Oxygen non-stoichiometry and thermodynamic properties of Bi2Sr2CoO6+δ ceramics

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ScienceDirect Journal of the European Ceramic Society 34 (2014) 1219–1225

Oxygen non-stoichiometry and thermodynamic properties of Bi2Sr2CoO6+δ ceramics O. Jankovsk´y a,∗ , D. Sedmidubsk´y a , Z. Sofer a , K. Rubeˇsová a , K. R˚uzˇ iˇcka b , P. Svoboda c a

Department of Inorganic Chemistry, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic Department of Physical Chemistry, Institute of Chemical Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic c Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, 121 16 Prague 2, Czech Republic b

Received 16 October 2013; received in revised form 1 November 2013; accepted 10 November 2013 Available online 3 December 2013

Abstract In the literature the structure Bi2 Sr2 CoO6+δ has been described with a variable oxygen nonstoichiometry δ. We identified δ parameter by thermogravimetric measurements and determined thermodynamic properties of Bi2 Sr2 CoO6+δ . The heat capacity and enthalpy increments of Bi2 Sr2 CoO6+δ were measured by the relaxation time method (PPMS) from 2 to 256 K, by differential scanning calorimetry (DSC) from 258 to 355 K and by drop calorimetry from 573 to 1123 K. Above-room temperature, the dependence of molar heat capacity under isodynamical (constant pO2 ) and conventional isoplethal conditions (constant composition) was derived from the experimental data by the least-squares method. The low temperature heat capacity was analyzed in terms of a combined Debye–Einstein model. The molar entropy Sm◦ (298.15) = 318.9 J K−1 mol−1 was evaluated from the low-temperature heat-capacity measurements. © 2013 Elsevier Ltd. All rights reserved. Keywords: Bismuth cobaltites; Heat capacity; Debye–Einstein model; Oxygen non-stoichiometry

1. Introduction Bi Sr Co O and Ca Co O systems have recently attracted attention of material scientists due to thermoelectric properties of some binary and ternary phases in these systems: the misfit layered compounds as [Ca2 CoO3 ][CoO2 ]1.62 (known as Ca3 Co4 O9 ) and [Bi2 Sr2 O3 ][CoO2 ]1.82 are the potential candidates for p-type cells in thermoelectric batteries for hightemperature energy recovery.1 The thermodynamic properties of Ca3 Co4 O9 and the corresponding phase diagram of the Ca Co O system are well known,2,3 while the phase diagram of Bi Sr Co O system has not been investigated yet. The pseudobinary system Bi Sr O was reported by Halstedt4 and we recently assessed the thermodynamic behavior of Costabilized ␥-Bi2 O3 and the liquid phase in the Bi Co O system.5 Bi Sr Co O based thermoelectrics and its thermoelectric properties were studied by Sotelo et al.6,7

∗

Corresponding author. Tel.: +420 220 444 416. E-mail address: [email protected] (O. Jankovsk´y).

0955-2219/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jeurceramsoc.2013.11.008

The Bi2 Sr2 CoO6+δ phase is a non-superconducting analog to the phase Bi2 Sr2 CuO6+δ (also called Raveau phase), representing the first member of the Bi2 Sr2 Can−1 Cun O4+2n+δ homologous series from the high Tc superconducting cuprates family (see Fig. 1). The structure is composed of sub-layers stacked in a sequence BiO SrO CoO2 SrO BiO . The double Bi2 O2+δ slabs are displaced along c axis, and the amplitude of the displacement oscillates sinusoidally along a.8 Moreover the cooperative displacement of the oxygen atoms in each BiO layer allows incorporation of an extra oxygen per modulation period giving rise to oxygen stoichiometry parameter δ ∼ 0.259 as shown in Fig. 2a. By contrast Hsu et al. performed an infrared and optical reflectance study10 on single crystals grown by traveling solvent floating zone method whose oxygen stoichiometry ranged from δ ∼ 0.4 to 0.5. It should be noted that various modulation patterns of BiO layers have been identified on structurally related cuprates,11 ferrates12 and manganates13 leading to different oxygen stoichiometries. Moreover the oxygen stoichiometry can be modified by a variation of modulation vector by controlling the cation composition or oxygen activity as has been demonstrated in the case of Raveau phase.9 The possible structure of

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capacity data in terms of a combined Debye–Einstein model and evaluated the molar entropy. This knowledge is essential for future phase-equilibria modeling in Bi Sr Co O system and tailoring the thermoelectric properties of Bi-based cobaltites ceramics. 2. Experimental

Fig. 1. Structure of the Bi2 Sr2 CoO6.25 .

