Refinement of the V-O phase diagram in the range 25-50 at % oxygen

Article abstract of DOI:10.1134/S0020168509010087

The boundaries of the V₁₄O₆ + V _x O _z two-phase region in the V-O system at temperatures from ? 1050 to ? 1650 K have been determined experimentally. The V-O phase diagram has been refined in the range 25-50 at

Full text of DOI:10.1134/S0020168509010087

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 1, pp. 47–54. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © D.A. Davydov, A.A. Rempel, 2009, published in Neorganicheskie Materialy, 2008, Vol. 45, No. 1, pp. 50–57.
Refinement of the V–O Phase Diagram
in the Range 25–50 at % Oxygen
D. A. Davydov and A. A. Rempel
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences,
Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russia
Received February 18, 2008; in final form, May 19, 2008
Abstract—The boundaries of the V O + V O two-phase region in the V–O system at temperatures from
1
4
6
x
z
ꢀ
1050 to ꢀ1650 K have been determined experimentally. TheV–O phase diagram has been refined in the range
2
5–50 at % oxygen using structural and microstructural data for vanadium oxides containing less than 50 at %
oxygen in conjunction with earlier results. The possibility of ordering of cubic vanadium monoxide has been
examined.
DOI: 10.1134/S0020168509010087
INTRODUCTION
γ-phase. Schönberg [13] and Andersson [14] were the
first to identify the δ-phase, a cubic vanadium monox-
ide, V O , with a broad homogeneity range, from VO
0.80
TheV–O system has variously been reported to con-
tain 21 to 25 compounds and phases [1–3]. In most of
them, the oxygen content is at least 60 at %: V O
x
z
to VO1.30
.
Ò
n
2n – 1
x
Vol’f et al. [15] confirmed the existence of V Oz
n = 2–8), VO , and V O Ò n = 2, 3, 7) [4, 5]. These
2
n
2n + 1
with a broad homogeneity range: from VO0.86 to VO1.27
oxides are well studied because many of them undergo
metal–insulator transitions and because they typically at 1870 K and from VO0.92 to VO1.24 at 1070 K. They
have no homogeneity range. In contrast, the vanadium found no phases between the cubic monoxide V O and
x
z
oxides containing less than 50–55 at % oxygen have V O . At the same time, in phase-equilibrium studies of
2
3
broad homogeneity ranges and are, therefore, highly
nonstoichiometric compounds [6, 7]. At present, three
the V–O system at 1070 K, Westman and Nordmark
16] identified a bcc phase of approximate composition
VO1.27 between V O and V O . This phase can be
[
oxide phases are known in the V–O system: V O
1
4
6
x
z
2
3
(
V O ꢀ ), V O (V ꢁ O ꢀ
≡ VO , y ≡ z/x), and
1 – z y
1
4
6
8
x
z
x
1 – x
z
thought of as an ordered V O phase of cubic vana-
204 256
V O (V ꢁ O ). They differ in the width of their
52
64
52 12 64
dium monoxide. In addition, they identified an oxide
homogeneity ranges and the concentrations of oxygen phase of composition VO0.53 (V O) with a narrow
2
(
ꢀ) and vanadium (ꢁ) vacancies. The first phase con-
tains structural vacancies only in the oxygen sublattice
ꢀ); vanadium monoxide, V O , has structural vacan-
homogeneity range and concluded that structural
vacancies were present in both the oxygen and vana-
dium sublattices of the V O monoxide [16]. Electrical
(
x
z
x
z
cies in both sublattices; and V O contains only vana-
52
64
conductivity data for vanadium oxides at 193 and
dium vacancies (ꢁ). The V–O phase diagram has been 373 K [17] also confirm the existence of the suboxide
studied in greater detail at oxygen contents above 55 at %. of composition ꢀVO . In later studies [18], the
0.5
In the composition range 25–50 at % oxygen, the phase
boundaries have been located only tentatively.
γ-phase (ꢀV O) was shown to have a monoclinic struc-
2
ture and to exist in the composition range VO0.45 to
−
VO0.54 (VO0.47 to VO0.53 at 1170 K).
