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The emerging phase change materials (PCMs) have attracted much attention due to their unique properties and applications. However, the optical properties of PCM at high pressures have not been well explored. In this work, the microstructure and optical properties of GeTe and Sn-substituted GeTe at high pressures up to 30 GPa have been systematically investigated. We find that the substitution of small amounts of Sn for Ge combined with the introduction of high pressure can induce an optical contrast of 117% in crystalline GeTe, which is even higher than the highest optical contrast reported so far (90%) caused by amorphous GeTe to crystalline GeTe. The pressure-induced contrast does not originate from any phase transition as previously, but from the combined effect of Peierls distortion elimination and Ge vacancy formation, which leads to an increase in carrier concentration. The relationship between pressure/atomic composition, Peierls distortion/vacancy formation, and optical contrast can be well explained by a combination of experiments and theoretical calculations. Therefore, this study reveals the effect of high pressure on the optical properties of PCMs and provides new ideas for the design of high-optical-contrast PCMs. [ABSTRACT FROM AUTHOR]

In view of the need to limit harmful emissions of chlorofluorocarbons into the atmosphere and sharp increase in life support areas that require cooling, the search for new magnetocaloric materials is very urgent at the present time. In this work, we suggest the ways for improving the magnetocaloric properties of the promising La 0.8- x K 0.2 Mn 1+ x O 3 (0 ≤ x ≤ 0.2) perovskites and compare the functional properties of nanocrystalline samples of about 40 nm with polycrystalline samples of about 1 μ m. The structure, morphology, magnetic properties and magnetocaloric effect of these compounds were studied as a function of manganese doping level. The main crystalline phase of all samples is a rhombohedral R 3 ‾ c perovskite structure. The maximum doping level of excess manganese for preserving single-phase structure of the nanocrystalline La 0.8- x K 0.2 Mn 1+ x O 3 compounds is x = 0.10. As x increases the Curie temperature T C is increased slightly. The best composition with the highest entropy change Δ S M max = −3.629 J/(kg⋅K) under 3 T at T C = 332 K is the La 0.8- x K 0.2 Mn 1+ x O 3 with x = 0.05. On the other hand, the La 0.8- x K 0.2 Mn 1+ x O 3 polycrystalline manganite with x = 0.10 exhibits the highest Δ S M max = −4.176 J/(kg⋅K) under 3 T at T C = 274 K. Additionally, the effect of hydrostatic pressure P up to 1.4 GPa has been investigated for the La 0.7 K 0.2 Mn 1.1 O 3 polycrystalline sample that leads to increasing T C to 287 K and reducing Δ S M max to −3.145 J/(kg⋅K) under 3 T. The obtained results demonstrate the ways for controlling magnetocaloric properties of the La 0.8- x K 0.2 Mn 1+ x O 3 compounds under internal and external conditions. [ABSTRACT FROM AUTHOR]