In recent years, heat accumulation occurs with the emergence of the "heat island effect". The high temperature can affects human living environment, and increases the energy consumption of refrigeration equipment. It is thus necessary to develop color thermal insulation materials with a high near-infrared reflectance. The existing components used for insulation coatings are mostly rare-earth transition metal composite oxides. However, a higher cost of using rare earth materials restricts the large-scale application to some extent. Transition metal high-entropy ceramic inorganic powder can be used as cheap raw materials, which has the advantages of multi-component characteristics (i.e., great component adjustment space, unique entropy effect, and adjustable material properties. However, little work on the use as high reflective pigments has been reported yet. In this paper, high-entropy ceramic carbonate powders were prepared and the color of the powders was innovatively adjusted via changing the transition metal components. In addition, the phase, color and optical reflection properties of the prepared high-entropy ceramic carbonate powders with different element combinations were also analyzed.

For the hydrothermal preparation of high-entropy carbonate powders (i.e., (Mg_0.2Co_0.2Ni_0.2Zn_0.2Cu_0.2)CO₃, (Mg_0.2Co_0.2Ni_0.2Zn_0.2Fe_0.2)CO₃, and (Mg_0.2Co_0.2Ni_0.2Zn_0.2Mn_0.2)CO₃), pure MgSO₄, CoSO₄, NiSO₄, ZnSO₄, FeSO₄, MnSO₄, CuSO₄ and Na₂CO₃ were used as raw materials according to a stoichiometric coefficient ratio. Firstly, a certain proportion of five metal sulfate solids was mixed with deionized water in a beaker under stirring until fully dissolved. The sulfate solution was put into a sodium carbonate solution, and mixed under stirring. Afterwards, the mixture in a high-pressure reactor with a PTFE lining was heated at 160 ℃ for 12 h for hydrothermal treatment. A high-entropy carbonate powder was obtained after centrifugation, washing, and drying.

The phase composition of the powder was determined by a model Empyrean X-ray diffractometer (XRD, PANalytical Co., The Netherlands). The microstructure of the powder was analyzed by a model SUPRA 55 field emission scanning electron microscope (FESEM, Carl Zeiss Co., Germany). The UV visible near-infrared diffuse reflectance of the powder was characterized by a model LAMBDA 750 ultraviolet-visible-near infra-red spectrometer (UV-VIS-NIR, PerkinElmer Co., USA). The near-infrared solar reflectance R* in the wavelength range of 780-2 500 nm was calculated according to the national standard ASTMG159-98. The L*a*b* color parameters of the powder were determined by a model CS-5960GX spectrophotometer.

Three types of five-component equimolar high-entropy carbonate solid solutions were prepared by a hydrothermal method. Based on the morphology characterizations, three high-entropy carbonates synthesized have the same crystal structure and the same spatial group as . The diffraction peak gradually shifts towards a lower diffraction angle with the order of the components Cu, Fe, and Mn. This is because the radius of the element ions gradually increases, resulting in a corresponding increase in the crystal plane spacing. Moreover, there are significant differences in the morphology characteristics of three high-entropy carbonates, which are the result of the multiple effects of factors such as the number of extranuclear electrons, the Jahn Teller distortion, magnetism, and bond length of Cu, Fe, and Mn ions. The Jahn Teller distortion and para-magnetism of Cu lead to the formation of irregular spherical particles with surface density and particle aggregation in the MgCoNiZnCu carbonate solid solution. The ferromagnetism of Fe causes MgCoNiZnFe carbonate solid solution to form the spheres with dense and regular surface morphology. The combination of ion bonding bond length and antiferromagnetism of Mn results in the formation of loose aggregates of MgCoNiZnMn carbonate solid solutions. In addition, the colorimetric characterization indicates that the color of three high-entropy powders is different (i.e., grayish green for MgCoNiZnCu, orange red for MgCoNiZnFe, and light purple for MgCoNiZnMn). The band gap value Eg gradually increases with the order of the component Fe, Cu, and Mn, and the absorption wavelength of the sample gradually decreases (i.e., the absorption wavelengths are 497.14, 393.99 nm and 242.71 nm, respectively), and the absorption edge gradually shifts blue. Three types of high-entropy carbonate solid solutions have a high near-infrared solar reflectance in the near-infrared band, with a maximum of 78.64%, which can be used as thermal insulation materials.

Three high-entropy carbonate powders with different elemental compositions were prepared by a hydrothermal method, and the phase composition, reflectance, and color properties of the synthesized powders were comprehensively investigated. The results showed that changing a single element in the compositions could change the color of the powder (i.e., grayish green for MgCoNiZnCu, orange red for MgCoNiZnFe, and light purple for MgCoNiZnMn), and the color coordinate L*a*b*c* value also changed accordingly. Meanwhile, the bandgap width of the powder gradually increased from 3.11 eV to 3.40 eV and 5.49 eV with the order of the component Fe, Cu, and Mn. The three compositions of high-entropy carbonate powders had three different shades in the visible light region, and a high near-infrared solar reflectance (R*) of 78.64%. Therefore, the high-entropy carbonate powders prepared via changing the component element had the characteristics of stable structure, tunable color, and high near-infrared reflectance, having broad application prospects in architecture, automotive, and chemical engineering.