Crystal structure and magnetic properties of K2CoV2O7

Materials Research Bulletin 71 (2015) 7–10

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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu

Crystal structure and magnetic properties of K2CoV2O7 Hamdi Ben Yahiaa,* , Rachid Essehlia , Ilias Belharouaka , Etienne Gaudinb a b

Qatar Environment and Energy Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France

A R T I C L E I N F O

A B S T R A C T

Article history: Received 9 February 2015 Received in revised form 30 May 2015 Accepted 23 June 2015 Available online 30 June 2015

The new compound K2CoV2O7 has been synthesized by a solid state reaction route. Its crystal structure has been determined using powder X-ray diffraction data (PXRD). The compound K2CoV2O7 crystallizes with the melilite-type structure with the tetragonal unit cell parameters a = 8.4574(1), c = 5.5729(1) Å and

Keywords: Inorganic compounds Layered compounds X-ray diffraction Crystal structure Magnetic properties

the space group P4 21 m. The structure consists of [CoV2O7]2 layers perpendicular to the c axis separated by K+ layers. The [CoV2O7]2 layers consist of corner-sharing CoO4 tetrahedra and V2O7 pyrovanadate units, the linkage of these tetrahedra forming five-membered rings. The K+ cations occupy distorted square antiprisms of oxygen atoms. The magnetic susceptibility of K2CoV2O7 follows the Curie law xT = C with an effective magnetic moment meff = 4.71 mB. ã 2015 Elsevier Ltd. All rights reserved.

1. Introduction The melilite compounds have been intensively studied through the years due to their interesting piezoelectric- [1], electrochemical- [2,3], luminescence- [4–8], magnetic- [9], and structuralproperties [10,11]. Most of the minerals of the melilite group are silicates with the general formula [8]A2[4]B[4]Si2O7 ([N] = coordination number). Among them, one can find the okayamalite Ca2B(BSi) O7, the akermanite Ca2MgSi2O7, the gehlenite Ca2Al(AlSi)O7, the gugiaite Ca2BeSi2O7, the hardystonite Ca2ZnSi2O7, and the melilite (Ca,Na)2(Mg,Fe,Al)(AlSi)O7 [12–17]. Many melilite-type compounds have been synthezised for a wide range of chemical compositions then the general formula becoming A2BC2X7 where A is a large cation such as Ca, Sr, Ba, Na, K, Y, lanthanides (La-Er), Pb, Bi; B is a small four coordinated cation such as Be, Mg, Mn, Fe, Co, Cu, Zn, Cd, Al, Ga, Si; C = Cr, Al, Ga, Si, Ge, B, V, and X = O, F, S, N. The melilite-type structure was first determined by Warren (1930) [12]. The structure has a tetragonal symmetry space group P4 21 m and consists of BC2O7 layers parallel to (0 0 1) made of cornersharing BO4 and CO4 tetrahedra. The A cations are lying between these layers in distorted square antiprisms of oxygen atoms. Besides Na2ZnV2O7, the first vanadate crystallizing with the melilite-type structure [18], only few vanadate compounds have structures closely related to the melilte-type. The compounds K2VOV2O7, Rb2VOV2O7, (NH4)2VOV2O7, and KBaCuClV2O7

* Corresponding author. Fax: +974 4454 0547. E-mail address: [email protected] (H.B. Yahia). http://dx.doi.org/10.1016/j.materresbull.2015.06.038 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.

crystallize with the fresnoite-type structure which is very similar to melilite-type, except that the V2O7 pyrogroups all point in the same direction and the V+4 and Cu2+ coordination polyhedra are square pyramids instead of the usual tetrahedra [19–22]. They all have tetragonal cell parameters with space group P4bm and with a ranging from 8.8581(13) to 8.9229(10) Å and c ranging from 5.215 (5) to 5.5640(5) Å. A different variation of the melilite-type structure is observed for K2MgV2O7 [23]. It crystallizes with the space group P42/mnm and cell parameters a = 8.38(2), c = 11.36(2) Å. The main difference between the K2MgV2O7 and the melilitetype structures is the doubling of the cell parameter c explained by the existence of a mirror perpendicular to the [0 0 1] direction between two MgV2O7 layers. This induces a change from P4 21 m to P42/mnm symmetry. This structural variation is only observed for phosphates and vanadates [23–26]. Notably, P4 21 m and P42/mnm are both subgroups of space group P4/mbm, however no compound of the melilite group crystallizes in this space group. It should be mentioned that structural modulation has been often observed in the melilite compounds. Bindi et al. reported the observation that the greater the size of the tetrahedral cations with respect to the cations A, the greater the structural misfit leading to the incommensurate structure is found [27]. During our study of the AMnVO4 (A = Cu, Ag, Na, K, Rb) series [28–32], the K2MnV2O7, KRbMnV2O7, and Rb2MnV2O7 phases have been discovered [33]. No modulation has been observed due to the partial or complete substitution of K+ for Rb+ on the A positions of the compounds of general formula A2MnV2O7. Recently, we have synthesised the K2BV2O7 (B = Fe2+, Co2+, Ni2+) compositions in order

