Crystal structure and properties of the new complex vanadium oxide K 2 SrV 3 O 9

Transcription

1 Materials Research Bulletin 40 (2005) Crystal structure and properties of the new complex vanadium oxide K 2 SrV 3 O 9 Alexander A. Tsirlin a, Victoria V. Chernaya a, Roman V. Shpanchenko a, *, Evgeny V. Antipov a, Joke Hadermann b a Department of Chemistry, Moscow State University, Moscow, Russia b EMAT University of Antwerp (RUCA), Groenenborgerlaan 171, 2020 Antwerp, Belgium Received 16 September 2004; received in revised form 24 December 2004; accepted 2 February 2005 Abstract The new complex vanadium oxide K 2 SrV 3 O 9 has been synthesized and investigated by means of X-ray powder diffraction (XPD), electron microscopy and magnetic susceptibility measurements. The oxide has an orthorhombic unit cell with lattice parameters a = (2) Å, b = (1) Å, c = (3) Å, space group Pnma and Z = 4. The crystal structure of K 2 SrV 3 O 9 has been refined by Rietveld method using X-ray powder diffraction data. The structure contains infinite chains built by V 4+ O 5 square pyramids linked to each other via VO 4 tetrahedra. The chains form layers and potassium and strontium cations orderly occupy structural interstices between these layers. Electron diffraction as well as high resolution electron microscopy confirmed the structure solution. Magnetic susceptibility measurements revealed an antiferromagnetic interaction with J of the order of 100 K inside the chains and no long-range magnetic order above 2 K. The origin of the magnetic exchange is likely a result of superexchange interaction through the two VO 4 tetrahedra linking the polyhedra with the magnetic V 4+ cations. # 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; D. Crystal structure; D. Magnetic properties 1. Introduction Complex oxides containing vanadium in the low oxidation state (d 1,d 2 ) are known as convenient objects for investigation in a field of magnetism. Many V-based oxides having magnetic V 4+ cations often * Corresponding author. Tel.: ; fax: address: shpanchenko@icr.chem.msu.ru (R.V. Shpanchenko) /$ see front matter # 2005 Elsevier Ltd. All rights reserved. doi: /j.materresbull

