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

: The rock-salt ordered A 2 CuWO 6 (A = Sr, Ba) with I 4 /m space group and disordered SrCu 0.5 M 0.5 O 3 − δ (M = Ta, Nb) with Pm 3 m space group perovskites were successfully obtained via a solid-state reaction route. Heat treatment of Ba 2 CuWO 6 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 ( ∆ W o ) exceeding 2.5% for Ba 2 CuWO 6 . At the same time, oxygen content reveals Cu 3+ content in SrCu 0.5 Ta 0.5 O 3 − δ . Under the following reoxidation of Ba 2 CuWO 6 , 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 M 5+ cations. The yellow color of Ba 2 CuWO 6 enables to measure the band gap 2.59 eV. SrCu 0.5 Ta 0.5 O 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.


Introduction
Complex oxides with perovskite structure ABO 3 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.
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% H 2 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% H 2 in Ar mixture at 900 • C for Ba 2 CuWO 6-δ or 950 • C for SrCu 0.5 Ta 0.5 O 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.
Thermal conductivity was measured in the temperature range 25-150 • C at the custommade 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]: 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].

Results and Discussion
3.1. Synthesis and High-Temperature Redox Behavior of Ba 2 CuWO 6−δ It is known that copper forms rock-salt ordered double perovskites with a tetragonal structure (S.G. I4/m) with W 6+ as A 2 CuWO 6 , where A = Ba or Sr [32][33][34][35][36][37]. The double perovskites A 2 CuWO 6 can be formed according to equation: The formation mechanisms as well as high-temperature redox behavior are illustrated on the example of Ba 2 CuWO 6 . The results of XRD (Figure 1a) show that the mixture after the firing at 870 • C in air consists of mostly barium tungstates, BaWO 4 , Ba 2 WO 5 , Ba 3 WO 6 , Ba 3 W 2 O 9 and copper-containing oxides Ba 2 CuWO 6 , CuO and Ba 2 Cu 2 O 5 . 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 Ba 2 CuWO 6 having the I4/m space group, indicating the single-phase formation. The refined values for unit cell parameters in Ba 2 CuWO 6 and Sr 2 CuWO 6 as well as perovskite structure predictors are collected in Table 1  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  Despite the literature data [34][35][36], it was found ( Figure 1a) that calcination in the range 950-1200 • C leads to the Ba 2 CuWO 6 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. Ba 2 WO 5 presence could be explained by the equation of Ba 2 CuWO 6 decomposition: Two phases Ba 2 CuWO 6 and Ba 2 WO 5 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 A 2 CuWO 6 phases within the synthesis process applied.
Ceramics 2023, 6, FOR PEER REVIEW 5 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: 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) 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]. Moreover, heating of the single-phase Ba 2 CuWO 6 up to 1000 • C results in BaWO 4 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 BaWO 4 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 BaCO 3 , resulting from Ba 2 WO 5 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 BaWO 4 were observed.
The following temperature enhancement yields Ba 2 WO 5 and BaWO 4 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 Ba 2 CuWO 6 , which reveals high stretching of the lattice [24].
Both oxygen content and high-temperature redox behavior were examined via TGA under gas media containing 5% H 2 in Ar (Figure 3a). Reducing atmosphere at 900 • C Ba 2 CuWO 6 easily decomposes through the following equation: 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 Ba2CuWO5.55, where copper cations nominally accord with Cu(I) state. A single-charged copper has a slightly bigger ionic radii compared with Cu 2+ 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 (Ba2CuWO5.58) 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).

Synthesis and High-Temperature Redox Behavior of SrCu0.5Ta0.5O3−δ
The effect of fivefold charged cations such as Nb 5+ , Ta 5+ 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 M 5+ are characterized by higher symmetry, since both SrCu0.5Ta0.5O3 and SrCu0.5Nb0.5O3 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). The TGA results show (Figure 3a) that in contrast to the other copper-containing mixed oxides [12,23], Ba 2 CuWO 6 reoxidizes with an immediate step corresponding to the Ba 2 CuWO 5.55 , where copper cations nominally accord with Cu(I) state. A singlecharged copper has a slightly bigger ionic radii compared with Cu 2+ 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 (Ba 2 CuWO 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, Ba 2 CuWO 6 fails to obtain a single-phase sample (Figure 3b). The effect of fivefold charged cations such as Nb 5+ , Ta 5+ on the copper-based perovskitelike 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 M 5+ are characterized by higher symmetry, since both SrCu 0.5 Ta 0.5 O 3 and SrCu 0.5 Nb 0.5 O 3 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). Ceramics 2023, 6, FOR PEER REVIEW 7 (a) (b) 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:

