Crystal structure of the pyrovanadate K4V2O 7

Article abstract of DOI:10.1134/S0036023606050159

The crystal structure of anhydrous K₄V₂O₇ (I) is determined by powder X-ray diffraction. The compound crystallizes in the monoclinic system (a = 10.222(1) ?, b = 6.2309(8) ?, c = 7.282(1) ?, β = 101.31(1)°, space group C2/m, Z = 2). The structure contains layers of isolated V₂O₇ pyrovanadate groups separated by layers of potassium cations. The hydration and dehydration of I are studied by thermal analysis and high-temperature X-ray diffraction. The dehydration is accompanied by decomposition of the starting crystal hydrate to give intermediate compounds. Anhydrous compound I undergoes a reversible phase transition at 740°C. The high-temperature phase is assumed to have a hexagonal unit cell (a = 6.169(4) ?, c = 15.72(1) ?, Z = 2). Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0036023606050159

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2006, Vol. 51, No. 5, pp. 779–784. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.V. Chernaya, R.V. Shpanchenko, Yu.A. Velikodnyi, M.L. Kovba, E.V. Antipov, 2006, published in Zhurnal Neorganicheskoi Khimii, 2006, Vol. 51, No. 5,
pp. 845–850.
PHYSICAL METHODS
OF INVESTIGATION
Crystal Structure of the Pyrovanadate K4V2O7
V. V. Chernaya, R. V. Shpanchenko, Yu. A. Velikodnyi,
M. L. Kovba, and E. V. Antipov
Moscow State University, Vorob’evy gory, Moscow, 119899 Russia
Abstract—The crystal structure of anhydrous K4V2O7 (I) is determined by powder X-ray diffraction. The com-
pound crystallizes in the monoclinic system (a = 10.222(1) Å, b = 6.2309(8) Å, c = 7.282(1) Å, β = 101.31(1)°,
space group C2/m, Z = 2). The structure contains layers of isolated V2O7 pyrovanadate groups separated by lay-
ers of potassium cations. The hydration and dehydration of I are studied by thermal analysis and high-temper-
ature X-ray diffraction. The dehydration is accompanied by decomposition of the starting crystal hydrate to give
intermediate compounds. Anhydrous compound I undergoes a reversible phase transition at 740°C. The high-
temperature phase is assumed to have a hexagonal unit cell (a = 6.169(4) Å, c = 15.72(1) Å, Z = 2).
DOI: 10.1134/S0036023606050159
The results of a study of phase relationships in the
K2O–V2O5 binary system are surveyed in [1]. However,
the existence of some double oxides in this system still
has not been proved. Six compounds have been struc-
turally characterized: KV3O8 [2], KVO3 [3], K3V5O14
[4], K5V3O10 [5], K3VO4 [6], and K2V8O21 [7]. For four
double oxides, K4V2O7 [8], K2V6O16 [9], K2V10O26
[10], and K4V10O27 [11], only X-ray diffraction patterns
have been reported, without determination of the struc-
ture and the unit cell parameters. Compound I has been
prepared by several authors; however, the X-ray dif-
fraction patterns given in two publications ([8] and
[12]) do not coincide. In another study [13], a phase
transition at 740°C was detected for this compound by
DTA but no structural data concerning the high-temper-
ature modification were reported. TheA4V2O7 pyrovan-
adates were found for all alkali metals [14–16] and for
Ag [17]; however, the structures were determined only
for Na and Ag derivatives.
EXPERIMENTAL
Anhydrous I was prepared in two steps. A stoichio-
metric mixture of K2CO3 andV2O5 was heated in a plat-
inum crucible in air at 800°C for 10 h. The X-ray dif-
fraction pattern of the sample recorded in air contained
lines from two phases, K4V2O7 · 2H2O and I. The final
annealing of the mixture was performed in a dynamic
vacuum (10–3 atm) in a quartz tube at 700°C until gas
evolution ceased. The tube was placed in a dry box and
unsealed. For X-ray powder diffraction analysis, a sam-
ple of I was sealed in a quartz capillary with an inner
diameter of 0.3 mm to avoid contact with air moisture.
The recording was performed on a STADI/P diffracto-
meter (CuKα1 radiation, linear PSD, capillary). The
X-ray diffraction pattern of the sample contained addi-
tional reflections (Imax < 5%) corresponding to quartz
and K5V3O10. This X-ray diffraction pattern was used
subsequently to solve and refine the structure of I. The
initial structural model was found by means of the CSD
program package [21]. At the first stage, the integrated
intensities found by profile analysis were used to calcu-
late the Patterson map and to determine heavy atom
positions. The oxygen atoms were located from a series
of differential Fourier syntheses. The final structure
refinement was carried out using the GSAS program
[22] for I and two impurity phases. For K5V3O10 and
quartz, only the unit cell parameters and the profile
parameters were refined.
The structures of pyrovanadates and pyrophos-
phates contain isolated M2O7 groups (M =V, P) consist-
ing of two corner-sharing tetrahedra. All the known
alkali metal pyrovanadates and pyrophosphates (except
for lithium compounds) form hydrates with different
contents of water molecules; only pyrovanadates of
large cations (K, Rb, Cs) form dihydrates A4V2O7 ·
2H2O [18–20]. As noted above, the crystal structures of
the dehydrated phases with larger alkali-metal cations
have not been determined as yet.
In this study, anhydrous K4V2O7 (I) was synthesized
and its crystal structure was determined. The structural
transformations taking place during hydration and
dehydration of I were studied.
The high-temperature powder X-ray diffraction was
done using a Guinier–Lenne camera (CuKα radiation)
in discrete and continuous modes in air.
779

