A new crystal Mg11(HPO3)8(OH)6 for daytime radiative cooling

Structures or materials with both high reflectivity in ultraviolet-visible-near infrared region (UV-Vis-NIR) and high emissivity in middle infrared region (MIR) can achieve radiative cooling. In this paper, a new crystal Mg11(HPO3)8(OH)6 was synthesized by hydrothermal method. Measurement results show that this crystal presents a reflectivity of 88.04% in 0.2-2.5 µm region and an emissivity of 87.57% in 2.5-25 µm region, suggesting that this material could reflect most of the solar light radiation, and in addition dissipate heat to the surroundings and outer space. The cell parameters and structure simulation diagram were obtained from the result of the Rietveld refinement. The properties and mechanisms were discussed based on X-ray diffraction, FTIR spectroscopy and electron microscopy results. The cooling test under strong sunshine showed that the temperatures in space below Mg11(HPO3)8(OH)6 coating were 2-4 °C lower than those below the coatings based on commercial solar heat reflection materials as TiO2 and CaCO3, and 6-8 °C lower than that below Al-foil, suggesting the potential application of the new crystal in radiative cooling.