Subsolidus phase relations in the systems M2+O-M 2+O-V2O5 (M+ = Li, Na, K, Rb, Cs; M2+ = Mg, Ca)

Article abstract of DOI:10.1023/B:INMA.0000016096.67759.41

Subsolidus phase relations in the M₂O(M₂CO ₃)-MgO-V₂O₅ and M₂O(M ₂CO₃)-CaO-V₂O₅ (M = Li, Na, K, Rb, Cs) systems are studied. Twenty mixed vanadat

Full text of DOI:10.1023/B:INMA.0000016096.67759.41

Inorganic Materials, Vol. 40, No. 2, 2004, pp. 188–194. Translated from Neorganicheskie Materialy, Vol. 40, No. 2, 2004, pp. 232–238.
Original Russian Text Copyright © 2004 by Slobodin, Surat.
Subsolidus Phase Relations in the Systems M+2 O–M2+O–V2O5
(M+ = Li, Na, K, Rb, Cs; M2+ = Mg, Ca)
B. V. Slobodin and L. L. Surat
Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences,
Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russia
e-mail: slobodin@ihim.uran.ru
Received July 2, 2003; in final form, September 2, 2003
Abstract—Subsolidus phase relations in the M2O(M2CO3)–MgO–V2O5 and M2O(M2CO3)–CaO–V2O5 (M =
Li, Na, K, Rb, Cs) systems are studied. Twenty mixed vanadates are obtained, of which Rb2CaV2O7,
Cs2CaV2O7, LiMg4(VO4)3, RbCaVO4, and CsCaVO4 are identified for the first time. Structural data are sum-
marized for all of the mixed vanadates: the space group and lattice parameters are indicated for 14 compounds
(for 6 compounds, such data are obtained for the first time), and I and d data are presented for 8 compounds.
Partial series of Ca3(VO4)2-based solid solutions with the general formula Ca3 – xM2x(VO4)2 (M = Na, K, Rb,
Cs) are identified in the range 0 < x ≤ 0.14. Six phase diagrams (M+ = Li, Rb, Cs; M2+ = Mg, Ca) are investigated
and are compared with the phase diagrams of the other ternary systems in question. The key features of the ter-
nary phase diagrams and, hence, the reactivity of the constituent oxides are shown to vary systematically in
going from Li2O to Cs2O and from MgO to SrO, which is interpreted in terms of the variation in the ionic radius
of the alkali and alkaline-earth metals.
INTRODUCTION
tem was shown to contain a compound of composition
KCa10V7O28 [12], isostructural with Ca3(VO4)2 [12].
By examining the effect of the M+ or M2+ metal on
the phase relations in M+2 O–M2+O–V2O5 systems, one
can establish relationships between the reactivities of
the M2+O or M+2 O oxides in such systems. This work,
dealing with the M2+ = Mg and Ca systems, continues
our earlier study [1], where we examined such relation-
ships for M+ = Li, Na, K, Rb, Cs and M2+ = Sr.
The objective of this work was to identify the phases
existing in the systems M+2 O–M2+O–V2O5 (M+ = Li,
Na, K, Rb, Cs; M2+ = Mg, Ca), to clarify a number of
controversial issues, to determine the structural param-
eters of several ternary vanadates, to systematize the
available information about these systems, and to
examine variations in reactivity across the M+2 O and
M2+O oxide series.
The Na2O(K2O)–MgO(CaO)–V2O5 phase diagrams
were reported in [2–6]. For the other systems, phase-
diagram data are missing. Data on mixed vanadates
existing in unexplored systems are given in [7]. In par-
ticular, the Li2O–MgO–V2O5 system was shown to
contain the orthovanadate LiMgVO4, solid-solution
series Li2xMg1.5 – xVO4 (0.10 < x < 0.15) [8], and
Li3Mg18V11O47 [7, card 46-0189] (Fig. 1, symbols 1–3).