2 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) contain chains, pipes or layers of magnetic cation polyhedra in their structures and form one- or twodimensional magnetic systems [1]. A series of V 4+ -containing oxides with the [VO(XO 4 ) 2 ] 4 group (X = V 5+,P 5+,orAs 5+ ) has been described during the last decade. It was shown that such oxides usually have a low-dimensional structure but the specific type of structure organization depends on X and the metallic cations that compensate the charge of the [VO(XO 4 ) 2 ] 4 group [2]. For example, M 2 V 3 O 9 (M = Sr, Ba, Pb) [3 6] structures are formed by chains of V 4+ O 6 -octahedra. These octahedra in Sr- and Pb-containing compounds are connected by common corners and VO 4 tetrahedra link the chains into layers. In the Ba 2 V 3 O 9 structure, the octahedra are edge-sharing and the chains are isolated. The exchange of one M 2+ cation for two M 1+ cations in the M 2 2+ VO(XO 4 ) 2 structure may result in the formation of new structures. So, we have recently reported a new complex vanadium oxide Na 2 Sr- VO(VO 4 ) 2 [7] which has a chain-like structure similar to that of Ba 2 VO(PO 4 ) 2 [8]. The temperature dependence of the susceptibility for Na 2 SrV 3 O 9 revealed a low dimensional magnetic behavior with a sizeable strength of the magnetic intra-chain exchange J of the order of 80 K, which is very likely due to superexchange through the two V 5+ O 4 tetrahedra linking the magnetic V 4+ cations. The Na 2 MVO(PO 4 ) 2 (M = Ca, Sr) compounds [9] have three-dimensional structures where the A-cations are situated in large cross-like tunnels. The latter result was explained by the small size of the tetrahedral PO 4 groups which does not allow the accommodation of Na and Sr (or Ca) cations in a chain-like structure. Both compounds are paramagnets. The aim of the present study is the synthesis and investigation of the K 2 SrVO(VO 4 ) 2 vanadate to understand the influence of the A-cation size on the structure and magnetic properties of the M 2 SrVO(XO 4 ) 2 (M = Na, K; X = V, P) compounds. 2. Experimental The bulk sample of K 2 SrV 3 O 9 was obtained by heating a stoichiometric mixture of K 4 V 2 O 7,Sr 2 V 2 O 7, V 2 O 3 and V 2 O 5 in an evacuated and sealed silica tube for 36 h at 650 8C. Sr 2 V 2 O 7 was synthesized from SrCO 3 and V 2 O 5 by annealing at 750 8C for 36 h in air. K 4 V 2 O 7 was obtained from K 2 CO 3 and V 2 O 5 by heating under vacuum at 650 8C for 24 h. All subsequent operations with highly hygroscopic K 4 V 2 O 7 were carried out in a glove box under an argon atmosphere. X-ray powder diffraction (XPD) data for the structure solution and refinement were collected on the STOE diffractometer (transmission mode, Cu Ka1-radiation, Ge-monochromator, linear-psd). The GSAS program package [10] was used for the full profile structure refinement. The XPD pattern of K 2 SrV 3 O 9 was indexed using the TREOR90 program [11]. The compound has an orthorhombic unit cell with the lattice parameters a = (2) Å, b = (1) Å, c = (3) Å, Z = 4. The sample contained minor admixtures of Sr 3 (VO 4 ) 2 and Sr 3 V 2 O 7 and a change of synthetic conditions did not result in a pure product. We failed to obtain single crystals of K 2 SrV 3 O 9 since a melt was accompanied by a decomposition of the compound. Therefore, the structure solution was done using powder diffraction data. Transmission electron microscopy was performed with a Philips CM20 microscope (ED and EDX) with a LINK-2000 attachment and with a JEOL 4000EX microscope (HREM and ED). The image simulations were made using the MacTempas software. Magnetic susceptibility measurements were performed on a Quantum Design MPMS SQUID magnetometer in the range between 2 and 400 K at fields of 0.1 and 1T.

