Delafossite oxides containing vanadium(III): Preparation and magnetic properties

Solid State Sciences 7 (2005) 710–717 www.elsevier.com/locate/ssscie

Delafossite oxides containing vanadium(III): Preparation and magnetic properties Khadija El Ataoui a , Jean-Pierre Doumerc b,∗ , Abdelaziz Ammar a , Jean-Claude Grenier b , Léopold Fournès b , Alain Wattiaux b , Michel Pouchard b a CERM, Faculté des Sciences Semlalia, Université Cadi Ayyad, BP 2390 Marrakech, Morocco b ICMCB-CNRS, 87, Ave. du Dr A. Schweitzer, 33608 Pessac Cedex, France

Received 4 November 2004; accepted 9 November 2004 Available online 27 April 2005 Dedicated to E.F. Bertaut

Abstract Although the pure delafossite CuVO2 does not exist, the present study well confirms that large amounts of vanadium(III) can be incorporated into copper based delafossites without reducing Cu+ into metallic copper, as several solid solutions CuM 1−x Vx O2 exist where M = Ga, Cr (x
0.50) or Fe (x
0.67). Vanadium(III) has a peculiar magnetic behavior. Actually V–V antiferromagnetic interactions appear rather large at high temperature, but as the temperature decreases magnetic data can be (at least qualitatively) interpreted assuming the formation of V–V pairs. The bonding involves one of the two electrons of the 3d2 (a1 1 e1 ) configuration of V(III)-atoms having a 6-fold D3d coordination, the one occupying the e orbital that lies in the (M, V) layers of the delafossite structure. The behavior changes from a rather 2D ferrimagnetic-like one to a 3D antiferromagnetic one with a Néel temperature larger than 4.5 K in the case of CuFe1−x Vx O2 as determined from a Mössbauer study.  2005 Elsevier SAS. All rights reserved. Keywords: Delafossite oxides; Magnetic properties; Mössbauer resonance; Copper vanadium(III) oxide

1. Introduction The first study on the mineral of formula CuFeO2 was published by Charles Friedel in 1873 [1] who named it “delafossite” as a tribute to the French crystallographer Gabriel Delafosse (1796–1878) (Fig. 1). In the fifties, Bertaut’s name became associated with the first works on this family of AMO2 double oxides showing that copper indeed was monovalent and iron trivalent, on the basis of the calculation of Madelung energy [2] (Fig. 1). Bertaut’s name is also attached to the study of magnetic oxides including the discovery of garnet ferrites [3]. In the * Corresponding author. Tel.: +33(0)5 4000 6264; Fax: +33(0)5 4000 8373. E-mail addresses: [email protected] (J.-P. Doumerc), [email protected] (A. Ammar).

1293-2558/$ – see front matter  2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2004.11.030

present paper, we shall mainly attempt to move on to this type of properties. Since this time several studies broadly extended the family of delafossites to the cases where A = Ag, Pd, Pt [4] and Hg [5], and, to a broad range of trivalent elements (from M = Al3+ to M = La3+ ) as well as to the so-called mixed delafossites, AM1−x Mx O2 , where M and M  can have oxidation states ranging from 2 to 5 [6,7]. Delafossite oxides are also interesting for applications such as catalysis [8], transparent conductors—especially of p-type [9]—and possibly thermoelectrics [10]. Recently, we prepared new mixed delafossites where one of the M-site cations is vanadium(III) (M  = V) whose strongly reducing character prevents any formation of CuVO2 [11]. For M = Fe3+ , using Mössbauer spectroscopy we have shown that the charge equilibrium Fe3+ + V3+ ↔ Fe2+ +

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Table 1 Delafossites giving a solid solution CuM1−x Vx O2 with the hypothetical [CuVO2 ] (a = 3.0176 A)

Fig. 1. The Charles Friedel’s 1873 paper title and the paragraph where the renowned chemist named the mineral “delafossite” from the name of the French crystallographer Gabriel Delafosse (a) and the title of E.F. Bertaut’s 1953 paper where this other renowned French crystallographer confirmed the valence state of copper and iron on the base of Madelung energy calculations (b).

V4+ was indeed shifted towards the left hand side and that, for x
0.67, V(III) no longer reduced Cu+ into Cu0 . In order to confirm this result we attempted to synthesize the solid solutions CuM1−x Vx O2 where M = Al3+ , Co3+ , Ga3+ , Cr3+ , Mn3+ and Sc3+ . This paper presents our results, and the magnetic behavior of the obtained phases leads us to discuss the role of V(III) in these oxides having a 2D structure, but where the strong Cu–O bonds of the (CuO2 )3− dumbbells linking the M 3+ layers could play a particular role.

