency
the
crystal
for vr. The ve modes also appear at considerably
oxygen
lower
frequencies.
bands
are shown
incompatible
with
of interstitial
determination
of
the
site.
crystal
number
occupied-i.e.
the
fabric
of
as in PNb,O,s,
is perfect-but
excess of cations in this inherently structure,
: oxygen that
over
ratio, provide
is
the
centred that the
nonstoichio-
formal
accommodated
tetrahedral
lo:25 in
sites.
the
Within
The
assignments
in Table
of bands
rather low (C,,).
channels
cm-l
(vJ
since it is in the same region as the bands due to an
symmetry
metal
ve > v1 by
of germanium
are
metric
cm-l
unless there are either vacant detailed
octahedra,
tetrahedra
660-680
assignment of the v1 band is difficult
structure [16] it is now clear that the oxygen sites fully
regions
There are bands in these regions in
the spectrum unequivocal
tetrahedral
GeO,
(ve), 450-520
from
This composition
sites or cations in some type a
be
in the
cm-l
[31].
occupied that
compound,
wss originally assigned
Ge0,.9NbeO,.
however,
structure From
bands due
are not unlike the spectrum of H-Nb,O,.
Germanium
is,
bands are
420 and 310(?) cm-l in the spec-
920 cm-l
germanium
expect
bands
709-830
SiO,
are generally found
cm-l.
give
%(T2). 330 cm-l.
in the regions 889-900,829-
and 250-310
would
cm-l
we should similarly see bands characteristic of VO, tetrahedra.
(vr),
of
preferentially
We
~300
to be little
and NbO,
atoms
NbOPO,
(just ss in the model compound
random
oxide
sites.
“phosphate”
the
giving rise to
if the
Ge
do indeed
with
niobium
of the polyhedra in the structure, we would expect We
consistent
should show bands due to GeO, tetrahedre
sites and the tetrahedra constitute $th the number in the Raman
be
of sites of each sub-set,
higher symmetry.
favour the 14/m structure.
qualitatively
to
occupation
1075
in the
3.
different
of the octahedra
‘polyrutile.’
rutile are multiplied, and
coupling [4].
Raman
would
the
appear to be as a
and BsP bands of
although there is a spread of
absence
of some modes
In the Ruddelsdon-Popper
(related to SrzTiO,),
the
regions,
We could consider B-Nb,O,
In trirutile, the A,,
frequencies
of
Considering
the Ai,
due to
structures
mode is multiplied,
these channels, cations also occupy highly distorted
but the number of observed bands does not account
octahedral sites. These are only partially occupied;
for all the degrees of freedom, due to ordering and
the relative number of cations in tetrahedral octahedral metry.
channel sites determines
Within
the stoichio-
any one channel, tetrahedral
octahedral cations may approximate
and and
to an ordered
the spectra bear no simple relation to the parent structure expect
[a].
simple
B-Nb,O,
By the same token, we would not multiplication
of rutile
since it has a much
bands
in
lower symmetry.
array, but there is little or no correlation between
BEATTIE and GILSON [8] have assigned the Raman
the
bands of B-Nb,O,
ordering in neighbouring
tetrahedral
octahedral
cations,
ordering, the structure
If only
the space group
would be 14/m; for ‘stuffed’
symmetry with
channels.
sites were occupied,
depending
channels, upon
may be CPn or S,.
the Since
there is no superlattice corresponding to correlation between
channels,
symmetry, Raman 11
a statistical
as for PNb,Oer,,
spectrum
model
with 14/m
is appropriate.
of germanium
niobium
The oxide
(OeNb& advantage However,
in terms of ONb,,
types of groupings. in this type
ONb,
and
We do not see any
of group decomposition.