3 802 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Results 3.1. Crystal structure of K 2 SrV 3 O 9 The lattice parameters for K 2 SrV 3 O 9 and Na 2 SrV 3 O 9 (a = 5.416(1) Å, b = (3) Å, c = Å, b = 97.03(3) 0 ) are rather close, therefore, we suggested that these compounds have similar structures. Indexing of the ED patterns (see below) resulted in two possible space groups Pn2 1 a and Pnma whereas the monoclinic P2 1 /c space group was found for Na 2 SrV 3 O 9. The multiplicity of the general positions in both P2 1 /c and Pn2 1 a is 4, therefore, one may use the atomic coordinates of Na 2 SrV 3 O 9 as an initial model in case of the Pn2 1 a space group. The multiplicity of the general position in Pnma is 8, therefore, one should move some the atoms to special positions to keep Z = 4. The special position 4c corresponding to y = 0.25 or y = 0.75 (mirror plane) was chosen for the Sr, K and V atoms and for five of the nine oxygen atoms in the Na 2 SrV 3 O 9 structure. The other oxygen atoms were situated in the general (8d) positions of the Pnma space group. The Rietveld refinement was carried out for both Pn2 1 a and Pnma space groups. The best fitting of the XRD profile was found for the structural model based on the Pnma space group. Moreover, Pnma is a supergroup of P2 1 /c showing the relation between K 2 SrV 3 O 9 and Na 2 SrV 3 O 9. Finally, the Rietveld refinement was carried out for the three phase mixture including Sr 3 (VO 4 ) 2 (1.8 wt.%) and Sr 3 V 2 O 7 (0.9 wt.%). The atomic coordinates for the admixture phases were taken from [12,13], respectively, and were not refined. The atomic displacement parameters of all oxygen atoms in the K 2 SrV 3 O 9 structure were constrained. The experimental and crystallographic parameters for K 2 SrV 3 O 9 are presented in Table 1. The atomic coordinates and displacement parameters are listed in Table 2. The experimental, calculated and difference X-ray patterns for K 2 SrV 3 O 9 are shown in Fig. 1. The projection of the K 2 SrV 3 O 9 crystal structure along the b-axis is shown in Fig. 2 (up). It contains infinite chains running along the b-axis. The chains are not connected to each other and form corrugated layers in the b c plane. Every chain is formed by VO 5 square pyramids linked via two VO 4 tetrahedra (Fig. 2 bottom). The square pyramids in the chain are isolated from each other and the apical V(1) O(1) vanadyl bonds in all pyramids have the same orientation within the chain while in the neighboring chains in the same layer pyramids are oriented in the opposite direction. Strontium and potassium cations are situated in the interstices between layers in an ordered manner. All V(1)O 5 square pyramids have one short apical bond (1.643(8) Å) that corresponds to the typical distance for a vanadyl bond in the V 4+ polyhedra. The V(2)O 4 and V(3)O 4 tetrahedra are not equivalent. The first one is almost regular with four close V(2) O distances of Å. The V(3)O 4 tetrahedron is noticeably distorted and has two pairs of discriminating bond lengths of 1.590(9) Å, 1.567(8) Å and 1.799(6) Å. Such a distortion may be explained by a different arrangement of the V(2)O 4 and V(3)O 4 tetrahedra with respect to K + and Sr 2+. Thus, each V(2) atom is surrounded by four potassium and four strontium atoms within the coordination sphere of 4 Å radius. The V(3) atom has six potassium and two strontium atoms within the same coordination sphere. K(1) and K(2) atoms have a different coordination arrangement (Fig. 3). The K(1) atom is situated in the center of a distorted single-capped octahedron. The coordination polyhedron for the K(2) atom is a distorted parallelepiped. The Sr atoms are situated in 10-vertex polyhedra. The main interatomic distances in the K 2 SrV 3 O 9 structure are listed in Table 3. The calculated bond valence sums (BVS) for the cations are presented in Table 2 and agree well with the valence determined from the composition K 2 SrVO(VO 4 ) 2.

4 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Table 1 Experimental and crystallographic parameters for K 2 SrV 3 O 9 Composition K 2 SrV 3 O 9 Formula weight Space group [no.] Pnma [62] a (Å) (12) b (Å) (2) c (Å) (2) Z 4 V (Å 3 ) (2) Calculated density (g/cm 3 ) m (mm 1 ) Color Brown Diffractometer, mode Stoe STADI/P, transmission Radiation, wavelength Cu Ka1, Å Detector Linear PSD Refinement method Full profile Program used GSAS Number of atomic sites 13 No. of variables 35 2u range, step (8) , 0.01 Total number of profile points 9100 Total number of reflections 550 Admixtures (wt.%) Sr 3 (VO 4 ) 2, 1.8%; Sr 3 V 2 O 7, 0.9% Reliability factors R P = 0.018, R wp = 0.023, x 2 = ED and HREM study EDX-analysis using TEM was performed to verify the composition of the crystals; it shows relative amounts of K:Sr:V of 34(3):17(9):49(7), resulting in a composition K 2 Sr 1 V 2.9 O x. The deviation from 3 of V is within measuring errors. Table 2 Atomic coordinates and displacement parameters for K 2 SrV 3 O 9 Atom Position x y z U iso 100 (Å 2 ) Valence from BVS [14] Sr 4c (2) 1/ (9) 0.72(5) 1.79 K(1) 4c (4) 1/ (2) 1.6(1) 1.13 K(2) 4c (4) 1/ (2) 3.6(1) 1.12 V(1) 4c (2) 1/ (2) 1.1(1) 3.90 V(2) 4c (3) 1/ (2) 1.1(1) 4.89 V(3) 4c (4) 1/ (2) 0.5(1) 5.17 O(1) 4c (8) 1/ (6) 1.0(1) O(2) 8d (5) (13) (4) 1.0 O(3) 4c (9) 1/ (5) 1.0 O(4) 8d (6) (9) (4) 1.0 O(5) 4c (8) 1/ (5) 1.0 O(6) 4c (8) 1/ (5) 1.0 O(7) 4c (8) 1/ (5) 1.0

