Crystal Structure and Magnetic Properties of the

Doklady Chemistry, Vol. 376, Nos. 1–3, 2001, pp. 20–24. Translated from Doklady Akademii Nauk, Vol. 376, No. 3, 2001, pp. 347–351. Original Russian Text Copyright © 2001 by Zubkov, Tyutyunnik, Berger, Bazuev.

CHEMISTRY

Crystal Structure and Magnetic Properties of the Quasi-One-Dimensional Oxide Ca3CuMnO6 V. G. Zubkov, A. P. Tyutyunnik, I. F. Berger, and G. V. Bazuev Presented by Academician G.P. Shveikin June 29, 2000 Received July 17, 2000

Recently, we reported the synthesis of a new series of complex oxides with the composition Ca3AMnO6 (A = Cu, Ni, Zn) [1–3]. The crystal structures of these oxides are related to the structures of the compounds Sr3APtO6 and Sr3AIrO6 [4, 5]. A specific feature of these structures is the presence of infinite chains consisting of face-sharing octahedra Pt(Ir)O6 and trigonal prisms AO6 (A = Ni, Zn) or square networks CuO4. These structural units run along the c axis and are surrounded by six columns of square antiprisms with strontium atoms inside. Such a structure enables us to classify the considered oxides as quasi-onedimensional structures. An analogous structure was found for the Ca3NiMnO6 oxide [2]. In this structure, the octahedral positions are occupied by Mn4+ cations and the trigonal-prismatic positions, by Ni2+ and Mn2+ cations. All compounds under consideration belong to the family of complex oxides of general composition A3n + 3 A 'n B3 + nO9 + 6n [6] with n = ∞. The quasi-one-dimensional structure of the compounds is responsible for their highly anisotropic physical properties and, especially, their magnetic properties. Substances with various magnetic properties (antiferromagnetic, ferromagnetic, and ferrimagnetic) are found among these compounds. This is determined by the electron configuration of transitionmetal cations incorporated in one-dimensional chains. The temperatures of magnetic ordering are low in these compounds. Magnetic susceptibility of the considered oxides can be described in terms of both the Heisenberg 1D paramagnetic model and the Ising 1D model. The magnetic behavior of the solid solution Sr3CuPt0.5Ir0.5O6 [4] was explained by the model of a disordered spin-chain paramagnetism. In the Ca3Co2O6 compound with a related structure, antiferromagnetic ordering occurs below 24 K [7]. This is a result of ferromagnetic exchange interaction of Co

Institute of Solid-State Chemistry, Ural Division, Russian Academy of Sciences, Pervomaiskaya ul. 91, Yekaterinburg, 620219 Russia

cations in the chains and of antiferromagnetic interaction between the chains. At the same time, the manganese analogues of the considered oxides (Ca3AMnO6, where A = Zn2+ or Ni2+) are characterized by antiferromagnetic interactions at low temperatures [1–3]. In this paper, we reported the refined structure (X-ray and neutron powder diffraction) and magnetic properties of the complex oxide Ca3CuMnO6. Calcium carbonate CaCO3 (special purity grade) and oxides MnO2 and CuO (99.9% chemical purity) were used as starting reagents for the synthesis of Ca3CuMnO6. The target compounds were obtained by the ceramic method. Mixtures of the starting reagents were pressed and annealed first at 950°ë for 30 h and then at 1130°ë for 18 h. The phase composition of the sintered products was monitored by X-ray powder diffraction (a DRON–2 diffractometer). The cell unit parameters were determined using certified standards on the basis of diffraction data obtained on a Stoe STADI-P automated diffractometer. Polycrystalline silicon (a = 5.43075(5) Å) was used as an external standard, and α-Al2O3 (NIST SRM 676, ah = 4.75919(44) Å, ch = 12.99183(174) Å) was used as an internal standard. The refinement of the crystal structure of Ca3CuMnO6 was carried out using X-ray and neutron powder diffraction data with the GSAS program [8] for full-profile analysis by the Rietveld method. X-ray powder diffraction data were obtained on a Stoe STADY-P diffractometer at room temperature with CuKα radiation. The X-ray diffraction pattern was taken pointwise in the 2θ range of 5°–120° with a step of 0.02°. Neutron powder diffraction data were collected on a D2A setup of the IVV 2M reactor (Zarechnyi) at the neutron wavelength λ = 1.8031 Å in the 2θ range from 5° to 120° with a step of 0.1°. These data were used in calculations. The correspondence of the oxygen content in the sample to the formula Ca3CuMnO6 was confirmed by thermogravimetric analysis upon reduction of the samples in a hydrogen flow at 900°ë. In calculations, we took into account that copper occurred in the reduction products as a metal, while manganese was reduced to the bivalent state.