The Rb2O–CaO–V2O5 system contains the ternary
metavanadate Rb2Ca(VO3)4 [9]. Structural data,
detailed or not, have been reported for only half of the
known vanadates in the systems in question.
V2O5
70
2
1
4
5
80
3
90
40
30
20
10
In addition, new data have recently been reported on
some of the systems studied earlier. According to the
structural analysis results reported by Martin and
Muller-Buschbaum [10], the composition of the mixed
metavanadate in the K2O–CaO–V2O5 system is
K3Ca(VO3)5, rather than K2Ca(VO3)4 [6, 11]. In addi-
tion to the ternary vanadates identified in [6], this sys-
mol % Li2O
MgO
Fig. 1. Portion of the Li O–MgO–V O phase diagram at sub-
2
2 5
solidus temperatures: (1) LiMgVO , (2) Li Mg
VO
4
2x
1.5 – x
4
(0.1 < x < 0.15), (3) Li Mg V O , (4) LiMg (VO ) ,
3
18 11 47
4
4 3
(5) Mg (VO ) .
3
4 2
0020-1685/04/4002-0188 © 2004 MAIK “Nauka/Interperiodica”

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEMS
189
EXPERIMENTAL
cium vanadates obtained in this work coincide with
those reported in [5].
Samples for this investigation were prepared by
solid-state synthesis (Naber furnace). Thoroughly
ground mixtures of extrapure-grade V2O5, MgO (or
CaCO3), and alkali carbonates were reacted at a tem-
perature 10–15°C below the melting point or the
decomposition temperature of the reaction products for
100–130 h with repeated intermediate grindings.
In addition, we reacted mixtures of presynthesized
magnesium, calcium, and alkali vanadates. We pre-
pared and characterized at least three samples for each
The Rb2O(Cs2O)–MgO–V2O5 and Li2O–CaO–
V2O5 systems contain no mixed vanadates. The phase
equilibria in these systems involve only alkali and alka-
line-earth vanadates. The other combinations of oxides
react to form mixed vanadates. In particular, our results
on the pseudobinary system Li3VO4–Mg3(VO4)2 in
conjunction with earlier data for Li2xMg1.5 – xVO4 and
Li3Mg18V11O47 [7] suggest that this system contains,
along with LiMgVO4, a new mixed orthovanadate of
assumed join to be studied and at least two samples for composition LiMg4(VO4)3 (Fig. 1, point 4). The sam-
each assumed three-phase region. The phase purity of
the synthesized ternary vanadates was checked care-
fully.
ples close in composition to this compound (Fig. 1,
symbols 2, 3) consist of LiMg4(VO4)3 and trace
amounts of Mg3(VO4)2 or MgO and Mg3(VO4)2.
LiMg4(VO4)3 is isostructural with NaMg4(VO4)3
(Table 1).
The phase composition of the samples was deter-
mined by x-ray diffraction (XRD) on a DRON-2 pow-
der diffractometer (ëuKα radiation), using JCPDS–
ICDD PDF 2 data [7]. Structural characterization of
new vanadates was carried out on a STADI-P (STOE,
Germany) automatic x-ray diffractometer (ëuKα radia-
tion, diffracted beam pyrolytic graphite monochroma-
tor, α-Al2O3 internal standard (NIST SRM 676, USA),
Si external standard with ‡ = 5.43075(5) Å). XRD pat-
terns were taken at room temperature in the angular
range 2° ≤ 2θ ≤ 120° with a step size of 0.02°. The
results were analyzed using STOE SOFTWARE. Struc-
tural analogs were sought for using ICDD PDF 2
(2002) data. Indexing schemes were checked with NBS
AIDS83.
The M2O–CaO–V2O5 (M = Na, K, Rb, Cs) systems
contain Ca3(VO4)2-based substitutional solid solutions
with the general formula Ca3 – xM2x(VO4)2 (0 < x ≤
0.14). The lattice parameters of the terminal solid solu-
tions (rhombohedral, sp. gr. R3c) are listed in Table 2.
