Spin–state transition in the layered barium cobaltite derivatives and their thermoelectric properties 26 D O I: 1 0. 15 82 6/ ch im te ch .2 02 0. 7. 1. 04 A. I. Klyndyuk*, E. A. Chizhova, S. V. Shevchenko Belarus State Technological University, 13a Sverdlova St., Minsk, 220006, Belarus Republic e-mail: klyndyuk@belstu.by Dedicated to the memory of professor L. A. Bashkirov (1930–2020) 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, ther- mal 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 oc- curs 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 coef- ficient; consequently, their power factor values decrease. Keywords: layered barium cobaltite; spin-state transition; thermal expansion; electrical conductivity; thermo-EMF; power factor Received: 29.02.2020. Accepted: 20.03.2020. Published: 31.03.2020. © Klyndyuk A. I., Chizhova E. A., Shevchenko S. V., 2020 Klyndyuk A. I., Chizhova E. A., Shevchenko S. V. Chimica Techno Acta. 2020. Vol. 7, no. 1. P. 26–33. ISSN 2409–5613 Introduction Layered sodium, calcium, or bismuth– calcium cobaltites (NaxCoO2, Ca3Co4O9, Bi2Ca2Co1.7Ox) are prospective materials for producing of p-branches for high-temper- ature 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 sta- bility at elevated temperatures in air [1–4]. The common elements of crystal structure, which are present in all of these compounds, are the conducting [CoO2] layers. Layered barium cobaltite Ba2Co9O14, which belongs to the Ba2n+1ConO3n+3(Co8O8) series [5], was also tested recently as a possi- ble thermoelectric oxide [6]. Crystal structure of Ba2Co9O14 consists of alternated [CoO2] layers (CdI2–type) and octahedral trimers Co3O12 that are interconnected by corner- shared CoO4 tetrahedra [6]. Ba2Co9O14 crystallizes in the rhombohedral syngony (space group R3m, Z = 3) with the unit cell parameters: a = 5.6958(4) Å, c = 28.909(4) Å [6], a = 5.6963(8)  Å, c = 28.924(6) Å [7], 27 a = 5.69464(3) Å, c = 28.9017(2) Å [8]. Lay- ered barium cobaltite is  stable in  air up to 1303 K; it decomposes to CoO and Ba- CoO2 at higher temperatures [7]. It was reported that at room tempera- ture Ba2Co9O14 is paramagnetic (TN = 39 K [7], ≈ 40 K [8]); its resistivity was evaluated as ρ300 > 2 Ω · cm [7], or 20 Ω · cm [8, 9]. The main charge carriers in Ba2Co9O14 are electronic holes [6, 7], so, similar to the oth- er layered cobaltites, it is a p-type conduc- tor [1–4]. Electrical conductivity of layered barium cobaltite sharply increases within the  temperature range of  473–673 K [7] due to the spin-state transition of Co3+ ions inside the Co3O12 octahedral trimers from low spin-state (LS) into high spin-state (HS) near 570 K (spin-state transition) [8, 10]. Studying of magnetic, electrotransport, and thermoelectric properties of Ba1.9M0.1Co9O14 (M = Ba, La, Na) materials below room tem- perature [9] allows us to conclude that they are p-type polaronic conductors; however, the values of their figure-of-merit are too small (ZT = S2 · σ · T/λ << 1) to  consider them as  potential thermoelectric mate- rials for low-temperature applications (at T < 300 K). It was also reported [11, 12] that Ba2Co9O14 can be used as a cathode material for intermediate-temperature solid oxide fuel cells in contact with various electro- lytes, 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 Ba2Co9O14 on crystal structure, microstructure, thermal expansion, electrotransport and thermoe- lectric properties of layered barium cobaltite derivatives above room temperature. Experimental Ceramic samples of the Ba1.9M0.1Co9O14 (M = Ba, Sr, Ca) composition have been prepared using solid-state reaction method from mixtures of starting materials BaCO3, SrCO3, CaCO3, Co3O4 (99.0%), taken in  appropriate stoichiometric composi- tions, in air within the temperature range 1173–1273 K during 40 h with few inter- mediate grindings according to the method described earlier [13]. Samples’ phase identification and de- termination 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 (ρrel) of the sintered ceramic samples was calculated as ρrel = (ρapp/ρXRD) · 100%, (1) where ρapp is apparent density, determined from the mass and dimensions of the sam- ples; ρXRD is calculated X-ray density. Thermal expansion, electrical conduc- tivity and thermo-EMF of the samples were measured within 300–1100 K in air accord- ing to the methods described in detail else- where [13–15]. Values of  average linear thermal expansion coefficient (LTEC, αav), activation energy of electrical conductivity (EA,av) and thermo-EMF (ES,av) were calcu- lated from the linear parts of Δl/l0 = f(T), ln(σ · T) = f(1/T), and S = f(1/T) plots, re- spectively. The true values of LTEC (α) and activation energy for electrical conductivity (EA) were calculated as follows: α = d(Δl/l0)/dT, (2) EA = (R/F) · dlnρ/d(1/T), (3) where R is  gas constant, F is  Faraday constant, ρ is  the  electrical resistivity of the sample. 