Synthesis of transparent mixed vanadia/niobia gels and their decomposition to a new metastable VNb9O25 phase

Article abstract of DOI:10.1016/j.tca.2006.06.003

Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)₃, and niobium pentaethoxide, [Nb(OEt)₅]₂, at 5 °C in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow down the rate of condensation processes. At 800 °C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically metastable, new phase of the composition VNb₉O₂₅. At lower temperatures, metastable solid solutions with TT-Nb₂O₅ structure (600 °C) and M-Nb₂O₅ structure (700 °C) are formed from the amorphous xerogel. The new VNb₉O₂₅ phase is structurally related to M-Nb₂O₅. The solid solution with M-Nb₂O₅ structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb₉O₂₅. The full transformation of this metastable phase to the well known tetragonal VNb₉O₂₅ requires a annealing temperature of about 1000 °C.

Full text of DOI:10.1016/j.tca.2006.06.003

Thermochimica Acta 447 (2006) 178–183
Synthesis of transparent mixed vanadia/niobia gels and their
decomposition to a new metastable VNb9O25 phase
T. Mayer-Uhma, H. Langbein∗
Department of Chemistry, Technische Universita¨t Dresden, Helmholtzstr. 10, D-01069 Dresden, Germany
Received 2 December 2005; received in revised form 1 June 2006; accepted 9 June 2006
Available online 16 June 2006
Abstract
Controlled hydrolysis and condensation of a mixture of vanadyl-tris-n-propoxide, VO(OPr)3, and niobium pentaethoxide, [Nb(OEt)5]2, at 5 ◦C
in propanol yields clear and transparent gels in which the ratio of V:Nb is 1:1, 1:4.5 or 1:9. Oxalic acid and low temperatures are used to slow
down the rate of condensation processes. At 800 ◦C, the thermal decomposition of a gel with the composition 1:9 forms a thermodynamically
metastable, new phase of the composition VNb9O25. At lower temperatures, metastable solid solutions with TT-Nb2O5 structure (600 ◦C) and M-
Nb2O5 structure (700 ◦C) are formed from the amorphous xerogel. The new VNb9O25 phase is structurally related to M-Nb2O5. The solid solution
with M-Nb2O5 structure acts structure directing, leading preferentially to a monoclinic low-temperature form of VNb9O25. The full transformation
of this metastable phase to the well known tetragonal VNb9O25 requires a annealing temperature of about 1000 ◦C.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Sol–gel synthesis; V2O5/Nb2O5 complex oxides; Thermal analysis; XRD
1. Introduction
aggregate to an open polymeric three-dimensional structure
[6].
Since the beginning of the nineties, it becomes more and
more important to produce complex oxides via so-called “soft
chemistry”routes(chimiedouce)[1]. Here, precursorswhichare
homogeneous as much as possible, are decomposed thermally
to the desired crystalline phases. In the best case, homogeneous
liquids can be solidified without any phase separation. Reac-
tion limitations of the conventional solid state reaction such as
the diffusion are thus avoided and the reaction temperature can
normally be lowered. Examples of precursors using this soft
chemistry are coprecipitated carboxylates, freeze-dried prod-
ucts, or as in our case, gels.
Intensive research on pure vanadia and niobia gels started
around 1985 [2,3]. The evaluated reaction mechanisms [4,5]
confirm completely different hydrolysis schemes for vana-
dium and niobium alkoxides. Monomeric dissolved vanadyl
alkoxides hydrolyze completely, forming ribbon-like poly-
vanadates. Dissolved niobium alkoxides are normally dimeric;
they first hydrolyze at the bridging alcoxy groups and then
Gelation of mixed components has scarcely been performed,
because of the different hydrolysis–condensation conditions.
Therefore, mixtures of NbCl5 and vanadyl alkoxides, which
have the disadvantage of chlorine content in the gels, have been
mostly used [7–9]. Catauro et al. [10] have prepared amorphous
vanadium–niobium mixed oxides with the composition V:Nb
of 1:6 by a sol–gel route starting from metal alkoxides. The
authors found that the synthesis of transparent, non-opaque gels
requires a pre-hydrolysis of the vanadium component and a mix-
ing process of single components at −20 ◦C in presence of HCl.
