High-purity (≥99.9%) powders of SrCO₃, La₂O₃, MnO₂, Fe₂O₃, Co₃O₄, and Al₂O₃ were used as starting materials. Stoichiometric quantities were calculated according to the nominal compositions SrLaMn_0.5Al_0.5O₄, SrLaFe_0.5Al_0.5O₄, and SrLaCo_0.5Al_0.5O₄. La₂O₃ was pre-dried at 900˚C for 12 h prior to weighing to remove adsorbed moisture and residual carbonate. The weighed powders for each composition were thoroughly mixed by mortar and pestle, pressed into pellet and calcined at 1100˚C for 24 h in air to initiate solid-state reaction and decompose the carbonate precursor. The calcined powders were reground, uniaxially pressed into pellets (diameter 13 mm, thickness ~2 mm) under 3 N force, and sintered at 1250˚C for 24 h in air with a ramping rate of 5˚C/min.

Phase identification was performed by powder X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5406 Å). Data were collected over a 2θ range of 20˚ - 80˚. Rietveld refinement was carried out using the GSAS 1 NS EXPIGUI interface to extract lattice parameters and confirm the crystal structure. Surface morphology and grain size were examined by field-emission scanning electron microscopy (GEOL) operating at 15 kV. Thermal conductivity (κ) was determined at room temperature using the equation given in Figure 1 as described for the Thermset TLS-100 Thermal conductivity meter [9][10]. The working principle of the Thermset TLS-100 Thermal conductivity meter is shown in Figure 1 (right) [9][10].

Figure 1. Thermal conductivity calculation equation (left) and low- and high-conductivity graphs as an example (right). It is used as given by the Thermset TLS-100 Thermal conductivity meter.

Figure 2 presents the Rietveld refinement profiles for all three sintered samples. The refinement plots show the observed data (black crosses), the calculated pattern (red solid line), the difference curve (blue line at the bottom), and the allowed Bragg reflection positions (pink tick marks). The close agreement between observed and calculated patterns, together with the flat, near-zero difference curves, confirms the excellent quality of the refinements. All diffraction peaks are indexed exclusively to the tetragonal K₂NiF₄-type Ruddlesden-Popper structure (space group I4/mmm, No. 139), consistent with the previously reported crystal structure for this system [11]. No secondary phases, such as perovskite SrMnO₃, LaAlO₃, or other oxide impurities, were detected within the sensitivity limits of the instrument, confirming the phase purity of all three compositions sintered at 1250˚C.

Figure 2. Rietveld refinement profiles of (left) SrLaMn_0.5Al_0.5O₄, (centre) SrLaCo_0.5Al_0.5O₄, and (right) SrLaFe_0.5Al_0.5O₄sintered at 1250˚C. Black crosses: observed data; red line: calculated pattern; blue line: difference curve; pink tick marks: allowed Bragg reflection positions. All reflections are indexed to the tetragonal I4/mmm Ruddlesden-Popper structure, confirming phase purity.

Rietveld refinement for SrLaMn_0.5Al_0.5O₄, SrLaCo_0.5Al_0.5O₄, and SrLaFe_0.5Al_0.5O₄ converged with good reliability factors (R-factors < 5%), and the refined lattice parameters are summarized in Tables 1-3, respectively. A systematic variation in unit-cell volume is observed across the series, following the trend Mn < Co < Fe, which correlates with the effective ionic radii of the B-site cations in octahedral coordination (Mn³⁺: 0.645 Å, Co³⁺: 0.61 Å, Fe³⁺: 0.645 Å at high spin), modulated by the mixed Al³⁺ (0.535 Å) co-occupancy [12]. The contraction in unit-cell volume relative to the unmixed end-members is consistent with the incorporation of the smaller Al³⁺ ions at the B-site.

Table 1. Refined structural parameters for SrLaCo_0.5Al_0.5O₄ using powder X-ray diffraction data. Space group: I4/mmm, a = 3.7856 (2) Å, c = 12.516 (1) Å Rp = 0.0346, wRp = 0.0458.

Table 2. Refined structural parameters for SrLaFe_0.5Al_0.5O₄ using powder X-ray diffraction data. Space group: I4/mmm, a = 3.8181 (3) Å, c = 12.7045 (5) Å Rp = 0.0336, wRp = 0.0438.

Table 3. Refined structural parameters for SrLaMn_0.5Al_0.5O₄ using powder X-ray diffraction data. Space group: I4/mmm, a = 3.7756 (3) Å, c = 12.5175 (2) Å Rp = 0.0475, wRp = 0.0654.

