Synthesis and characterization of Ca2CoTaO6, a new monoclinically distorted double perovskite

Synthesis and characterization of Ca₂CoTaO₆, a new monoclinically distorted double perovskite

Araceli Elisabet Lavat^I; Enrique José Baran^II;*

^IDepartamento de Ingeniería Química, Facultad de Ingeniería, Universidad Nacional del Centro de la Provincia de Buenos Aires, 7400, Olavarría, Argentina

IICentro de Química Inorgánica - CEQUINOR/CONICET, Facultad de Ciencias Exactas, Universidad Nacional de La Plata - UNLP, CP 962, 1900, La Plata, Argentina

ABSTRACT

The new Ca₂CoTaO₆ double perovskite has been synthesized by a conventional solid state reaction and its unit cell parameters determined by X-ray powder diffractometry. It crystallizes in the monoclinic space group P2₁/n. The unit cell parameters are: a = 5.507(2) Å; b = 5.564(3) Å; c = 7.798(3) Å; β = 89.99(4)° and Z = 2. The IR spectrum of the material was recorded and is briefly discussed. Some comparisons with Ca₂CoNbO₆ and other isostructural perovskites are also performed.

Keywords: Ca ₂ CoTaO ₆ , double perovskite, X-ray diffraction, IR spectrum

1. Introduction

It is well-known that mixed oxides with the perovskite structure present an important number of interesting physicochemical properties and high potential for technological applications^1-3. Double perovskites of the type A₂BB'O₆ containing Nb(V), Mo(VI), W(VI) or Te(VI) associated with first-row transition metal cations have shown to posses interesting magnetic properties^4-8 and are potentially useful as materials for oxide fuel cells and other similar applications^9-15. In this context, a new perovskite of this type, Ca₂CoNbO₆, has been recently prepared and characterized¹⁶.

As an extension of this work we have now prepared a similar double perovskite containing Ta(V) instead of Nb(V) and performed some comparisons between the two materials.

2. Experimental

Polycrystalline samples of Ca₂CoTaO₆ and Ca₂CoNbO₆ were prepared by mixing stoichiometric amounts of CaCO₃, Co₃O₄ and Ta₂O₅ (or Nb₂O₅). The mixtures were heated in air, in alumina crucibles, initially at 1000 °C during 8 hours. followed by multiple heatings, during other 8 hours more at 1250 °C with intermediate grinding after each step. Finally, the samples were furnace cooled to room temperature.

The obtained mixed oxides were characterized by X-ray powder diffractometry, using a continuous step scanning procedure (step size: 0.020° (in 2θ); time per step: 0.5 seconds), with a Philips PW 1710 diffractometer and monochromatic Cu-K_Α radiation (λ = 1.54186 Å), using NaCl as an external calibration standard. The indexation of the powder diagrams and calculation of unit cell parameters were carried out using a locally modified version of the program PIRUM of Werner¹⁷.

The infrared spectra were recorded with a Nicolet-Magna 550 FTIR instrument, using the KBr pellet technique. Spectral resolution was 4 cm^-1. Unfortunately, attempts to record the corresponding Raman spectra, using the FRA 106 Raman accessory of a Bruker IFS 66 FTIR instrument and the 1046 nm line of a solid state Nd:YAG laser for excitation, failed due to the darkness of the samples.

3. Results and Discussion

3.1. Crystallographic data and structural aspects

The prepared Ca₂CoTaO₆ and Ca₂CoNbO₆ perovskites show identical powder diagrams indicating the formation of a pair of isostructural materials. Besides, the diagram of Ca₂CoNbO₆ was identical to that previously published¹⁶ and also to that of the isostructural Ca₂CrTaO₆^[18]. The powder diagrams of the three materials present some clearly splitted reflections as well as a number of weak superstructure reflections (cf. also^16,18).

The powder diagram of Ca₂CoTaO₆ could be clearly indexed in the monoclinic system. The refined unit cell parameters, together with other relevant crystallographic data, are shown in Table 1 and the complete indexed powder diagram is presented in Table 2. By comparison with the Rietveld refined structures of Ca₂CoNbO₆^[16] and Ca₂CrTaO₆^[18], one can admit that the space group of the new perovskite is also P2₁/n, with the Co^III and Ta^V ions distributed randomly over Wykoff positions 2c and 2d whereas Ca^II and all the O-atoms are at general Wykoff positions 4e. Briefly, the structure is built up by two types of octahedral MO₆ polyhedra, running along the c- axis of the unit cell, over which the Ta(V) and Co(III) ions are distributed in a disordered way. The Ca(II) ions are located in the holes generated by this arrangement of octahedra, coordinated by twelve O-atoms.

On the other hand, and as suggested in the case of Ca₂CoNbO₆ and other related systems¹⁶, it is possible that the material presents a slight oxygen deficiency, derived from the presence of a low percentage of Co^II ions at the Co^III sites. This supposition is supported additionally by the presence of weak bluish spots on the crucible walls after the final heating step, due probably by generation of small amounts of the CoAl₂O₄ spinel.

The unit cell parameters of Ca₂CoTaO₆ are very close to those of Ca₂CoNbO₆, as expected from the fact that Shannon and Prewitt's radii for both Nb(V) and Ta(V) in octahedral coordination are identical¹⁹.

3.2. Infrared spectra

The FT-IR spectra of both, Ca₂CoTaO₆ and Ca₂CoNbO₆ samples, are also totally similar and present a very simple spectral pattern, as usually found in perovskite materials^1,20. The spectrum of one of the prepared Ca₂CoTaO₆ samples is shown in Figure 1.

IR and Raman spectra for Sr₂LnTaO₆ materials (with Ln = trivalent lanthanides, Y(III) and In(III)), which are also isostructural to Ca₂CoTaO₆ and Ca₂CoNbO₆, have recently been investigated²¹ and also theoretically analyzed²².

In all cases, two groups of bands, together with a certain number of weak shoulders, could be clearly identified. Measured band positions for Ca₂CoTaO₆ and Ca₂CoNbO₆ are shown in Table 3.

The very strong higher energy band is assigned to the antisymmetric stretching vibration (ν₃ of an O_h-symmetry species) of the MO₆ octahedra containing the Co(III) and Ta(V) ions and is surely dominated by the Ta-O motions, which involves the stronger metal-oxygen bonds. The second strong band, at 370 cm^-1, can be assigned to the antisymmetric deformation (ν₄ of an O_h-symmetry species) of these same octahedra.

Only slight energy differences are observed between the band positions of Ca₂CoTaO₆ and Ca₂CoNbO₆, in agreement with their practically identical unit cell dimensions and M-O bond strengths. The determined band positions are also comparable to those measured in the Sr₂LnTaO₆ materials²¹.

4. Conclusions

Ca₂CoTaO₆ constitutes a new example of a monoclinically distorted double perovskite. Its unit cell parameters are close to those of the recently reported isostructural Ca₂CoNbO₆ material. The very simple two-band infrared spectra of both compounds are also totally similar and resemble that of other perovskite materials. In order to go deeper and to provide more details about this compound, further studies should be conducted.

Acknowledgements

This work has been supported by the Universidad Nacional del Centro de la Provincia de Buenos Aires, the Universidad Nacional de La Plata and the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina. E.J.B. is a member of the Research Career from this organization.

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Received: January 17, 2011

Revised: August 2, 2011

Publication Dates

• Publication in this collection
  30 Sept 2011
• Date of issue
  Dec 2011

History

• Accepted
  02 Aug 2011
• Received
  17 Jan 2011