Spin-state transition in the layered barium cobaltite derivatives and their thermoelectric properties

Ba1.9Me0.1Co9O14 (Me = Ba, Sr, Ca) (BCO) layered cobaltites were prepared by means of solid-state reactions method. Crystal structure, microstructure, thermal expansion, electrical conductivity, and thermo-EMF for the obtained oxides were studied; the values of their linear thermal expansion coefficient, activation energy of electrical transport, and power factor values were calculated. It was found that BCO are p-type semiconductors, in which the spin-state transition occurs within 460–700 K temperature interval due to change in spin state of cobalt ions, which accompanied the sharp increase in electrical conductivity, activation energy of electrical conductivity, and linear thermal expansion coefficient, while thermo-EMF coefficient decreased. Partial substitution of barium by strontium or calcium in BCO leads to the increase in spin-state transition temperature and electrical conductivity of the samples, and, at the same time, thermo-EMF coefficient; consequently, their power factor values decrease.


Introduction
Layered sodium, calcium, or bismuthcalcium cobaltites (Na x CoO 2 , Ca 3 Co 4 O 9 , Bi 2 Ca 2 Co 1.7 O x ) are prospective materials for producing of p-branches for high-temperature thermoelectric generators, since they possess high values of electrical conductivity (σ) and thermo-EMF coefficient (S) and low thermal conductivity (λ), as well as high stability at elevated temperatures in air [1][2][3][4]. The common elements of crystal structure, which are present in all of these compounds, are the conducting [CoO 2 ] layers.
It was also reported [11,12] that Ba 2 Co 9 O 14 can be used as a cathode material for intermediate-temperature solid oxide fuel cells in contact with various electrolytes, such as yttria stabilized zirconia (YSZ) or cerium doped gadolinium oxide (GCO).
The aim of this work was to study the effect of partial substitution of barium by strontium or calcium in Ba 2 Co 9 O 14 on crystal structure, microstructure, thermal expansion, electrotransport and thermoelectric properties of layered barium cobaltite derivatives above room temperature.

Experimental
Ceramic samples of the Ba 1.9 M 0.1 Co 9 O 14 (M = Ba, Sr, Ca) composition have been prepared using solid-state reaction method from mixtures of starting materials BaCO 3 , SrCO 3 , CaCO 3 , Co 3 O 4 (99.0%), taken in appropriate stoichiometric compositions, in air within the temperature range 1173-1273 K during 40 h with few intermediate grindings according to the method described earlier [13].
Samples' phase identification and determination of their unit cell parameters were performed using X-ray diffraction analysis (XRD) with a Bruker D8 Advance diffractometer (Cu Kα radiation, Ni filter). The microstructure of sintered ceramics was studied by means of a JSM -5610 LV scanning electron microscope (JEOL, Japan). Relative density (ρ rel ) of the sintered ceramic samples was calculated as where ρ app is apparent density, determined from the mass and dimensions of the samples; ρ XRD is calculated X-ray density. Thermal expansion, electrical conductivity and thermo-EMF of the samples were measured within 300-1100 K in air according to the methods described in detail elsewhere [13][14][15]. Values of average linear thermal expansion coefficient (LTEC, α av ), activation energy of electrical conductivity (E A,av ) and thermo-EMF (E S,av ) were calculated from the linear parts of Δl/l 0 = f(T), ln(σ · T) = f(1/T), and S = f(1/T) plots, respectively. The true values of LTEC (α) and activation energy for electrical conductivity (E A ) were calculated as follows: where R is gas constant, F is Faraday constant, ρ is the electrical resistivity of the sample.
Power factor values for the ceramics studied were found using equation (4)