our assignments in Table 3 show general
correspondence to those of Beattie and Gilson. EXPERWNTAL Raman spectra were recorded on a Gary 81 Raman spectrometer employing 6328 A He-Ne laser excitation and on a Spex Ramalog 4 with Coherent Radiation
1076
A. A. MCCONNELL,J. S. ANDERSONand C. N. R. RAO
52G argon ion laser, using the 4580 A and 5145 A lines. Infrared spectra were recorded using a PerkinElmer 457 spectrometer in the 4000-250 cm-’ region, powdered samples being dispersed in TlBr pellets. Infrared spectra in the 400-40 cm-’ region wore measured on a Beckman-RllC FS620 interferometer with FTC100 computer, powdered samples being dispersed in 1” Rigidox discs. Polarized i.r. spectra were recorded on a Perkin-Elmer 557 spectrometer using a rotatable gold wire grid polariser in the common beam and also on the interferometer. Single crystals were mounted on a Wilks specular reflectance attachment with an angle of incidence of 15”. Single crystal studies were possible only with N- and B-Nb,O, and polarized data could be obtained only in the case of N-Nb,O,. Acknowledgement-Our thanks are due to Dr. A. P. Lava of the University of Glasgow for the use of the Spex Ramalog, for assistance in obtaining polarized IR data and for much helpful discussion. REFERENCES [l] A. D. WADSLEY and S. ANDERSSON,Perspectives in Structural Chemistry, (edited by J. D. DUNITZ and J. A. IBERS,) John Wiley, New York, Vol. 3, 1970. [Z] J. S. ANDERSONin Surface and Defect Properties of Solids (Specialist Periodical Report), (edited by M. W. ROBERTSand J. M. TIIOMAS,)Chemical Society, London, 1972. [3] J. S. ANDERSONin Solid State Chemistry, (edited by R. S. ROTE and S. J. SCHNEIDER,JR.) NBS Special Publication 364, National Bureau of Standards, Washington, page 295 1972. [4] W. B. WHITE and V. G. KERMADASin Solid State Chemistry, (edited by R. S. ROTH and S. J. SCHNEIDER,JR.), NBS Special Publication 364, National Bureau of Standards, Washington, page 113 1972. [5] G. BLASSEand A. F. CORS~IIT, J. Solid State Chem.. 8, 513 (1973); ibid., 10,39 (1974). [6] G. BLASSE and G. P. M. VAN DEN HEWEL, 2. Phyeik. Chem. (N.P.), 84, 114 (1973); J. Solid State Chem., 10,206 (1974). [7] F. MATOSSI, J. Chem. Phya., 19, 1543 (1951). [S] I. R. BEATTIE and T. R. GILSON, J. Chem. Sot. A, 2322 (1969).
PI J. T. LAST, Phys. Rev., 105, 1740 (1967). [lOI P. S. NARAYANAN and K. VEDAM, 2. Physik. 163, 168 (1961). [Ill C. H. PERRY and N. E. TORNBERG in Light Scattering Spectra of Solids, (edited by G. B. WRIGHT), Springer, New Tork, 1969 page 467. WI S. AXDERSSON, 2. anorg. allgem. them., 351, 106 (1967). r131 R. B. VON DREELE and A. K. CHEETAM, Proc. Roy. Sot. A (London), 338, 311 (1974). P41 B. M. GATEHOUSEand A. D. WADSLEP, Acta &yet., 17,1546 (1956). WI R. S. ROTH, A. D. WADSLEY and S. ANDERSSON, Acto &?/8t.,18, 643 (1965). [16] J. S. ANDERSON,J. M. BROWXE, A. K. CHEETAM, R. VON DREELE, J. L. HUTCEISON,F. J. LINCOLN and D. J. M. BEVAN, J. STRAEHLE,Nature, 243, 81 (1973); J. S. ANDERSON, D. J. M. BEVAN, A. K. CHEETAM,J. L. HUTSHISON,J. STRAEHLE and R. VON DREELE, Proc. Roy. Sot., In Publication. [17] S. ANDERSSONand J. GALY, J. Solid State Chem., 1, 576 (1970); Also see F. LAYES, W. PETER and H. WULF, Naturwias., 51, 633 (1964). [lS] R. LOUDON, Advances in Physics, 13, 423 (1964). [19] L. E. ORGEL, An Introduction to Transition Metal Chemistry, page 174. John Wiley, New York, 1960. [20] L. PAULING, The Nature of the Chemical Bond, Chapter 48. Oxford University Press, 1952. [Zl] G. BLASSE,J. Inorg. Nucl. Chem., 26, 1191 (1964). [22] J. B. GOODENOUGH and J. A. KAF~LAS, J. Solid State Chem., 6, 493 (1973). [23] L. A. WOODWARDand J. A. CREIOHTON,Spectrochim. Acta, 17, 594 (1961). [24] W. VAN BRONSU’IJCK,R. J. H. CLARK and L. ~MAREscA,Inorg. Chem., 8, 1396 (1969). [25] L. K. FREVEL and H. W. RINN, Acta CT@., 9, 626 (1956). [ZS] _ - C. H. PERRY, J. H. FERTELand T. H. MCNELLT, J. C&m. Phye., 47, 1619 (1967). 1271 J. M. LONGO and P. KIERKEGAA~D, Acta Chem. Stand., 20, 72 (1966). [28] Ii. NAKAMOTO,Infrared Spectra of Inorganic and Coordination. Compounds. John Wilev, New York, 1970. [29] B. MARINDER,A&iv Kemi, 19,436 (1962). [30] G. BLASSE,J. Solid State Chem., 7, 169 (1973). [31] J. F. SCOTT,Phy8. Rev., Bl,3488(1970). 1,