2. The hypothetical phase CuVO2 viewed as the limit of the solid solution CuFe1−x Vx O2 (x = 1) [12] The linear variation with x of the a (a(x) = 3.0344– 0.0168x A) and c (c(x) = 17.167 + 0.016x A) parameters ¯ allows an acof CuFe1−x Vx O2 (0
x
0.67) (S.G.: R3m) curate extrapolation for x = 1 giving: a(1) = 3.0176 A and c(1) = 17.183 A. These values are close to those of CuFeO2 (a = 3.0345 A and c = 17.166 A) according to the very small difference in ionic radii of Fe3+ (0.645 A) and V3+ (0.64 A) [13]. Two effects are able to limit or even to prevent the formation of a solid solution with the hypothetical [CuVO2 ]: – a steric effect, that is to say a too large difference of size between the V3+ cation and the substituted cation, – a large difference of redox power of the involved atoms.

CuMO2

a lattice constant (A)

x-value range

CuFeO2 CuGaO2 CuCrO2

3.0345 3.005 2.997

0
x
0.67 0
x
0.50 0
x
0.50

Both effects are indeed linked by the so-called chemical pressure: when it is positive—i.e., for compressed cations— it favors its oxidation and then its reducing power. When it is negative—i.e., for cations in an expanded environment—it favors its reduction and then its oxidizing power. In the case of CuAlO2 and CuCoO2 with small lattice constants (a ≈ 2.85 A) the large chemical pressure acting on the substituting V3+ cation strongly increases its reducing power and thus does not allow a sufficient stability for Cu+ . The monoclinic distortion of the crednerite unit cell of CuMnO2 resulting from the Jahn-Teller ion Mn3+ (3d4 ) also yields an effect unfavorable for the formation of a solid solution. On the other hand the large unit cell of CuScO2 (a = 3.21 A) should stabilize the V3+ ion, but it is not appropriate for a large Madelung energy, although solid solutions have been already observed between delafossites with large lattice constant a and with ionic radius relatively different [11]. Actually, we found that only three delafossites with a lattice constant close to that of the hypothetical [CuVO2 ] led to a solid solution with a significant domain of composition. They are listed in Table 1. This work mainly focuses on the synthesis and structural characterizations of the selected systems CuGa1−x Vx O2 and CuCr1−x Vx O2 , and on the magnetic properties of the three systems including the series CuFe1−x Vx O2 , the crystal chemistry of which was previously reported [12].

3. Sample preparation and X-ray study of CuM1−x Vx O2 (M = Ga, Cr) The preparation of solid solutions CuM1−x Vx O2 (M = Ga, Cr) was carried out by direct reaction of stoichiometric mixtures of Cu, CuO, M2 O3 and V2 O5 according to the equation: (1 + 2x)Cu + (1 − 2x)CuO + (1 − x)M2 O3 + xV2 O5 → 2CuM1−x Vx O2

(0
x
0.5).

After homogenisation, the starting mixture is pressed into φ = 8 mm discs under a pressure of 5 × 108 Pa. Heating is performed in vacuum sealed silica tubes with the exception of CuCrO2 that is left in air. For CuGa1−x Vx O2 , samples are heated at 1100 K for 10 days. In the case of CuCr1−x Vx O2 reaction temperature and time depend on the x-value and are given in Table 2. To achieve complete reaction several annealings and grindings

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Table 2 Experimental conditions for the synthesis of CuCr1−x Vx O2

Table 5 Atomic position parameters for CuM0.5 V0.5 O2 (M = Cr, Ga)

x value

Heating temperature (K)

Heating time (days)

Atom

0.00 0.20 0.33 0.50

1300 1350 1320 1320

1 3 3 2

Multiplicity and Wyckoff site

x

y

z

M = Cr Cu M V O

Table 3 Experimental conditions for data collection; conditions and results of the structure refinement for CuM0.5 V0.5 O2 (M = Cr, Ga) M

Cr

Diffractometer Radiation Temperature 2θ range (◦ ); step (◦ ); counting time (s) Rietveld program Profile function Number of reflections Number of fitted parameters Rpa Rwpa cRpb cRwpb R-Bragg χ2

Philips X’Pert MPD Cu Kα RT 5-120; 0.02; 20 Fullprof [14] pseudo-Voigt 40 17 0.031 0.029 0.048 0.042 0.186 0.156 0.151 0.121 0.069 0.061 8.1 6.3

Ga

3a 3b 3b 6c

0 0 0 0

0 0 0 0

0 1/2 1/2 0.108(1)