Additionally, the minimum content of niobia is limited to the
given composition.
Metal oxides, synthesized through hydrolysis and conden-
sation of metal alkoxides, are generally amorphous. The crys-
tallization of different phases at higher temperatures depends
on the precursor. Even small variations of the conditions dur-
ing the gel preparation as for example the change of the solvent
have a large influence [11]. So, as it seems, the composition and
preparation of the precursor determines the phase formation.
In this way, Yamaguchi et al. [12,13] have synthesized
metastable V4Nb18O55 and VNbO5 as well as solid solutions of
V2O5 (up to 10 mol%) in TT-Nb2O5 by thermal treatment of pre-
∗
Corresponding author. Tel.: +49 351 463 34366; fax: +49 351 463 37287.
E-mail address: Hubert.Langbein@chemie.tu-dresden.de (H. Langbein).
0040-6031/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2006.06.003

T. Mayer-Uhma, H. Langbein / Thermochimica Acta 447 (2006) 178–183
179
Table 1
cipitated alkoxide hydrolysis products. Polte et al. [14] obtained
Conditions of formation of pure and mixed gels in propanol
the same compounds by thermal decomposition of freeze-dried
alkoxide hydrolysis products. At about 800 ◦C, V4Nb18O55 and
VNbO5 are decomposed forming VNb9O25 and some V2O5.
On annealing between 650 and 1000 ◦C, the single phase solid
solution of V2O5 (10 mol%) in TT-Nb2O5 transforms to mix-
tures of solid solutions with T-, M- and H-Nb2O5 structure
(and VNb9O25). However, no single phase compound could be
obtained. The transformation of these mixtures to single phase
VNb9O25 requires an annealing temperature of about 1050 ◦C
[13,14].
Substance
ca (mol l−1
)
hb
Additive
tgel (min−1
)
V(O)(OPr)3
V(O)(OPr)3
1.00
0.50
0.71
1.06
2.6
3.1
3.7
2.6
–
–
–
<1
<1
<1
∼1
1/2 [Nb(OEt)5]2
V(O)(OPr)3 +
1/2[Nb(OEt)5]2
V(O)(OPr)3 + 2.25
[Nb(OEt)5]2
Oxalic acid
1.06
1.06
2.6
2.6
Oxalic acid
Oxalic acid
<1
<1
V(O)(OPr)3 + 4.5
[Nb(OEt)5]2
Occurring for the first time, the tetragonal VNb9O25 was syn-
thesized in 1960 by solid state reaction [15]. It is the only known
thermodynamically stable phase in the system VxNb(2−x)O5.
Using the mineralizing aid of molten V2O5, tetragonal VNb9O25
can be formed already at about 700 ◦C [16]. Besides this stable
phase and the metastable solid solution with TT-Nb2O5 structure
no further pure phases with the composition of VNb9O25 have
been synthesized up to now. Nevertheless, it cannot be excluded
that sol–gel synthesis may lead to the formation of further new
phases. For their preparation, it seems to be necessary to obtain
a gel as homogeneous as possible.
In this paper, clear and transparent gels with the ratio
V:Nb = 1:1, 1:4.5 and 1:9 were obtained by hydrolysis of
vanadyl-tris-propylate and niobium pentaethylate. Chelating
agents, which control the reaction rates by modifying the reactiv-
ity of the precursors, and a special temperature program prevent
spontaneous precipitation and allow the sequential formation of
sols and gels. From a so-formed gel of appropriate composition,
a metastable bulk vanadium niobium oxide with the composi-
tion VNb9O25 has been prepared and characterized by thermal
analysis and X-ray powder diffractometry.
a
c is the total starting concentration of both alkoxides.
h is defined as described in the text.
b
Xerogel samples were processed by allowing the gel to
dry under reduced pressure at 50 ◦C. After that, it was pre-
decomposed for 2 h after increasing the temperature with
2 ◦C/min to 400 ◦C. During this annealing process water and
alcohol were released, resulting into a hygroscopic, amorphous
complexV–Nb-oxide. Thefollowingcrystallizationprocesswas
carried out at the given temperature for 4 h in air.