According to Seybolt and Sumsion [8] and Gurevich
and Ormont [9], the vanadium-rich region of the V–O
system contains a bcc solid solution of oxygen (no
more than 3 at % O) in vanadium (α-phase) and a body-
centered tetragonal solid solution of oxygen in vana-
dium, V(O), in the range 15 to 22–25 at % oxygen
Gel’d et al. [19, 20] found that at 1670 K phase-pure
cubic vanadium monoxide was formed in the range
VO0.85 to VO1.25. Their results confirm that the cubic
monoxide V O contains defects in both sublattices. At
the same time, they obtained neither an ordered bcc
phase of composition VO1.27 nor a suboxide of compo-
sition ꢀVO0.5 [20], which led them to conclude that in
theV–O phase diagram there is a broad δ + β two-phase
region to the left of the vanadium monoxide field.
x
z
(
β-phase). As shown by neutron diffraction [10], the
β-phase of composition VO0.53 has a tetragonal struc-
ture (sp. gr. I4/mmm). The phase equilibria in the com-
position range of the α- and β-phases were studied in
detail by Henry et al. [11]. Rostoker [12] reported per-
itectoid formation of the suboxide containing no more
The V–O phase diagram was first presented by Ros-
than 37 at % O, which was later referred to as the toker [12] and later refined by Stringer [21], who took
4
7

4
8
DAVYDOV, REMPEL
into account the experimental data reported for the rect evidence that this phase exists in the range VO1.20
V O2n – 1, VO2 and V O2n + 1oxides before 1964. to VO1.30
.
n
n
According to Stringer [21], the γ-phase has the compo-
On the whole, the literature data on the position of
phase boundaries in theV–O system at oxygen contents
below 60 at % are insufficient and contradictory. This
led us to refine the V–O phase diagram in the range 25–
sition VO0.53, has no homogeneity range, and is stable
below ꢀ1430 K (Fig. 1a). Adjoining to the γ-phase field
are the β + γ and γ + δ two-phase fields. Above 1430 K,
there is a β + δ two-phase region. In the temperature
range ꢀ1000–1800 K, the homogeneity range of the
5
0 at % oxygen using our experimental data and earlier
results.
cubic (δ) phase varies little: ꢀVO0.85–VO1.25
.
Alexander and Carlson [22] studied the phase equi-
libria in the V–VO system. Vanadium oxide samples
containing up to 50 at % oxygen were prepared by arc-
melting a mixture of vanadium and V O and were out
of equilibrium. Accordingly, because of the stress
relaxation, transformation temperatures could be esti-
EXPERIMENTAL
VO vanadium oxide samples with oxygen contents
in the range 0.43 ≤ y ≤ 1.29 were prepared by solid-state
sintering of a mixture of VH1.5 and V O in an SShV-
y
2
5
2
3
1
.2.5/25I1 furnace. The process was run in a vacuum of
–5
mated only roughly. According to Alexander and Carl- 0.0013 Pa (10 mm Hg) at 1770 K for 10 h, with an
son [22], the γ-phase has a rather broad homogeneity intermediate grinding after 5 h. The synthesis proce-
range (Fig. 1b), in contrast to earlier reports that it has dure was described in detail elsewhere [34]. We
no homogeneity range [16, 17, 21]. Moreover, in the obtained VO samples with y = 0.43, 0.57, 0.81, 0.86,
y
phase diagram constructed by Alexander and Carlson 0.87, 0.89, 0.90, 0.96, 0.97, 0.98, 0.99, 1.00, 1.02, 1.03,
22] (Fig. 1b), the boundaries of the β-, δ, β + γ, and γ + 1.04, 1.07, 1.11, 1.29, and 1.30. The samples with y =
[
δ phase fields differ markedly from those in the phase 0.43, 0.57, 0.86, 0.96, 0.97, 1.29, and 1.30 were
diagram presented by Stringer [21] (Fig. 1a). On the annealed for 2 h in evacuated silica ampules at 1470,
whole, the data reported in [22] are insufficient for 1370, 1270, 1170, and 1070 K and then rapidly cooled
accurately determining the position of phase bound- (quenched) to room temperature. TheVO0.57 sample
aries.
