Materials with excellent nonlinear optical properties, especially the second-harmonic-generation (SHG) ability that can double the frequency of a laser ω to 2ω, are greatly important to modern optical technology. Recently, a temperature-induced SHG “on–off” phenomenon has been reported in dozens of metal–organic complexes, which have emerged as candidates in advanced optoelectronic applications. As a counterpart to temperature, pressure is also expected to trigger a SHG “on–off” phenomenon; nevertheless, such material is relatively limited. Herein, we report the observation of a pressure-induced SHG “on–off” phenomenon accompanied by a structural phase transition and piezochromism in CdTeMoO6. Under ∼10 GPa compression, the material exhibits a sharp transition from the “SHG-on” state to the “SHG-off” state. Coupled with the SHG “on–off” phenomenon, a structural transition occurs from two-dimensional to three-dimensional due to the dramatic change in the coordination environments of Te4+ and Mo6+ from [TeO4] and [MoO4] tetrahedra to [TeO6] and [MoO6] octahedra, respectively. The suppression of stereoactive lone-pair electrons on Te4+ is considered to be responsible for the SHG-off state under high pressure. Piezochromism from colorless to black is also observed coupled with the SHG “on–off” phenomenon, which can be attributed to the hybridization between Mo 4d and O 2p orbitals. Moreover, we demonstrate that the “three-in-one” transition (structural transition, SHG on–off, and piezochromism) is irreversible at room temperature but reversible upon annealing at increased temperatures (230–350 °C) with SHG, lattice dimension and color returning to those of the initial state. All of these behaviors make CdTeMoO6 a maneuverable pressure- and temperature-regulated SHG switching material with pragmatic multifunctionality.

Materials with zero-linear compression (ZLC) and zero-area compression (ZAC) have great promise for specific applications retaining constant in specific directions or planes under external impaction. To date, no more than ten ZLC/ZAC materials have been reported, most of which have very limited working pressure ranges (< 10 GPa). Herein, we report the observation of ZLC and ZAC in Li 2 Ti(IO 3 ) 6 with a gear-spring type structure over an ultra-wide pressure range (0~40 GPa). Structural analysis from the experiment and theoretical calculation reveals that the rotatable metal coordination polyhedra (gears) and extremely compressible metal chains (springs) work together to form an exquisite mechanical unloading device with intrinsic ZLC and ZAC behavior. Moreover, Li 2 Ti(IO 3 ) 6 sets a record-wide ZLC/ZAC working pressure range (up to 40 GPa) among anisotropic compression materials. The demonstration of intrinsic and long-lasting ZLC/ZAC with a gear-spring mechanism allows shock-resistant precision optics to be applied under extreme conditions.

Anomalous compression behaviors such as negative linear compressibility (NLC) and negative area compressibility (NAC) are emerging functionalities essential for utilities under extreme conditions. Metal–organic frameworks (MOFs) present a large family of NLC/NAC materials under the well-recognized “wine-rack” mechanism. Nevertheless, the working pressure of MOFs is limited to typically no more than 3 GPa. Herein, we report the observation of a rare reentrant NLC behavior of MIL-53­(Al) over an ultrawide pressure region up to 41.6 GPa. MIL-53­(Al) with the classic wine-rack topology first exhibits an NLC behavior along the c-axis in the pressure region of 0–2.7 GPa, followed by a normal compression process between 2.7 and 12.9 GPa. After that, it is surprising to find that the reentrant NLC property remains at a record-high pressure (>40 GPa) among the previously reported MOFs. The mechanisms of the distinct compression behaviors of MIL-53­(Al) in the three stages are studied by combining the X-ray diffraction analyses, Raman spectra, and DFT calculation. Particularly, the photoluminescence of Cr3+ is used as a unique probe for the subtle changes of the local coordination environments of Al3+ under compression. Both the absorption spectra and photoluminescence of Cr3+-doped MIL-53­(Al) not only verify the three-stage compression processes but also reveal a hidden structural/electrical phase transition around 22.0 GPa. These findings provide an in-depth understanding of the structure–property relationship of abnormal compression behavior at the local-structure level.

Materials with multi-stabilities controllable by external stimuli have potential for high-capacity information storage and switch devices. Herein, we report the observation of pressure-driven two-step second-harmonic-generation (SHG) switching in polar BiOIO 3 for the first time. Structure analyses reveal two pressure-induced phase transitions in BiOIO 3 from the ambient noncentrosymmetric phase (SHGhigh) to an intermediate noncentrosymmetric phase (SHGintermediate) and then to a centrosymmetric phase (SHGoff). The three-state SHG switching was inspected by in situ high-pressure powder SHG and polarization-dependent single-crystal SHG measurements. Local structure analyses based on the in situ Raman spectra and X-ray absorption spectra reveal that the SHG switching is caused by the stepwise suppression of lone-pair electrons on the [IO 3 ] À units. The dramatic evolution of the functional units under compression also leads to subtle changes of the optical absorption edge of BiOIO 3 . Materials with switchable multistabilities provide a state-of-art platform for next-generation switch and information storage devices.

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