Evolutions of structural and optical properties of lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub> under high pressure
Authors: Yao Pan-Pan , Wang Ling-Rui , Wang Jia-Xiang , Guo Hai-Zhong ,   School of Physics and Microelectronics, Zhengzhou University, Zhengzhou 450052, China,   Collaborative Innovation Center of Light Manipulations and Applications, Shandong Normal University, Jinan 250358, China
Published: December 2020

In recent years, organic-inorganic hybrid perovskite materials have been widely used in solar cells, photodetectors, and light-emitting diodes due to their advantages such as high light absorption coefficient, good carrier mobility, and long carrier diffusion length. However, the high toxicity of lead and poor stability still restrict the application and promotion of such materials. The lead-free double perovskite material derived from the concept of “heterovalent substitution”, while maintaining the high symmetrical structure of perovskite, avoids using the toxic lead elements, which has the advantages of environmental friendly, stable structure, and suitable band gap. At present, the limited research on lead-free double perovskite materials still leaves a big room to researchers, and such a limited research seriously restricts the development and promotion of such materials. Therefore, the relationship between the structure and performance of lead-free double perovskite materials needs further exploring in order to provide theoretical basis for the practical application of such materials. Here in this work, the lead-free double perovskite material Cs<sub>2</sub>TeCl<sub>6</sub> is prepared by the solution method. The crystal structure and optical properties of the lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub> under high pressure are investigated by using diamond anvil cell combined with <i>in-situ</i> high-pressure angle-dispersive X-ray diffraction and ultraviolet-visible absorption technology. The results show that the crystal structure of Cs<sub>2</sub>TeCl<sub>6</sub> is not changed within the experimental pressure range of 0－50.0 GPa, and the structural symmetry of <i>Fm-</i>3<i>m</i> is still maintained, indicating the sample has good stability. The lattice constant and volume of Cs<sub>2</sub>TeCl<sub>6</sub> gradually decrease within the pressure range of 0－50.0 GPa. The volume and pressure of Cs<sub>2</sub>TeCl<sub>6</sub> are fitted using the third-order Birch-Mumaghan equation of state, the bulk elastic modulus is obtained to be <i>B</i><sub>0</sub> = (18.77 ± 2.88) GPa. The smaller bulk elastic modulus indicates that the lead-free double perovskite material Cs<sub>2</sub>TeCl<sub>6</sub> has higher compressibility. The optical band gap of Cs<sub>2</sub>TeCl<sub>6</sub> is 2.68(3) eV at 1 atm and its optical band gap gradually decreases with the increase of pressure, which is related to the shrinkage of octahedral [TeCl<sub>6</sub>]<sup>2–</sup> under high pressure. The calculation results show that the Cs<sub>2</sub>TeCl<sub>6</sub> possesses an indirect band gap, the valence band maximum is mainly composed of Cl 3p orbits, and the conduction band minimum is mainly composed of Te 5p and Cl 3p orbits. After the pressure is completely relieved, Cs<sub>2</sub>TeCl<sub>6</sub> returns to the initial state. The above conclusions further deepen the understanding of the crystal structure and optical properties of lead-free double perovskite Cs<sub>2</sub>TeCl<sub>6</sub>, and provide a theoretical basis for designing and optimizing the lead-free double perovskite materials.