Fractional occupancy M = Ga

0.109(1)

1 1/2 1/2 1

Table 6 Main interatomic distances (A) and bond angles (◦ ) in CuM0.5 V0.5 O2 (M = Cr, Ga) Cu–O (M,V)b –O O–Oa O–Od O–(M,V)b –Oa O–(M,V)b –Od Cu–O–(M,V)b (M,V)b –O–(M,V)c

×2 ×6 ×6 ×3

M = Cr

M = Ga

1.86(1) 2.00(1) 2.9971(1) 2.64(1) 97.2(1) 82.8(5) 120.0(4) 97.2(1)

1.86(1) 2.00(1) 3.0048(2) 2.64(1) 97.4(2) 82.6(5) 119.7(4) 97.4(2)

Symmetry code: none (x, y, z). a (x, y − 1, z); b (x + 1/3, y + 2/3, z − 1/3); d (x + 2/3, y + 1/3, 1/3 − z).

c (x + 1/3, y −

1/3, z − 1/3);

a R-factors uncorrected for background [14]. b Conventional Rietveld R-factors corrected for background [14].

Table 4 Crystallographic data for CuM0.5 V0.5 O2 (M = Cr, Ga) M

Cr

Ga

Crystal system Space group Z a (Å) (hexagonal cell) c (Å) (hexagonal cell) Unit cell volume (Å3 ) (h.c.) Molar mass (g mol−1 ) Calculated density (g cm−3 )

rhomboedral ¯ (N◦ 166) R3m 3 2.9971(2) 17.131(1) 133.27(1) 147.01 5.50

3.0048(2) 17.122 (1) 133.88(1) 155.87 5.80

are required. Better results were obtained with a cooling rate of 2 K/mn. Powder XRD data were collected using the experimental conditions given in Table 3. All the prepared samples are isostructural with the socalled 3R delafossite polytype. For x > 0.50, CuM0.5 V0.5 O2 coexists with metallic copper and VO2 . Cell and structural parameters were refined using the Rietveld method. Program reference [14] and reliability factors for x = 0.5 are also given in Table 3. Crystal system, space group, lattice constants and other relevant crystallographic data are given in Table 4. Atomic positions are reported in Table 5 and selected interatomic distances and bond angles in Table 6.

Fig. 2. Measured (discrete points) and calculated (solid line) XRD intensities and their difference for CuCr0.5 V0.5 O2 at RT. Peak positions are indicated by vertical lines.

Experimental and calculated diffractograms are displayed in Figs. 2 and 3 for CuCr0.5 V0.5 O2 and CuGa0.5 V0.5 O2 , respectively. The Cu–O distance (1.86 A) is in good agreement with the sum of ionic radii (1.84 A) assuming a twofold coordination for Cu+ (0.46 A) and a fourfold coordination for O2− (1.38 A) [13]. The (M, V)–O distance (2.00 A in both Cr and Ga cases) is also in good agreement with the sum of ionic radii taking for the atoms occupying the 3b-site the average of the ionic radii of M 3+ and V3+ giving 2.007 A and 2.010 A for M = Cr and M = Ga respectively. As no su-

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Fig. 3. Measured (discrete points) and calculated (solid line) XRD intensities and their difference for CuGa0.5 V0.5 O2 at RT. Peak positions are indicated by vertical lines.

perstructure peak was observed, at the present stage of our investigations we shall assume that there is no long range ordering of the M 3+ and V3+ cations. The unit cell volume of CuM1−x Vx O2 (M = Ga, Cr) increases with x (Figs. 4 and 5). This behavior mainly results from a significant increase of the a lattice constant despite, in the Ga case, a small decrease of the c constant. The increase of the molar volume with x can be attributed to the change of ionic radii from that of chromium (0.615 A) or gallium (0.62 A) to that of vanadium (0.64 A). This result agrees well with the usual behavior of delafossite oxides. Actually, increasing the size of the M 3+ cation leads to a decrease of

Fig. 5. Composition dependence of the lattice constants of CuCr1−x Vx O2 .

the c/a ratio. As the Cu–O distance does not change much, this behavior reveals an increasing distortion of the MO6 octahedra that become more and more flattened as the size of the M cation increases.

4. Mössbauer study of CuFe1−x Vx O2 at 4.2 K

Fig. 4. Composition dependence of the lattice constants of CuGa1−x Vx O2 .