was annealed at 1070 K for 1000h in an evacuated
quartz cell with further tempering. The residual pres-
In the β- and γ-phases, the oxygens and ꢀ structural
vacancies (vacant sites in the nonmetal sublattice) form
a substitutional solution. As shown in detailed struc-
sure in the ampules was 0.0013 Pa. The VO0.81, VO0.87
,
and VO0.98 samples were annealed during slow cooling
tural studies [23–27], the γ-phase has a monoclinic lat- from the synthesis temperature (1770 K) to 1070 K at a
tice and results from ordering of oxygens and oxygen rate of 25 K/h. Oxygen content was determined thermo-
vacancies (ꢀ) in the parent body-centered tetragonal gravimetrically, from the weight gain upon the com-
phase (β). The boundaries of the γ-phase, however, plete oxidation of the sample to the higher oxide V O .
2
5
received very little attention in [23, 26, 27]. According The samples were oxidized for 100 h in air at ꢀ820 K.
to Westman [18], the homogeneity range of the γ-phase The absolute uncertainty in the oxygen content of VOy
at 1170 K is VO0.47–VO0.53. Hiraga and Hirabayashi was ±0.2 wt %, or ±0.01 in y: VOy ± 0.01. The equilibrium
[
24] reported the phase V O (V O ) to exist in the
14 6 7 3
oxygen partial pressure pO2 over the vanadium oxides
studied does not exceed 10–17 Pa [35]. Accordingly,
annealing slightly increased the oxygen content of all
the samples.
composition range VO0.44 to VO0.54, whereas Galkin et
al. [25] found the monoclinic phase V O in oxide
samples from VO0.39 to VO0.49
14
6
.
Cubic vanadium monoxide has been studied by
many groups [13–16, 19, 20, 28–30], but its phase
boundaries have not yet been accurately determined. In
those studies, samples were prepared at high tempera-
tures and then quenched by rapid cooling. With
decreasing temperature, however, the homogeneity
range of nonstoichiometric compounds becomes nar-
rower [6, 7], and ordered phases may form.
The phase composition of the samples and the lat-
tice parameters of the phases present were determined
by x-ray diffraction (XRD) on a DRON-UM1 auto-
matic diffractometer (Bragg–Brentano geometry,
CuKα1,2 radiation, 2θ = 20°–140°, step-scan mode with
a step size ∆2θ = 0.03° and a counting time per data
point of 2 s). In the final structure refinement and quan-
titative phase analysis, we used the GSAS package [36].
The background was represented by a fifth degree Cheby-
shev polynomial, and diffraction line profiles were fitted
Indeed, long-term annealing of the vanadium mon-
oxide VO1 at 1070 K [16] and VO1.2–1.3 at tempera-
.27
tures from 870 to 1070 K [31] led to the formation of an with pseudo-Voigt functions. The accuracy in the phase
ordered phase. An ordered phase was also found in compositions obtained was ±2 wt % or better.
annealed VO1.19, VO
1.25
[32], and VO1.30 [33]. Accord-
Microstructures and phase compositions were also
studied on a Leica DM2500 M polarizing microscope,
offering a resolution of down to 500 nm. Sintered sam-
ples for optical examination were polished to mirror
ing to Andersson and Gjonnes [31] and Morinaga and
Cohen [33], the ordered phase has a tetragonal struc-
ture, and its composition is ꢀV O . The phase bound-
52
64
aries of ꢀV O have not been located. There is indi- finish using a Buehler metallographic system.
