SrTiO₃-based oxides have been investigated as a promising n-type thermoelectric material at high temperatures; however, the relatively high thermal conductivity results in inferior thermoelectric performance. The lattice thermal conductivity can be significantly reduced by high-entropy engineering via severe lattice distortion. However, high configuration entropy also causes the deterioration of carrier mobility and restrains electron transport resulting in low electrical conductivity. In this work, the low lattice thermal conductivity of 1.7 W/(m·K) at 1 073 K and significantly improved electrical conductivity of 112 S/cm from 60 S/cm can be achieved in n-type (Sr_0.25Ca_0.25Ba_0.25La_0.25)TiO₃/Pb@Bi composites ceramics with core-shell grains of all-scale hierarchical microstructure. The effects of the complex microstructure of core-shell grains as well as the precipitated Pb@Bi particles on electrons and phonons transport properties were systematically explored. ZT_max of 0.18 was obtained for the SPS-1200, which was 1.5 times that of pure high-entropy (Ca_0.2Sr_0.2Ba_0.2La_0.2Pb_0.2)TiO₃ samples prepared by a solid-state method. This improvement in thermoelectric performance contributes to the addition of Bi₂O₃ into the high-entropy (Sr_0.2Ca_0.2Ba_0.2Pb_0.2La_0.2)TiO₃ matrix resulting in multiphase core-shell grain structure combined with well-dispersed nano-sized metal Pb@Bi precipitates in the matrix. This feasible strategy of in-situ constructing all-scale hierarchical nanostructures can also be applied to enhance the performance of other thermoelectric systems.

Reversible luminescence modulation behavior upon the photochromic effect endows the photochromic ceramics with great potential in anti-counterfeiting and data storage applications. Here, Sm³⁺-doped KSr₂Nb₅O₁₅ photochromic ceramics exhibit superior anti-counterfeiting ability: good covertness and considerable modulation ratio of luminescent emission intensity after photochromic reaction. The results show that the photochromism originated from oxygen and cation vacancies, which were directly identified by electron paramagnetic resonance and positron annihilation lifetime spectra. Unexpectedly, oxygen vacancies work more effectively than cation vacancies during photochromic reactions. Moreover, the extraordinary anti-counterfeiting ability was attributed to the high energy transfer rate, which was particularly caused by the short mean distance below 1 nm between the Sm³⁺ and vacancies. The work here has provided atomic-scale structural evidence and made a progress in understanding the photochromic origins and mechanism in color-center theory.

The luminescence modulation behaviour under the in-situ electric field of rare-earth doped KSr₂Nb₅O₁₅ ceramics opened a new door for the development of dielectric materials. Where the understanding the effect of rare-earth doping on the electric properties of host, especially at the similar doping concentration with luminescence researches (low concentrations) is very important for the exploration of mechanism of electric-luminescent coupling effect. In this work, Nd³⁺-doped KSr₂Nb₅O₁₅ (KSN-xNd) ceramics were synthesized, and the electric properties were investigated systematically. Our results suggest that the Nd³⁺ doping slightly increased the phase transition temperatures and improved the piezoelectric response of KSr₂Nb₅O₁₅. Most importantly, a bidirectional dielectric tunability is revealed in KSr₂Nb₅O₁₅. The dielectric permittivity can be adjusted by the DC electric bias, with tunability ranging from −12.3% to 21.9%. The related mechanism and relationship between the bidirectional dielectric tunability and ferroelectricity are revealed by temperature-dependent dielectric and ferroelectric characterization. The researches of electric properties and bidirectional dielectric tunability of KSN-based ceramics paved the way to further exploration of electric-luminescent coupling mechanism.

Secondary phase Bi₂O₃, which could promote the sintering densification of ceramics, was used to prepare (Ca_0.85Ag_0.1La_0.05)₃Co₄O₉ thermoelectric ceramics. The mechanism of liquid−phase sintering process revealed that the diffusion rate of particles originated from the dissolved–precipitated processing for samples and suppressed the coarsening of grain growth. The effects of Bi₂O₃ on the microstructure and thermoelectric properties were investigated. The results showed that the relative density of samples increased from 92.1% to 95.5% through the liquid−phase sintering mechanism. The band gaps were tuned and it had the profound impact on the transport of charge carriers. Electrical resistivity decreased while Seebeck coefficient increased from 110 μV/K to 190 μV/K with increasing Bi₂O₃. Furthermore, the peak ZT value of 0.25 for 6 wt% Bi₂O₃ sample at 1073 K was obtained, resulting from low thermal conductivity of 0.92 W/(m·K). It suggests that Bi₂O₃ additive can dramatically improve the thermoelectric properties of Ca₃Co₄O₉.