Ongoing research endeavors aim to enhance the thermal insulation properties of materials crucial for heat-based equipment like aerospace turbines and airplane blades, thereby ensuring safety and longevity. Various materials, including titanate-based ceramics, 1 composite materials, 2 zirconia, yttria-based materials, 3 and others, have been extensively studied to achieve low thermal conductivity. Materials with low thermal conductivity are widely used in thermoelectrics as well. 4,5 This quest for low thermal conductivity extends to diverse material classes such as perovskite oxides, pyrochlore, 1 nanophase, 6 and Zintl phases of intermetallics. 7 Numerous methods, including coating, alloy formation, and introducing twin interfaces, have been employed to reduce thermal conductivity. Ceramic-based oxides, particularly perovskite oxides, have garnered significant attention due to their high melting points. 4 Perovskite oxides, denoted by the general formula ABO 3 , where A and B represent alkaline Earth or rare Earth metal and transition metal, respectively, possess an ideal cubic structure. Strategies for reducing the thermal conductivity of perovskite oxides involve inducing oxygen vacancies in the crystal lattice, lattice distortions, grain size reduction, and introducing grain boundaries. Oxygendeficient perovskites are expressed as ABO 3-x or A 2 B 2 O 6-δ , where x and δ represent oxygen vacancy concentration. The arrangement of vacancies in these materials can be ordered or disordered, influencing their structural flexibility and resulting in different crystal structures. [8][9][10][11] Phonons are quantized vibrational modes of the crystal lattice, and they are the primary carriers of heat in insulating materials. In perovskite oxides, the lattice structure and phonon interactions strongly influence thermal conductivity. Defects, grain boundaries, and other structural features can scatter phonons, affecting their ability to transport heat. Phonon scattering, which can be increased through methods like vacancy generation, boundary increment, impurity introduction, and lattice distortion, plays a crucial role in reducing thermal conductivity. 12,13 Notably, the generation of oxygen vacancies has proven effective in lowering thermal conductivity, and its impact is often more pronounced than other contributions. 4,[13][14][15] some common strategies applied to suppress the lattice thermal conductivity of thermoelectric materials include not only introducing defects 16,17 but also alloying, 18 creating nanostructures, 19 and forming substructures. 20 One example is the ultralow thermal conductivity captured in Na 2 TlSb due to the soft Tl-Sb bonding and resonant bonding in the pseudo cage composed of the Na and Sb atoms interaction. 21 The report explains the strong quartic anharmonicity and temperature dependence of the Tl atom with rattling behavior playing an important role in the lattice stability of Na 2 TlSb. The report mentioned the finding that soft Tl-Sb bonding and resonant bonding in the pseudo cage composed of the Na and Sb atoms interaction was responsible for ultralow thermal conductivity. 21 Strong anharmonic phonon scattering was reported in Li 2 NaBi also as a causes for the lattice thermal conductivity. 22 This study focuses on oxygen-deficient perovskites, specifically Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ , to explore their thermal insulation properties. The compounds were synthesized using solid-state reaction at high temperatures, and their crystal structures were determined through Rietveld refinement of powder X-ray diffraction. Both materials exhibit different arrangements of oxygen vacancies, leading to distinct coordination geometries and crystal structures. Ca 2 FeMnO 6-δ features tetrahedral layers alternating with octahedral layers, forming a brownmillerite-type structure, 23 while Sr 2 FeMnO 6-δ showcases random vacancy distribution resembling cubic perovskite structure. 23

The synthesis of Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ involved the utilization of SrCO 3 , CaCO 3 , Mn 2 O 3 , and Fe 2 O 3 powders. These powders were meticulously weighed and thoroughly mixed in stoichiometric proportions using an agate mortar and pestle. Subsequently, the resulting mixture was compressed into pellets under a pressure of 3 tons. The pellets underwent calcination at 1000 °C for 24 h [MTI furnace], followed by cooling to room temperature. After cooling, the pellets were reground and repelletized. Sintering of the pellets was carried out at 1200 °C for 24 h, and the heating and cooling rate for both calcination and sintering processes was maintained at 100 °C/h.

