Phase Instability, Oxygen Desorption and Related Properties in Cu-Based Perovskites Modified by Highly Charged Cations

Abstract

The rock-salt ordered A₂CuWO₆ (A = Sr, Ba) with I4/m space group and disordered SrCu_0.5M_0.5O_3−δ (M = Ta, Nb) with Pm3m space group perovskites were successfully obtained via a solid-state reaction route. Heat treatment of Ba₂CuWO₆ over 900 °C in air leads to phase decomposition to the barium tungstate and copper oxide. Thermogravimetric measurements reveal the strong stoichiometric oxygen content and specific oxygen capacity (ΔW_o) exceeding 2.5% for Ba₂CuWO₆. At the same time, oxygen content reveals Cu³⁺ content in SrCu_0.5Ta_0.5O_3−δ. Under the following reoxidation of Ba₂CuWO₆, step-like behavior in weight changes was observed, corresponding to possible Cu⁺ ion formation at 900 °C; in contrast, no similar effect was detected for M⁵⁺ cations. The yellow color of Ba₂CuWO₆ enables to measure the band gap 2.59 eV. SrCu_0.5Ta_0.5O_3−δ due to high oxygen valance concentration has a low thermal conductivity 1.28 W·m⁻¹·K⁻¹ in the temperature range 25–400 °C.

1. Introduction

Complex oxides with perovskite structure ABO₃ are well known because of the variety of properties enabling applications in different industrial and research areas. Such perovskites could be considered as semiconductors [1,2], dielectrics [3], thermoelectrics [4], thermal barrier coatings [5], optical and luminescence materials [6] and others. Perovskites with oxygen nonstoichometry and effective oxygen transport could be used as materials for heat storage or combustion. Due to dramatic climate changes, ecological friendly technologies are in great demand.

The green energy part in total heat power production is projected to increased annually until 2040 [7]. Nevertheless, in 2022 more than 60% of electricity was generated by fuel combustion: coal (34%), oil (38%) and natural gas (28%) [8]. Therefore, material design for ecologically friendly power generation technologies such as chemical looping with oxygen uncoupling (CLOU) is still currently required. The CLOU approach is based on the use of oxygen carriers (OCs) that uptake oxygen from the air for subsequent fuel combustion [9]. Then, the oxygen-depleted OC particles are transferred to the air reactor for reoxidation.

OC properties significantly affect the fuel conversion within the CLOU process; as a consequence, several requirements were considered [9,10,11,12,13]:

• The combination of temperature and equilibrium values of oxygen partial pressure should be sufficient to provide a complete conversion in a fuel reactor and fast oxygen saturation of OCs under oxidation.
• The reaction in the fuel reactor should be exothermic, providing some temperature increase, which promotes the release rate of gas-phase oxygen.
• Rather high oxygen capacity (>3%).
• Thermodynamic stability under reducing atmospheres specifically for the fuel reactor.
• Elevated kinetic parameters of oxygen exchange with ambient gas.

Cu-based OCs are of special interest due to the combination of high values of oxygen storage with fast oxygen exchange with the gas phase. However, reducing conditions are known to promote metallic copper formation, which tends to melt and agglomerate at elevated temperatures. Such an objectionable effect leads to a reduction in effective surface area of the particles used and as a consequence a decrease in combustion efficiency. Therefore, most research has paid attention to the design of composite materials where CuO is applied as the active component deposited on the inert material support such as alumina, zirconia, ceria [9,11,14,15].

Among the copper-based complex oxides, Ruddlesden–Popper compounds related to a K₂NiF₄-type structure are well recognized. These materials have demonstrated elevated catalytic activity, oxygen exchange rate and electrochemical performance as cathodes for solid oxide fuel cells with unusual magnetic properties [16]. Special attention is paid to the copper-based phase La₂CuO₄ [17]. These materials are also characterized by having enough stability with respect to crystal structure and functional characteristics at high temperatures but they can be completely decomposed in reducing media. On the other hand, complex oxides with perovskite structure are suggested as promising OCs because of their long-term mechanical stability, good fluidization properties, and appreciable reactivity with gaseous fuels [9,12,18,19,20]. It is interesting to combine the properties of copper-based oxides in a structured perovskite-like motif. For example, pure copper perovskite LaCuO_3−δ can be obtained only when an oxygen-rich atmosphere is applied; moreover, in CLOU processes it tends to decompose and mostly fails to reassemble within the reoxidation cycle [21,22]. Such an effect is connected with the presence of unstable Cu³⁺ cations. The structure and charge stabilization could be achieved by a partial doping with high valence cation as Ta⁵⁺, Nb⁵⁺, W⁶⁺. However, there is a lack of high temperature redox behavior and thermal stability of such perovskites.