780
CHERNAYA et al.
Table 1. Crystal parameters for K4V2O7 and X-ray experiment details
Formula
Space group
a, Å
β-K4V2O7
C2/m [12]
10.2147(5)
6.2280(3)
b, Å
c, Å
7.2758 (4)
101.2875(7)
2
β, deg
Z
V, Å3
453.91(4)
ρ
calcd, g/cm–3
2.709
Diffractometer
Temperature, °C
Radiation, wavelength, Å
Detector
STOE STADI/P
20
CuKα1, 1.5406
Linear PSD
10 ≤ 2θ ≤ 100, 0.01
0.026, 0.019, 0.87
2θ angle, step (2θ)
wRp, Rp, χ2
Impurities
SiO2
1.5
K5V3O10
4.0
Content, wt %
Space group
P41212 [92]
Fd3 m [227]
Thermal analysis was carried out on a Perkin-Elmer group was chosen. The thermal displacement parame-
ters for atoms Uiso were fixed to be 0.01 Å3. Refinement
details and crystal data for the structure of I are listed in
Table 1, the atom coordinates and the key interatomic
distances are in Tables 2 and 3, respectively. The exper-
imental, calculated, and difference X-ray diffraction
patterns are shown in Fig. 1.
derivatograph.
RESULTS AND DISCUSSION
The X-ray diffraction pattern of anhydrous I was
indexed using the TREOR software [23] in the mono-
clinic system with the unit cell parameters a =
10.222(1) Å, b = 6.2309(8) Å, c = 7.282(1) Å, β =
101.31(1)°, Z = 2. Analysis of systematic extinctions
unambiguously revealed the C-centered cell without
Two projections of the structure of I are shown in
Fig. 2. Pairs of VO4 tetrahedra share a corner to form
isolated V2O7 groups. These groups are packed in lay-
any additional extinctions. The structure solution was ers in the (011) plane and are separated by layers
performed for three possible space groups (C2, Cm, and
C2/m). After the final refinement, the last-mentioned
formed by potassium atoms (Fig. 2). The distance
between the V atom and the O(1) atom, which is com-
Table 2. Atomic coordinates for K4V2O7*
Table 3. Selected interatomic distances (Å) in the structure
of β-K4V2O7
Atom Position
x
y
z
K(1)–O(2) 2.578(5) × 4
–O(3) 3.089(6) × 2
K(2)–O(3) 2.678(5) × 2
–O(2) 3.008(5) × 4
–O(1) 3.1140(3) × 2
V–O(3) 1.609(7)
K(3)–O(2) 2.850(5) × 2
–O(1) 3.008(3)
K(1)
2c
2a
4i
0
0
0
0
0
0
0
1/2
K(2)
K(3)
V
0
–O(2) 3.063(6) × 2
–O(3) 3.114(6)
0.3250(3)
0.6328(2)
1/2
0.2878(4)
0.1986(3)
0
4i
–O(2) 3.200(5) × 2
–O(3) 3.289(2) × 2
O(1)
O(2
O(3)
2b
8j
0.1135(4) 0.2844(8) 0.3265(7)
0.7781(6) 0.1443(8)
4i
0
–O(2) 1.667(5) × 2
–O(1) 1.778(2)
* The thermal displacement parameters U for all atoms were
taken to be 0.01 Å .
iso
2
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 5 2006

