Obtaining crystalline materials possessing any one of these three properties—second-harmonic generation (SHG) switchable behavior, ultra-wide bandgap (deep-ultraviolet transparency), or strong nonlinear optical (NLO) effects—is highly challenging. Here, we report a fully inorganic NLO material RbHSeO4, which simultaneously exhibits all the three properties. It features an ultra-wide bandgap (6.56 eV), a large room-temperature SHG effect (about 5.8 times that of KH2PO4), and reversible SHG switching behavior over a wide temperature range near room temperature. Variable-temperature experimental dynamic charge density analysis and ab initio molecular dynamics (AIMD) jointly confirm the proton hopping between O atoms at specific lattice sites during heating and the resulting phase transition. Theoretical calculations suggest that the strong SHG effect and SHG switching property may mainly originate from the ordered arrangement of [(HSeO4)−]∞ chains involving hydrogen bonds and the disruption of this order by lattice thermal vibrations, respectively.

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