5 804 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Fig. 1. Experimental, calculated and difference X-ray patterns for K 2 SrV 3 O 9. Fig. 4 shows the ED patterns of the [1 0 0] *,[010] * and [0 0 1] * zones of K 2 SrV 3 O 9. These patterns can be indexed using the cell parameters and space group obtained from X-ray diffraction. The appearance of the forbidden reflections h00:h = 2n + 1 and 00l:l = 2n + 1 on the [0 1 0] ED pattern is due to double diffraction; this is seen by the fact that the reflections disappear when rotating the crystal away from the perfect orientation around these axes. It is also evident from their absence on the other two ED patterns. Fig. 2. The projection of the K 2 SrV 3 O 9 crystal structure along the b-axis (a) and a chain of VO 5 pyramids connected via VO 4 tetrahedra (b).

6 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Fig. 3. Coordination polyhedra for K(1) (left), K(2) (center) and Sr (right) cations in the K 2 SrV 3 O 9 structure. A HREM image of the K 2 SrV 3 O 9 structure projected along the [0 1 0] direction is shown in Fig. 5.A calculated image made using the cell parameters and atomic coordinates obtained from the refinement of the XRD data is shown in the image, within the largest white rectangle. The smaller white rectangle indicates the size of a unit cell. The calculated image has a width of 4 unit cells along a and 2 unit cells along b, it has a defocus value of 30 nm and a thickness of 10 nm. Under the intense electron beam used Table 3 Main interatomic distances (Å) in the K 2 SrV 3 O 9 structure Sr O(2) (6) Sr O(2) (6) Sr O(3) (9) Sr O(4) (6) Sr O(6) (9) Sr O(7) (2) K(1) O(1) (9) K(1) O(3) (4) K(1) O(4) (6) K(1) O(5) (9) K(1) O(7) (9) K(2) O(1) (9) K(2) O(1) (4) K(2) O(3) (9) K(2) O(5) (9) K(2) O(5) (4) K(2) O(6) (9) V(1) O(1) (8) V(1) O(2) (7) V(1) O(4) (6) V(2) O(2) (7) V(2) O(5) (8) V(2) O(7) (8) V(3) O(3) (9) V(3) O(4) (6) V(3) O(6) (8)

7 806 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) for high resolution electron microscopy the material rapidly becomes amorphous from the edge of the crystal inwards, explaining the disorder at the left side of the image. The white dots in the simulated image are the empty columns between the columns of atoms, the darkest dots show the position of the vanadium atoms Magnetic properties Fig. 4. Electron diffraction patterns along the three main directions of K 2 SrV 3 O 9. In spite of the fact that the K 2 SrV 3 O 9 structure does not contain directly connected V 4+ O 5 polyhedra it demonstrates a x(t) behavior typical for low-dimensional magnetic systems. A susceptibility curve (Fig. 6) follows the Curie Weiss law at high temperature and shows a Bonner Fisher dependence [15] at low temperature. An increase of the susceptibility below 25 K may be attributed to the paramagnetic admixture Sr 3 V 2 O 7 and defects in K 2 SrV 3 O 9. We did not find any evidence for a long-range magnetic order above 2 K. Fitting of the high-temperature part of the curve by the Curie Weiss law resulted in m = 1.92m B with u = 112 K. The negative u value indicates the presence of a significant antiferro- Fig. 5. High resolution electron microscopy image of K 2 SrV 3 O 9. The small white rectangle shows the outlines of a unit cell, the larger white rectangle shows the image simulation.