Differential thermoanalysis (DTA) and thermo gravimet-
ric measurements (TG) were performed with a Netzsch STA
409 analyzer. X-ray powder diffraction patterns were col-
lected by a Siemens D5000 diffractometer (Cu K␣-radiation;
10◦ ≤ 2θ ≤ 80◦; step scan mode width 0.02◦; 1 s per step).
3. Results and discussion
3.1. Gel synthesis
The gelation of pure and mixed alkoxides varies with
the hydrolysis ratio and the concentration of the compo-
nents. Therefore, pure and mixed gels are prepared under dif-
ferent experimental conditions using various mole ratios of
VO(OPr)3:[Nb(OEt)5]2:PrOH:H2O:additive as shown graph-
ically in Figs. 1–3. In Table 1 the results are summa-
rized for characteristic samples. The hydrolysis ratio h is
defined as the ratio of moles of water nH2O to the moles of
2. Experimental
VO(OPr)3 (Pr = n-C3H7) has been synthesized under
moisture-free conditions from V2O5 and PrOH, distilling off
the water with benzene. [Nb(OEt)5]2 (Et = C2H5) from H.C.
Starck GmbH (Germany) and oxalic acid have been used with-
out further purification. The water content in propanol has been
determined (0.23%) and considered for the calculations.
Mixed gel samples with V:Nb = 1:1 have been prepared at
room temperature in the presence of air using a solution of
0.252 g H2C2O4·2H2O (2 mmol) in 1.5 ml propanol, adding
0.502 ml [Nb(OEt)5]2 (1 mmol) as starting agent. To this solu-
tion 0.470 ml VO(OPr)3 (2 mmol) and subsequently 0.108 ml
H2O (6 mmol) in 1 ml propanol were added. The gel was formed
during 1 min. After that, the gel was kept in normal air atmo-
sphere at 5 ◦C for 12 h, giving a liquid with lower viscosity. At
room temperature, this sol reacts to a transparent, brownish gel
again, which is stable without any precipitation for about 4 h.
Gels containing V:Nb ratios of 1:4.5 and 1:9 were prepared with
the same total concentration of metals, adapted ratios of alkox-
ides and same amounts of water and oxalic acid. Pure vanadia
and niobia gels were prepared adding water containing propanol
(10% H2O) to alkoxide solutions of different concentration in
propanol, as given in Table 1.
Fig. 1. Hydrolysis of V(O)(OPr)3 in propanol to gels and precipitates (pr.) of
V2O5. Additives:(*)1 molaceticacid/molV;(**)1 molHNO3/molV. c:starting
concentration of vanadyl alkoxide.

180
T. Mayer-Uhma, H. Langbein / Thermochimica Acta 447 (2006) 178–183
additive. To our knowledge, up to now no clear niobia gel has
been prepared without any additives.
The formation of a gel from two metal alkoxides is difficult,
if the components hydrolyze and condensate differently. More-
over, the condensation process to small stable oligomers like
Nb8O6(OEt)20 has to be suppressed. Thus, a quick hydrolysis
connecting a slow condensation is the preferred way.
Dimeric [Nb(OPr)5]2 hydrolyzes slower than monomeric
VO(OPr)3 [20]. [Nb(OEt)5]2 should react similarly, because of
the transesterification process in propanol. During the prepara-
tion, [Nb(OEt)5]2 is allowed to hydrolyze first. After addition
of the vanadium component the condensation process must be
slowed down, otherwise a precipitate forms immediately. There-
fore oxalic acid is added, chelating probably both transition
metals, as found for vanadium alkoxide [21]. Additionally, the
condensation rate was reduced by allowing the gel to become
equilibrated at low temperature.
Fig. 2. Hydrolysis of 1/2 [Nb(OC2H5)5]2 in propanol to gels and precipitates
(pr.) of Nb2O5. Additives: (*) 1 mol acetic acid/mol Nb; (**) HNO3 in different
amounts. c: starting concentration of monomeric niobium alkoxide.
The hydrolysis of mixed V- and Nb-alkoxides (V:Nb = 1:1)
to sols, gels and precipitates is shown in Fig. 3. Although
with and without acidic additives pure VO(OPr)3 and pure
[Nb(OEt)5]2 form gels in a similar concentration/hydrolysis
ratio field, the application of these conditions to the prepa-
ration of mixed gels usually results in a precipitation. Obvi-
ously, the hydrolysis–condensation process of one component
is strongly influenced by the presence of the other component.