Bi O sub-layers with resulting oxygen stoichiometry δ ∼ 0.5 is shown in Fig. 2b and corresponds to that found in Bi2 Sr2 MnO6.5 .13 Magnetic and calorimetric studies were carried out on singlecrystal Bi2 Sr2 CoO6+δ by Shi et al.14 Magnetic susceptibility data were measured from 5 to 300 K and the heat capacity was measured between 78 and 285 K. Raman spectrum of Bi2 Sr2 CoO6+δ was measured by Farrow et al.15 giving seven bands at 41, 77, 92, 141, 303, 561 and 669 cm−1 , the lower four bands can be attributed to vibrational modes of Bi. In this work, we prepared a single phase polycrystalline sample by the ceramic route from pure oxides and carbonates. We measured the variation of δ parameter by thermogravimetric measurements and we compare it with other available data from literature. Having identified the oxygen non-stoichiometry, we determined thermodynamic properties of Bi2 Sr2 CoO6+δ by measuring the heat capacity by the relaxation time method and differential scanning calorimetry, as well as the relative enthalpies by the drop calorimetry. After the fitting of the experimental data we obtained above-room temperature dependence of the molar heat capacity. We analyzed low-temperature heat

Fig. 2. Structure of the Bi O sub-layers in Bi2 Sr2 CoO6.25 (A) and Bi2 Sr2 CoO6.5 (B).

The sample with the overall stoichiometry Bi2 Sr2 CoO6+δ was prepared by a solid state reaction from pure powders of SrCO3 (99.9%, Aldrich), Bi2 O3 (99.9%, Aldrich) and Co2 O3 (99.9%, Riedel-de Haen). The mixture of powders was homogenized in an agate mortar and twice calcined in a platinum crucible at 1003 and 1053 K in air for 48 h. Calcined powder was milled manually and pressed for 0.5 GPa. The sample was sintered in air atmosphere at 1123 K for 100 h. Pellets were manually milled and the procedure of sintering was two times repeated (at 1133 and 1153 K). Let us note that a very slow kinetics of Bi2 Sr2 CoO6+δ formation is, similarly to the superconductive cuprates, caused by the layered character of the structure and presumably by a small Gibbs energy difference between Bi2 Sr2 CoO6+δ and the competing phases. The sample was analyzed by X-ray powder diffraction (XRD) on Bruker AXS D8 θ–θ powder diffractometer with parafocusing Bragg–Brentano geometry using CoK␣ radiation ˚ U = 34 kV, I = 20 mA). X-ray diffraction patterns (λ = 1.79021 A, were compared with theoretical diffractogram which was published by Tarascon et al.8 Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) were performed simultaneously from 293 K to 1350 K on Setaram STA Setsys Evolution with a heating rate of 10 K min−1 . The PPMS equipment 14 T-type (Quantum Design, USA) was used for the heat capacity measurements in the lowtemperature region similarly to work.16 The measurements were performed by the relaxation method17 with fully automatic procedure under high vacuum (pressure ∼10−2 Pa) to avoid heat loss through the exchange gas. The sample was a compressed powder plate of ∼25.0 mg. The density of the pressed sample was around 75% of the theoretical density. The sample was mounted to a calorimeter platform with cryogenic grease Apiezon N (supplied by Quantum Design). Prior to the sample run, a blank sample holder with the Apiezon only was measured in the same temperature regime to obtain background data that involve the heat capacity of Apiezon grease exhibiting a sol–gel transition below room temperature. The sample heat capacity was then obtained from the difference between the two data sets acquired on sample and blank run. The manufacturer claims the precision of this measurement better than 2%. A Micro DSC III calorimeter (Setaram, France) was used for the heat capacity determination in the temperature range of 258–355 K. First, the samples were preheated in a continuous mode from room temperature up to 355 K. Then the heat capacity was measured in the incremental temperature scanning mode consisting of a number of 5 K steps (heating rate 0.3 K min−1 ) followed by the isothermal delays of 40 min. Two subsequent step-by-step heating runs were recorded for