Considering the aforementioned data, it is interesting to examine properties of copper-containing complex oxides and to conduct an initial assessment for its implementation as the OC material for the CLOU process [23]. In addition, some basic properties of the studied compounds such as thermal conductivity and expansion coefficient as well as a band gap value were estimated in order to extend existing characteristics. As the stable copper compound, the rock-salt ordered double A₂CuWO_6-δ where A = Ba or Sr and disordered SrCu_0.5M_0.5O_3-δ where M = Nb, Ta perovskite-type phases were chosen for the study.

2. Materials and Methods

2.1. Materials and Sample Preparation

The A₂CuWO_6−δ and SrCu_0.5M_0.5O_3−δ powders where A = Sr, Ba and M = Ta, Nb were synthesized via a conventional solid-state route. High-purity tungsten (WO₃ 99.999%), copper (CuO 99.99%), tantalum (Ta₂O₅, 99.999%), niobium (Nb₂O₅, 99.999%) oxides and barium (BaCO₃ 99.99%) and strontium (SrCO₃ 99.99%) carbonates were selected as raw materials. The starting reagents were weighed in required ratios and mixed for 30 min in ethanol media. After thorough grinding in agate mortar, the compositions were fired for 12 h at 870 °C in the air in an alumina crucible. The temperature was chosen because of the endothermic peak corresponding to the solid-state reaction observed at DSC earlier. The resulting powder was ground, pelletized with 50 MPa uniaxial pressure within 30 s at Hydraulic Press 660 (Silfradent, Italy) and annealed in a platinum crucible at 900–1150 °C for 12 h in the air. At each temperature, the sintered body was crushed and grinded in ethanol media in agate mortar, analyzed via XRD-technique, pelletized and annealed at a temperature 50 °C above the previous one.

2.2. Characterization

The phase composition and structural studies were conducted using a Shimadzu XRD 7000 (Shimadzu, Japan) diffractometer (Cu_Kα radiation (λ = 1.5418 Å) between angles from 10 to 80°, with a step of 0.03° and a scanning rate of 5 s per point. Diffraction patterns were collected from the polished surface of the sintered tablet form sample.

Thermogravimetric analysis of the sample (weighted with an accuracy of 0.01 mg) was carried out at Setaram TG-92 (Setaram, France) in air and argon flows. Isothermal dependence of mass change in Cu-based perovskites at variable atmosphere was recorded at consistent switching of gaseous fluxes as follows: air → (5% H₂ in Ar) → air. Switching of the gas medium was carried out via vacuuming the measuring cell and triple purging with gas in which measurements were to be taken. Oxygen content in the synthesized samples was determined via total reduction in the 5% H₂ in Ar mixture at 900 °C for Ba₂CuWO_6-δ or 950 °C for SrCu_0.5Ta_0.5O_3−δ.

The scanning electron microscopy (SEM) images were obtained from JEOL JSM 6390LA (Jeol, Japan). Chemical composition of the obtained materials was controlled via the energy-dispersive X-ray spectroscopy (EDX) technique using a Jeol JED2300 EDX analyzer implemented to the microscope specified.

Perovskite structural predictors, such as tolerance (t), new tolerance (τ), octahedral (µ) factors and octahedral mismatch (Δµ) were calculated by approaches thoroughly described earlier [24,25,26]. Electronegativity and chemical hardness mismatch of B-cations were calculated using the data [27,28].

HSC Chemistry 7.0 was used to assess thermodynamical probability of the decomposition reactions.

Thermal conductivity was measured in the temperature range 25–150 °C at the custom-made installation IT-λ-400 discussed in detail earlier [29]. The experimentally obtained value was adjusted for the porosity value according to the known equation [30]:

$$\frac{{\lambda }_{exp}}{{\lambda }_{dense}}=1-\frac{4}{3}·P$$

(1)

where λ_exp and λ_dense are experimental and porous free sample thermal conductivity, respectively, W·m⁻¹·K⁻¹, P is porosity of the specimen.