Mixed sols can be prepared in a relatively large concentration
range, but only below room temperature (−10 ◦C) in acidic
solution. Because of the obviously low condensation rate, no
gel is formed. At room temperature these sols become unsta-
ble and precipitates are formed. Acidic agents, suitable for the
synthesis of niobia gels, as HCl, HNO3 [22,23] or acetic acid
cannot prevent the spontaneous precipitation of mixed oxide
hydrates.
A transparent gel can only be formed with oxalic acid as
chelating agent in a narrow range of alkoxide concentrations.
The hydrolysis ratio to form the gel is h = 2.6, which shows
an incomplete exchange of the alkoxide groups. Independently
of the V:Nb ratio, the gels turn in opaque gels during 15 min
because of a rapid condensation process. Therefore, clear gels
obtained after mixing the components at room temperature, were
aged at lower temperature (5 ◦C). After 12 h of aging, the gels
stay clear at room temperature for 4 h and can be dried and
annealed.
monomeric metal alkoxides (n(VO(OPr)3 + nNb(OPr)5)) : h =
nH2O/(n(VO(OPr)3 + nNb(OPr)5)).
Pure VO(OPr)3 can be hydrolyzed and condensated at a
hydrolysis ratio h = 2.6 (Fig. 1). The literature confirms this
result indicating that the factor h in 1 M alcoholic vanadium
alkoxide solution varies between 3 and 100 [17]. With low
amounts of water (h = 3), an orange gel is obtained. This
gel should have other properties as a red gel obtained with
h = 100 or via ion exchange of aqueous vanadate solutions
[17].
Niobia gels can be formed with acidic additives. With addi-
tion of HCl, the hydrolysis factor h in ethanol was found between
3 [18] and 25 [19]. According to our results, [Nb(OEt)5]2 is
hydrolyzed and condensated to gels below a hydrolysis ratio
of h = 7 (Fig. 2). The gels often become slightly opaque after
15 min. Gelation to a transparent gel is favoured by HNO3 or
acetic acid as additive. In a very small concentration and hydrol-
ysis ratio field clear niobia gels can be formed without any
3.2. Pyrolysis of gels and phase formation
A dried xerogel of the composition V:Nb = 1:9 was heated
at various temperatures for 4 h. Fig. 4 shows the obtained
X-ray powder diffraction patterns. At 600 ◦C, a crystalline solid
solution with the composition nine Nb2O5·V2O5 in the pseudo
hexagonal TT-modification of Nb2O5 (also called ␦-Nb2O5) is
formed. At 700 ◦C, a tetragonal M-Nb2O5 (␤-Nb2O5) or a solid
solution of V2O5 in this phase is the main product. The excess
of V2O5 forms V4Nb18O55, a thermodynamically metastable
(V, Nb)2O5 phase. Heating the xerogel between 800 and 900 ◦C,
the result obtained reveals a new, unknown phase beside some
traces of unreacted M-Nb2O5. The composition should be
Fig. 3. Hydrolysis of V(O)(OPr)3 and 1/2 [Nb(OC2H5)5]2 in propanol to gels,
sols and precipitates (pr.) of V2O5 and Nb2O5. Sols are prepared at −10 ◦C.
Additives: (*) 1 or 2 mol acetic acid/mol V; (**) 1 mol HNO3/mol V; (***)
oxalic acid. c: total starting concentration of monomeric alkoxides.

T. Mayer-Uhma, H. Langbein / Thermochimica Acta 447 (2006) 178–183
181
Fig. 5. DTA/TG of a gel of the composition V:Nb = 1:9, pretreated at 400 ◦C.
Heating rate: 10 K/min, atmosphere: air.