8 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Fig. 6. Fitting of the temperature dependence of the magnetic susceptibility by Curie Weiss and Bonner Fisher equations. The insertion demonstrates a linear fit of 1/x(T) in the high temperature range. magnetic interaction between the V 4+ (S = 1/2) cations inside the chains; the absence of long-range magnetic order shows that the interaction between chains is very weak. The low-temperature part of the curve was fitted by a sum of Bonner Fisher (for K 2 SrV 3 O 9 ) and two Curie Weiss (with different C and u values corresponding to paramagnetic Sr 3 V 2 O 7 and defects) equations. The fitting resulted in the exchange integral of J = 105 K that is slightly larger than the value of 80 K found for Na 2 SrV 3 O 9 [7]. 4. Discussion Reduced vanadates with the [V 3 O 9 ] 4 group often have low dimensional structures and contain chains of V 4+ O n polyhedra [2]. This stoichiometry supposes a presence of one V 4+ cation (usually six or five coordinated) and two V 5+ cations having a tetrahedral coordination leading to the structural formula of the group as [VO(VO 4 ) 2 ] 4. The charge of the group may be compensated by two divalent or by two monovalent and one divalent metal cations. Na 4 V 3 O 9 with four alkali metal atoms was reported in [16] but the structure of this oxide has not been solved. Three types of chains were found in the A n V 3 O 9 oxides. One may consider the morphology of these chains depending on the size and number of the A- cations. A 2 V 3 O 9 compounds were not reported for small divalent cations: Mg and Ca. These cations appear to be too small to form low dimensional structures with this composition. An increase of the A-cation size (Sr, Pb) results in the formation of structures containing chains of corner-linked V 4+ O 6 octahedra. One type of the VO 4 tetrahedra additionally links octahedra within the chains whereas another type connects neighboring chains into layers (Fig. 7a) [3,6]. The lone pair of the lead atom in Pb 2 V 3 O 9 results in a decrease of the symmetry but the structural motif is kept. Sr and Pb atoms may substitute each other forming a solid solution Sr 1 x Pb x V 3 O 9 with 0 x 1 [17]. A partial substitution (up to 50%) of Sr atoms for Ba ones keeps two-dimensional Sr 2 V 3 O 9 structure [18]. However, a further increase of the size of the divalent cation requires larger structural interstices

9 808 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) Fig. 7. Comparison of chains in the structures of (a) Sr 2 V 3 O 9 ; (b) Ba 2 V 3 O 9 ; and (c) K 2 SrV 3 O 9. between the layers. Therefore, the dimensionality of the Ba 2 V 3 O 9 structure decreases in comparison with that for Sr 2 V 3 O 9 due to breaking of the bonds between the chains. Ba 2 V 3 O 9 contains infinite rutile-like chains. V 4+ O 6 octahedra are connected via a common edge. Half of the VO 4 tetrahedra link two neighboring octahedra whereas another half shares only one vertex with the octahedra (Fig. 7b) [4,5]. One can suppose that an exchange of one divalent cation for two monovalent ones will significantly change the structure. It happens in the case of Na 2 SrV 3 O 9 but the structure maintains its low dimensionality. The chains are formed by V 4+ O 5 square pyramids connected by two VO 4 tetrahedra (Fig. 2 bottom) [7]. Large structural interstices between the chains which are required to accommodate three A-cations make the structure less sensitive to a change in cation size. Indeed, both K 2 SrV 3 O 9 and Na 2 SrV 3 O 9 structures are closely related. An increase of the average A-cation size in the K-containing compound results in a higher symmetry but it does not change the structural motif. Potassium cations require more space between the layers causing a more regular orientation of the VO 4 tetrahedra which are noticeably tilted in the Na 2 SrV 3 O 9 structure. The strontium polyhedra are similar in both compounds while the alkali metal polyhedra significantly differ due to their variation in size.