The NaVO3–Ca(VO3)2 system contains two partial solid-
solution series in the composition ranges 0–5 mol %
Ca(VO3)2 and 0–17 mol % NaVO3 [11, 19, 21].
By firing a 1.5 : 1 : 2.5 mixture of K2CO3, CaCO3,
and V2O5 at 350°C for 200 h, followed by slow cooling,
or at 450°C for 50 h, followed by quenching, we
obtained K3Ca(VO3)5, in accordance with earlier data
[10]. In contrast to what was reported by Glazyrin et al.
[11], we detected no polymorphic transformations of
this compound. Heat treatment of a 1 : 1 : 2 mixture of
the same reagents (K2Ca(VO3)4 stoichiometry) under
the same conditions led to the formation of K3Ca(VO3)5
and Ca(VO3)2.
RESULTS AND DISCUSSION
The M+2 O–M2+O–V2O5 phase diagrams studied in
this work at subsolidus temperatures are displayed in
Figs. 2 and 3. The phase diagrams of six systems (M+ =
Li, Rb, Cs; M2+ = Mg, Ca) were mapped out for the first
It was, therefore, reasonable to expect that a ternary
metavanadate of the same stoichiometry exists in the
time, and those of three systems (M+ = Na, M2+ = Mg; Rb system. However, reacting 1 : 1 : 2 and 1.5 : 1 : 2.5
M+ = Na, M2+ = Ca; M+ = K, M2+ = Ca) were refined.
mixtures of Rb2CO3, CaCO3, and V2O5, we obtained
phase-pure Rb2Ca(VO3)4 in the former case and a mix-
ture of this vanadate with RbVO3 in the latter case.
Thus, the vanadates existing in the joins (systems)
NaVO3–Ca(VO3)2, KVO3–Ca(VO3)2, and RbVO3–
Ca(VO3)2 are Na2Ca(VO3)4, K3Ca(VO3)5, and
Rb2Ca(VO3)4, respectively. The fact that the K-contain-
ing vanadate differs in stoichiometry from the other two
vanadates correlates with the specific behavior of
potassium and its compounds, which often do not fit in
with general trends exhibited by the other alkali metals
and their compounds. In particular, this refers to the
systems M3VO4–Ca3(VO4)2 (Fig. 3) and MVO3–
Sr(VO3)2 [1] with M = Li, Na, K, Rb, and Cs.
Strictly speaking, none of the systems studied are
pseudoternary because they contain vanadium bronzes
(VBs) [13], which form at pO = 0.21 × 105 Pa via par-
2
tial reduction of vanadium (accompanied by oxygen
loss) and belong to the corresponding M2O–V2O4–
V2O5 systems. It is, however, common practice to
include VBs in the M2O–V2O5 systems (Figs. 2, 3).
Another conventional simplification is that the compo-
sitions of VBs are represented in phase diagrams by
points, even though the alkali-metal content of these
compounds may vary in a narrow range [13].
