Crystal structures of new pyrovanadates A2MnV2O 7 (A = Rb, K)

Article abstract of DOI:10.1515/znb-2007-0701

The new compounds A₂MnV₂O₇ (A - K, Rb) with structures related to the melilite-type have been synthesized by a solid state reaction route. The crystal structures of K₂MnV ₂O₇, Rb₂MnV₂O₇ and KRbMnV₂O₇ have been determined using X-ray single crystal diffraction data. The compound K₂MnV₂O₇ crystallizes with a melilite-type structure with tetragonal unit cell parameters a = 8.609(3), c = 5.538(4) ?and space group P4?2₁m. The structures of Rb₂MnV₂O₇ and KRbMnV ₂O₇ are derived from the mehlite-type structure with space group P4₂mnm and unit cell parameters a = 8.577(6), c=11.809(6) ?, and a = 8.530(6), c = 11.466(5) ?, respectively. The three structures consist of [MnV₂O₇]^2- layers perpendicular to the c axis separated by A⁺ layers. The [MnV ₂O₇]^2- layers feature corner-sharing MnO ₄ tetrahedra and V₂O₇ pyrovanadate units, the linkage leading to rings of five tetrahedra. The doubling of the c parameter for Rb₂MnV₂O₇ or Rb₂MnV ₂O₇ is explained by the existence of a mirror plane perpendicular to the [001] direction between two [MnV₂O ₇]^2- layers. The A⁺ alkali cations occupy distorted square antiprisms of oxygen atoms in K₂MnV ₂O₇ and distorted square prisms of oxygen atoms in Rb ₂MnV₂O₇ and Rb₂MnV₂O ₇.

Full text of DOI:10.1515/znb-2007-0701

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Crystal Structures of New Pyrovanadates A2MnV O7 (A = Rb, K)
2
Hamdi Ben Yahia, Etienne Gaudin, and Jacques Darriet
Institut de Chimie de la Matie`re Condense´e de Bordeaux-CNRS, Universite´ Bordeaux 1,
87 avenue du docteur A. Schweitzer, 33608 Pessac Cedex, France
Reprint requests to Dr. E. Gaudin. Fax: (+33)540002761. E-mail: gaudin@icmcb-bordeaux.cnrs.fr
Z. Naturforsch. 2007, 62b, 873 – 880; received March 3, 2007
Dedicated to Dr. Bernard Chevalier on the occasion of his 60th birthday
The new compounds A2MnV2O7 (A = K, Rb) with structures related to the melilite-type have
been synthesized by a solid state reaction route. The crystal structures of K2MnV2O7, Rb2MnV2O7
and KRbMnV2O7 have been determined using X-ray single crystal diffraction data. The com-
pound K2MnV2O7 crystallizes with a melilite-type structure with tetragonal unit cell parameters a =
˚
¯
8.609(3), c = 5.538(4) A and space group P421m. The structures of Rb2MnV2O7 and KRbMnV2O7
are derived from the melilite-type structure with space group P42/mnm and unit cell parameters a =
˚
˚
8.577(6), c = 11.809(6) A, and a = 8.530(6), c = 11.466(5) A, respectively. The three structures consist
of [MnV2O7]2− layers perpendicular to the c axis separated by A+ layers. The [MnV2O7]2− layers
feature corner-sharing MnO4 tetrahedra and V2O7 pyrovanadate units, the linkage leading to rings
of five tetrahedra. The doubling of the c parameter for Rb2MnV2O7 or RbKMnV2O7 is explained
by the existence of a mirror plane perpendicular to the [001] direction between two [MnV2O7]2−
layers. The A+ alkali cations occupy distorted square antiprisms of oxygen atoms in K2MnV2O7 and
distorted square prisms of oxygen atoms in Rb2MnV2O7and RbKMnV2O7.
Key words: Vanadate, Melilite, Crystal Chemistry, Single Crystal X-Ray Diffraction, Oxides
Introduction
Most of the minerals of the melilite group are
thesized Na2ZnV2O7, the first vanadate crystallizing in
the melilite-type structure [7].