10 A.A. Tsirlin et al. / Materials Research Bulletin 40 (2005) In spite of different structures the A n V 3 O 9 compounds demonstrate similar magnetic behavior typical for low-dimensional systems with an antiferromagnetic exchange interaction between S = 1/2 spins (V 4+ cations). So, Ba 2 V 3 O 9 and M 2 SrV 3 O 9 (M = Na, K) do not exhibit any magnetic order down to 2 K while Sr 2 V 3 O 9 undergoes an antiferromagnetic transition at 3 5K[7,19]. The values of the exchange integral J of K are close for all compounds. M 2 SrV 3 O 9 (M = Na, K) compounds demonstrate a significant interaction in spite of the absence of a direct connection between two V 4+ O 5 pyramids. In [7], we referred this interaction to a V O O V superexchange through each of the VO 4 tetrahedra. A similar superexchange via the PO 4 tetrahedra has been recently studied for (VO) 2 P 2 O 7 [20] and Ba 2 VO(PO 4 ) 2 H 2 O [21]. K 2 SrV 3 O 9 presents a new example for this mechanism of superexchange interaction. Acknowledgements The authors are grateful to RFBR (grant ) and ICDD (GiA APS91-05) for financial support, M. Kovba for help in the synthetic experiment, P. Chizhov for the magnetic susceptibility measurements, A. Gippius and A. Vasiliev for useful discussion. Part of this work has been performed within the framework of the IAP 5-1 of the Belgian government. References [1] Y. Ueda, Chem. Mater. 10 (1998) [2] P. Zavalij, M. Whittingham, Acta Crystallogr. B 55 (1999) 627. [3] J. Feldmann, Hk. Muller-Buschbaum, Z. Naturforsch. B 50 (1995) 43. [4] J. Feldmann, Hk. Mueller-Buschbaum, Z. Naturforsch. B 51 (1996) 929. [5] A.-C. Dhaussy, F. Abraham, O. Mentre, H. Stainfink, J. Solid State Chem. 126 (1996) 328. [6] O. Mentre, A.C. Dhaussy, F. Abraham, E. Suard, H. Stainfink, Chem. Mater. 11 (1999) [7] R.V. Shpanchenko, V.V. Chernaya, E.V. Antipov, J. Hadermann, E.E. Kaul, C. Geibel, J. Solid State Chem. 173 (2003) 244. [8] C. Wadewitz, Hk. Mueller-Buschbaum, Z. Naturforsch. B 51 (1996) [9] V.V. Chernaya, A.A. Tsirlin, R.V. Shpanchenko, E.V. Antipov, A.A. Gippius, E.N. Morozova, V. Dyakov, J. Hadermann, E.E. Kaul, C. Geibel, J. Solid State Chem. 177 (2004) [10] A.C. Larson, R.B. Von Dreele, Los Alamos National Laboratory Report LAUR, , 2000.; B.H. Toby, J. Appl. Cryst. 34 (2001) 210. [11] P.E. Werner, L. Eriksson, M. Westdahl, J. Appl. Crystallogr. 18 (1985) 367. [12] W. Carrillo-Cabrera, H.G. von Schnering, Z. Kristallogr. 205 (1993) 271. [13] K.-J. Range, F. Rau, U. Klement, Z. Naturforsch. B 46 (1991) [14] I.D. Brown, D. Altermatt, Acta Crystallogr. B 41 (1985) 244. [15] J.C. Bonner, M.E. Fisher, Phys. Rev. 135 (1964) A640. [16] V. Volkov, B. Golovkin, Zh. Neorgan. Khimii 34 (1989) 1785 (in Russian). [17] O. Mentre, A.-C. Dhaussy, F. Abraham, H. Stainfink, J. Solid State Chem. 140 (1998) 417. [18] ICDD PDF2 [52 476]; R.V. Shpanchenko, E.E. Kaul, unpublished data. [19] E.E. Kaul, H. Rosner, V. Yushankhai, J. Sicherschmidt, R.V. Shpanchenko, C. Geibel, Phys. Rev. B 67 (2003) [20] A.W. Garret, S.E. Nagler, D.A. Tennant, B.C. Sales, T. Barnes, Phys. Rev. Lett. 79 (1997) 745; D.A. Tennant, S.E. Nagler, A.W. Garret, T. Barnes, C.C. Toradi, Phys. Rev. Lett. 78 (1997) 4998; J. Kikuchi, K. Motoya, T. Yamauchi, Y. Ueda, Phys. Rev. B 60 (1999) [21] W.T.A. Harrison, S.C. Lim, J.T. Vaughey, A.J. Jacobson, D.P. Goshorn, J.W. Johnson, J. Solid State Chem. 113 (1994) 444.