28 Power factor values for the  ceramics studied were found using equation P = S2 · σ. (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 [6–9]. The unit cell parameters for Ba1.9M0.1Co9O14 (M = Sr, Ca) solid solutions do not differ much from those reported for undoped Ba2Co9O14 (Table  1), despite of  the  large difference in ionic radii between dopants and barium (for coordination number of 6 RBa2+ = 1.36 Å, RSr2+ = 1.16 Å, RCa2+ = 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 Ba1.9M0.1Co9O14 ceramics were equal to 75%, 68%, and 66% for M = Ba, Sr, and Ca, respectively. This fact let us to  con- clude that partial isovalent substitution of barium by other alkaline-earth elements in Ba2Co9O14 essentially decreases its sin- terability. Crystallites of  BCO ceramics had a plate-like form, which is typical for ce- ramics 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 rela- tive elongation of  the  studied samples demonstrate three obvious regions (Fig. 2, Table 1 The unit cell parameters for the Ba1.9M0.1Co9O14 cobaltites M a, Å c, Å c/a V, Å3 Ba 5.697 ± 0.006 28.97 ± 0.09 5.09 ± 0.02 814.4 ± 4.1 Sr 5.696 ± 0.007 28.98 ± 0.11 5.09 ± 0.02 814.4 ± 4.9 Ca 5.703 ± 0.005 29.03 ± 0.07 5.09 ± 0.02 817.7 ± 3.2 Fig. 1. Electron microscopy image of the Ba1.9Sr0.1Co9O14 ceramics surface Table 2 Average values of LTEC (αav) for the sintered Ba1.9M0.1Co9O14 ceramics M αav, ppm/K T1, K T2, K300 – T1 T1 – T2 T2 – 1000 Ba 10.9 ± 0.7 36.6 ± 0.5 21.3 ± 0.2 475 650 Sr 13.9 ± 0.6 43.9 ± 1.1 19.7 ± 0.2 500 640 Ca 17.3 ± 0.5 28.1 ± 0.2 21.7 ± 0.1 500 700 29 Table  2); the  second one characterized by  larger values of  LTEC, corresponded to the broad spin-state transition of cobalt ions from low spin-state into high spin- state inside Co3O12 trimers in the crystal structure [8, 10]. Partial substitution of bar- ium by strontium or calcium in Ba2Co9O14 raises αav values in  the  first region (be- fore 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 (Ta- ble  2, Fig.  2, light-gray rectangle area). The temperature of Co3+(LS) → Co3+(HS) transition, which was determined as a peak temperature on the  α = f(T) dependenc- es for the  materials studied, was equal to 560 K, 575 K, and 600 K for Ba2Co9O14, Ba1.9Sr0.1Co9O14, and Ba1.9Ca0.1Co9O14, re- spectively (Fig. 2, insets). The values of αav in the third region of the Δl/l0 = f(T) de- pendences (T > T2) were essentially larger compared to those in the first one (T < T1). The  latter was caused by  destroying of charge-ordered state of Co2+/Co3+ ions in the CoO2 layers after the spin-state tran- sition in the layered barium cobaltite had completed [8]. It is worth noting that simi- lar shape of Δl/l0 = f(T) dependences was observed by us earlier for Nd1–xGdxCoO3 solid solutions in  which a  semiconduc- tor–metal phase transition took place within the temperature range 370–790 K due to the spin-state transition of Co3+ ions from the low spin-state to the intermediate spin-state (IS) [17]. Ba1.9M0.1Co9O14 (M = Ba, Sr, Ca) com- pounds were p-type semiconductors (Fig. 2), which is in good agreement with the results of [5, 7–9]. The values of elec- trical 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 coeffi- cient (Fig.  2) due to  the  change in  spin state of cobalt ions Co3+(LS) → Co3+(HS) Fig. 2. Temperature dependences of relative elongation (Δl/l0), electrical conductivity (σ), and thermo-EMF coefficient (S) for Ba2Co9O14 (a), Ba1.9Sr0.1Co9O14 (b), and Ba1.9Ca0.1Co9O14 (c). Insets show the temperature dependences of true values for LTEC (α) and for electrical conductivity activation energy (EA). Light-gray rectangle area shows the temperature interval (T1 – T2), in