In the M2O–V2O5 (M = Li, Na, K, Rb, Cs) systems
(Figs. 2, 3), we obtained the same phases as in earlier
studies [1]. The MgO–V2O5 system contains magne-
sium meta-, pyro-, and orthovanadates [14]. The cal-
Of the compounds listed in Table 1, the mixed van-
adates Rb2CaV2O7, Cs2CaV2O7, LiMg4(VO4)3,
RbCaVO4, and CsCaVO4 and the solid solutions
INORGANIC MATERIALS Vol. 40 No. 2 2004

190
SLOBODIN, SURAT
V2O5
V2O5
(b)
(a)
Na0.33V2O5 (VB)
Na2.5V6O16 (VB)
Li2.5V6O16 (VB)
NaVO3
LiVO3
Mg(VO3)2
Mg(VO3)2
Na5V3O10
Na4V2O7
Na3VO4
Mg2V2O7
Mg2V2O7
3
Li3VO4
Mg3(VO4)2
Mg3(VO4)2
2
2
1
Li2O(Li2CO3)
MgO
Na2O(Na2CO3)
MgO
V2O5
V2O5
(c)
(d)
Rb2V8O21-δ (VB)
Na2V8O21-δ (VB)
K2V6O16
K3V5O14
KVO3
K5V3O10
Rb2V6O16
Rb3V5O14
RbVO3
Mg(VO3)2
Mg(VO3)2
Mg2V2O7
4
5
Rb5V3O10
Rb4V2O7
Rb3VO4
K4V2O7
K3VO4
Mg2V2O7
Mg3(VO4)2
Mg3(VO4)2
K2O(K2CO3)
MgO
Rb2O(Rb2CO3)
(e)
MgO
V2O5
Cs2V8O21-δ (VB)
Cs2V6O16
Cs2V4O11
CsVO3
Mg(VO3)2
Cs5V3O10
Cs4V2O7
Cs3VO4
Mg2V2O7
Mg3(VO4)2
Cs2O(Cs2CO3)
MgO
Fig. 2. Schematic phase diagrams of M O(M CO )–MgO–V O systems at subsolidus temperatures: M = (a) Li, (b) Na, (c) K,
2
2
3
2 5
(d) Rb, (e) Cs: (1) LiMgVO , (2) MMg (VO ) , (3) Na Mg (VO ) , (4) K Mg(VO ) , (5) K MgV O .
4
4
4 3
6
3
4 4
2
3 4
2
2 7
Ca3 − xM2x(VO4)2 (M = K, Rb, Cs), 0 < x ≤ 0.14 (Table 2) XRD (Tables 1, 2). The powder XRD data for
LiMg4(VO4)3, Cs2CaV2O7, and Cs2CaVO4 are pre-
sented in Table 3. We also confirmed the existence of
some other mixed vanadates (Figs. 2, 3; Table 1).
were synthesized for the first time. Some of these com-
pounds, and also Na2Ca(VO3)2, KCaVO4, and the
Ca3(VO4)2-based solid solution, were characterized by
INORGANIC MATERIALS Vol. 40 No. 2 2004

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEMS
V2O5
191
V2O5
(b)
(a)
Na0.33V2O5 (VB)
Na2.5V6O16 (VB)
Li2.5V6O16 (VB)
11
10
1
LiVO3
Ca(VO3)2
NaVO3
Ca(VO3)2
Na5V3O10
Na4V2O7
Ca2V2O7
Ca2V2O7
Ca3(VO4)2
Ca7V4O17
4
3
9
Li3VO4
Na3VO4
Ca3(VO4)2
6
7
Ca7V4O17
Na2O(Na2CO3)
CaO
Li2O(Li2CO3)
CaO
(c)
V2O5
V2O5
(d)
K2V8O21-δ (VB)
Rb2V8O21-δ (VB)
Rb2V6O16
K2V6O16
K3V5O14
KVO3
K5V3O10
Rb3V5O14
2
5
1
5
RbVO3
Ca(VO3)2
Ca2V2O7
Ca(VO3)2
Ca2V2O7
Rb5V3O10
Rb4V2O7
K4V2O7
K3VO4
9
9
Ca3(VO4)2
Rb3VO4
Ca3(VO4)2
Ca7V4O17
6
8
6
Ca7V4O17
K2O(K2CO3)
CaO
Rb2O (Rb2CO3)
(e)
CaO
V2O5
Cs2V8O21-δ (VB)
Cs2V6O16
Cs2V4O11
Ca(VO3)2
Ca2V2O7
CsVO3
Cs5V3O10
Cs4V2O7
Cs3VO4
5
9
Ca3(VO4)2
6
Ca7V4O17
Cs2O (Cs2CO3)
CaO
Fig. 3. Schematic phase diagrams of M O(M CO )–CaO–V O systems at subsolidus temperatures: M = (a) Li, (b) Na, (c) K,
2
2
3
2 5
(d) Rb, (e) Cs: (1) M Ca(VO ) , (2) K Ca(VO ) , (3) Na Ca (V O ) , (4) Na Ca V O , (5) M CaV O , (6) MCaVO ,
2
3 4
3
3 5
2
7
2
7 4
3
2
3
11
2
2
7
4
(7) NaCa (VO ) , (8) K Ca(VO ) , (9) Ca (VO ) -based solid solution, (10) NaVO -based solid solution, (11) Ca(VO ) -based
4
4 3
4
4 2
3
4 2
3
3 2
solid solution.