0012-5008/01/0001-0020$25.00 © 2001 åÄIä “Nauka /Interperiodica”

CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES

Magnetic properties were studied by measuring magnetic susceptibility and magnetization in a temperature range of 4.2–320 K on a SQUID magnetometer. Magnetic susceptibility was measured in magnetic fields of 8.9 kOe after the sample had been cooled in the field to be measured. Magnetization was measured in magnetic fields of 1–20 kOe at temperatures below and above the magnetic ordering point. The structure of the compound Sr3NiIrO6 [4, 6] was first taken as the initial model for the structural refinement of the crystal structures of both Ca3CuMnO6 and Ca3NiMnO6 [1, 2]. In this model (space group R 3 c, z = 6), the copper atoms are located in the centers of trigonal prisms (positions 6a), and manganese atoms are sited in octahedra (6b). The eight-coordinate calcium atoms are located in square antiprisms (18e), and oxygen atoms are in the positions 36f. The refinement of the structure using these data was inefficient, because the difference X-ray pattern contained reflections forbidden for space group R 3 c. The refinement parameters were considerably improved when the structure of Sr3CuPtO6 was used as the initial model. This compound [9, 10] has a monoclinic lattice (space group C2/c, z = 4). In this compound, strontium atoms are located in the 4e and 8f positions, copper atoms are in the 4e position, platinum is in the 4c position, and oxygen atoms of three types are located in the 8f positions. The trigonal-prismatic coordination of copper atoms reduces in Sr3CuPtO6 to a squareplanar one due to the shift of these atoms from the prism center into one of the rectangular faces of the coordination polyhedron. The manganese atoms remain in the octahedral positions. Nevertheless, the use of this model as the initial structure for Ca3CuMnO6 also did not lead to success. Despite a minimum being reached, the calculated and experimental diffraction patterns considerably differed. The latter patterns contained additional lines incongruous with both space groups C2/c and P2/c (a subgroup of group C2/c) and P2/m. The positions of these lines are in agreement with space group P 1 (Z = 4). This space group is a subgroup of space group C2/c. The triclinic unit cell parameters are a = 8.8243(4) Å, b = 9.1565(4) Å, c = 6.3635(3) Å, α = 90.203(3)°, β = 92.899(3)°, and γ = 90.156(3)°. This triclinic unit cell is a derivative of the monoclinic unit cell of Sr3CuPtO6, and all atoms are in the general positions 2i. A sharp global minimum in isotropic approximation of the thermal factor for identical atoms was found at the following ratios between the difference factors of X-ray and neutron diffraction profiles: Rwp = 3.47/2.28, Rp = 2.53/1.77, DWd = 0.846/1.243, R(F 2) = 8.56/1.58, and CHI 2 = 1.355. A total of 154 variable parameters, including profile ones, were used in the calculations. Because of the large number of the parameters, determination of the values of individual thermal parameters was impossible. Figures 1a and 1b show the experimental (crosses), calcuDOKLADY CHEMISTRY

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Table 1. Interatomic distances Cu–O and Mn–O in Ca3CuMnO6 Atom

