3.1 Compound synthesis

The composition of this new compound was found starting from the pyrochlore-type structure compound Bi_1.524Sb_1.524Cu_0.952O_7+δ we synthesized recently [8] when we looked for a possibility to obtain a solid solution by partial substitution of bismuth [9], antimony or copper for other cations. Fig. 1 shows the existence of various phases when copper is totally substituted for cobalt. The stoichiometry Bi_1.524Sb_1.524Co_0.952O_7+δ resulted in a three-phase mixture of pyrochlore and small amounts of BiSbO₄ [JCPDS/30-177] and CoSb₂O₆ [JCPDS/18-403]. As it was impossible to eliminate simultaneously both impurities, we managed to eliminate them successively. So, we eliminated CoSb₂O₆ by adding gradually a small amount of Bi₂O₃ and then we proceeded to the elimination of the BiSbO₄ impurity following these steps:

The earlier Mössbauer study of the copper pyrochlore (Bi_1.524−xM_xCu_0.476)(Sb_1.524Cu_0.476)O_7+δ (M = Ca, Pb) compound [9] showed that antimony exhibits a +5 oxidation valence. Observed, indexed X-ray powder diffraction data for the pyrochlore with composition Bi_1.56Sb_1.48Co_0.96O₇ are given in Table 1. The 442 reflection (2θ = 52.504°), forbidden for an ideal pyrochlore structure [10], was easily observed in this pattern. Similar to that of Bi–Zn–Nb–O pyrochlores [10,11], the observation of this diagnostic reflection indicates the presence of displacements from ideal crystallographic sites. The consistent observation of this reflection indicates that the Bi–Co–Sb–O pyrochlore exhibits positional displacements. Furthermore, the stoichiometric location of the pyrochlore phase field, as also found for the analogous Bi–M–Nb–O systems (M = Zn, [10] Fe, [12] Mn [13]), occurs at substantially lower Bi concentrations than conventional formulations placing only Bi on the A sites and the smaller M/Nb cations on the B sites. The pyrochlore field includes compositions with “excess” B cations that require mixing of some Co on the A sites with Bi³⁺, as found for Zn in Bi–Zn–Nb–O pyrochlore [10].

The scanning electron micrograph of the sample (Fig. 2) sintered at 1100 °C shows that the external area of the pellet is under spherical grain and porous shape. This compound remains stable until 1100 °C. But above this temperature, a new unidentified peak appears at 2θ = 26.7°.

3.2 Structural refinement

The highly symmetrical ideal pyrochlore structure A₂B₂O₇ crystallizes in space group Fd-3m with A and B cations on two special positions (16d, 16c) and oxygen atoms on two sites, 48f O and 8b O′, resulting in a single positional variable (x for the 48f-site oxygen atoms in the B₂O₆ octahedral network). As expected, this model resulted in anomalously large thermal parameters for the A-site cations and the O′ oxygen atoms in an initial refinement. The disordered structure was therefore modelled assuming static displacements of the A and O′ atoms, similar to that of analogous Bi–M–Nb–O pyrochlores (M = Zn [11], Fe [12], Mn [13]) whose structures were refined using neutron powder diffraction data. The refinement of the positional parameters for the disordered metal sites was carried out assuming displacements of the A₂O′ network, as found for a number of other pyrochlores [14–17]. Refinement with only displacement of Bi gave bad residuals for the observed reflections; improved results were obtained when O′ was also displaced. The A-site Bi/Co metals were displaced to six partially filled 96g sites; however, the corresponding 96g position for O′ was not stable. Refinement of the O′ isotropic thermal parameter diverged, so a displaced position of 32e site was chosen. Other combinations were not stable. Theoretical, calculated and difference X-ray powder diffraction spectra are represented in Fig. 3. The final refinement results are given in Tables 2 and 3.

The results suggest to write the initial composition as (Bi_1.56Co_0.44)(Sb_1.48Co_0.52)O₇, where the cobalt is distributed between A and B sites. Polyhedral representation of this structure is given in Fig. 4. Every octahedron forms the B-site and the arrangement of the octahedral, sharing vertices, generates hexagonal cavities which are the A sites. The value of the isotropic thermal agitation factor 0.487(49) Ų in the A-site can be interpreted by the good model used for the resolution following the presence of two kinds of cations with different radius: bismuth (R_VIII = 1.17 Å) and cobalt (R_VIII = 0.90 Å) [18]. This result is in agreement with works made by Withers et al. [19]. They showed in their neutron diffraction studies of (Bi_1.5Zn _0.5)(Nb_1.5Zn_0.5)O₇ pyrochlore-like compound that for bismuth and zinc, 96g-site (with O′ in 96g) as well as 96h-site (with O′ in 32e) is more stable than 16d-site. Lately Vanderah et al. [20] synthesized an isotype phase of formula (Bi_1.56Co_0.44)(Nb_1.48Co_0.52)O₇ which confirms the accuracy of the chemical formula of our compound.