Results and discussion
All the BCO samples after the final stage of annealing were found to be single phase within XRD accuracy; they possessed structure of layered barium cobaltite with unit cell parameters a ≈ 5.7 Å, c ≈ 29.0 Å ( Table 1). The obtained values are in good agreement within the experimental error with the data given in the literature [6][7][8][9]. The unit cell parameters for Ba 1.9 M 0.1 Co 9 O 14 (M = Sr, Ca) solid solutions do not differ much from those reported for undoped Ba 2 Co 9 O 14 ( Table 1), despite of the large difference in ionic radii between dopants and barium (for coordination number of 6 R Ba 2+ = 1.36 Å, R Sr 2+ = 1.16 Å, R Ca 2+ = 1.00 Å [16]), which is probably due to the small substitution degree of strontium or calcium (x = 0.1) in these solid solutions.
The values of relative density of the Ba 1.9 M 0.1 Co 9 O 14 ceramics were equal to 75%, 68%, and 66% for M = Ba, Sr, and Ca, respectively. This fact let us to conclude that partial isovalent substitution of barium by other alkaline-earth elements in Ba 2 Co 9 O 14 essentially decreases its sinterability.
Crystallites of BCO ceramics had a plate-like form, which is typical for ceramics of layered cobaltites; their sizes varied within 2-5 μm and the thickness changed within 0.5-1 μm (Fig. 1).
The temperature dependences of relative elongation of the studied samples demonstrate three obvious regions (Fig. 2 [8,10]. Partial substitution of barium by strontium or calcium in Ba 2 Co 9 O 14 raises α av values in the first region (before spin-state transition). This is caused by the increase in anharmonicity degree of metal-oxygen vibrations in the lattice, and resulted in the shift of the spin-state transition to the higher temperatures (Table 2, Fig. 2, light-gray rectangle area). The temperature of Co 3+ (LS) → Co 3+ (HS) transition, which was determined as a peak temperature on the α = f(T) dependences for the materials studied, was equal to 560 K, 575 K, and 600 K for Ba 2 Co 9 O 14 , Ba 1.9 Sr 0.1 Co 9 O 14 , and Ba 1.9 Ca 0.1 Co 9 O 14 , respectively (Fig. 2, insets). The values of α av in the third region of the Δl/l 0 = f(T) dependences (T > T 2 ) were essentially larger compared to those in the first one (T < T 1 ). The latter was caused by destroying of charge-ordered state of Co 2+ /Co 3+ ions in the CoO 2 layers after the spin-state transition in the layered barium cobaltite had completed [8]. It is worth noting that similar shape of Δl/l 0 = f(T) dependences was observed by us earlier for Nd 1-x Gd x CoO 3 solid solutions in which a semiconductor-metal phase transition took place within the temperature range 370-790 K due to the spin-state transition of Co 3+ ions from the low spin-state to the intermediate spin-state (IS) [17]. Ba 1.9 M 0.1 Co 9 O 14 (M = Ba, Sr, Ca) compounds were p-type semiconductors (Fig. 2), which is in good agreement with the results of [5,[7][8][9]. The values of electrical conductivity sharply (by more than two orders of magnitude) increased within 460-665 K temperature interval, which was accompanied by essential (up to three-five times) decrease in the Seebeck coefficient (Fig. 2) (Fig. 2, insets); they coincided with those found from the dilatometry results. The values of room temperature electrical conductivity in BCO ceramics increase when barium is partially substituted by strontium or calcium. Ba 1.9 Sr 0.1 Co 9 O 14 solid solution reveals highest conductivity within entire temperature interval studied (Fig. 2).
The average values of electrical conductivity activation energy for the BCO ceramics were maximal within the middle temperature (T 1 -T 2 ) region (Fig. 3, Table 3) where spin-state transition took place. At the same time, average E A values for the Ba 1.9 M 0.1 Co 9 O 14 (M = Sr, Ca) solid solutions within the high temperature region (T 2 -1000 K) were larger than those at low temperatures (300 K -T 1 ), which is in good agreement with the results obtained earlier [7,8]. Thermo-EMF values for the Ba 1.9 M 0.1 Co 9 O 14 (M = Sr, Ca) solid solutions were smaller than those for Ba 2 Co 9 O 14 cobaltite, especially in the vicinity of room temperature (Fig. 2). The shape of the σ = f(T) and S = f(T) dependences for BCO ceramics as well as the fact that E S,av < E A,av let us conclude that charge carriers in the studied materials can be described by the small polaron model [19].
The shape of power factor temperature dependences for the studied compounds were similar to that of the S = f(T) curves (Fig. 2, 4). The maximum on the P = f(T) dependence for the Ba 1.9 Sr 0.1 Co 9 O 14 solu-  Although the power factor values for the layered barium cobaltite ceramics synthesized in this work are too small to consider these oxide materials prepared using conventional solid-state reactions method as possible high-temperature thermoelectrics, they can be improved using special sintering methods (spark plasma sintering, hot pressing etc.), which can help to obtain low-porous textured ceramics with essentially larger values of electrical conductivity and, consequently, higher power factors.

Conclusions
The ceramic samples of Ba 2 Co 9 O 14 and its derivatives Ba 1.9 Me 0.1 Co 9 O 14 (Ba, Sr, Ca) were prepared using solid-state reactions method. The values of unit cell parameters and microstructure, thermal expansion, electrotransport and thermoelectric properties were determined. It was shown that synthesized materials are p-type semiconductors, in which the spin-state transition occurs within 460-700 K temperature interval that caused by change of spin state of cobalt ions. It was accompanied with sharp increase in electrical conductivity value, activation energy of electrical conductivity, and linear thermal expansion coefficient, while the Seebeck coefficient decreased. It was found that partial substitution of barium by strontium or calcium in Ba 2 Co 9 O 14 leads to the increase of spinstate transition temperature and electrical conductivity of the oxides, while thermo-EMF coefficient and power factor values were decreased.