Mössbauer study at room temperature was reported elsewhere [12]. Experimental conditions are the same as those reported in Ref. [15]. Spectra recorded at 4.2 K are displayed in Fig. 6. All the spectra can be described by a single sextuplet. Therefore we may conclude that below 4.2 K all the samples are magnetically ordered. Spectrum fitting was carried out in two stages. In a first calculation, assuming Lorentzian peak profile allowed us to determine averaged Mössbauer parameters. The analysis clearly shows that there is a single site for Fe3+ ions. In addition no trace of Fe2+ was detected. These results confirm the RT study [12]. The increase of the isomer shift with respect to the RT values is due to the second order Doppler shift. The half-height width is larger than what is usually observed (ca. 0.30 mm s−1 ). We have attributed this broadening to a distribution of hyperfine field. In a second calculation, the value of the half-height width was fixed at 0.40 mm s−1 and the isomer shift (δ) values are those determined in the first calculation. A better fitting is thus obtained (Fig. 6). The corresponding Mössbauer parameters are displayed in Ta-

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Fig. 6. Experimental and calculated (solid line) Mössbauer spectra of CuFe1−x Vx O2 recorded at 4.2 K for x = 0 (a), 0.33 (b), 0.50 (c), 0.67 (d).

Table 7 Mössbauer parameters for CuFe1−x Vx O2 at 4.2 K x

δ (mm s−1 )

Γ (mm s−1 )

ε (mm s−1 )

H (T)

x = 0.00 x = 0.33 x = 0.50 x = 0.67

0.496 0.425 0.430 0.435

0.40 0.40 0.40 0.40

+0.296 −0.116 −0.133 −0.123

51.5 49.3 50.3 50.4

come less covalent under the influence of the more covalent V–O bonds. However, as the vanadium content further increases, a weak opposite tendency is observed. The electric field parameter ε undergoes a drastic change at the lowest substitution rates (x = 0.33) even changing its sign.

5. Magnetic properties ble 7. The substitution of vanadium for iron does not modify significantly the hyperfine field. Reversely, a small vanadium substitution acts on the values of δ and ε. Normally the evolution of the isomer shift reflects that of the covalence of the Fe–O bond. The decrease of δ as x changes from 0 to 0.33 suggests that the Fe–O bonds be-

The temperature dependences of the reciprocal mo−1 ) are given in Figs. 7–9 lar magnetic susceptibility (χM for CuGa1−x Vx O2 , CuCr1−x Vx O2 and CuFe1−x Vx O2 , respectively. Obviously, they do not follow a Curie–Weiss law, at least over the whole temperature range. For the

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Fig. 7. Temperature dependence of the reciprocal magnetic susceptibility of CuGa1−x Vx O2 (x = 0.20, 0.33, 0.50) for one vanadium mole.

715

Fig. 9. Temperature dependence of the reciprocal molar magnetic susceptibility of CuFe1−x Vx O2 (x = 0.25, 0.33, 0.50, 0.67).

Fig. 8. Temperature dependence of the reciprocal molar magnetic susceptibility of CuCr1−x Vx O2 (x = 0.33 and 0.50).

CuGa1−x Vx O2 system the displayed susceptibility is calculated for one vanadium mol. It clearly shows that the behavior changes with the vanadium concentration x unlike what is observed in the case of iron where, for a similar plot (i.e., magnetic susceptibility normalized to 1 iron atom), only a simple shift of the paramagnetic Curie temperatures is observed with |θp | normally increasing with the iron concentration x (Fig. 10). Two explanations can a priori be proposed for the non −1 linear behavior of χM vs. T : – for the V3+ ion characterized by a 3 T1g term in Oh symmetry, a significant spin-orbit coupling is expected, which could give rise to a T dependence of the effective

Fig. 10. Temperature dependence of the reciprocal magnetic susceptibility of CuGa1−x Fex O2 (x = 0.33 and 0.50) for one iron mole.

moment. However, such a behavior is rather rare in inorganic compounds where the V concentration is large. In addition lifting the degeneracy of t2g orbitals in D3d symmetry—flattened octahedron—into a a1 singlet and a e doublet will strongly reduce the effect that can finally be discarded in the present case, – the V3+ ion with a a1 1 e1 configuration presents two electrons having a very different localization in space: the a1 -electron occupying an orbital oriented along the c-axis has no neighbor in this direction whereas the e-electron lying perpendicularly to the c-axis has six neighboring cations in the V-layer.