52
64
INORGANIC MATERIALS Vol. 45 No. 1 2009

REFINEMENT OF THE V–O PHASE DIAGRAM
49
y = O/V
0
.2
0.3
0.4
0.5
0.6 0.7 0.8 0.9 1.0
1.2 1.4
T, K
000
(a)
2
β + δ
α + β
δ + ζ
1
1
500
000
δ
γ + δ
β
β + γ
≅
1430 K
5
00
ε + ζ
δ + ε
(b)
2
1
000
500
β + δ
α + β
≅
1460 K
δ + ζ
δ
β
1
000
β + γ
γ
γ + δ
500
δ'
1
2
3
(c)
2
1
000
500
≅
1620 K β + δ
α + β
β
δ
δ
β + γ
1
000
γ + δ
δ + ζ
γ
500
δ'
2
0
30
40
50
60
O, at %
Fig. 1. V–O phase diagram in the range 15–60 at % oxygen: (a) proposed by Stringer [21]; (b) portion of the generalized phase
diagram constructed by Wriedt [1, 2] with the use of data from [4, 5]; (c) this work.α = bcc solid solution of oxygen in vanadium,
β = body-centered tetragonal (sp. gr. I4/mmm) solid solution of oxygen in vanadium, γ = monoclinic (sp. gr. C2/m) ordered phase
V O , δ = cubic monoxide V O , ε ≡ δ' = tetragonal (sp. gr. I4 /amd) ordered phase V O , ζ = V O ; (1) two-phase (γ + δ) sam-
1
4
6
x
z
1
52 64
2 3
ples, (2) single-phase samples (δ-phase), (3) boundaries of the γ + δ two-phase region (with error bars).
INORGANIC MATERIALS Vol. 45 No. 1 2009

5
0
DAVYDOV, REMPEL
1
1
070 K
170 K
1
270 K
As-synthesized
4
0
50
0
70
80
2
θ, deg
Fig. 2. XRD patterns of the two-phase sample VO0.86 after annealing at different temperatures. The as-synthesized VO0.86 sample
contained ꢀ13 wt % V O and ꢀ87 wt % V O (cubic monoxide V O ); annealing at 1070 K increased the V O content to
14
6
x
z
x
z
14 6
ꢀ
32 wt %. The long and short tick marks show the positions of reflections from the ordered oxide V O and cubic vanadium mon-
14 6
oxide, respectively, in the XRD pattern of the sample annealed at 1070 K ( CuKα1,2 radiation).
RESULTS AND DISCUSSION
in position. In view of this, we first calculated the XRD
patterns of the β- and γ-phases. The results indicate that
the signatures of the monoclinic phase (γ) are the reflec-
tions 113 and 114, situated in the 2θ ranges 47.9°-48.0°
and 57.7°-57.8°, whereas the β-phase has no reflections
in these angular ranges. The observed XRD patterns of
the as-synthesized and annealed VO0.57, VO0.81, and
VO0.86 samples showed a reflection in the range 2θ =
XRD characterization showed that the synthesized
samples with y > 0.87 were single-phase and consisted
of cubic (sp. gr. Fm3 m) vanadium monoxide. The
VO0.43 sample consisted of a monoclinic ordered phase
(
γ). The VO0.57, VO0.81, and VO0.86 samples consisted of
cubic vanadium monoxide and the γ-phase (86, 30, and
1
3 wt %, respectively). The structure of the γ-phase was
57.7°–57.8°, characteristic of the γ-phase. A detailed
described in detail by Gusev and Davydov [27].
GSAS analysis [36] provided conclusive evidence that
the VO0.43, VO0.57, VO0.81, and VO0.86 samples contained
the γ-phase but not the β-phase and that annealing pro-
Annealing increased the amount of the γ-phase in
the VO0.57, VO0.81, and VO0.86 samples. Moreover, the
γ-phase appeared in the VO samples with VO (0.87 ≤
y
y
duced the γ-phase in the VO samples with 0.87 ≤ y ≤
y
y ≤ 0.98) which consisted of cubic vanadium monoxide
0.98.
before annealing. The amount of the γ-phase after
annealing was ꢀ33 wt % in VO0.87, ꢀ20 wt % in VO0.96
,
The γ-phase was shown to have a monoclinic struc-
and ꢀ3 wt % in VO0.97; in the VO0.97 sample annealed
for 1000 h, the ammount of the γ-phase reached 90 wt %.
Figure 2 shows the XRD patterns of the VO0.86 sample
before and after annealing at different temperatures,
followed by quenching. In neither the as-synthesized
nor annealed samples was the β-Ù‡Á‡ (tetragonal struc-
ture, sp. gr. I4/mmm) detected.
ture (sp. gr. C2/m). The formula of an ideal, fully
ordered γ-phase can be written in the form V O
6
14
(
V O ꢀ ). The unit-cell parameters of the γ-phase
14 6 8
V O (VO0.44) contained in the VO0.86 sample annealed
14
6.16
at 1070 K for 2 h studied here are a = 0.95536(6) nm,
M
b = 0.29184(2) nm, c = 0.77622(9) nm, and β =
M
M
M
9
0.32(4)°. In the sample VO0.57 annealed for 1000 h at
The XRD patterns of the β- and γ-phases differ very
little: most of the reflections from these phases coincide 1070 K, the γ-phase had the composition V14
O6.48 (VO0.46).