Distance, Å

Atom

Distance, Å

Mn1–O1

1.954(24)

Cu1–O3

2.101(22)

Mn1–O3

1.883(20)

Cu1–O4

2.399(23)

Mn1–O5

1.984(26)

Cu1–O5

2.206(24)

Mn1–O7

1.869(25)

Cu1–O6

2.080(20)

Mn1–O9

1.975(23)

Cu1–O11

2.319(22)

Mn1–O11

1.911(24)

Cu1–O12

2.101(22)

Mn2–O2

1.909(25)

Cu2–O1

2.569(19)

Mn2–O4

1.863(23)

Cu2–O2

1.914(23)

Mn2–O6

1.982(26)

Cu2–O7

2.1543(25)

Mn2–O8

1.748(21)

Cu2–O8

2.101(23)

Mn2–O10

1.937(22)

Cu2–O9

1.946(26)

Mn2–O12

1.984(22)

Cu2–O10

2.540(19)

Note: Standard deviations are parenthesized.

lated (solid line) and difference patterns derived from X-ray and neutron diffraction data, respectively. Figure 2 displays a projection of the crystal structure onto the (010) plane. The presence of two positions for both Cu and Mn is characteristic of the Sr3CuPtO6 structure, unlike the structure of Ca3CuMnO6. The calculation of site occupancies shows that the Cu1 position is filled with 90% copper and 10% manganese. The Cu2 positions are occupied 100% by copper. Selected interatomic cation–anion distances in Ca3CuMnO6 are listed in the table. The copper atoms of two types (Cu1 = 0.9Cu + 0.1Mn and Cu2 = Cu) are located inside the trigonal prisms. However, analysis of interatomic copper–oxygen distances shows that the copper atoms of both types actually have the coordination number (4 + 2). With the supposition of a squareplanar environment (the coordination number is four), the average copper–oxygen distances are 2.122 Å and 2.026 Å for the Cu1 and Cu2 positions, respectively. This environment is a form of manifestation of the Jahn–Teller effect for bivalent copper atoms. Because the Cu1 position is 10% occupied with manganese, the Jahn–Teller effect is partially suppressed. This results in the lengthening of the average cation–anion distance (2.122 Å) for the coordination number of four and in the shift of these atoms to the center of the trigonal prism. The average Cu1–O distance is 2.201 Å for this case (the coordination number is (4 + 2)). At the same time, the Cu2 atoms are actually in the center of the lateral face of the trigonal prism. Manganese atoms of two

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Intensity, abs. units × 104 (a)

1.0

0.8

0.6

0.4

0.2

0

0.2

0.4

0.6

0.8

1.0

1.2

3.0 (b)

2.0

1.0

0 0.1

0.2

0.3

0.4

0.5 0.6 2Θ, deg × 102

0.7

0.8

0.9

1.0

Fig. 1. Experimental (crosses), calculated (solid line), and difference patterns of Ca 3CuMnO6 derived from (a) X-ray and (b) neutron diffraction data. DOKLADY CHEMISTRY

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CRYSTAL STRUCTURE AND MAGNETIC PROPERTIES χ, cm3 g–1 2.0 × 10–4

c

χ, cm3 g–1

1.6 × 10–4

1.6 × 10–4

1.2 × 10–4

b

a

23

1.2 × 10–4

8.0 × 10–5

0

9

18 T, K

4.0 × 10–5 0 Ca Mn Cu O

100

200

300 T, K

Fig. 3. Specific magnetic susceptibility χ of Ca3CuMnO6 vs. temperature in the magnetic field of 8.9 kOe.