O. Jankovsk´y et al. / Journal of the European Ceramic Society 34 (2014) 1219–1225

Fig. 3. Powder X-ray diffraction pattern of Bi2 Sr2 CoO6.5 analyzed by Rietveld refinement. Impurities in the sample are marked ().

each sample. Synthetic sapphire, NIST Standard reference material No. 720, was used as a reference. The uncertainty of heat capacity measurements is estimated to be better than ±1%. Enthalpy increment determinations were carried out by drop method using high temperature calorimeter, Multi HTC 96 (Setaram, France). All measurements were performed in air by the alternating dropping of reference material (small pieces of synthetic sapphire, NIST Standard reference material No. 720) and of the sample (small pieces of pellets) being initially held at room temperature, through a lock into the working cell of the preheated calorimeter. Endothermic effects are detected and the relevant peak area is proportional to the heat content of the dropped specimen. The measurements were performed in the temperature range 573–1123 K on samples of 200–250 mg. The delays between two subsequent drops were 20 min to stabilize the heat flow. Estimated overall accuracy of the drop measurements is ±3%. 3. Results and discussion XRD measurements were performed after each sintering step. After the first sintering the sample contained a mixture of three phases: the misfit layered cobalatite [Bi2 Sr2 O3 ][CoO2 ]1.82 , Bi2 Sr2 O5 and Bi2 Sr2 CoO6+δ . After the final sintering only the phase Bi2 Sr2 CoO6+δ was present except for a small amount of impurity with a major reflexion at 2θ ∼ 20.6 (see XRD pattern in Fig. 3). This impurity can be attributed to misfit cobaltite [Bi2 Sr2 O3 ][CoO2 ]1.82 in an amount of ∼1%. Considering the accuracy of heat capacity measurement and similar phonon spectra of Bi2 Sr2 CoO6+δ and [Bi2 Sr2 O3 ][CoO2 ]1.82 we do believe that such impurity has no apparent effect on the resulting data. By means of simultaneous DTA and TGA analysis the melting temperature was determined at 1185 K; hence the enthalpy increments were measured only up to 1123 K. The main peak on Fig. 4 is the melting effect, but there is another smaller peak at lower temperature connected to the main peak. It can be explained as a swift oxygen release from BiO sublayers likely associated with a disruption of their modulation a few degrees below melting or, alternatively, as a melting of

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Fig. 4. Results of DTA (blue line) and TG analysis (black line) of Bi2 Sr2 CoO6+δ . The TGA curve was recalculated to oxygen non-stoichiometry coefficient δ (δ = 0 corresponds to Co2+ , δ = 0.5 to Co3+ ). The dashed (red) line represents the fitted curve according to Eq. (2). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

[Bi2 Sr2 O3 ][CoO2 ]1.82 impurity phase. Thermogravimetric analysis showed that the sample lost 0.45% of its weight during the heating prior to melting and another 0.06% during the melting process. We can assume that all the weight loss can be attributed to an oxygen release. Assuming the maximum oxygen stoichiometry δ = 0.5 corresponding to oxygen saturation in low temperature limit, which was confirmed by previous TGA measurements by Tarascon et al.8 and Hsu et al.,10 then the oxygen content corresponds to δ = 0.29 just before melting and δ = 0.25 after the melting. Let us note that Tarascon et al. found by chemical analysis the formal valence for Co ∼ 2.3, resulting in δ ∼ 0.15. Further mass decrease on heating above 1200 K indicates the incongruent melting into a liquid containing Co2+ , Sr2+ and Bi3+ and O2– ions and a solid phase with Co valency above Co2+ . The tubular structure cobaltite Bi4 Sr12 Co8 O28 18 with ∼Co2.5+ or non-modulated Bi221 with approximate stoichiometry BiSr3 CoO6 and ∼Co3+ 19 are the possible candidates. As the temperature increases the liquidus surface moves from Bi toward Sr Co, the melt content increases (the solid fraction becomes smaller) and oxygen is released from the condensed system. The TGA data can be analyzed in terms of a simple oxygen non-stoichiometry model based on an ideal mixing of Bi4 Sr4 CoO12 (x) and Bi4 Sr4 CoO13 (1 − x) end-members and the oxygen exchange reaction Bi4 Sr4 CoO13 → Bi4 Sr4 CoO12 + ½O2 whose equilibrium constant is defined as K=e