To assess the crystal structure and phase purity of the samples, Bruker X-ray diffractometer D2 Phaser was employed for powder X-ray diffraction (PXRD), using Cu Kα1 and Kα2 radiations with a wavelength (λ) of 1.54056 Å. Rietveld refinements of the XRD data were conducted using GSAS software (Larson and Von Dreele 1994) along with the EXPEGUI interface (Toby 2001). Microstructure analysis was performed through scanning micrographs.

z E-mail: rhona@uttc.edu

The concentration of oxygen vacancies in the materials was determined through iodometric titrations. In this process, 2 g of potassium iodide was mixed with 50 mg of the sample in 100 ml of 1 M HCl. A portion of the resulting solution was titrated against 0.025 M Na 2 S 2 O 3 , with the addition of 0.2 ml of a starch solution as an indicator. The entire experiment was conducted under an argon atmosphere.

The conductivities of the materials were studied using a Thermtest thermal conductivity meter, MP-2 with TPS-4, based on the Transient Plane Source Method. Circular discs with diameters of 13 mm and/or 25 mm and a thickness of 3 mm were placed on the flat sensor, and the thermal conductivity was measured at room temperature.

Structural analysis.-The crystal structures of the two materials, Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ have been previously documented. As reported, Ca 2 FeMnO 6-δ exhibits a brownmillerite type compound characterized by tetrahedral layers alternating with octahedral layers, as depicted in Fig. 1. Within each tetrahedral layer, the orientation is opposite to that of the nearest layer, both above and below. This results in the alternating arrangement of tetrahedral layers, as illustrated in the inset of Fig. 1. The space group for Ca 2 FeMnO 6-δ is Pnma, as detailed in Fig. 1 and Table I, which present the Rietveld refinement profile and parameters, respectively.

A structural variation arises when Sr 2+ replaces Ca 2+ , leading to differences in the ionic radii. This substitution induces distinct oxygen vacancy arrangements, thereby altering the coordination geometry around the B cation. In Sr 2 FeMnO 6-δ , octahedra form around Fe/Mn, as depicted in the inset of Fig. 2, deviating from the tetrahedral coordination geometry observed in Sr 2 FeMnO 6-δ . The cubic crystal structure of Sr 2 FeMnO 6-δ is characterized by the Pm-3m space group, as shown in Fig. 2 and detailed in Table II.

Figure 3 presents scanning electron microscope (SEM) micrographs of Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ , illustrating their microstructures. These images highlight distinct grain geometries and growth styles in the materials, consistent with the anticipated structural variations. The grain size is significantly smaller for Sr 2 FeMnO 6-δ than Ca 2 FeMnO 6-δ which increases the boundaries in Sr 2 FeMnO 6-δ .

The thermal conductivities of both materials were investigated, revealing values below 0.50 Wm -1 K -1 at room temperature. Sr 2 FeMnO 6-δ demonstrated a lower thermal conductivity with the value of 0.36 Wm -1 K -1 , representing one of the lowest reported values for perovskite oxide materials. Figure 4 and Table III show the comparative thermal conductivities of the materials. This finding underscores the relationship between the grain size and reduced thermal conductivity. The synthesis method and environment were consistent with those used for previously reported perovskite oxides. 23 This study contributes to the understanding of the role of vacancy and boundary in the context of thermal insulation properties, a dimension that has received limited attention thus far.

The evaluation of thermal conductivity (K) was conducted using the Thermtest thermal conductivity meter, MP-2 with TPS-4, based on the Transient Plane Source Method. This method employs a plane sensor, a mathematical model describing heat conductivity, and electronic components to measure Thermal Transport Properties effectively. To ensure the reliability of the equipment, the thermal conductivity of a standard Pyrex verification sample was measured, yielding a value of 1.117 Wm -1 K -1 , consistent with the instruction manual and validating the thermal conductivity data for the studied materials.