INORGANIC MATERIALS Vol. 45 No. 1 2009

REFINEMENT OF THE V–O PHASE DIAGRAM
51
5
µm
5 µm
5 µm
(
‡)
(b)
(c)
Fig. 3. Microstructures of (a) VO0.43, (b) VO0.86, and (c) VO1.0 samples (the VO0.43 sample consisted of the γ-phase V O in the
14
6
two-phase sample of composition V O , the γ-phase precipitated in the form of needle inclusions in a cubic-structured matrix; the
1
4 6
VO1.00 sample consisted of cubic vanadium monoxide).
An important question is whether the γ-phase exists therefore reasonable to assume that the β + δ two-phase
at high temperatures. In a number of studies [12, 21, region, shown in the phase diagrams in Figs. 1a and 1b
2], the γ-phase was observed at temperatures below [21, 22] at T > 1430–1460 K, actually begins at a higher
1430–1460 K, and the higher temperature phase field temperature of ꢀ1620 K (Fig. 1c).
2
ꢀ
was assumed to be two-phase: β + δ. That assumption
appears invalid: the data points lay in a two-phase
region, and the reflections from the γ-phase were misat-
tributed to the β-phase. Indeed, the structure of the γ-
phase (V O ) was accurately determined much later
The increase in the content of the monoclinic subox-
ide V O upon quenching of the VO (0.81 ≤ y ≤ 0.98)
14
6
y
samples from 1470 K and lower temperatures is due to
the oxygen redistribution from the V O monoxide to
x
z
14
6
V O in response to the narrowing of the homogeneity
1
4
6
[
[
24, 25]. Hiraga and Hirabayashi [24] and Galkin et al.
25] presented, however, relatively low-temperature
range of V O with decreasing temperature. Since the
x
z
oxygen concentration in the monoclinic suboxide
V O is lower than that in cubic vanadium monoxide,
data (below 1270 K), which, therefore, cannot be used
to validate or refute an assumption as to the high-tem-
perature stability limit of the γ-phase. In this study, the
samples were characterized at temperatures of up to
1
4
6
the formation of V O upon annealing and quenching
14
6
x
is accompanied by a rise in the oxygen content of V Oz
and, as a consequence, by an increase in the lattice
1
770 K. Over the entire temperature range studied, no
reflections from the β-phase were detected, whereas
reflections from the γ-phase and cubic vanadium mon- phase compositions of the samples quenched from dif-
parameter a of cubic vanadium monoxide. From the
B1
oxide were identified with certainty.
ferent temperatures and containing the γ- and δ-phases
and the compositions of the γ- and δ-phases in all of the
two-phase samples (table), one can infer the boundaries
of the γ + δ two-phase region (Fig. 1c).
Does the presence of the γ-phase (V O ) in the
14
6
VO0.43, VO0.57, VO0.81, and VO0.86 samples synthesized
at 1770 K mean that it forms starting at this tempera-
ture? After heat treatment, the synthesized samples
were furnace-cooled: the temperature dropped rapidly
to ꢀ1400–1500 K, and subsequent cooling took about
The present results indicate that the γ-phase has a
very narrow homogeneity range located at considerably
lower oxygen contents than was assumed previously.At
ꢀ
ꢀ
ꢀ
1000 K, the vanadium-rich phase boundary is at
VO0.42 [24], and the oxygen-rich phase boundary is at
2
h. Thus, the synthesized samples were partially
annealed during cooling. The highest quenching tem-
perature in our experiments was 1470 K. Quenching
from this temperature increased the content of the
γ-phase in the VO0.57, VO0.81, and VO0.86 samples and
VO0.46 (V O6.48). As the temperature is raised from
14
1
000 K to the peritectoid temperature, the homogeneity
range of the γ-phase becomes narrower (Fig. 1c).
