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 Cs₂TeCl₆ is prepared by the solution method. The crystal structure and optical properties of the lead-free double perovskite Cs₂TeCl₆ 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 Cs₂TeCl₆ 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 Cs₂TeCl₆ gradually decrease within the pressure range of 0－50.0 GPa. The volume and pressure of Cs₂TeCl₆ are fitted using the third-order Birch-Mumaghan equation of state, the bulk elastic modulus is obtained to be <i>B</i>₀ = (18.77 ± 2.88) GPa. The smaller bulk elastic modulus indicates that the lead-free double perovskite material Cs₂TeCl₆ has higher compressibility. The optical band gap of Cs₂TeCl₆ 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 [TeCl₆]^2– under high pressure. The calculation results show that the Cs₂TeCl₆ 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, Cs₂TeCl₆ returns to the initial state. The above conclusions further deepen the understanding of the crystal structure and optical properties of lead-free double perovskite Cs₂TeCl₆, and provide a theoretical basis for designing and optimizing the lead-free double perovskite materials.

Recent advance in highly efficient solar cells based on organic-inorganic hybrid perovskites has triggered intense research efforts to ascertain the fundamental properties of these materials. In this work, we utilize diamond anvil cell to investigate the pressure-induced structural and optical transformations in methylammonium lead iodide (CH3NH3PbI3) at pressures ranging from atmospheric pressure to 7 GPa at room temperature. The synchrotron X-ray diffraction experiment shows that the sample transforms from tetragonal (space group I4cm) to orthorhombic (space group Imm2) phase at 0.3 GPa and amorphizes above 4 GPa. Pressure dependence of the unit cell volume of CH3NH3PbI3 shows that the unit cell volume undergoes a sudden reduction at 0.3 GPa, which can prove the observed phase transition. We provide the high-pressure optical micrographs obtained from a diamond anvil cell. Upon compression, we can visually observe that the opaque black sample gradually transforms into a transparent red one above 4 GPa. We analyze the pressure dependence of the band gap energy based on the optical absorption and photoluminescence (PL) results. As pressure increases up to 0.25 GPa, the absorption edge and PL peak move to the longer wavelength region of 9 nm. However, abrupt blueshifts of the absorption edge and PL peak occur at 0.3 GPa, followed by a gradual blueshift up to 1 GPa, these phenomena correspond to the previously observed phase transitions. Phase transition increases the band gap energy of CH3NH3PbI3 as a result of reductions in symmetry and tilting of the[PbI6]4- octahedral. Upon further compression, the sample exhibits pressure-induced amorphization at about 4 GPa, which significantly affects its optical properties. Further high pressure Raman and infrared spectroscopy experiments illustrate the high pressure behavior of organic CH3NH3+ cations. Owing to the presence of hydrogen bonding between organic cations and the inorganic framework, all of the bending and rocking modes of CH3 and NH3 groups are gradually red-shifted with increasing pressure. The transition of NH stretching mode from blueshift to redshift as a result of the attractive interactions between hydrogen atoms and iodine atoms is gradually strengthened. Moreover, all the observed changes are fully reversible when the pressure is completely released. In situ high pressure studies provide essential information about the intrinsic properties and stabilities of organic-inorganic hybrid perovskites, which significantly affect the performances of perovskite solar cells.

Perovskite transition-metal oxides can undergo significant structural topological phase transition between perovskite structure, brownmillerite structure, and infinite-layer structure through the gain and loss of the oxygen ions under the external fields, accompanied with significant changes in physical properties such as transportation, magnetism, and optics. Topotactic phase transformation allows structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for utilizing the redox reaction and oxygen diffusion within transition metal oxides, establishing great potential applications in solid oxide fuel cells, oxygen sensors, catalysis, intelligent optical windows, and neuromorphic devices. In this work, we review the recent research progress in manipulating the topological phase transition of the perovskite-type oxide films and the regulation of their physical properties, which mainly focuses on tuning the novel physical properties of these typical films through strong interaction between the lattice and electronic degrees of freedom by the external fields such as strain, electric field, optical field, and temperature field. For example, a giant photoinduced structure distortion in SrCoO_2.5 thin films excited by photons was observed, higher than any previously reported data in the other transition metal oxide films; SrFeO₂ films undergo an insulator-to-metal transition when the strain state changes from compressive to tensile; It is directly observed that perovskite SrFeO₃nanofilaments were formed under electric fields and extend almost through the brownmillerite SrFeO_2.5 matrix in the ON state and are ruptured in the OFF state, unambiguously revealing a filamentary resistance switching mechanism; Utilizing <i>in situ</i> electrical scanning transmission electron microscopy, the transformation from brownmillerite SrFeO_2.5 to infinite-layer SrFeO₂under electric field can be directly visualized with atomic resolution. We also clarify the relationship between the microscopic coupling mechanism and the macroscopic quantum properties of charges, lattices, orbits, spin etc. Related research is expected to provide a platform for new materials, new approaches and new ideas for the development of high-sensitivity and weak-field response electronic devices based on functional oxides. These findings about the topological phase transition in perovskite oxide films have expanded the research space of material science, and have important significance in exploring new states of matter and studying quantum critical phenomena.