In the context of the M+2 O–M2+O–V2O5 (M+ = Li,
Na, K, Rb, Cs; M2+ = Mg, Ca, Sr) phase diagrams
(Figs. 2 and 3 in this work and Fig. 2 in [1]), several
important points warrant mention. In going from Li2O
to Cs2O or from MgO to SrO, the key features of these
ternary phase diagrams vary systematically. This is
associated primarily with the effect of the cation size on
the reactivity of the constituent oxides and on the nature
INORGANIC MATERIALS Vol. 40 No. 2 2004

Table 1. Crystallographic data for ternary vanadates in M2O–MgO(CaO)–V2O5 systems
Compound
Structure
Source
Compound
Structure
Source
Metavanadates
Orthovanadates
K2Mg(VO3)4
I, d*
[3]
LiMgVO4
Orthorhombic, Cmcm, a = 5.6283 Å,
b = 8.6123 Å, c = 6.2381 Å
[7, card. 78-2263]
Tetragonal, P4/nbm, a = 10.43849 Å,
c = 4.93875 Å
Tetragonal, I4 2d, a = 6.86707(2) Å,
c = 18.95455(1) Å
Na2Ca(VO3)4
K3Ca(VO3)5
[7, card. 52-0705] LiMg4(VO4)3
This work
[2]
Orthorhombic, Cmma, a = 25.953 Å,
b = 15.688 Å, c = 7.804 Å
[10]
[9]
Na6Mg3(VO4)4 I, d*
Rb2Ca(VO3)4
I, d*
NaMg4(VO4)3
NaCaVO4
[17]
Tetragonal, I42d, a = 6.89 Å, c = 19.292 Å
Pyrovanadates
Low-temperature form: orthorhombic, [7, card. 75-2310] [18]
Cmcm, a = 5.8726 Å,
b = 9.3028 Å, c = 7.1526 Å
High-temperature form: hexagonal,
[18]
[19]
P3 m1, a = 5.57 Å, c = 7.33 Å
NaCa4(VO4)3 I, d*
K2MgV2O7
Tetragonal, P42/mnm,
a = 8.38 Å, c = 11.376 Å
[15]
Na2Ca7(V2O7)4
K2CaV2O7
I, d*
I, d*
[5]
[6]
K4Ca(VO4)2
KCaVO4
I, d*
[6]
Low-temperature form: monoclinic sub-
cell**,
This work
a' = 5.6821(3) Å, b' = 9.9953(6) Å,
c' = 7.6883(4) Å, β = 91.92°
High-temperature form: hexagonal,
P63/mmc, a = 5.718(7) Å, c = 7.528(9) Å
Rb2CaV2O7
Cs2CaV2O7
Na3Ca2V3O11
Monoclinic, C2/c, a = 10.3419(1) Å,
b = 5.9656(1) Å, c = 13.8865(2) Å,
β = 104.953°
This work
This work
[16]
RbCaVO4
CsCaVO4
Monoclinic subcell**, a' = 5.729(7) Å,
b' = 10.10(1) Å, c' = 7.758(9) Å,
β = 91.92°
This work
This work
Monoclinic, C2/c, a = 10.59218(7) Å,
b = 6.09025(4) Å, c = 14.06766(6) Å,
β = 104.629°
I, d*
Monoclinic, C2/c, a = 23.791 Å,
(Na3Ca2(VO4)(V2O7)) b = 8.706 Å, c = 10.891 Å, β = 109.74°
* The structure was not determined; only powder XRD data were presented.
** The true unit cell was not identified; data are presented for a subcell derived from the high-temperature form: a'm ah, bm ah 3, cm' ch.
