In situ structural measurements

Chapter 10. Properties of Co- and Fe-containing perovskite-type oxides 146 the disordered perovskite-type materials is used. It is worth mentioning, that the oxygen change, calculated using data from thermogravimetric experiments, is quite low for this material, being equal to 0.3 (or 0.6 per double perovskite formula unit). On the other hand it can be much higher, and for example for La0.6Sr0.4Co0.8Fe0.2O3-δ canexceed 0.55 (or 1.1 per double perovskite formula unit, see Tab. 10.1). This behavior is different comparing to the studied manganese-containing oxides, and may originate from difference of the intrinsic properties of manganese, iron and cobalt elements, which prefer different oxidation states, while present in the oxides. For example, presence of high amounts of Co⁴⁺ and Fe⁴⁺ cations is not very common in most of the perovskite-type oxides, and even if so, very high oxygen partial pressure or electrochemical intercalation of the oxygen into the materials are required for all Co and Fe to adopt +4 oxidation state [195]. On the other hand, Mn⁴⁺ oxidation state is well known and is often observed.

It seems that crystal structure of the considered materials also plays an important role regarding oxygen storage capacity, with the ordered structure hindering the incorporation of the oxygen up to the fully oxidized state. At the same time, the disordered structure of other studied in this chapter perovskites, which allows for statistical formation of the oxygen vacancies, seems more suitable for Fe- and Co-containing perovskites from the point of view of the oxygen storage.

brownmillerite phase already ongoing. With further increase of the temperature, oxidation takes place and is visible on the Fig. 10.2a as a disappearance of the brownmillerite-related peak, and at the same time, as an appearance of the reflection associated with the perovskite-type phase. In the vicinity of the transition temperature intensity of visible peaks is low, showing that the ongoing oxidation affects crystal structure of the material, lowering structural coherence length. In this region, for the XRD data refinements presence of two phases (reduced Icma and oxidized I12/c1 phase) was assumed. In the temperature range of 230-275 °C additional oxidation of the perovskite phase likely takes place. Data of the temperature behavior of the oxidized La0.6Sr0.4Co0.8Fe0.2O3 is presented in Fig.

10.2b, showing only a slight shift of the main reflection towards lower angles, which is associated with thermal expansion of the material. If results for both of the cases are compared at the temperature of 300 °C, it can be noticed that the diffractograms are almost the same, which indicates that the starting La0.6Sr0.4Co0.8Fe0.2O2.42 material is fully oxidized at this temperature, and the actual oxygen content in the material is equal to 3.

a) b)

Fig. 10.2. Structural evolution and temperature dependence of normalized unit cell parameters and volume, together with calculated thermal expansion coefficients for a) reduced La0.6Sr0.4Co0.8Fe0.2O2.42 and b) oxidized La0.6Sr0.4Co0.8Fe0.2O3 sample during heating in air. Data shown for selected angular range.

Similar behavior was observed for the reduced La0.5Sr0.5Co0.5Fe0.5O2.53 (Fig. 10.3a).

At low temperatures no changes apart from thermal expansion are recorded, then, a partial oxidation takes place, and in the intermediate region, with low intensity of the peaks, brownmillerite-perovskite transformation occurs. Finally, strong shift of the main peak of the perovskite-type phase in temperature range of 125-190 °C takes place, which can be

Chapter 10. Properties of Co- and Fe-containing perovskite-type oxides 148 linked to the oxidation of the perovskite phase. At higher temperatures only thermal expansion-related shift of the peaks towards lower angles is visible. Temperature dependence of the normalized unit cell parameters and volume (also in the two-phase range), together with calculated thermal expansion coefficient are also presented in Fig.

10.3a.

Equivalent data for the oxidized La0.5Sr0.5Co0.5Fe0.5O3 are presented in Fig. 10.3b, and the obtained results are the same like for the described above La0.6Sr0.4Co0.8Fe0.2O3

oxide. What is more, thermal expansion coefficients of oxidized La0.5Sr0.5Co0.5Fe0.5O3 and La0.6Sr0.4Co0.8Fe0.2O3 materials are alike.

a) b)

Fig. 10.3. Structural evolution and temperature dependence of normalized unit cell parameters and volume, together with calculated thermal expansion coefficients for a) reduced La0.5Sr0.5Co0.5Fe0.5O2.53 and b) oxidized La0.5Sr0.5Co0.5Fe0.5O3 sample during heating in air. Data shown for selected angular range.

In the case of Sm0.5Sr0.5Co0.5Fe0.5O3-δ (Figs. 10.4a and b) the observed changes are of the same nature as described above for La0.5Sr0.5Co0.5Fe0.5O3-δ and La0.6Sr0.4Co0.8Fe0.2O3-δ

oxides, and at 300 °C the sample can be considered as fully oxidized.

For barium-containing material (Figs. 10.5a and b), La0.5Ba0.5Co0.5Fe0.5O3-δ, there is no evidence of a phase transition occurring upon oxidation, and the space group remains the same (I4/mcm). Above 190 °C main peak shifts towards higher angles, and at the same time its intensity decreases in a region where the oxidation takes place, indicating shorter structural coherence length.

a) b)

Fig. 10.4. Structural evolution and temperature dependence of normalized unit cell parameters and volume, together with calculated thermal expansion coefficients for a) reduced Sm0.5Sr0.5Co0.5Fe0.5O2.53 and b) oxidized Sm0.5Sr0.5Co0.5Fe0.5O3 sample during heating in air. Data shown for selected angular range.

a) b)

Fig. 10.5. Structural evolution and temperature dependence of normalized unit cell parameters and volume, together with calculated thermal expansion coefficients for a) reduced La0.5Ba0.5Co0.5Fe0.5O2.55 and b) oxidized La0.5Ba0.5Co0.5Fe0.5O3 sample during heating in air. Data shown for selected angular range.

Another Ba-containing material, Sm0.5Ba0.5Co0.5Fe0.5O2.54, also does not exhibit phase transition upon oxidation, with cation-ordered structure and P4/mmm space group in the reduced and the oxidized material. This compound oxidizes above 230 °C (Fig. 10.6a).

Its unit cell volume changes are much smaller than for the other considered oxides, and position of the main peak changes only slightly while oxidation process proceeds.

However, the ratio, is changing significantly upon oxidation (Tab. 10.1).

Chapter 10. Properties of Co- and Fe-containing perovskite-type oxides 150

a) b)

Fig. 10.6. Structural evolution and temperature dependence of normalized unit cell parameters and volume, together with calculated thermal expansion coefficients for a) reduced Sm0.5Ba0.5Co0.5Fe0.5O2.54 and b) oxidized Sm0.5Ba0.5Co0.5Fe0.5O3 sample during heating in air. Data shown for selected angular range.

The presented and discussed above unique high-temperature in situ studies allowed to directly observe structural changes that occurs during oxidation of the reduced Co- and Fe- containing perovskite-type materials.