Figure 3 shows micrographs of a single-phase
VO0.43 sample (consisting of the γ-phase), two-phase
VO0.86 sample (consisting of the γ- and δ-phases), and
VO1.00 sample (consisting of cubic vanadium monox-
caused this phase to appear in the VO samples with
y
0
.87 ≤ y ≤ 0.98. Thus, the γ phase in VO samples
y
appears below 1770 K and is present at temperatures
above 1570 K. Taking into account the exact synthesis
and annealing conditions, we think that the formation ide). In reflected polarized white light, the grains of the
of the γ-phase begins at ꢀ1620 K, which is close to the VO0.43 sample exhibit interference colors, which means
temperature reported by Seybolt and Sumsion [8]. It is that they are anisotropic, in accordance with the mono-
INORGANIC MATERIALS Vol. 45 No. 1 2009

5
2
DAVYDOV, REMPEL
Effect of heat-treatment temperature on the phase composition of two-phase VO samples and the compositions of the γ- and
y
δ-phases
wt % in
the sample
wt % in the
sample
Composition
at % oxygen
Composition
at % oxygen
VOy
Quenching
sample temperature, K
Monoclinic phase (γ)
Cubic phase (δ)
VO0.57
VO0.57
VO0.57
VO0.81
VO0.81
VO0.86
VO0.86
VO0.86
VO0.86
VO0.86
VO0.87
VO0.87
VO0.96
VO0.96
VO0.96
VO0.97
VO0.98
*1600
**1470
****1070
*1600
86
88
90
30
41
13
26
29
34
32
10
33
11
12
20
3
V O (VO0.43)
30.00
30.00
31.64
30.00
30.56
30.00
30.56
30.56
30.56
30.56
30.00
30.56
30.56
30.56
30.56
30.56
30.56
12
10
10
70
59
87
74
71
66
68
90
67
89
88
80
97
96
V0.86O0.815 (VO0.95
)
48.66
48.48
49.08
47.47
49.70
48.34
48.53
48.85
49.33
50.98
48.45
48.75
49.40
49.79
49.79
49.42
49.37
14
6
V O
(VO0.44
)
)
V0.85O0.80 (VO0.94
V0.84O0.81 (VO0.96
V0.83O0.75 (VO0.90
V0.85O0.84 (VO0.99
V0.79O0.74 (VO0.94
)
)
)
)
)
14
6.16
V O
(VO0.46
14
6.48
V O (VO0.43)
14
6
**1070
*1600
V O
(V0.44)
14
6.16
V O (VO0.43)
14
6
**1370
**1270
**1170
**1070
*1600
V O
(VO0.44
(VO0.44
(VO0.44
(VO0.44
)
)
)
)
V0.79O0.745 (VO0.94
V0.78O0.745 (VO0.955
)
14
6.16
6.16
6.16
6.16
V O
)
14
V O
V0.76O0.74 (VO0.97
V0.75O0.78 (VO1.04
V0.83O0.78 (VO0.94
V0.82O0.78 (VO0.95
V0.85O0.83 (VO0.97
V0.85O0.84 (VO0.99
V0.85O0.84 (VO0.99
V0.87O0.85 (VO0.98
)
)
)
)
)
)
)
)
14
V O
14
V O (VO0.43)
14
6
***1070
**1270
**1170
**1070
**1070
***1070
V O
(VO0.44
(VO0.44
(VO0.44
(VO0.44
(VO0.44
(VO0.44
)
)
)
)
)
)
14
6.16
6.16
6.16
6.16
6.16
6.16
V O
14
V O
14
V O
14
V O
14
4
V O
V0.80O07.8 (VO0.975)
14
Notes: *Effective quenching temperature of 1600 K: cooling from the synthesis temperature (1770 K) to 1500 K, followed by cooling
to 300 K in 2 h.
*
*Vacuum annealing at the specified temperature for 2 h, followed by quenching from the annealing temperature to 300 K in 5 min.
*
**Annealing during cooling from the synthesis temperature (1770 K) to 1500 K at a rate of 25 K/h, followed by quenching from 1070
to 300 K in 15 min.
