Pressure Controls the Structure and Nonlinear Optical Properties of Piezochromic CdTeMoO6

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.