which the spin-state transition of Co3+ ions takes place 30 (spin-state transition) inside Co3O12 trimers [8, 10]. We have observed earlier [17, 18] similar behavior of  σ = f(T) and S = f(T) dependences for the  Nd1–xGdx- CoO3, LnCo1–xGaxCoO3 (Ln = La, Nd) solid solutions, in which semiconductor- metal phase transitions occur within tem- perature intervals of 325–860 K and 550– 950 K, respectively, due to change of spin state of Co3+ ions form the low spin-state to  the  intermediate spin-state. The  tem- peratures of  spin-state transition, which were determined as peak temperatures on the EA = f(T) dependences for the cobal- tites studied, were equal to 560 K, 575 K, and 600 K for Ba2Co9O14, Ba1.9Sr0.1Co9O14, and Ba1.9Ca0.1Co9O14, respectively (Fig. 2, insets); they coincided with those found from the  dilatometry results. The  values of room temperature electrical conductiv- ity in BCO ceramics increase when bar- ium is partially substituted by strontium or calcium. Ba1.9Sr0.1Co9O14 solid solution reveals highest conductivity within entire temperature interval studied (Fig. 2). The  average values of  electrical con- ductivity activation energy for the  BCO ceramics were maximal within the  mid- dle temperature (T1–T2) region (Fig.  3, Table 3) where spin-state transition took place. At the same time, average EA values for the Ba1.9M0.1Co9O14 (M = Sr, Ca) solid solutions within the high temperature re- gion (T2–1000 K) were larger than those at  low temperatures (300 K – T1), which is in good agreement with the results ob- tained earlier [7, 8]. Thermo-EMF values for the  Ba1.9M0.1Co9O14 (M = Sr, Ca) sol- id solutions were smaller than those for Ba2Co9O14 cobaltite, especially in the vicin- ity 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 ES,av < EA,av let us conclude that charge car- riers in  the  studied materials can be de- scribed 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 Ba1.9Sr0.1Co9O14 solu- Fig. 3. Dependences of ln(σ · T) = f(1/T) and S = f(1/T) for Ba1.9Sr0.1Co9O14. The average values for electrical conductivity activation energy (EA,av) and thermo-EMF activation energy (ES,av) are given near the corresponding lines Table 3 Values of average activation energy of electrical conductivity (EA,av) and thermo-EMF (ES,av) of Ba1.9M0.1Co9O14 cobaltites M EA,av, eV T1, K T2, K ES,av, eV300 — T1 T1 – T2 T2–1000 Ba 0.166 ± 0.011 0.711 ± 0.035 0.143 ± 0.006 495 650 0.083 ± 0.003 Sr 0.164 ± 0.012 0.884 ± 0.032 0.311 ± 0.003 525 665 0.065 ± 0.001 Ca 0.119 ± 0.012 0.470 ± 0.020 0.128 ± 0.008 460 640 0.058 ± 0.001 31 tion was shifted towards the larger tem- perature, while that for Ba1.9Ca0.1Co9O14 was shifted down to smaller temperature, as compared to the Ba2Co9O14 parent phase. The  maximal value of  power factor ob- tained for the Ba2Co9O14 cobaltite was equal to 3.36 μW/(m · K2) at 600 K; all P values of this phase within the entire temperature interval studied were larger than those for the Ba1.9M0.1Co9O14 (M = Sr, Ca) solid solu- tions, except the temperature interval 700– 950 K, in which Ba1.9Sr0.1Co9O14 possessed maximal of the power factor values, mainly due to the fact that its Seebeck coefficient was essentially smaller in comparison with unsubstituted Ba2Co9O14 phase. Although the power factor values for the layered barium cobaltite ceramics syn- thesized in this work are too small to con- sider these oxide materials prepared using conventional solid-state reactions method as possible high-temperature thermoelec- trics, they can be improved using special sintering methods (spark plasma sintering, hot pressing etc.), which can help to obtain low-porous textured ceramics with essen- tially larger values of electrical conductivity and, consequently, higher power factors. Conclusions The ceramic samples of Ba2Co9O14 and its derivatives Ba1.9Me0.1Co9O14 (Ba, Sr, Ca) were prepared using solid-state reactions method. The values of unit cell parameters and microstructure, thermal expansion, electrotransport and thermoelectric prop- erties were determined. It was shown that synthesized materials are p-type semicon- ductors, in which the spin-state transition occurs within 460–700 K temperature in- terval that caused by change of spin state of  cobalt ions. 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