Synthesis and
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 (Tm.p. (CuO) = 975 °C, Tm.p. (SrCuO2) = 1080 °C [39], points 006-010 in Figure 6), which in contact with Sr2M2−xO7 (M = Ta, Nb) forms the desirable compound. The samples containing only Cu-based perovskite phase SrCu0.5M0.5O3 were obtained after 1150 °C and their crystal cell parameters were refined via the Rietveld method (Table 1).  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: 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 (Tm.p. (CuO) = 975 °C, Tm.p. (SrCuO2) = 1080 °C [39], points 006-010 in Figure 6), which in contact with Sr2M2−xO7 (M = Ta, Nb) forms the desirable compound. The samples containing only Cu-based perovskite phase SrCu0.5M0.5O3 were obtained after 1150 °C and their crystal cell parameters were refined via the Rietveld method (Table 1). 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 Sr 2 M 2 O 7 as impurities. The formation mechanism could be expressed by the following reactions:  (Table 1).
Ceramics 2023, 6, FOR PEER REVIEW According to the EDX results (Table 2), there is a slight deviation from th ometric ratio of copper and tantalum (3:2), which was not observed in [40]. At time, the presence of tantalum in the binding phase of SrO·CuO can be explain formation of Sr2Ta2O7, the amount of which is insufficient for its identificatio XRD method. It has been shown [40] that under equilibrium conditions at 950 SrO-CuO-TaO2.5 system with an atomic ratio of 2:1:1, two phases are present: SrC Sr3Ta2−xCu1+xO9+ δ . This is confirmed by the proposed mechanism (7): there is a dissolution of complex copper and strontium oxide in strontium tantalate. Ho contrast to the presented work, the formation of cubic perovskite is observed fo and Nb, including at 950 ° C, which was also presented in a number of papers [ significant deviation in the content of elements (Sr, Cu, Ta) at points 006-010 is c the migration of cations in the SrO·CuO melt at the synthesis temperature. Sr2T ticles in the crystallized melt were not detected via the SEM method. Their pr only indirectly confirmed via EDX and XRD data.  According to the EDX results (Table 2), 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 Sr 2 Ta 2 O 7 , 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: SrCuO 2 and Sr 3 Ta 2−x Cu 1+x O 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. Sr 2 Ta 2 O 7 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 0.5 Ta 0.5 O 3−δ (Figure 7) also completely differs from Ba 2 CuWO 6 . 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 0.5 Ta 0.5 O 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 Cu 2+ state, while 2.81 suggests the appearance of Cu 3+ . 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 Cu 2+ state, while 2.81 suggests the appearance of Cu 3+ . 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 Ba2CuWO5.545 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 W 6+ and Ta 5+ or Nb 5+ , 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.

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. The lowest thermal stability of Ba 2 CuWO 6 among considered compounds could be connected with a high tolerance factor (t,  (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 W 6+ and Ta 5+ or Nb 5+ , 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.

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 Ba 2 CuWO 6 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 SrCu0.5Ta0.5O3−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.

•
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 copper-based perovskite compounds with M 5+ have to consist of Cu 3+ according to the oxygen content. A rather high thermal stability of SrCu 0.5 Ta 0.5 O 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.

•
The rock-salt ordered double perovskites A 2 CuWO 6−δ (A = Sr, Ba) with the I4/m space group and disordered perovskites SrCu 0.5 M 0.5 O 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 Ba 2 CuWO 6 was studied at 900 • C and a step was found within reoxidation which should contribute to the presence of Cu(I). • Ba 2 CuWO 6 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 Ba 2 CuWO 6 band gap was 2.59 eV.

•
The disordered perovskite SrCu 0.5 M 0.5 O 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 M 5+ have to consist of Cu 3+ according to the oxygen content. • Thermal properties of SrCu 0.5 Ta 0.5 O 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.