Thermal expansion measurements were carried out with a Linseis L75/1250 dilatometer (Linseis Messgerate, Germany) using rectangular cuts of dense samples with a heating/cooling rate of 5 K/min at 25–1000 °C.

Details for band gap measurements were thoroughly discussed in the paper [31].

3. Results and Discussion

3.1. Synthesis and High-Temperature Redox Behavior of Ba₂CuWO_6−δ

It is known that copper forms rock-salt ordered double perovskites with a tetragonal structure (S.G. I4/m) with W⁶⁺ as A₂CuWO₆, where A = Ba or Sr [32,33,34,35,36,37]. The double perovskites A₂CuWO₆ can be formed according to equation:

$$2\mathrm{A}\mathrm{C}{\mathrm{O}}_3+\mathrm{C}\mathrm{u}\mathrm{O}+\mathrm{W}{\mathrm{O}}_3={\mathrm{A}}_2\mathrm{C}\mathrm{u}\mathrm{W}{\mathrm{O}}_6+2\mathrm{C}{\mathrm{O}}_2^{\mathrm{g}}$$

(2)

where A = Ba, Sr.

The formation mechanisms as well as high-temperature redox behavior are illustrated on the example of Ba₂CuWO₆. The results of XRD (Figure 1a) show that the mixture after the firing at 870 °C in air consists of mostly barium tungstates, BaWO₄, Ba₂WO₅, Ba₃WO₆, Ba₃W₂O₉ and copper-containing oxides Ba₂CuWO₆, CuO and Ba₂Cu₂O₅. Despite the observed solid-state reaction initiation temperature at DSC analysis, it was suggested to slightly increase the synthesis temperature to accelerate the reaction rate. Hence, the following manual grinding in ethanol media and pressing with heat treatment at 900 °C contributes to active interplay of the components enabling the desired oxide to be obtained after holding at the synthesis temperature for 50 h. The XRD pattern of the as-prepared sample is shown in Figure 1b. All the reflections observed can be reliably ascribed to tetragonal Ba₂CuWO₆ having the I4/m space group, indicating the single-phase formation. The refined values for unit cell parameters in Ba₂CuWO₆ and Sr₂CuWO₆ as well as perovskite structure predictors are collected in

Table 1. Structural and thermodynamical parameters of Cu-based perovskites.

	Ba2CuWO6	Sr2CuWO6	SrCu0.5Ta0.5O3−δ	SrCu0.5Nb0.5O3−δ
a ± 0.03, Å	5.562	5.433	3.976	3.978
c ± 0.03, Å	8.630	8.382	-	-
t	1.039	0.979	0.969	0.969
µ	0.493	0.493	0.507	0.507
Δµ	0.048	0.048	0.033	0.033
τ	3.636	3.506	3.630	3.630
Δχ (Pauling)	0.46	0.46	0.4	0.3
Δχ (Mulliken)	0.3	0.3	0.2	0.5
Δχ (Allen)	0.38	0.38	0.51	0.44
Δ (Chemical hardness)	0.2	0.2	0.7	0.5

.

Despite the literature data [34,35,36], it was found (Figure 1a) that calcination in the range 950–1200 °C leads to the Ba₂CuWO₆ phase partial decomposition: the black shade at the yellow-colored sample is clearly observed, which may result from the formation of copper (II) oxide on the surface. The image of blackened sample after 950 °C annealing is presented in Figure 1c. Interestingly, the sample cross-section is also black, which suggests that neither air nor crucible material affects the decomposition.

Ba₂WO₅ presence could be explained by the equation of Ba₂CuWO₆ decomposition:

$$\mathrm{B}{\mathrm{a}}_2\mathrm{C}\mathrm{u}\mathrm{W}{\mathrm{O}}_6=\mathrm{B}{\mathrm{a}}_2\mathrm{W}{\mathrm{O}}_5+\mathrm{C}\mathrm{u}\mathrm{O}$$

(3)

Two phases Ba₂CuWO₆ and Ba₂WO₅ were observed to provide after the calcination at 950 °C; barium tungstate forms according to decomposition reaction (3) and is indirectly proved by the image (Figure 1c). The formed copper oxide was not detected via the XRD technique, but a combination of SEM with EDX data partially confirms the mechanism suggested (Figure 2). As can be seen from Figure 2b, dark color grains presumably correspond to copper oxide, while light grains apparently are barium tungstate. The decomposition process (Equation (3)) takes place in the whole volume (Figure 2a) of the cross-section view of the sample. Such an effect reveals pretty low thermal stability of A₂CuWO₆ phases within the synthesis process applied.