'

SUBSOLIDUS PHASE RELATIONS IN THE SYSTEMS
193
Table 2. Lattice parameters of Ca3 – xM2x(VO4)2 solid solu- Na, K, Rb, Cs) in the ternary phase diagrams under dis-
tions with x = 0.14
cussion. In the former case, there are three types of
phase relations: (1) The join contains two mixed vana-
M
a, Å
c, Å
Source
dates; the orthovanadates are stable over the whole
composition range (Fig. 2a, Li system). (2) The join
also contains two mixed vanadates, but the Mg-contain-
ing orthovanadates are only stable in the composition
range 50–100 mol % Mg3(VO4)2 (Fig. 2b, Na system).
–(Ca3(VO4)2)
10.809
10.815
10.830
10.881
10.922
38.028
38.075
37.860
37.924
37.912
[7, card 70-0790]
[20]
Na
K
[12]
Rb
Cs
This work
This work
(3) The vanadates M+3 VO4 (M+ = K, Rb, Cs) and
Mg3(VO4)2 do not coexist and decompose into other
compounds (Figs. 2c–2e). In the latter case, M3VO4 and
Sr3(VO4)2 always coexist, independent of the alkali
metal, but only in the Li3VO4–Sr3(VO4)2 system do the
constituent orthovanadates not interact, while the Na,
K, Rb, and Cs systems contain mixed orthovanadates
(Fig. 2 in [1]).
and structure of the reaction products. It is, therefore,
reasonable to analyze the effect of the difference in
ionic radius between M+ and M2+ [22].
As an illustration, consider the pseudobinary joins
M3VO4–Mg3(VO4)2 and M3VO4–Sr3(VO4)2 (M = Li,
Table 3. XRD data for ternary vanadates
hkl
I, %
d, Å
hkl
I, %
d, Å
I, %
d, Å
LiMg4(VO4)3
Cs2CaV2O7
CsCaVO4
101
004
3
6.4564
4.7387
002
111
8
3
6.8054
5.1124
7
3.870
3.136
10
13
103
112
105
202
211
204
213
116
220
215
008
18
7
4.6502
4.3219
3.3186
3.2283
3.0315
2.7803
2.7620
2.6478
2.4279
2.3862
2.3696
17
25
1
4.7045
4.6884
4.4343
3.9122
3.6707
3.4029
3.2365
3.2286
3.0544
3.0450
2.9795
34
29
12
100
10
9
3.112
3.081
3.060
2.972
2.161
2.138
1.958
1.723
202
111
31
8
112
112
2
54
100
17
12
1
53
29
59
100
86
75
2
113
004
8
204
113
13
311
020
2
7
310
021
311
022
114
206
301
310
224
17
1
2.3248
2.2727
2.1715
2.1606
4
11
11
2
2.9713
2.7852
2.7795
2.6981
1
2
303
312
217
5
2
4
2.1521
2.1165
2.0310
2
13
27
2.6342
2.5622
2.5542
221
400
115
INORGANIC MATERIALS Vol. 40 No. 2 2004

194
SLOBODIN, SURAT
Ca(VO3)2, Zh. Neorg. Khim., 1974, vol. 19, no. 3,
pp. 840–842.
ACKNOWLEDGMENTS
We are grateful to V.G. Zubkov and A.P. Tyutyunnik
for performing the x-ray diffraction characterization of 12. Schrandt, O. and Muller-Buschbaum, Hk., K+ auf einer
mit Ca2+ unterbesetzen Punktlage in Ca3(VO4)2: Ein
Beitrag uber KCa10V7O28, Z. Naturforsch., B: Chem.
Sci., 1996, vol. 51, pp. 473–476.
the new phases, supported by the International Center
for Diffraction Data, grant no. 93-09 (2002).
13. Fotiev, A.A., Volkov, V.L., and Kapustkin, V.K., Oksid-
nye vanadievye bronzy (Vanadium Oxide Bronzes),
Moscow: Nauka, 1978.
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