¯ ¯ ΔH O ΔSO RT − R

1/2

=

xpO2

1−x

(1)

¯ O and S¯ O are, respectively, the relative partial molar where H enthalpy and entropy of oxygen. The oxygen non-stoichiometry coefficient δ in Bi2 Sr2 CoO6+δ then reads δ = 0.5 −

x K = 0.5 − 1/2 2 2pO2 + 2K

(2)

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Fig. 5. Low-temperature heat capacity of Bi2 Sr2 CoO6.5 (compared with data reported by Shi et al.) and the Cp /T vs T2 plot (for T < 10 K) in the inset.

and its temperature dependence for a given value of pO2 = 0.21 can be described by means of two parame¯ O = −26.22 ± 0.24 kJ mol−1 and S¯ O = −13.19 ± ters, H 0.25 J mol−1 K−1 . The resulting model curve is shown in Fig. 4 together with the experimental data. The measured thermodynamic data used for further analysis involve 93 Cp values from relaxation time method (2–256 K), 40 points from DSC (258–355 K) and 24 values of enthalpy increments from the drop calorimetry measurements (573–1123 K). All Cp data are plotted in Figs. 5 and 6 and the enthalpy increment data are shown in Fig. 7. The thermodynamic data published by Shi et al.14 are plotted in Fig. 5 for comparison. The fit of the low-temperature heat capacity data (LT fit) consists of two steps. Assuming the validity of the phenomenological formula Cp = βT3 + ␥el T at T → 0 where β is proportional to the Debye temperature θ D and γ el T represents

Fig. 7. Enthalpy increments of Bi2 Sr2 CoO6+δ corrected for the enthalpy needed to release oxygen from the stoichiometry δ = 0.5 to a particular δ at a given temperature. Solid line – simultaneous fit of relative enthalpy (drop) and heat capacity (DSC).

the Sommerfeld electronic term, we plotted the Cp /T vs. T2 dependence for T < 10 K (see the inset in Fig. 5). The resulting linear fit yielded a negligible value of γ el which is consistent with the insulating nature of the cobaltite under study. The Cp /T vs. T2 plot provides simultaneously the estimation of the Debye temperature θ D , as in this temperature range only three acoustic phonons are populated. From the fitted cubic term parameter β = 1.54 ± 0.02 mJ mol−1 K−4 we obtain the value θ D = 108 K, which was used as an initial guess parameter in the following analysis. The analysis of the lattice heat capacity in the temperature range 2–355 K was performed similarly to work.3 It is based on an additive combination of Debye and Einstein modes. The phonon spectrum of a polyatomic compound contains three acoustic branches and 3n − 3 optical ones, where n is number of atoms per primitive unit cell. In our case, considering the structure formula Bi2 Sr2 CoO6.5 (a complete saturation with oxygen at low temperatures) this represents 31.5 optical branches. The acoustic part of the phonon heat capacity is then described using the Debye model in the form 

CphD

Fig. 6. High temperature heat capacity of Bi2 Sr2 CoO6+δ . Solid lines represent the D–E fit of low temperature data and the isoplethal heat capacity simultaneous fit of relative enthalpies (drop) and DSC data. Dashed line – heat capacity at constant oxygen activity (pO2 = 0.21) involving the saturation contribution, dashed-dotted line – NKR. Inset – difference between isodynamical and isoplethal heat capacity (saturation contribution) and the difference between Cp,δ = 0.5 and the real Cp,δ .

