Synthesis and Characterization of Ca 2 CoTaO 6 , a New Monoclinically Distorted Double Perovskite

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.


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][2][3] .Double perovskites of the type A 2 BB'O 6 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][5][6][7][8] and are potentially useful as materials for oxide fuel cells and other similar applications [9][10][11][12][13][14][15] .In this context, a new perovskite of this type, Ca 2 CoNbO 6 , has been recently prepared and characterized 16 .
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.

Experimental
Polycrystalline samples of Ca 2 CoTaO 6 and Ca 2 CoNbO 6 were prepared by mixing stoichiometric amounts of CaCO 3 , Co 3 O 4 and Ta 2 O 5 (or Nb 2 O 5 ).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 = 1.54186Å), 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 Werner 17 .
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.

Crystallographic data and structural aspects
The prepared Ca 2 CoTaO 6 and Ca 2 CoNbO 6 perovskites show identical powder diagrams indicating the formation of a pair of isostructural materials.Besides, the diagram of Ca 2 CoNbO 6 was identical to that previously published 16 and also to that of the isostructural Ca 2 CrTaO 6 [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 Ca 2 CoTaO 6 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 Ca 2 CoNbO 6 [16]   and Ca 2 CrTaO 6 [18]   , one can admit that the space group of the new perovskite is also P2 1 /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 MO 6 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 Ca 2 CoNbO 6 and other related systems 16 , 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 CoAl 2 O 4 spinel.
The unit cell parameters of Ca 2 CoTaO 6 are very close to those of Ca 2 CoNbO 6 , as expected from the fact that Shannon and Prewitt's radii for both Nb(V) and Ta(V) in octahedral coordination are identical 19 .

Infrared spectra
The FT-IR spectra of both, Ca 2 CoTaO 6 and Ca 2 CoNbO 6 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 Ca 2 CoTaO 6 samples is shown in Figure 1.
IR and Raman spectra for Sr 2 LnTaO 6 materials (with Ln = trivalent lanthanides, Y(III) and In(III)), which are also isostructural to Ca 2 CoTaO 6 and Ca 2 CoNbO 6 , have recently been investigated 21 and also theoretically analyzed 22 .
In all cases, two groups of bands, together with a certain number of weak shoulders, could be clearly identified.Measured band positions for Ca 2 CoTaO 6 and Ca 2 CoNbO 6 are shown in Table 3.
The very strong higher energy band is assigned to the antisymmetric stretching vibration (ν 3 of an O h -symmetry species) of the MO 6 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 (ν 4 of an O h -symmetry species) of these same octahedra.
Only slight energy differences are observed between the band positions of Ca 2 CoTaO 6 and Ca 2 CoNbO 6 , 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 Sr 2 LnTaO 6 materials 21 .

Conclusions
Ca 2 CoTaO 6 constitutes a new example of a monoclinically distorted double perovskite.Its unit cell parameters are close to those of the recently reported isostructural Ca 2 CoNbO 6 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.

Figure 1 .
Figure 1.FTIR spectrum of one of the prepared Ca 2 CoTaO 6 samples in the spectral range between 900-300 cm -1 .