****Annealing at a temperature of 1070 K for 1000 h in evacuated quartz cell with further tempering to 300 K for 5 min.
clinic symmetry of the γ-phase. In the two-phase sam- vanadium monoxide exists in the composition range
ple of composition VO0.86, the γ-suboxide precipitated VO0.90 to VO1.30, or, with consideration for the vana-
in the form of interfering elongated inclusions in a dium vacancies, V0.83O0.75 to V–V0.77O .
1.00
cubic-structured matrix exhibiting no interference col-
The vacancy concentration in cubic vanadium mon-
oxide has been the subject of several studies [28–30].
The only attempt to experimentally determine the
homogeneity range of the δ-phase (Fig. 1b) was made
by Alexander and Carlson [22], but their conclusions
ors. The VO1.00 sample did not interfere in polarized
light, which indicated that it was isotropic and con-
sisted of a cubic phase, in agreement with diffraction
data. The δ-phase (cubic vanadium monoxide), V O , has
x
z
the widest homogeneity range. Its most interesting feature were drawn from limited experimental information.
is the high concentration of both metal and oxygen vacan- Later results [28–30] and the present experimental data
cies. According to the present results, at 1600 K cubic lead us to conclude that the vanadium-rich boundary of
INORGANIC MATERIALS Vol. 45 No. 1 2009

REFINEMENT OF THE V–O PHASE DIAGRAM
53
the δ-phase near its melting point lies at somewhat 11. Henry, J.L., O’Hare, S.A., McCune, R.A., and Krug, M.P.,
The Vanadium–Oxygen System: Phase Relations in the
higher oxygen contents than was reported byAlexander
Vanadium-Rich Region below 1200°C, J. Less-Common
and Carlson [22] and was indicated in the phase dia-
grams presented by Wriedt [1, 2]. With decreasing tem-
perature, the homogeneity range of this phase becomes
narrower, and it seems likely that, below 800 K, it lies
Met., 1970, vol. 21, no. 2, pp. 115–135.
1
1
2. Rostoker, W., Metallurgy of Vanadium, New York:
Wiley, 1958.
3. Schönberg, N., X-Ray Studies of Vanadium and Chro-
mium Oxides with Low Oxygen Content, Acta Chem.
Scand., 1954, vol. 8, no. 1, pp. 221–225.
4. Andersson, G., Studies ofVanadium Oxides, Acta Chem.
Scand., 1954, vol. 8, no. 9, pp. 1599–1606.
at oxygen-enriched VO compositions (y > 1.0), as
y
shown in Fig. 1c. The present experimental data are
inconsistent with the assumption made earlier [34] that,
at relatively low temperatures, an ordered V O -type
1
5
5
phase (V O (V ꢁ O ꢀ )) may be formed in cubic vana- 15. Vol’f, E., Tolkachev, S.S., and Kozhina, I.I., X-Ray Dif-
5
5
5
1
5
1
fraction Study of Titanium and Vanadium Monoxides,
dium monoxide. According to a number of reports [31–
3], ordering takes place at oxygen-enriched composi-
tions and leads to the formation of the δ'-phase V O .
The oxygen-rich phase boundary of vanadium monox-
ide in Fig. 1c was inferred from those data.
Vestn. Leningr. Univ., 1959, no. 10, pp. 87–92.
3
1
1
6. Westman, S. and Nordmark, C., Phase Analysis Studies
on the Vanadium–Oxygen System within VO0.25–VO1.5
Region at 800°C, Acta Chem. Scand., 1960, vol. 14,
no. 2, pp. 465–470.
52
64
7. Bogdanova, N.I. and Ariya, S.M., Electrical Conductiv-
ity of Lower Vanadium Oxides, Vestn. Leningr. Univ.,
ACKNOWLEDGMENTS
1
961, no. 16, pp. 143–147.
We thank A.I. Gusev for helpful discussions. This
study was supported by the Russian Fund for Basic
Research (grant no. 09-03-00010a).
1
1
8. Westman, S., On the Lower Oxides of Vanadium, Acta
Chem. Scand., 1963, vol. 17, no. 3, pp. 749–752.
9. Gel’d, P.V., Alyamovskii, S.I., and Matveenko, I.I.,
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the β-, δ-, and ζ-Phases in the Vanadium–Oxygen Sys-
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