Moreover, heating of the single-phase Ba₂CuWO₆ up to 1000 °C results in BaWO₄ impurity formation. Interestingly, no considerable heat effect of the decomposition process at DSC equipment (Setsys Evo-18, Setaram, France) was observed. The formation mechanism of BaWO₄ within heating over 900 °C could be represented by Equation (4). To estimate barium carbonate presence, EDX analysis was applied. Unfortunately, the EDX technique fails to assess light element quantity, such as carbon, but it could help to find barium-containing phase without the presence of copper or tungsten. The upper right corner of the EDX mapping (Figure 2b) shows high barium content, while the copper and tungsten amount are low. That area is supposed to be BaCO₃, resulting from Ba₂WO₅ decomposition. Barium oxide formation within any decomposition reaction is not thermodynamically favorable in air. According to the thermodynamical data, reaction (4) could take place only on cooling of the sample lower than 442 °C. The hypothesis was proved via rapid cooling of the sample in the liquid nitrogen: no peaks at XRD pattern of the sample corresponding to the BaWO₄ were observed.

The following temperature enhancement yields Ba₂WO₅ and BaWO₄ impurity accumulation from 2.3% at 950 °C to 6.9% at 1250 °C. The observed phenomenon evidences higher thermodynamical stability of barium tungstates than copper-based double perovskite. This could be connected with a high value of tolerance factor (1.039, Table 1) for Ba₂CuWO₆, which reveals high stretching of the lattice [24].

$$\mathrm{B}{\mathrm{a}}_2\mathrm{W}{\mathrm{O}}_5+\mathrm{C}{\mathrm{O}}_2^{\mathrm{g}}=\mathrm{B}\mathrm{a}\mathrm{W}{\mathrm{O}}_4+\mathrm{B}\mathrm{a}\mathrm{C}{\mathrm{O}}_3\hspace{0.33em}\hspace{0.33em}\hspace{0.33em}\Delta {\mathrm{G}}^0=-11.315 \mathrm{kJ}\hspace{0.33em} \mathrm{at} 400 °\mathrm{C}$$

(4)

Both oxygen content and high-temperature redox behavior were examined via TGA under gas media containing 5% H₂ in Ar (Figure 3a). Reducing atmosphere at 900 °C Ba₂CuWO₆ easily decomposes through the following equation:

$$\mathrm{B}{\mathrm{a}}_2\mathrm{C}\mathrm{u}\mathrm{W}{\mathrm{O}}_6+{\mathrm{H}}_2^{\mathrm{g}}=\mathrm{B}{\mathrm{a}}_2\mathrm{W}{\mathrm{O}}_5+\mathrm{C}\mathrm{u}+{\mathrm{H}}_2{\mathrm{O}}^{\mathrm{g}}$$

(5)

The TGA results show (Figure 3a) that in contrast to the other copper-containing mixed oxides [12,23], Ba₂CuWO₆ reoxidizes with an immediate step corresponding to the Ba₂CuWO_5.55, where copper cations nominally accord with Cu(I) state. A single-charged copper has a slightly bigger ionic radii compared with Cu²⁺ 0.77 and 0.73 Å correspondingly [38]. That leads to reducing the tolerance factor value to 1.029 and a partial lattice distortion decrease. Moreover, due to high charge of the tungsten (+6), the overall B-site charge remains at the sufficient level (+3.5) to maintain the perovskite-like structure. Together, both the above-mentioned factors result in appearance of the step during the reoxidation process.

The same nonstoichiometry (Ba₂CuWO_5.58) could be achieved in an argon atmosphere: the value of media oxygen partial pressure allowing one to uncouple the oxygen and reduce the copper from +2 to +1 state. After one reoxidation cycle, Ba₂CuWO₆ fails to obtain a single-phase sample (Figure 3b).