CRYSTAL STRUCTURE OF THE PYROVANADATE K4V2O7
781
Amount
8000
2000
6000
4000
2000
1000
50
60
70
20
30
40
50
60
70
80
90
2θ, deg
Fig. 1. Experimental, calculated, and difference X-ray diffraction patterns for K V O . The positions of peaks: (upper) K V O ,
4
2
7
4 2 7
(middle) SiO , (lower) K V O . The inset shows a magnification of the 50 < 2θ < 70 range.
2
5 3 10
mon for two tetrahedra, is the longest (1.778(2) Å). The a Guinier–Lenne chamber. After heating from room
other two distances V–O(2, 3), are much shorter temperature to 350°C, the X-ray diffraction pattern
(1.609(7) and 1.667(5) Å). As in the vast majority of exhibits only reflections that refer to KVO3 (Fig. 4a)
pyrovanadate and pyrophosphate structures, the V2O7
groups in the structure of I are centrosymmetrical.
(the weak peaks corresponding to KOH appear only at
high temperatures).
At 350°C, KVO3 reacts with KOH to give an inter-
mediate compound K5V3O10, which is stable in the
temperature range of 350–440°C. Further increase in
the temperature induces the reaction between K5V3O10
and the remaining alkali to yield I. These processes can
be represented as follows:
The potassium cations have different coordination
(Fig. 3). The K(1) atom is bound to the O atoms of six
anions, which are situated in the vertices of a distorted
octaherdron in which the four equatorial K(1)–O(2)
distances are 0.511 Å shorter than the two apical
K(1)−O(3) distances. The K(2) coordination polyhe-
dron is a hexagonal bipyramid, four K(2)–O(2) and two
K(2)–O(1) distances, which form the hexagonal base of
the bipyramid, being nearly equal (3.00(8) and
3.1140(3) Å, respectively). However, the K(2)–O(3)
distances are much shorter (2.678(5) Å). The coordina-
tion polyhedron of K(3) is a 10-vertex polyhedron
(K(3)–O, 2.850(5)–3.289(2) Å).
3äVé3 + 2KOH = ä5V3O10 + H2O,
(2)
K5V3O10 + 2KOH + KVO3 = 2K4V2O7 + H2O. (3)
Dehydration of II was studied using a freshly pre-
pared sample containing II and a minor amount of
KVO3, having formed by reaction (1) (Fig. 4b). On
gradual heating, dehydration proceeded simultaneously
with decomposition of the oxide. No other hydrates
containing less than two water molecules were
detected.
Compound I reacts with air moisture to give the
hydrate K4V2O7 · 2H2O (II). After several days in air,
dihydrate II loses water to give the metavanadate KVO3
and KOH:
K4V2O7 · 2H2O = 2KVO3 + 2KOH + H2O.
(1)
The dehydration scheme we propose was confirmed
by the results of thermogravimetric analysis. The
The completely decomposed sample containing no weight is lost in two stages. The weight lost at the first
traces of pyrovanadate I or dihydrate II was studied in stage (below 350°C) is twice as great as that lost at the
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 5 2006

782
CHERNAYA et al.
a
c
b
a
c
Fig. 2. Three projections of the structure of β-K V O . The circles correspond to the K atoms.
4
2
7
O(3)
O(3)
O(3)
O(2)
O(3)
O(2)
O(2)
O(2)
O(2)
O(2)
O(2)
O(2)
O(1)
O(1)
O(3)
O(1)
O(2)
O(2)
O(2)
K(3)
O(2)
O(3)
K(1)
O(2)
O(3)
K(2)
O(2)
Fig. 3. Coordination of potassium atoms in β-K V O .
4
2 7
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 5 2006