In the past only few vanadate compounds having
silicates with the general formula [8]A[4]B[4]Si2O7 structures closely related to the melilte-type have been
([N] = coordination number). Among the2m, one can prepared. The compounds (NH4)2VOV2O7 [8], K2VO-
find the akermanite Ca2MgSi2O7 [1], the gehlenite V2O7 [9], Rb2VOV2O7 [10], and KBaCuClV2O7 [11]
Ca2Al(AlSi)O7 [2], the okayamalite Ca2B(BSi)O7 [3] crystallize in the fresnoite-type structure which is very
the hardystonite Ca2ZnSi2O7 [4], the gugiaite Ca2- similar to the melilite-type, except that the V2O7 py-
BeSi2O7 [5], and the melilite (Ca,Na)2(Mg,Fe,Al)- rogroups all point in the same direction and the V4+
(AlSi)O7 [6]. Many melilite-type compounds have and Cu2+ coordination polyhedra are square pyramids
been synthezised with a wide range of chemical com- instead of the usual tetrahedra. They all have tetrago-
positions, the general formula becoming A2BC2X7 nal cell parameters with space group P4bm and with a
˚
where A is a large cation such as Ca, Sr, Ba, Na, K, ranging from 8.8581(13) to 8.9229(10) A and c rang-
˚
Y, lanthanides (La–Er), Pb, Bi; B is a small four co- ing from 5.215(5) to 5.5640(5) A. A different variation
ordinated cation such as Be, Mg, Mn, Fe, Co, Cu, Zn, of the melilite-type structure is observed for K2Mg-
Cd, Al, Ga, Si; C = Cr, Al, Ga, Si, Ge, B, V, and X = V2O7 [12], which crystallizes in space group P42/mnm
˚
O, F, S, N. The melilite-type structure was first deter- with cell parameters a = 8.38(2) and c = 11.36(2) A.
mined by Warren (1930) [1]. The structure has a tetrag- The main difference between the K2MgV2O7 and the
¯
onal symmetry, space group P421m, and consists of melilite-type structures is the doubling of the lattice
BC2O7 layers parallel to (001) made of corner-sharing constant c explained by the existence of a mirror
BO4 and CO4 tetrahedra. The cations A are lying be- plane perpendicular to the [001] direction between two
¯
tween these layers in distorted square antiprisms of MgV2O7 layers. This induces a change from P421m to
oxygen atoms. Recently, Tyutyunnik et al. have syn- P42/mnm symmetry. This structural variation is only
0932–0776 / 07 / 0700–0873 $ 06.00 © 2007 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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H. B. Yahia et al. · Crystal Structures of New Pyrovanadates A2MnV2O7 (A = Rb, K)
observed for phosphates and vanadates [12 – 15]. No- ture of yellow, orange and green crystals corresponding
¯
to KMnVO4, K2MnV2O7, and MnO, respectively, was ob-
tained.
Rb2MnV2O7 powder and crystals were prepared exactly
in the same way as K2MnV2O7. We used RbVO3 instead of
KVO3. RbVO3 was obtained by heating a 1 : 1 mixture of
Rb2CO3 and V2O5 at 700 ◦C for 10 h.
tably, P421m and P42/mnm are both subgroups of
space group P4/mbm. No compound of the melilite
group crystallizes in this space group.
Crystals of the new phase K2MnV2O7 were obtained
during fast melting of a vanadate powder with the com-
position KMnVO4. In a recent paper the crystal struc-
ture of KMnVO4 has been solved and shown to be an
oxygen-deficient perovskite [16].