Fig. 2. Projection of the crystal structure of Ca3CuMnO6 onto the (010) plane. The unit cell of the compound and isolated chains consisting of MnO6 octahedra of two types and CuO6 trigonal prisms of two types are shown.

types (Mn1 and Mn2) are located inside the octahedra with average manganese–oxygen distances of 1.929 Å and 1.904 Å, respectively. The lattice of this compound contains calcium atoms of six types, which are located inside the distorted square antiprisms (the coordination number being eight). The average Ca–O distances vary from 2.425 Å to 2.513 Å. As follows from Fig. 2, the octahedra and trigonal prisms share faces to form isolated zigzag chains of copper and manganese atoms with a …Mn2Cu1Mn2Cu2Mn1… repeat motif. These chains are separated from one another by columns of distorted oxygen square antiprisms with Ca atoms located inside these antiprisms. Low-temperature magnetic measurements showed that the temperature dependence of the magnetic susceptibility χ of Ca3CuMnO6 has a distinctive maximum at 6 K (Fig. 3), which may point to the magnetic ordering (most likely, an antiferromagnetic one). The magnetic susceptibility χ of Ca3CuMnO6 follows the Curie– Weiss law χ = C /(T – θ) at temperatures above 50 K (Fig. 4). The constants C and θ in the Curie–Weiss law are equal to 2.28 and –85 K, respectively. The effective magnetic moment (4.27 µB) is close to the theoretical value for the combination of cations Cu2+–Mn4+ (4.24 µB). The magnetization versus magnetic field curve is linear both above and below 6 K, which indicates that Ca3CuMnO6 lacks a spontaneous magnetic moment. Therefore, the foregoing shows that Ca3CuMnO6 is the first from the compounds of the A3n + 3 A n' B3 + nO9 + 6n family (n = inf, the Sr4PtO6 structure type) to have a triclinic lattice (space group P-1, z = 4). Studying the magnetic susceptibility tentatively suggests the occurrence of a long-range magnetic order DOKLADY CHEMISTRY

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χ–1, g cm–3 6.0 × 104 4.0 × 104 2.0 × 104

0 – 100

0

100

200

300 T, K

Fig. 4. Reciprocal specific magnetic susceptibility χ–1 of Ca3CuMnO6 in the magnetic field of 8.9 kOe vs. temperature.

(evidently, antiferromagnetic) in this compound at temperatures below 6 K. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project nos. 00–15–97418 and 98– 03–32697a. REFERENCES 1. Bazuev, G.V., Zubkov, V.G., Krasil’nikov, V.N., et al., Dokl. Akad. Nauk, 1998, vol. 363, no. 5, p. 634 [Dokl. Chem. (Engl. Transl.), vol. 363, nos. 4–6, pp. 239–242]. 2. Bazuev, G.V., Zubkov, V.G., Berger, I.F., and Arbuzova, T.I., Solid State Sci, 1999, vol. 1, p. 365. 3. Bazuev, G.V., Zubkov, V.G., Berger, I.F., and Arbuzova, T.I., Zh. Neorg. Khim., 2000, vol. 45, no. 7, pp. 1204–1210.

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4. Nguen, T.N. and Lee, P.A., Zue Loye, H.-C., Science, 1996, vol. 271, no. 5248, p. 489. 5. Nguen, T.N., Zur Loye H.-C., J. Solid State Chem., 1995, vol. 117, no. 1, p. 300. 6. Darriet, J. and Subramanian, M.A., J. Mater. Chem., 1995, vol. 5, no. 4, p. 543. 7. Fjellvag, H., Gulbransen, E., Aasland, S., et al., J. Solid State Chem., 1996, vol. 124, no. 1, p. 190.

8. Larson, A.C. and Von Dreele, R.B., GSAS Lansce MSH805, Los Alamos: Los Alamos Nat. Lab., NM 87545. 9. Wilkinson, A.P., Cheetam, A.K., Kunnman, W., and Kvick, A., Eur. J. Solid State Inorg. Chem., 1999, vol. 28, p. 453. 10. Hodeau, J.L., Tu, H.Y., Bordet, P., et al., Acta Crystallogr., Sect. B: Struct. Sci., 1992, vol. 48, p. 1.

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