T = 9R ΘD

3 xD 0

x4 exp(x) dx (exp(x) − 1)2

(3)

where R is the gas constant, θ D is the Debye characteristic temperature and xD = θ D /T. Similarly, the individual optical branches are described in terms of Einstein model using the characteristic Einstein temperatures θ Ei as parameters

CphEi = R ·

2 exEi wi xEi , (exEi − 1)2

(4)

O. Jankovsk´y et al. / Journal of the European Ceramic Society 34 (2014) 1219–1225 Table 1 Parameters of Debye–Einstein model for lattice heat capacity (θ D and θ Ei ) evaluated by non-linear least-square fit.

θ i /K wi α

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by the linear least-squares method (HT-fit). The temperature dependence of Cp was considered in the form

D

E1

E2

E3

Cp = A + B · T + C · T −2

96.0 ± 5.4 3 0.000475

168.2 ± 8.8 10 0.000404

357.5 ± 6.6 10 0

655.2 ± 11.0 11.5 0

thus the related temperature dependence H(T) = H(T) − H(T0 ) is given by equation

(6)

ΔH(T ) = H(T ) − H(T0 )

where xEi has an analogous meaning as in the previous case and wi refers to a degeneracy of the corresponding Einstein mode. The heat capacity then reads

Cp = Cel + CphD +

3 

CphEi ,

(5)

i=1

where the sum runs over three Einstein modes whose degeneracies are specified in Table 1. Let us note that the grouping of several optical phonon modes into a single Einstein mode with a degeneracy wi is arbitrary and is only qualified by the requirement of reasonable number of free parameters and the best achievable fit to the experimental data. Furthermore, a correction for anharmonicity was considered, which is responsible for an additive term at higher temperatures and which accounts for the discrepancy between isobaric and isochoric heat capacity. According to literature,20 the term 1/(1 − αT) was considered as a multiplication factor and applied on the Debye and the first Einstein mode. A fit with lower standards errors can be obtained by considering a single anharmonicity parameter αD (smaller off-diagonal elements in correlation matrix), however, in that case a relatively large value of αD = 0.00127 results in a divergence of the fitted Cp (T) curve at 780 K. The model parameters given in Table 1 and the resulting fitting curve shown in Fig. 5 are based on a non-linear least-square applied on the measured heat capacity data (from both the relaxation method and DSC). Note the agreement between the Debye temperature obtained from the low temperature (T < 10 K) and the entire range of heat capacity data. The absolute entropy of Bi2 Sr2 CoO6.5 at the reference temperature T = 298.15 K was evaluated by integrating the experimental data of heat capacity divided by the thermodynamic temperature from lowest available temperature T = 2 K up to ambient temperature as S◦ (298.15) = 318.4 J mol−1 K−1 . If we integrate the model curve resulting from Debye–Einstein fit we obtain S◦ (298.15) = 318.9 ± 2.7 J mol−1 K−1 . Similarly the integration of the experimental heat capacity data and the fitted Cp (T) function yields the relative enthalpy H◦ (298.15) − H◦ (0) = 48.46 kJ mol−1 and H◦ (298.15) – H◦ (0) = 48.55 ± 0.07 kJ mol−1 , respectively. For the assessment of temperature dependence of Cp above room temperature, the heat capacity data from DSC (11 selected points form the interval 258–355 K) and the enthalpy increment data from drop calorimetry were treated simultaneously

1 = A · (T − T0 ) + B · (T 2 − T02 ) − C · 2



1 1 − T T0

of

 (7)

The sum of squares which is minimized has the following form  N(C p )  C F = w2i Cp,i − A − B · Ti − 2 Ti i=1  N(H) 2 )  (Tj2 − T0,j 2 wj Hj A(Tj − T0,j ) − B + 2 j=1

 +C ·

1 1 − Tj T0,j

2 → min

(8)

where the first sum runs over the Cp experimental points and the second sum over the H experimental points. Different weights wi (wj ) were assigned to individual points calculated as wi = 1/δi (wj = 1/δj ) where δi (δj ) is the absolute deviation of the measurement estimated from overall accuracies of measurements (1% for DSC and 3% for drop calorimetry). Both types of experimental data thus gain comparable significance during the regression analysis. The temperature dependence of heat capacity of Bi2 Sr2 CoO6+δ in the temperature region from 298 K to 1123 K can be expressed as: Cp = [(256.3 ± 18.2) + (0.09708 ± 0.02813) · T − (2.2284 ± 0.8903) · 106 · T −2 ] J K−1 mol−1