Nb2O5 structure are formed at first. At 700 ◦C, the occurrence
of traces of V4Nb18O55 beside the solid solution with M-Nb2O5
structure shows that the solubility V2O5 in M-Nb2O5 is lower
than in TT-Nb2O5. At 800 ◦C, the M-Nb2O5 solid solution
and V4Nb18O55 react to the new metastable low-temperature
VNb9O25 phase (lt-VNb9O25). In Fig. 4, connection lines show
the possible relationship between the powder pattern of M-
Nb2O5 and the X-ray pattern of the new phase. A single or
double splitting of the peaks is found, which indicates structural
relatedness between both phases and a lowering of symmetry
during the phase transformation. A comparison with patterns
of H-Nb2O5 (␣-Nb2O5) and thermodynamically stable high-
temperature VNb9O25, as shown in Fig. 6, confirms the for-
mation of a new phase. Structural relationships between both
VNb9O25 phases are also pointed out.
Another experiment confirms the essential role of M-Nb2O5
forming lt-VNb9O25. Annealing a nearly carbon free mixture
9:1 of reactive TT-Nb2O5 and V2O5 in oxygen-reduced atmo-
sphere (argon with 10−4 bar oxygen) at 800, 900 or 1000 ◦C
leads to a M-Nb2O5 phase. Some traces of V4Nb18O55 are only
found after annealing at 800 ◦C. A controlled reducing atmo-
sphere therefore acts as a stabilizer on the M-Nb2O5 phase. This
is confirmed by the fact that the M-Nb2O5 sample, formed in a
reducing atmosphere, is also transformed to the new lt-VNb9O25
by an annealing process at 800 ◦C in the presence of air atmo-
sphere. The following scheme summarizes the phase formation
processes described above:
Fig. 4. X-ray powder diffraction patterns of a gel in the composition V:Nb = 1:9
after annealing in air at different temperatures. TT-Nb2O5 (᭹), M-Nb2O5 (+),
V4Nb18O55 (♦), high temperature VNb9O25 (ꢀ). Metastable VNb9O25 is not
marked.
VNb9O25, since no other phases are detectable by XRD. The
new phase can be regarded as a low temperature modification
of VNb9O25 (see below). The well known, thermodynamically
stable high-temperature form of VNb9O25 with a tetragonal
block structure [24] forms on annealing at 1100 ◦C. Here,
an oxygen atmosphere is necessary to yield a slightly yellow
product and to avoid partial reduction, since the substance
generally has a high oxygen partial pressure. As function of
the temperature and the surrounding oxygen partial pressure,
a reversible release of oxygen can be observed [20]. The
compound can therefore be used excellently as an oxidative
catalyst. That will be the scope of another paper.
Togiveanoverviewofthephaseformation, athermalanalysis
of the precursor after heating it at 400 ◦C has been made. DTA
and TG curves are shown in Fig. 5. Up to 530 ◦C a small amount
of water is released because of the hygroscopic nature of the pre-
treated sample. The exothermic peak between 530 and 670 ◦C
in the DTA curve is usually connected to the burn-out of traces
of carbon [14,20], which stems from the chemically bonded
alcohol groups. This carbon is firmly bonded, much more than
expected, and a further small decrease of mass can be noticed up
to 1400 ◦C. Because of the obviously low rate of transformation
between the different phases the DTA curve does not reveal clear
informations concerning phase transitions.
The alkoxide-derived carbon is oftenly found to induce the
formation of a metastable phase instead of the expected ther-
modynamically stable phase. It can be therefore explained that,
instead of VNb9O25, the metastable phases with TT- and M-
Fig. 6. Overlay of X-ray powder diffraction patterns of H-Nb2O5, ht-VNb9O25
and lt-VNb9O25
.

182
T. Mayer-Uhma, H. Langbein / Thermochimica Acta 447 (2006) 178–183
a X-ray powder diffractogram for a hypothetical compound
The X-ray powder pattern of the metastable lt-VNb9O25
V(Nb11.7V0.3)O32.5 of the composition VNb9O25 was calcu-
lated. The result is shown in the lower part of Fig. 7. The high
degree of agreement between the experimental powder diffrac-
togram of lt-VNb9O25 and the calculated diffractogram speaks
for the existence of a solid solution phase V(Nb12−xVx)O32.5
with x = 0.3 and WNb12O33-structure. The solid solution phase
contains some vanadium on niobium sites and vacancies in the
oxygen sublattice.