FE-SEM micrographs of the surfaces of sintered pellets are shown in Figure 3, acquired at 15 kV. The SrLaMn_0.5Al_0.5O₄ sample displays comparatively the smallest and most uniform microstructure, consisting of well-rounded, compact grains indicative of a high degree of sintering densification. The SrLaCo_0.5Al_0.5O₄ sample exhibits somewhat coarser and more irregular grain morphology and slightly increased intergranular porosity compared to the Mn analogue. The SrLaFe_0.5Al_0.5O₄ sample shows the largest microstructure among the three compositions, with angular to plate-like grains and more heterogeneous surface texture, suggesting a different sintering behavior for the Fe-containing phase at 1250˚C.

Figure 3.SEM micrographs of sintered surfaces of (a) SrLaMn_0.5Al_0.5O₄, (b) SrLaCo_0.5Al_0.5O₄ and (c) SrLaFe_0.5Al_0.5O₄. Grain size increases in the order Mn < Co < Fe.

The progressive increase in grain size and grain growth uniformity variation following the order Mn < Co < Fe directly correlates with the observed thermal conductivity trend. The SrLaMn_0.5Al_0.5O₄ compound’s finest and most uniform grain size implies the highest grain boundary density per unit volume, providing abundant phonon-scattering interfaces that impede heat conduction. Grain boundaries are well-established phonon-scattering centers that reduce the effective phonon mean free path and, consequently, the overall thermal conductivity. The coarser, more angular morphology of the Fe compound, by contrast, results in a lower grain boundary density, offering less resistance to phonon transport and contributing to its highest κ value in the series. This microstructural evidence, combined with the B-site disorder arguments presented in Section 3.3, provides a coherent mechanistic picture of the thermal conductivity trend across the three compositions.

Room-temperature thermal conductivity values for all three compositions are listed in Table 4 and depicted graphically in Figure 3. The measured values are strikingly low compared to conventional ceramic oxides such as Al₂O₃ (~30 W/mK) and even relative to the benchmark TBC material YSZ (~2.2 W/mK), placing all three compounds in the ultralow thermal conductivity regime.

Table 4. Room-temperature thermal properties of the three compositions.

The thermal conductivity follows the order SrLaMn_0.5Al_0.5O₄ (0.204 W/mK) < SrLaCo_0.5Al_0.5O₄ (0.326 W/mK) < SrLaFe_0.5Al_0.5O₄ (0.496 W/mK). It is compared in the bar chart in Figure 4 for a quick glance. Several concerted mechanisms are proposed to account for this trend:

1.B-sitemass-contrastdisorder. The degree of phonon scattering by point defects is governed by the mass-fluctuation scattering parameter Γ, which scales with (ΔM/M⁻)², where ΔM is the mass difference between co-occupying species and M⁻ is the average B-site mass. The atomic mass of Mn (54.94 u) differs most strongly from that of Al (26.98 u) compared to Fe (55.85 u)-Al and Co (58.93 u)-Al pairs. Although the Fe-Al and Mn-Al mass contrasts are similar in absolute terms, the Mn-Al pair yields the highest relative contrast (ΔM/M⁻ ≈ 0.68) compared to Fe-Al (≈0.69) and Co-Al (≈0.74). Combined with the Mn-compound’s finer grain size and smaller unit-cell volume (enhanced lattice strain), the net phonon scattering is most pronounced for the Mn analogue.

2.Grainboundaryscattering. As noted in Section 3.2, the Mn compound exhibits the finest grain size (~1 - 2 μm). For phonons whose mean free path is comparable to or larger than the grain size, grain boundaries act as strong scattering centres, effectively reducing the mean free path and hence κ. The Fe compound’s coarser microstructure explains, at least in part, its higher thermal conductivity.

3.AnharmonicityandUmklappscattering. The layered RP structure intrinsically promotes anharmonic lattice dynamics due to the mismatch in bonding character between the rigid BO₆ octahedral layers and the more ionic AO rock-salt interlayers. The degree of anharmonicity, quantified by the Grüneisen parameter γ, tends to increase with the level of B-site compositional disorder, further suppressing phonon group velocity and contributing to the low κ values across the entire series.

4.Intrinsiclayered-structurephononblocking. The natural superlattice of RP phases imposes periodic acoustic impedance mismatches along the c-direction, suppressing cross-plane phonon group velocities in a manner analogous to engineered superlattices. All three compounds benefit from this intrinsic mechanism, which is responsible for their collectively low κ values relative to simple perovskites.

The 0.204 W/mK recorded for SrLaMn_0.5Al_0.5O₄ is among the lowest reported for polycrystalline oxide ceramics at room temperature, competitive with glass-like conductors and comparable to values reported for optimized RP chalcogenides. This result positions the Mn compound as a highly promising candidate for thermal barrier and thermal insulation applications.

Figure 4. Bar chart comparing room-temperature thermal conductivity (κ) of the three compositions: SrLaMn_0.5Al_0.5O₄ (red = 0.204 W/mK), SrLaCo_0.5Al_0.5O₄ (green = 0.326 W/mK), and SrLaFe_0.5Al_0.5O₄ (blue = 0.496 W/mK). Error bars represent ±1 standard deviation from three independent measurements.