KRbMnV2O7 was prepared by solid state reaction from
a stoichiometric mixture of KVO3, RbVO3 and MnO. The
mixture was put in a gold tube which was sealed under vac-
uum in a silica tube and then heated at 500 ◦C for 48 h with
intermittent grinding. This led to a mixture of KRbMnV2O7,
K1−xRbxVO3, and MnO. The heating of this mixture at
950 ◦C for 2 h and the slow cooling at the rate of 5 ◦C h−1 to
The aim of this work is to discuss the structural ef-
fects of the partial or complete substitution of K+ for
Rb+ on the A positions of the compounds of general
formula A2MnV2O7, in order to contribute to a bet-
ter insight into the crystal chemistry of the divanadates 700 C and at 10 C h−1 to r. t. enabled us to obtain several
◦
◦
crystals of KRbMnV2O7.
A2BV2O7.
X-Ray diffraction measurements
Experimental Section
Crystals of the title compounds suitable for single crys-
tal X-ray diffraction were selected on the basis of the size
and the sharpness of the diffraction spots. In the case of
Rb2MnV2O7, the quality of the single crystals was really
poor, and this explains the high internal R value (see Table 1).
A similar problem was encountered for KRbMnV2O7. The
data collections were carried out on an Enraf-Nonius Kappa
CCD diffractometer using MoKα radiation. Data processing
and all refinements were performed with the JANA2000 pro-
gram package [17]. A Gaussian-type absorption correction
was applied, and the shapes were determined with the video
microscope of the Kappa CCD. Details of data collection are
summarized in Table 1.
K2MnV2O7 was prepared by solid state reaction from
a stoichiometric mixture of KVO3 and MnO. KVO3 was
obtained by heating a 1 : 1 mixture of K2CO3 and V2O5
at 550 ◦C for 6 h. The mixture was put in a gold tube
which was sealed under vacuum in a silica tube and then
heated at 450 ◦C for 24 h and at 500 ◦C for 12 h. Af-
ter grinding a further heating of the mixture at 500 ◦C
for 18 h led to a mixture of three phases: K2MnV2O7,
KVO3, and MnO. Different treatments (time and temper-
ature) did not improve the results. As KVO3 is solu-
ble in water, the powder sample was washed to obtain
K2MnV2O7 as major phase (Fig. 1). Attempts to prepare
single crystals of K2MnV2O7 by melting the sample pow-
der were unsuccessful. Subsequently, a 1 : 1 mixture of
KVO3 and MnO was prepared. By fast heating of this
Structure refinement
◦
starting mixture at 950 C followed by slowly decreasing
The extinction conditions observed for K2MnV2O7 agree
the temperature at the rate of 5 ◦C h−1 to r. t., a mix-
¯
with the space group P421m. Most of the atomic posi-
tions were found by Direct Methods using SIR97 [18].
With anisotropic displacement parameters, the final resid-
ual factors converged to R(F) = 0.025 and wR(F2) = 0.055
for 34 refined parameters, 832 observed reflections and
difference-Fourier residues in the range between −0.33 and
+0.32 e A−3. The Flack parameter refined to 0.01(4).
˚
The lattice of Rb2MnV2O7 has tetragonal geometry. The
observed systematic extinctions agree with space group
P42/mnm. The structure was solved by Direct Methods using
SHELXS-97 [19], which revealed the heavy atom positions.
Several difference-Fourier syntheses allowed us to localize
the oxygen atom positions. This refinement led to the resid-
ual factors R(F) = 0.042 and wR(F2) = 0.088 for 36 refined
parameters, 455 observed reflections and difference−-F3ourier
˚
residues in the range between −1.06 and +1.43 e A
.
Fig. 1. Final observed, calculated and difference plots for the
XRPD profile refinement of K2MnV2O7 (peaks marked with
an asterisk indicate impurities).
The extinction conditions observed for RbKMnV2O7
agree with the space group P42/mnm already used for the
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H. B. Yahia et al. · Crystal Structures of New Pyrovanadates A2MnV2O7 (A = Rb, K)
875
Table 1. Crystallographic data
and structure refinement for
K2MnV2O7, Rb2MnV2O7, and
RbKMnV2O7.