CRYSTAL STRUCTURE OF THE PYROVANADATE K4V2O7
783
440∞
250∞
(a)
100∞
20∞
460∞
350∞
250∞
(b)
(c)
800∞
600∞
Fig. 4. X-ray diffraction patterns obtained in the high-temperature Guinier–Lenne camera: (a) formation of K V O from a mixture
4
2 7
of KVO and KOH (one can see the formation of the intermediate oxide, K V O ), (b) dehydration of K V O · H O, and (c) phase
3
5
3
10
4
2
7
2
transition in K V O (600°C, low-temperature β phase; 800°C, high-temperature α phase).
4
2 7
second stage (350–450°C) (Fig. 5). The initial decrease in the structure of the anhydrous compound. Low C.N.s
in the weight corresponds to reactions (1) and (2) and are unusual for large cations; upon hydration, they
results in the loss of two water molecules, whereas only change into seven-vertex (pentagonal bipyramid) and
one water molecule is lost at the second stage (3). This nine-vertex polyhedra, respectively. The coordination
result is consistent with the above scheme of the pro- environment of the third potassium atom is also supple-
cesses.
mented by one atom (C. N. = 11, a distorted three-cap
cube). Similar structural changes are induced by inser-
tion of water molecules into the Ba2ZrO4 oxide struc-
ture [25]. The barium coordination number changes,
and the ZrO6 octahedra remain unchanged. The b
parameter of the unit cell in the crystal of II approxi-
mately doubles upon hydration, and the structure
becomes orthorhombic. Note that the resulting struc-
ture is unstable and spontaneously decomposes to give
the denser K5V3O10 structure. This accounts for the
presence of an impurity in the studied sample, which
apparently contained traces of moisture.
At 740°C, compound I undergoes a reversible phase
transition, which is in line with the previous data [13]
(Fig. 4c). The X-ray diffraction pattern of the high-tem-
perature phase of I was indexed in the hexagonal sys-
tem with the unit cell parameters a = 6.169(4) Å, c =
15.792(8) Å, Z = 2 (Table 4). Note that the indexing
results cannot be interpreted as unambiguous, due to
the low quality of the X-ray diffraction pattern. How-
ever, it is quite probable that the phase transition results
in a structure resembling the high-temperature Tl4V2O7
phase [24] having a trigonal cell. To solve these prob-
lems, an additional high-temperature X-ray diffraction
experiment is required.
Due to the lack of structural data for the high-tem-
perature modification α-K4V2O7 , it is impossible to
suggest a mechanism for this transition. However, it
should be noted that phase transitions of this type near
Compound I is the first pyrovanadate containing a
large alkaline-earth metal cation for which the crystal
structure has been determined. One reason why this
class of compound has been poorly studied is their very
high hygroscopicity, resulting in substantial experi-
mental difficulties. Unlike the anhydrous compound I,
the structure of dihydrate II has long been known (a =
11.2156(5) Å, b = 12.3711(5) Å, c = 8.0751(4) Å, space
group Cmcm, Z = 4) [18]. The insertion of water mole-
cules into the structure of anhydrous pyrovanadate I
results in slight shifts of the pyrovanadate groups rela-
tive to one another (Fig. 6). This is accompanied by a
substantial redistribution of potassium atoms in the
cell. The coordination of the vanadium atoms remains
unchanged, but the coordination of the potassium
atoms changes. First of all, this is true for the K(1) and
K(2) atoms, which have C.N.s of 6 and 8, respectively,
∆m, %
150
350
450
T, °C
Fig. 5. TG curve for K V O · H O (two weight losses cor-
4
2
7
2
respond to different reactions (1 + 2 and 3).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 51 No. 5 2006

784
CHERNAYA et al.
2H O
2
K V O
K V O · 2H O
4 2 7 2
4
2
7
Fig. 6. Structural changes upon the insertion of water molecules into K V O . The water oxygen atoms are shown by large white
4
2 7
circles.
10. M. Gasior, B. Grzybowska, J. Haber, et al., J. Catal. 58,
the melting points have also been found for rubidium
and cesium pyrovanadates.
15 (1979).
11. V. V. Illarionov, R. P. Ozerov, and E. V. Kil’disheva, Zh.
Neorg. Khim. 2 (4), 883 (1957).
ACKNOWLEDGMENTS
12. PDF2 (International Centre for Diffraction Data), Card
No 52-1580.
The authors are grateful to I. V. Arkhangel’sky for
conducting thermal analysis.
This work was supported by the Russian Foundation
for Basic Research (project no. 04-03-32787) and the
ICDD (GiA 91-05APC).
13. F. Holtzberg, A. Reisman, M. Berry, and M. Berkenblit,
J. Am. Chem. Soc. 78 (8), 1536 (1956).
14. K. Kato and E. Takayama-Muromachi, Acta Crystal-
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15. M.Ya. Khodos, V. N. Krasil’nikov, and A. A. Fotiev, Izv.
Akad. Nauk SSSR, Neorg. Mater. 18, 1846 (1982).
16. J. Perraud, Rev. Chim. Miner. 11, 302 (1974).
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