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to study the effect of the size of the transition metal on the crystal structure, however only K2CoV2O7 was formed. In this paper we report on the crystal structure and the magnetic properties of the new divanadate K2CoV2O7. The aim of this work is to contribute to a better insight into the crystal chemistry of the divanadates A2BV2O7, since up to now only three vanadates crystallise with the melilite-type structure (Na2ZnV2O7, K2MnV2O7, and K2CoV2O7). 2. Experimental 2.1. Synthesis K2CoV2O7 was prepared by solid state reaction from a stoichiometric mixture of KVO3 and CoO. 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. After grinding, a further heating of the mixture at 500  C for 18 h led to mainly K2CoV2O7 and traces of unidentified impurities (Fig. 1). Attempts to prepare single crystals of K2CoV2O7 by melting the sample powder were unsuccessful even with very slow cooling rates.

Table 1 Crystallographic data and structure refinement for K2CoV2O7. Rietveld Refinement Crystal data Chemical formula Mr Crystal system Space group Temperature (K) a (Å) c (Å) V (Å3) Z Data collection Diffractometer Radiation type 2 umin, 2 ustep, 2 umax values ( ) Refinement Rp Rwp Rexp R(F) RBragg goodness of fit x2 No. of data points No. of parameters Profile function Background

K2CoV2O7 351 Tetragonal P4 21 m 300 8.45740 (9) 5.57287 (8) 398.61 (1) 2 Panalytical Cua1,a2 5, 0.02, 110 0.079 0.115 0.049 0.049 0.075 5.429 5251 41 Pseudo-Voigt Chebyshev function with15 terms

2.2. X-Ray diffraction measurement To check the purity of K2CoV2O7 powder, high precision PXRD measurements were performed. The data were collected at room temperature over the 2 u angle range of 5  2 u  110 with a step size of 0.02 using a Panalytical diffractometer operating with CuKa radiations. Full pattern matching refinement was performed with the Jana2006 program package [34]. The background was estimated by a Legendre function, and the peak shapes were described by a pseudo-Voigt function. The refinement of peak asymmetry was performed using four Berar-Baldinozzi parameters. The preferred orientation with respect to the [0 0 1] axis, according to March-Dallase, was applied. Evaluation of these data revealed the refined cell parameters listed in Table 1.

2.3. Magnetic measurements Magnetic susceptibility measurements of K2CoV2O7 were carried out with a Quantum Design SQUID magnetometer. The susceptibility of K2CoV2O7 was recorded at 5 kOe. The diamagnetic corrections were carried out on the basis of Pascal’s tables [35]. 2.4. Electron microprobe analysis Semiquantitative EDX analyses of the powder, were carried out with a Leica 420i scanning electron microscope. The experimentally observed compositions were close to the ideal one, K2CoV2O7. No impurities could be detected. An SEM image is depicted in Fig. S1.

Fig. 1. Final observed, calculated and difference plots for the XRPD profile refinement of K2CoV2O7 (* indicates peak positions of the impurities).

H.B. Yahia et al. / Materials Research Bulletin 71 (2015) 7–10 Table 2 Atom positions and isotopic displacement parameters for K2CoV2O7. Atom

Wyck.

x

y

z

Uiso(Å2)

K Co V O1 O2 O3

4e 2a 4e 8f 2c 4e

0.8382(2) 0 0.14225(17) 0.0880(6) 0 0.1463(7)

0.3382(2) 0 0.35775(17) 0.1799(6) 1/2 0.3537(7)

0.5047(4) 0 0.0526(3) 0.1714(9) 0.1530(17) 0.7536(9)

0.0268(8) 0.0112(8) 0.0122(6) 0.031(2) 0.022(3) 0.014(2)