Formula
K2MnV2O7
orange block
347
Rb2MnV2O7
yellow block
439.7
tetragonal
P42/mnm
8.577(6)
11.809(6)
868.7(10)
4
RbKMnV2O7
yellow block
393.4
Crystals
MW, g mol−1
Crystal system
space group
¯
P421m
P42/mnm
8.530(6)
11.466(5)
834.3(9)
4
˚
a = b, A
8.609(3)
5.538(4)
410.4(4)
2
˚
c, A
3
˚
V, A
Z
Density calc, g cm−3
Temperature, K
Diffractometer
2.81
3.36
3.13
293(1)
Enraf-Nonius KappaCCD
Oriented graphite
MoKα ; 0.71069
CCD scan
−12 < h < 11
−12 < k < 12
−17 < l < 17
2.94 / 31.95
14.7
Monochromator
Radiation; λ, A
Scan mode
˚
h k l range
−13 < h < 13
−12 < k < 13
−8 < l < 8
3.35 / 34.99
4.8
−12 < h < 12
−12 < k < 12
−17 < l < 16
4.78 / 2.02
θ
min / θmax, deg
Linear absorption coeff. (mm−1
Absorption correction
Tmin / Tmax
)
10.0
Gaussian
0.499 / 0.816
14505
0.176
849
0.477 / 0.716
8856
0.067
976
0.379 / 0.837
12515
0.140
819
540
No. of reflections
Rint
No. of independent reflections
Reflections used [I ≥ 3σ(I)]
Refinement
832
455
F2
F(000), e
330
34
0.025 / 0.055
0.01(4)
1.09
804
36
0.042 / 0.088
–
732
37
0.044/0.100
–
No. of refined parameters
R factors R(F) / wR(F2)
x (Flack)
g. o. f.
Weighting scheme
1.23
1.52
w = 1/(σ2(I)+0.0009I2)
−1.06 / +1.43
−3
˚
Diff. Fourier residues, e A
−0.33 / +0.32
−0.76 / +0.85
2
˚
Table 2. Atom positions and iso-
topic displacement parameters for
K2MnV2O7, Rb2MnV2O7, and
KRbMnV2O7.
Atoms
K2MnV2O7
K
Mn
V
O1
O2
O3
Occupancy
Site
x
y
z
Ueq (A )
1
4e
2a
4e
8f
2c
4e
0.83871(5)
0
0.14046(4)
0.0876(2)
0
1/2+x
0
0.35954(4)
0.18543(18)
1/2
0.50737(14)
0
0.05876(8)
0.1802(3)
0.1610(6)
0.7616(4)
0.02491(13)
0.01567(12)
0.01252(8)
0.0215(4)
0.0188(6)
0.0283(5)
1
1
1
1
1
0.1452(2)
0.3548(2)
Rb2MnV2O7
Rb1
Rb2
Mn
V
O1
O2
O3
1
1
1
1
1
1
1
4g
4 f
4d
8j
16k
4e
8j
0.32119(10)
0.85079(10)
1/2
0.63961(11)
0.5881(5)
1/2
x
1/2
1/2
1/4
0.28601(15)
0.3395(5)
0.3380(4)
0.1472(7)
0.0375(3)
0.0310(3)
0.0245(4)
0.0224(3)
0.0299(16)
0.0268(13)
0.034(2)
−x
0
−x
0.1826(5)
1/2
0.6411
−x
KRbMnV2O7
K1
Rb1
K2
Rb2
Mn
V
O1
O2
O3
0.572(7)
0.428
0.428
0.572
1
1
1
1
1
4g
4g
4f
4 f
4d
8j
16k
4e
8j
0.31980(13)
0.3198
0.85335(11)
0.8534
x
x
1/2
1/2
1/2
1/2
0.0342(5)
0.0342
0.0271(4)
0.0271
0.0161(3)
0.0136(2)
0.0201(9)
0.0191(15)
0.0266(12)
−x
−x
1/2
0
1/4
0.64028(9)
0.5888(4)
1/2
−x
0.28842(10)
0.3446(3)
0.3432(6)
0.1458(4)
0.1815(3)
1/2
0.6410(4)
−x
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H. B. Yahia et al. · Crystal Structures of New Pyrovanadates A2MnV2O7 (A = Rb, K)
Table 3. Final displacement
parameters for K2MnV2O7,
Rb2MnV2O7, and KRbMnV2O7.