3.3 Infrared spectroscopy

The IR absorption bands of solids in the range of 100–1000 cm⁻¹ are usually assigned to vibrations of ions in the crystal lattice [21]. The infrared lattice vibration frequencies of some pyrochlore compounds with a formula A₂B₂O₇ have been studied. It is known that there are seven IR-active optic modes in the infrared spectra of a pyrochlore compound, and the highest frequency and the most intense band (ν₁) at about 600 cm⁻¹ is from the B–O stretching vibration in the BO₆ octahedron; the weak band (ν₂) at about 500 cm⁻¹ is from the A–O′ stretching vibration and the weak band (γ₃) at about 400 cm⁻¹ is from the A–O stretching vibration in the AO₆O′₂ polyhedron in A₂B₂O₇, respectively [1]. As Fig. 5 shows, there are five absorption bands in the range 400–1000 cm⁻¹ of the infrared spectra of the (Bi_1.56Co_0.44)(Sb_1.48Co_0.52)O₇ compound. These bands are at 845, 677, 600, 582 and 412 cm⁻¹, respectively. The weak and strong bands are at 412, 582, 600 cm⁻¹, respectively. These bands are just in the region of the A–O and the A–O′ stretching vibrational frequency in the AO₆O′₂ polyhedron of the pyrochlore compound, respectively. Hence, it may be reasonable to assign the band (ν₁ = 412 cm⁻¹) to the (Bi/Co)–O stretching vibrations and the band (ν₂ = 582 cm⁻¹) and (ν′₂ = 600 cm⁻¹) to the Bi–O′ and the Co–O′ stretching vibrations in the (Bi/Co)O₆O′₂ polyhedron, respectively. This is because the stretching vibration frequency (ν) of the bond relates to the mass of the bonding atoms [22]. The more the mass of the bonding atoms, the lower the stretching vibration frequency of the bond. The band (ν₃ = 677 cm⁻¹) may be from the B–O stretching vibration in the BO₆ octahedron. McCauley [23] measured the infrared absorption spectra of several pyrochlore-structure materials. The data showed a number of very weak absorption bands in the 800–1100 cm⁻¹ region, leading to the suggestion that this lattice mode is an indication of an additional structural complexity. As stated before, Withers et al. [19] indicated the bond length difference for the A–O′ bond in the A₂O′ substructure. Their data show the longer bond length is 2.351 Å and the shorter bond length is 1.961 Å, about a 20% difference. Hector and Wiggin [24] showed that the displacements of both the O′ anion and the A-site cation must be cooperative within domains and this may lead to one A–O′ bond being shortened and the other being lengthened. According to this picture, the vibration of the shorter A–O′ bond may correspond to the phonon mode around 845 cm⁻¹ and the vibration of the longer bond may lead to a phonon mode around 582 and 600 cm⁻¹. It has been recently proposed that the disorder of A and O′ ions is due to static displacements in all pyrochlores in which the A cation has active lone pairs [15]. And it has been noted [25] that in some pyrochlores the lone pair character of Bi³⁺ is reduced by mixing of the Bi 6s electron pairs and the d orbitals (of Bi-site cations).

3.4 Magnetic susceptibility

Magnetic data obtained for the compound indicate that the field dependence of the magnetization confirmed that the sample was overall paramagnetic in nature with an effective moment of 5.04μ_β for the cobalt. The observed effective magnetic moments (μ_eff/(mol) Co), calculated from the slopes of the Curie–Weiss fits, are similar of those observed for other Co-containing pyrochlores [26] giving the range of values reported for high-spin Co²⁺ (4.6μ_β/Co–5.2μ_β/Co)[27]. The temperature dependence of the magnetic susceptibility and its inverse are shown in Fig. 6. Above 100 K the inverse magnetic susceptibility was linear and that range was fitted to the Curie–Weiss law. The extrapolation of the linear fits indicated small negative temperature intercepts −49.48 (θ (K)), suggesting the presence of weak antiferromagnetic cooperative interactions.

3.5 Electrical characterization

The typical complex impedance diagram Z″ = f(Z′) of the compound obtained at 420 K is presented in Fig. 7. It shows a non-deformed unique circle arc passing by the origin and having a centre displaced under the abscissa axis. The resistance R of the sample at each temperature was determined from the intersection of abscissa axis (zero phase-shift angle) with the circle arc at low frequencies [28]. Fig. 7 shows the variation log(σ) versus the reciprocal temperature (1/T). The electrical conductivity obeys the Arrhenius relation: σ = σ₀ exp(−E_a/K_BT) where σ₀ represents a pre-exponential factor, K_B the Boltzmann constant, E_a was estimated from the slope of log(σ) versus T⁻¹. The activation energy (E_a) of the compound is 0.41 ± 0.02 eV. These results reveal a semiconducting behaviour of the material.