3.2. Synthesis and High-Temperature Redox Behavior of SrCu_0.5Ta_0.5O_3−δ

The effect of fivefold charged cations such as Nb⁵⁺, Ta⁵⁺ on the copper-based perovskite-like structures formation and their properties was studied. The obtained compounds obey disordered perovskite structure contrary to rock-salt ordered double perovskite observed for tungsten-based oxide. However, the cuprates containing M⁵⁺ are characterized by higher symmetry, since both SrCu_0.5Ta_0.5O₃ and SrCu_0.5Nb_0.5O₃ are found to crystallize with a cubic Pm3m crystal structure. An example of such a Rietveld refined XRD pattern is illustrated in Figure 4a. The grains with a cubic symmetry could also be clearly observed at the SEM images (Figure 5).

Moreover, Ta and Nb containing perovskites possess higher thermal stability. The cubic phase appears at 900 °C (Figure 4b) with a mixed oxide CuO·SrO and Sr₂M₂O₇ as impurities. The formation mechanism could be expressed by the following reactions:

$$\mathrm{S}\mathrm{r}\mathrm{C}{\mathrm{O}}_3+\mathrm{C}\mathrm{u}\mathrm{O}=\mathrm{S}\mathrm{r}\mathrm{O}·\mathrm{C}\mathrm{u}\mathrm{O}+\mathrm{C}{\mathrm{O}}_2^{\mathrm{g}}$$

(6)

$$2\mathrm{S}\mathrm{r}\mathrm{C}{\mathrm{O}}_3+\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_5=\mathrm{S}{\mathrm{r}}_2\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_7+2\mathrm{C}{\mathrm{O}}_2^{\mathrm{g}}$$

(7)

$$\mathrm{S}{\mathrm{r}}_2\mathrm{N}{\mathrm{b}}_2{\mathrm{O}}_7+2\mathrm{S}\mathrm{r}\mathrm{O}·\mathrm{C}\mathrm{u}\mathrm{O}=4\mathrm{S}\mathrm{r}\mathrm{C}{\mathrm{u}}_{0.5}\mathrm{N}{\mathrm{b}}_{0.5}{\mathrm{O}}_{2.75}$$

(8)

The proposed mechanism is partially confirmed via EDX (Figure 6) analysis applied for the sample preheated at 1000 °C. The final perovskite phase grains are separated via melted phase CuO·SrO + CuO (T_m.p. (CuO) = 975 °C, T_m.p. (SrCuO₂) = 1080 °C [39], points 006–010 in Figure 6), which in contact with Sr₂M_2−xO₇ (M = Ta, Nb) forms the desirable compound. The samples containing only Cu-based perovskite phase SrCu_0.5M_0.5O₃ were obtained after 1150 °C and their crystal cell parameters were refined via the Rietveld method (Table 1).

According to the EDX results (

Table 2. Sr-Cu-Ta-O system calcinated at 1000 °C EDX analysis results.

N	Sr, at. %	Cu, at. %	Ta, at. %
001	47.66	30.73	21.61
002	47.03	29.58	23.39
003	45.76	32.03	22.21
004	46.1	32.31	21.59
005	48.7	28.66	22.64
006	52.66	40.59	6.75
007	64.1	25.37	10.53
008	45.71	41.01	13.27
009	55.8	29.27	14.94
010	45.47	44.54	9.89

), there is a slight deviation from the stoichiometric ratio of copper and tantalum (3:2), which was not observed in [40]. At the same time, the presence of tantalum in the binding phase of SrO·CuO can be explained by the formation of Sr₂Ta₂O₇, the amount of which is insufficient for its identification via the XRD method. It has been shown [40] that under equilibrium conditions at 950 °C of the SrO-CuO-TaO_2.5 system with an atomic ratio of 2:1:1, two phases are present: SrCuO₂ and Sr₃Ta_2−xCu_1+xO_9+δ. This is confirmed by the proposed mechanism (7): there is a gradual dissolution of complex copper and strontium oxide in strontium tantalate. However, in contrast to the presented work, the formation of cubic perovskite is observed for both Ta and Nb, including at 950 °C, which was also presented in a number of papers [41,42,43]. A significant deviation in the content of elements (Sr, Cu, Ta) at points 006–010 is caused by the migration of cations in the SrO·CuO melt at the synthesis temperature. Sr₂Ta₂O₇ particles in the crystallized melt were not detected via the SEM method. Their presence is only indirectly confirmed via EDX and XRD data.