(9)

However, since the oxygen stoichiometry varies in the high temperature region, the measured heat capacity does not correspond to isoplethal conditions (constant composition of all components) but to isodynamical conditions (constant activity of a component being exchanged with the surrounding atmosphere). As recently shown by Holba and Sedmidubsk´y21 the excess (saturation) contribution associated with oxygen release can be found in the form   ¯ O ∂δ sat Cp = Cp,pO2 − Cp,δ = H (10) ∂T p,pO 2

¯ O = 1/2H ◦ + H ¯ O is the partial molar enthalpy of where H O2 ¯ O and substituting it oxygen. Thus taking the fitted value of H to Eqs. (2) and (10) (along with S¯ O and pO2 = 0.21) we obtain the sat Cp (T) function which is plotted in the inset of Fig. 6.

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Apparently such a behavior can be hardly described by threeterm polynomial in the form of Eq. (9) and the measured relative for the enthalpy associated enthalpy data have to be corrected

with oxygen release, sat H = sat Cp (T)dT, before the threeparameter fit can be applied. The resulting heat capacity (both isoplethal, Cp,δ , and isodynamical, Cp,pO2 = Cp,δ + sat Cp ) as well as the relative enthalpy are shown in Figs. 6 and 7, respectively. The corrected heat capacity Cp,δ in the temperature region from 298 K to 1123 K was assessed by applying the same simultaneous regression procedure and can be expressed as Cp,δ = [(264.9 ± 18.9) + (0.07662 ± 0.02916) · T − (2.4544 ± 0.9288) · 106 · T −2 ] J K−1 mol−1

(11)

In Fig. 6 it is compared to Neumann–Kopp rule which is given as: Cp (Bi2 Sr 2 CoO6+δ ) = Cp (Bi2 O3 )+2Cp (SrO) + δCp (Co3 O4 ) + (1 − 3δ)Cp (CoO)

(12)

where only the nonmagnetic contribution of Cp (Co3 O4 ) was considered and all other values of heat capacity for binary oxides were taken from the FactSage database [22]. Let us note that the isopletal heat capacity, though defined as Cp,δ = (∂H/∂T)p,δ=const , still refers to different oxygen stoichiometries at different temperatures. It would be thus interesting to recalculate Cp,δ to molar heat capacity defined for a fixed oxygen stoichiometry (e.q. δ = 0.5) at any temperature. This can be done by the following correction: Cp,δ=0.5 = Cp,δ + δCO

(13)

where CO is the heat capacity associated with the phonon modes of released oxygen atoms. The corresponding correction curve given in the inset of Fig. 6 was calculated from the Einstein term, Eq. (4), considering the weight wi = 3 (three independent vibration modes of a single oxygen atom) and the highest Einstein temperature θ E3 = 655 K. A more general form of Eq. (13) ¯ O (T, δ) dependence,21 but its application would involves the H require a more detailed assessment of relative partial molar enthalpy of oxygen. 4. Conclusion In this study, we explored the thermochemical properties of the cobalt analog of high temperature superconducting cuprate sometimes referred to as Raveau phase. The Bi2 Sr2 CoO6+δ phase reveals a variable oxygen stoichiometry δ ∼ 0.5–0.29 which likely results from a combination of several modulation patterns and/or a variable modulation vector in Bi–O double layers. The exchange of oxygen with surrounding atmosphere is manifested by an excess term in heat capacity which was assessed from δ(T, pO2 = 0.21) dependence measured by thermogravimetry. Using this correction we were able to derive the conventional heat capacity defined for isoplethal conditions from the measured data of heat capacity and relative enthalpies obtained under isodynamical conditions (fixed oxygen activity).