3. Results and discussion 3.1. Structure refinement The crystal structure of K2CoV2O7 was solved using the crystal structure of K2MnV2O7 as starting model [33]. The Rietveld analysis of the PXRD data collected at 300 K led to the reliability factors [Rp = 7.9%, wRp = 11.5%, RFobs = 4.67%, RFwobs = 4.9%, RB = 7.5% (GOF = 2.33)]. Fig. 1 shows a good agreement between the experimental and calculated patterns. The final atomic positions are given in Table 2. 3.2. Crystal structure The crystal structure of K2CoV2O7 is isotypic to the melilite-type structure consisting of alternating CoV2O7 and K2 layers (Fig. 2a). The CoV2O7 layers contain corner-sharing CoO4 tetrahedra and V2O7 pyrovanadate units (two corner-sharing VO4 tetrahedra) that form five-membered rings (Fig. 2b). The eight coordinated K cations are positioned between these layers. The interatomic distances for the CoO4, VO4, and KO8 polyhedra are listed in Table 3. The CoO4 tetrahedron contains four regular Co-O bonds of 1.944 (5) Å very close to the value of 1.96 Å, estimated from the effective ionic radii of Co2+ and O2 [36]. A comparable Co–O distance (1.956 Å) has been reported for the pyrophosphate Na2CoP2O7 [25]. The large O–Co–O angles of 121.2(2) indicate that the CoO4 tetrahedron is flattened along the [0 0 1] direction. Two neighboring VO4 units share an O2 atom to form the pyrovanadate unit [V2O7]4 . This induces a stretched V–O2 distance of 1.791(3) Å, typical of divanadate entities in which the longer distance characterizes the V–O–V bridge [18–23]. In the VO4 tetrahedron the average V–O distance of 1.718 Å is slightly lower than the value of 1.735 Å, expected from the sum of the effective ionic radii. No significant deviation from the ideal

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Table 3 Interatomic distances (in Å), angles (in degrees) and BVS for K2CoV2O7. distance Co-O1 (4)

1.944(5)

V-O3 V-O1 (2) V-O2

1.667(5) 1.706(5) 1.791(3) <1.718>

K-O3 K-O2 K-O1 (2) K-O3 (2) K-O1 (2)

2.709(6) 2.754(7) 2.786(6) 2.955(6) 3.115(6) <2.897>

O1-Co-O1 (4) O1-Co-O1 (2)

angle 104.0(2) 121.2(2) <109.7>

O1-V-O2 (2) O1-V-O1 O2-V-O3 O1-V-O3 (2)

106.9(2) 108.9(3) 109.9(4) 112.0(3) <109.4>

V-O2-V V-O1-Co

143.6(6) 127.0(3)

Co V K

BVS 2.02 5.08 1.09

* B.V. = e(r0-r)/b with the following parameters [37,38]: b = 0.37 and r0 (CoII–O) = 1.692, r0 (VV– O) = 1.803, r0 (KI–O) = 2.132.

tetrahedral-angle of 109.5 is observed. The O–V–O angles range from 106.9(2) to 112.0(3) with an average value of 109.4 . The coordination polyhedron around the A-site K atom is a distorted square antiprism (Fig. 3). There are four short K–O distances ranging from 2.709(6) to 2.754(7) Å and four stretched distances ranging from 2.955(6) to 3.115(6) Å giving an average value of 2.897 Å consistent with the Shannon Table (dK–O = 2.93 Å) [36]. The calculations of the bond valence sums (BVS) confirm the charge balance (i.e., BVS = 2.02, 5.08 and 1.09 for Co2+, V5+, and K+, respectively) [37,38].

Fig. 2. Projection of crystal structure of K2CoV2O7 (a) onto the ac plane and (b) onto the ab plane.

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pyrovanadate units, separated by K+ layers. K2CoV2O7 exhibits a paramagnetic behavior in the temperature range 2–300 K with a magnetic susceptibility following the Curie law xT = C (C = 2.77 mol 1 cm3 K and meff = 4.71 mB). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull. 2015.06.038. Fig. 3. a) Perspective view of the square antiprismatic environment of the potassium atoms in K2CoV2O7 and b) a projection view along [0 0 1].

Fig. 4. Magnetic susceptibility x vs. Temperature and the corresponding x plots of K2CoV2O7 measured with the applied field H = 5000 Oe.

1

vs. T

3.3. Magnetic properties The magnetic susceptibility x vs. T and the corresponding x 1 vs. T for K2CoV2O7 are shown in Fig. 4. The x 1 vs. T plot reveals that K2CoV2O7 exhibits a paramagnetic behavior in the temperature range 2–300 K. The susceptibility follows the Curie law xT = C with C = 2.77 mol 1 cm3 K. The effective magnetic moment meff calculated from the Curie constant 4.71 mB is a typical value obtained for divalent cobalt atoms, although higher than the spin only value of 3.87 mB expected for a high-spin Co2+ (d7) ion. This is due to contribution from the orbital angular momentum. 4. Conclusion The title compound K2CoV2O7 has been synthesized by a solid state reaction route from a stoichiometric mixture of KVO3 and CoO. Its crystal structure has been solved using the Rietveld method using the structure parameters of the analogous compound K2MnV2O7 as the initial structural model. K2CoV2O7 crystallizes with the melilite-type structure and consists of [CoV2O7]2 layers of corner-sharing CoO4 tetrahedra and V2O7

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