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Many examples of very strong e–e interactions can be found in the literature even leading to electronic pairing within clusters of vanadium atoms that can be ordered as in LiVO2 [16]. In these conditions, at low temperature the onset of such a pairing destroys the antiferromagnetic (AF) interactions between in-plane near neighbors and only lets survive non interacting a1 -electrons. In this case, a behavior of the type of that shown in Fig. 11 can be predicted. Such a behavior is very close to that of Fig. 7. In delafossites containing a single cation such as CuFeO2 and CuCrO2 , a 3D AF order sets at relatively low temperature (below ca. 25 K) despite the large magnitude of the AF interactions that are revealed by θp -values ranging from −90 to −200 K, respectively. This departure from the predictions of molecular field theory originates in frustration effects typical of the triangular lattice and in the weakness of superexchange interactions along the c-axis. These interactions involve e(V3+ ), sp3 (O2− ), 4pz (Cu+ ) and hybridized (3dz2 4s)(Cu+ ) orbitals. At low temperature, when the e-electrons become paired within V–V bonds these interactions along the c-axis tend to vanish, giving rise to a progressive change from a 3D to a 2D character. Therefore, in the solid solutions CuFe1−x Vx O2 and CuCr1−x Vx O2 , mainly non-interacting (001) planes are observed, at least at large enough temperatures with respect to the low Néel temperature that is only 14 K for CuFeO2 and 27 K for CuCrO2 . In the non-substituted delafossites short range ordering gives rise to a typical 2D AF behavior [17]. As vanadium is substituted for the M-cation (M = Fe, Cr), layers no longer behave as simple AF triangular lattices as a complex interplay of three kind of interactions (M–M, M–V and V–V) is operating. As a result a 2D ferrimagnetic like behavior is expected, leading, while short range order sets, to a non linear temperature dependence of the reciprocal susceptibility that decreases strongly as T drops, in a similar way as classical 3D ferrimagnets do (Fig. 12). Such a behavior is indeed observed for CuFe1−x Vx O2 and CuCr1−x Vx O2 phases for which experimental values of the Curie constant C smaller than the theoretical spin-only values have been found. Like for classical ferrimagnets this trend can be as-

Fig. 12. Behavior of a 2D metaferrimagnet. The magnetic susceptibility does not diverge at Tc as for a classical 3D ferrimagnet according to the molecular field theory (solid line). Instead, an AF 3D ordering takes place at low temperature. In the case of CuFe0.33 V0.67 O2 (symbols), TN is larger than 4.5 K as deduced from the Mössbauer study. The dashed lines corresponds to a Curie–Weiss law asymptotic to the solid line at high temperature.

cribed to the fact that the theoretical C value actually is the reciprocal slope of the asymptote that merges with the curve only at large enough temperatures (Fig. 12). The magnetic behavior of these phases is thus quite original as it appears of metaferrimagnetic type since χM does not diverge at Tc , which can result from an interlayer coupling that remains very weakly AF as long as Fe3+ and Cr3+ ions are involved. High magnetic field measurements at low temperature are planned to check the possibility of spin flip in order to stabilize ferrimagnetic 3D ordering against the AF 3D ordering stable in fields smaller than 0.3 T. The model also explains the non linear shape of the −1 vs. T plot observed for the CuNi1/3 V2/3 O2 delafossite χM where AF interactions between Ni2+ (3d8 ) ions should be weak as all the t2g orbitals are fully occupied (t2g 6 eg 2 ) [18]. −1 at low T can be described by a The T dependence of χM Curie law with Cexp = 0.55. This value is close to that corresponding to 1/3 of Ni2+ (S = 1) and 2/3 of V3.5+ with SV3.5+ = 1/2 instead of an average between V3+ (S = 1) and V4+ (S = 1/2) as found at high temperature (Cexp = 0.86). 6. Conclusions

Fig. 11. Theoretical temperature dependence of the magnetic susceptibility accounting for the progressive formation of V–V pairs as the temperature decreases, according to the model described in the text. The dashed lines represent the asymptotic behavior at law and high temperature.

Although the pure delafossite CuVO2 does not exist, the present study well confirms that large amounts of vanadium(III) can be incorporated into copper based delafossites without reducing Cu+ into metallic copper as several solid solutions CuM1−x Vx O2 exist where M = Ga, Cr (x
0.50) or Fe (x
0.67). Vanadium(III) has a peculiar magnetic behavior. Actually V–V antiferromagnetic interactions appear rather large at high temperature, but as the temperature decreases magnetic data can be (at least qualitatively) interpreted assuming

K. El Ataoui et al. / Solid State Sciences 7 (2005) 710–717

the formation of V–V pairs. The bonding involves one of the two electrons of the 3d2 (a1 1 e1 ) configuration, the one occupying the e orbital that lies in the V layers. The behavior changes from a rather 2D ferrimagnetic-like to a 3D antiferromagnetic with a Néel temperature larger than 4.5 K in the case of CuFe1−x Vx O2 as determined from the Mössbauer study.

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