Previous attempts to reduce thermal conductivity in perovskite oxides involved A-site or B-site cation substitution. 4,14,15 In our study, A-cations were substituted, leading to a structural transformation. The lower thermal conductivity of our materials can be attributed to various factors, including oxygen vacancy, lattice distortion, grain size, and boundary effects. Structural transformation due to A-cation substitution rules out lattice distortions as a significant contributor to phonon scattering. Scanning electron microscopy (SEM) images indicated well-sintered materials with micrometer-sized grains, which are essential for low lattice thermal conductivity due to increased phonon scattering. Since grain boundaries can contribute to phonon scattering, the observed clear grain boundaries in both materials were focused for a significant difference of thermal conductivity. As seen in the SEM images, the grains are smaller for Sr 2 FeMnO 6-δ which has relatively lower value of thermal conductivity. Table I. Rietveld refinement parameters of Ca 2 FeMnO 6-δ . Element x y z Uiso Multiplicity Occupancy Ca 0.495981 0.133570 -0.051958 0.042380 8 1 Fe1 0.0000 0.0000 0.0000 0.014069 4 0.5 Mn1 0.0000 0.0000 0.0000 0.014069 4 0.5 Fe2 0.465613 0.2500 0.533668 0.046658 4 0.5 Mn2 0.465613 0.2500 0.533668 0.046658 4 0.5 O1 0.289757 -0.002798 0.174856 0.026796 8 1 O2 0.049616 0.165703 0.081423 0.097407 8 1 O3 0.687468 0.2500 -0.307780 0.002865 4 1 Space group = Pnma, a = 5.334244 Å, b = 14.969558 Å, c = 5.461733 Å, wRp = 0.0376, Rp = 0.0296

The materials, Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ , have been extensively studied for their structures and charge transport properties, demonstrating different charge transport behaviors at room temperature. 23 However, the thermal conductivity difference between the two materials suggests an insignificant contribution of charge carriers to thermal conductivity. Fe 3+ and Fe 4+ or Mn 3+ and Mn 4+ spins may play a role, but their contribution insignificant compared to the dominant effect of phonon scattering.

The total thermal conductivity (K) is the sum of contributions from phonons (K p ) and radiation (K R ). 24,25 As discussed earlier, phonon scattering contributes to a decrease in thermal conductivity. Local lattice distortions, void spaces, and random vacancies act as sites for phonon scattering, leading to reduced thermal conductivity. The low thermal conductivity of our materials can be attributed to the suppression of lattice thermal conductivity and increased phonon scattering due to oxygen vacancies. The oxygen vacancy (δ) values for Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ are 1.0 and 0.22, respectively, emphasizing the role of oxygen vacancies in lowering thermal conductivity. 23,25 In conclusion, the low thermal conductivity of our compounds is primarily attributed to the significant impact of oxygen vacancies on increasing phonon scattering, emphasizing their crucial role in modulating thermal conductivity. The observed difference in thermal conductivity between Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ suggests that random vacancy distribution and smaller grain size or increased grain boundaries may further contribute to this difference in thermal conductivity, leading to the lower value of Sr 2 FeMnO 6-δ .

In conclusion, the study on Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ highlights their low thermal conductivity, a crucial attribute for effective thermal insulation. A-cation substitution induces structural transformations, ruling out lattice distortions as a significant contributor to phonon scattering. Well-sintered materials with micrometer-sized grains, observed through SEM, contribute to increased phonon scattering and lower thermal conductivity. Charge carriers, Fe 3+ and Fe 4+ spins, play a limited role, with the dominant effect of phonon scattering evident in the observed thermal conductivity difference. Table II. Rietveld refinement parameters of Sr 2 FeMnO 6-δ . Element x y z Uiso Multiplicity Occupancy Sr1 0.5000 0.5000 0.5000 0.023990 1 1.000 Mn1 0.0000 0.0000 0.0000 0.026154 1 0.500 Fe1 0.0000 0.0000 0.0000 0.026154 1 0.500 O1 0.5000 0.5000 0.5000 0.033145 3 0.963 Space group = Pm-3m, a = 3.85051(5) Å, wRp = 0.0387, Rp = 0.0300 The study underscores the interplay of phonons and radiation in determining thermal conductivity, with oxygen vacancies identified as a key factor. The presence of oxygen vacancies increases phonon scattering, suppressing lattice thermal conductivity. The difference in thermal conductivity between Ca 2 FeMnO 6-δ and Sr 2 FeMnO 6-δ suggests a role for vacancy arrangements and grain boundaries in shaping their thermal properties. Random vacancy distribution and increased grain boundaries help reduce thermal conductivity. Overall, the findings provide crucial insights into the design of materials with tailored thermal properties, with implications for applications requiring efficient thermal insulation.

Table III. Thermal conductivity data of Ca 2 Fe 2 O 6-δ , and Sr 2 Fe 2 O 6-δ at room temperature. Ca 2 FeMnO 6-δ Sr 2 FeMnO 6-δ 0.45 ± 0.098 (Wm -1 K -1 ) 0.36 ± 0.095 (Wm -1 K -1 )