Atoms U11
K2MnV2O7
U22
U33
U12
U13
U23
K
Mn
V
0.0267(2)
U11
0.01199(15) U11
0.01164(11) U11
0.0213(3)
0.0230(3)
−0.0081(2) 0.00382(16) 0.00382(16)
0
0
0
0.01428(19) 0.00102(14) 0.00057(11) −0.00057(11)
O1
O2
O3
0.0236(8)
0.0184(8)
0.0343(8)
0.0128(6)
U11
U11
0.0282(9)
0.0195(15)
0.0161(9)
−0.0043(6) −0.0038(6)
−0.0006(6)
0
−0.0025(7)
0.0080(11)
0.0080(13)
0
0.0025(7)
Rb2MnV2O7
Rb1
Rb2
Mn
V
O1
O2
O3
0.0409(5)
U11
U11
U11
U11
0.020(2)
U11
U11
0.0307(7)
0.0302(7)
0.0306(10)
0.0244(7)
0.038(4)
0.027(3)
0.021(5)
−0.0150(6)
0.0057(5)
0
0
0
0
0
0
0
0.0314(4)
0.0215(6)
0.0214(5)
0.032(2)
0.027(2)
0.041(3)
−0.0003(5) 0.0007(4)
−0.0007(4)
0.0041(19)
0
−0.002764
−0.006(3)
0.0060(19)
0.009(4)
0.0003(19)
0
0.002764
KRbMnV2O7
K1/Rb1 0.0374(7)
K2/Rb2 0.0269(5)
U11
U11
U11
U11
0.0279(9)
0.0275(8)
0.0242(7)
0.0172(5)
−0.0175(7)
0.0050(5)
0
0.0009(4)
−0.004(2)
0.0027(14)
0.003(3)
0
0
0
0
0
0
Mn
V
O1
O2
O3
0.0121(4)
0.0118(3)
0.0217(17)
0.0000(3)
0.0000(3)
0.0115(17) 0.027(3)
−0.0037(15) 0.0009(15)
0.0173(16) U11
0.0297(19) U11
0.0226(19)
0.021(3)
0
0
0.002149
−0.002149
Fig. 2. Projection of the crystal
structure of K2MnV2O7 (a) onto the
ac plane and (b) onto the ab plane.
refinement of the homologous compound Rb2MnV2O7. The atomic positions and anisotropic displacement parameters
atomic positions of the latter were used as a starting model (ADPs) of the different phases are given in Tables 2 and 3,
for the refinement. The rubidium atom in the 4 f position respectively.
was replaced by a potassium atom to get the correct formula
Further details may be obtained from: Fachinformation-
szentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen
(Germany), by quoting the Registry No’s. CSD–417890
(Rb2MnV2O7), CSD–417891 (KRbMnV2O7), and CSD–
417892 (K2MnV2O7).
RbKMnV2O7. At this stage the refinement led to residual fac-
tors of R(F) = 0.194 and wR(F2) = 0.430 with large negative
2
˚
displacement parameters for Rb (0.067 A ) and K, respec-
tively. Attempts to solve the structure with fully ordered sites
failed; therefore the structure was refined with introduction
of K/Rb disorder on the 4g and 4f positions. With anisotropic
displacement parameters, the final residual factors converged
to R(F) = 0.044 and wR(F2) = 0.100 for 37 refined parame-
ters, 540 observed reflections and difference-Fourier residues
Results and Discussion
The crystal structure of K2MnV2O7 is isotypic to
the melilite-type structure consisting of alternating
MnV2O7 and K layers (Fig. 2a). The MnV2O7 lay-
in the range between −0.76 and +0.85 e A−3. The refined
˚
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