The crystallization of the lt-VNb9O25 phase with the struc-
ture of WNb12O33 should be a kinetically controlled process and
can be explained as follows: M-Nb2O5 as a necessary precursor
phase crystallizes with an irregular block structure. First of all,
a tetragonal structure with 4 × 4 blocks of NbO6 octahedrons
(space group I4/mmm) was suggested [27]. Afterwards, it was
shown by TEM that the XRD pattern with a relatively small
number of broad peaks is the result of an averaging of differ-
ent block sizes and block connections [28]. On annealing the
M-(Nb, V)2O5 phase in air atmosphere a rearrangement of the
irregularblockstructuretakesplacein whichthealreadyexisting
3 × 4 blocks act as crystallization centers. This process results
in the transformation into lt-VNb9O25 with a regular 3 × 4 block
structure. Only above a temperature of 900 ◦C the further trans-
formation into the thermodynamically stable VNb9O25 phase
takes place.
phase is very similar to that of WNb12O33 described by Roth and
Wadsley (PDF 44–465). The structure of WNb12O33 consists of
3 × 4 × 4 blocks of NbO6 octahedrons, which are joined by shar-
ing edges, and with tetrahedrally coordinated W atoms ordered
at the junctions of every four blocks. The unit cell parameters
◦
˚
are a = 22.37, b = 3.825, c = 17.87 A, β = 123.6 , space group C2
[25]. All the peaks of the X-ray powder pattern of lt-VNb9O25
can also be indexed in the monoclinic crystal system. The (h k l)-
values are given in the upper part of Fig. 7. The u◦nit cell parame-
˚
ters (a = 22.22, b = 3.839, c = 17.75 A, β = 123.3 ) are very sim-
ilar to these of WNb12O33. Using these cell parameters and the
metal atomic parameters (x, y, z) given in [25] for WNb12O33,
Thermodynamically stable VNb9O25 formed in oxygen
atmosphere has a slightly yellow colour and shows ther-
mochromism, as all mixed phases in the system V2O5/Nb2O5.
At 1100 ◦C in air atmosphere VNb9O25 loses some oxygen. This
leads to the formation of a substoichiometric brown substance,
designated in literature [24] as VNb9O24.9. Thus we obtained
for the first time the pale yellow non-reduced phase.
Fig. 7. X-ray powder diffraction pattern of lt-VNb9O25 (upper part) and cal-
culated powder diffraction pattern of V(Nb11.7V0.3)O32.5 using a Powder Cell
program [26] (lower part).

T. Mayer-Uhma, H. Langbein / Thermochimica Acta 447 (2006) 178–183
183
4. Conclusion
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Vanadium and niobium alkoxides can be used as precur-
sors to get transparent, homogeneous (V, Nb)2O5 gels. Different
hydrolysis and condensation rates can be equalized by control-
ling the hydrolysis, condensation process with chelating agents
and a special temperature regime. Because of the high homo-
geneity, these gels are good candidates for the preparation of
new, metastable phases in the system V2O5/Nb2O5. This was
shown for the synthesis of a new lt-VNb9O25 phase via an
M-Nb2O5 phase. Obviously, the formation of a nearly single
phaseproductwithM-Nb2O5 structureisinducedbya“reducing
agent” within the precursor (unburned carbon). The M-Nb2O5
phase transforms preferentially to the new low-temperature lt-
VNb9O25 phase with a 4 × 3 × 4 block structure. Its transfor-
mation to the well known thermodynamically stable tetragonal
form with a 3 × 3 × 4 block structure requires an annealing pro-
cess at about 1100 ◦C.
This result differs from results of Amountza et al. [29] who
obtained tetragonal VNb9O25 by a controlled hydrolysis of
VO(OC2H5)3/Nb(OC2H5)5 mixtures without additives and their
annealing at only 680 ◦C for 100 h. Possibly, some molten V2O5
is formed intermediately from the slightly inhomogeneous pre-
cursor, whichactsasmineralizingaid. TheX-raypowderdiffrac-
togram shown in [29] gives some signs for this assumption.
It can be concluded that the variation of the carbon content,
the composition of a sol gel precursor and the variation of the
reaction atmosphere change the nature of intermediate phases.
Therefore, these take control of the out coming products. In this
context, the large variety of modifications of Nb2O5 plays an
important role, because they can “direct” the phase formation
and the structure of complex oxide phases formed in the system
V2O5/Nb2O5. More of this structure directing effect will be the
scope of another paper.
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