Redox behavior of SrCu_0.5Ta_0.5O_3−δ (Figure 7) also completely differs from Ba₂CuWO₆. After applying a single red-ox cycle, no weight changes compared with initial state were clearly seen. No intermediate steps within sample reoxidation were observed. Moreover, there is considerable oxygen nonstoichometry, which corresponds to SrCu_0.5Ta_0.5O_2.81 wherein the oxygen loss rate is twice as low (time for almost complete reduction) but twice as high as the reoxidation rate. Finally, oxygen content 2.75 corresponds to the presence of copper in Cu²⁺ state, while 2.81 suggests the appearance of Cu³⁺.

The lowest thermal stability of Ba₂CuWO₆ among considered compounds could be connected with a high tolerance factor (t, Table 1) close to the perovskite stability edge. Even large Cu+ cation (0.77Å) in the intermediate state Ba₂CuWO_5.545 does not permit considerable reduction in lattice distortions (t = 1.029). The other structural parameters (Table 1) are close for all synthesized Cu-based perovskites and lay into the region of stable perovskite structure. The electronegativity mismatch in all considered scales shows similar results for W⁶⁺ and Ta⁵⁺ or Nb⁵⁺, revealing alike kinds of bonds in nature. Interestingly, a low chemical hardness mismatch between copper and tungsten shows closer affinity to electrons of Cu and W than with Nb or Ta.

3.3. Some Properties of Cu-Based Perovskites

Due to the low thermal stability, which makes it complicated to produce well sintered ceramics, tungsten-based cuprates could be considered as luminescence materials and coatings [44]. The yellow color of Ba₂CuWO₆ allows one to suggest a considerable band gap value which amounted to 2.59 eV (Figure 8a), so this material should be characterized as a wide gap semiconductor with promising base for the photocatalytic water splitting approach and can be suggested as an alternative for titania in this field.

A rather high thermal stability of SrCu_0.5Ta_0.5O_3−δ for Cu-based perovskite enables one to investigate high temperature properties such as thermal expansion. It was found that TEC (Figure 8b) gradually increases with a temperature growth and its value has average values of 10.8–11.6·10⁻⁶ K⁻¹. The values obtained are believed to be appropriate for the use of tantalum-doped oxide in contact with other oxide materials at elevated temperatures. In addition, low thermal conductivity 1.28 W·m⁻¹·K⁻¹, attributed to high oxygen vacancy concentration, at temperatures 25–400 °C looks promising for thermoelectrical materials. Considerable oxygen exchange parameters and good thermal stability could assess the material for oxygen adsorption from gaseous media.

Finally, it should be stressed here that a light doping of copper-based perovskites with highly charged compounds does not sufficiently increase phase stability. As can be seen even a simple heating in air is accompanied mostly by a partial decomposition with copper oxide crystallization. This fact constrains the application of the studied materials in high-temperature devices, but taking into account elevated rate of oxygen exchange these materials could be proposed as oxygen carrier materials and oxygen sorbents at lower temperatures.

4. Conclusions

• The rock-salt ordered double perovskites A₂CuWO_6−δ (A = Sr, Ba) with the I4/m space group and disordered perovskites SrCu_0.5M_0.5O_3−δ (M = Nb, Ta) with a Pm3m space group were synthesized via a solid-state reaction route.
• The structural predictors for the synthesized Cu-based perovskites were calculated and the cell parameters were refined via the Rietveld method.
• Redox behavior of Ba₂CuWO₆ was studied at 900 °C and a step was found within reoxidation which should contribute to the presence of Cu(I).
• Ba₂CuWO₆ decomposes after 900 °C with the formation of copper oxide and barium tungstanate; the suggested mechanism was approved via EDX analysis.
• The value of the measured Ba₂CuWO₆ band gap was 2.59 eV.
• The disordered perovskite SrCu_0.5M_0.5O_3−δ (M = Nb, Ta) forms within the reaction of the liquid complex strontium-copper oxide and strontium niobate (tantalate) at temperatures in the range 900–1150 °C.
• The copper-based perovskite compounds with M⁵⁺ have to consist of Cu³⁺ according to the oxygen content.
• Thermal properties of SrCu_0.5Ta_0.5O_3−δ were investigated: average TEC value at 1000 °C 11.6·10⁻⁶ K⁻¹ and a low thermal conductivity 1.28 W·m⁻¹·K⁻¹ in the temperature range 25–400 °C.