Moreover, the temperature dependence of heat capacity for the stoichiometric Bi2 Sr2 CoO6.5 end-member was estimated from the Debye–Einstein analysis of low temperature Cp data. This approach can be applied in the analysis of thermochemical properties of other ceramic systems exhibiting a variable oxygen stoichiometry. Acknowledgment This work was supported by Czech Science Foundation, Grant No. 13-17538S, and by Ministry of Education of the Czech Republic, Grant No. 20/2013 for specific university research. Experiments were performed in MLTL (http://mltl.eu/), which is supported within the program of Czech Research Infrastructures (project no. LM2011025). The work of P. S. was supported by Czech Science Foundation of the Czech Republic, Grant No. P108-10-1006. References 1. Fergus JW. Oxide materials for high temperature thermoelectric energy conversion. J Eur Ceram Soc 2012;32:525–40. 2. Sedmidubsky D, Jakes V, Jankovsky O, Leitner J, Sofer Z, Hejtmanek J. Phase equilibria in Ca-Co-O system. J Solid State Chem 2012;194: 199–205. 3. Jankovsky O, Sedmidubsky D, Sofer Z, Simek P, Hejtmanek J. Thermodynamic behavior of Ca3 Co3.93+x O9+δ ceramics. Ceram-Silikaty 2012;56:139–44. 4. Hallstedt B, Gauckler LJ. Revision of the thermodynamic descriptions of the Cu–O, Ag–O, Ag–Cu–O, Bi–Sr–O, Bi–Ca–O, Bi–Cu–O, Sr–Cu–O Ca–Cu–O and Sr–Ca–Cu–O systems. Calphad 2003;27:177–91. 5. Jankovsky O, Sedmidubsky D, Sofer Z. Phase diagram of the pseudobinary system Bi–Co–O. J Eur Ceram Soc 2013;33:2699–704. 6. Sotelo A, Guilmeau E, Rasekh Sh, Madre MA, Marinel S, Diez JC. Enhancement of the thermoelectric properties of directionally grown Bi–Ca–Co–O through Pb for Bi substitution. J Eur Ceram Soc 2010;30:1815–20. 7. Sotelo A, Rasekh Sh, Madre MA, Guilmeau E, Marinel S, Diez JC. Solutionbased synthesis routes to thermoelectric Bi2 Ca2 Co1.7 Ox . J Eur Ceram Soc 2011;31:1763–9. 8. Tarascon JM, Miceli PF, Barboux P, Hwang DM, Hull GW, Giroud M, et al. Structure and magnetic properties of nonsuperconducting doped Co and Fe Bi2 Sr2 Cu1−x Mx Oy phases. Phys Rev B 1989;39:11587–98. 9. Tarascon JM, Le Page Y, McKinnon WR. Synthesis and structural effects of doped and undoped high Tc Bi phases. Eur J Solid State Inorg Chem 1990;27:81. 10. Hsu HC, Chou FC, Koyama K, Watanabe K, Liu HL. Spin-phonon coupling in antiferromagnetic Bi2 Sr2 CoO6+␦ : an infrared reflectance study. Phys Rev B 2009;79:155109. 11. Beskrovnyi AI, Dlouhá M, Jirák Z, Pollert E. Neutron diffraction study of the modulated structure of Bi2 (Sr Ca)3 Cu2 O8+y . Physica C 1990;166:79–86. 12. Le Page Y, Mc Kinnon WR, Tarascon JM, Barboux P. Origin of the incommensurate modulation of the 80-K superconductor Bi2 Sr2 CaCu2 O8.21 derived from isostructural commensurate Bi10 Sr15 Fe10 O46 . Phys Rev B 1989;40:6810–6. 13. Jirák Z, Pollert E, Sedmidubsk´y D, Dlouhá M, Vratislav S. Neutron diffraction study of Bi2 Sr2 MnO6.5 . Physica C 1992;196:68–72. 14. Shi JB, Ho JC, Lee TJ, Chiou BS, Ku HC. Cobalt ordering in layered Bi2 Sr2 CoO6+δ single crystal. Physica C 1993;205:129–32. 15. Farrow LA, Ramesh R, Tarascon JM. Raman spectra of bismuth cuprate high-Tc superconductors and 3d-metal-substituted phases. Phys Rev B 1991;43:418–23. 16. Leitner J, Sedmidubsk´y D, R˚uzˇ iˇcka K, Svoboda P. Heat capacity, enthalpy and entropy of SrBi2 O4 and Sr2 Bi2 O5 . Thermochim Acta 2012;531: 60–5.

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