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Two new polar polyselenides Rb 4 Ge 4 Se 12 (1) and Cs 4 Ge 4 Se 12 (2) with rarely reported dimeric [Ge 2 Se 4 (μ-Se 2 )] 4− units were synthesized. They present large second-harmonic generation (SHG) intensities of 7.5 and 6.5 times that of the benchmark AgGaS 2 with type I phase-matching behavior, high laser-induced damaged thresholds, a wide transmission region and congruently melting behavior, making them excellent candidates for IR nonlinear optical (NLO) applications. The SHG functional motifs of both compounds are determined to be [Ge 2 Se 4 (μ-Se 2 )] 4− unit by time-dependent density functional theory calculation, which further reveals that charge transfers from the lone pairs of terminal Se atoms to the five σ* orbitals of five-membered ring Ge 2 Se 3 have a predominant contribution to the total SHG effect.

We have systematically investigated the electronic structures and activation capacities of BiOBr {001} facets with different atomic terminations by means of DFT methods. Our calculations reveal that oxygen vacancies (OVs) give a significant boost in band edges of the O-terminated BiOBr {001} facets, and excess electrons induced by OVs could exceed the reduction potentials of high-energy N intermediates. Interestingly, the Bi-terminated BiOBr {001} facets may be good candidates for photocatalytic nitrogen fixation due to the stronger activation ability of N molecules comparing with O-terminated BiOBr {001} facets with OVs. Moreover, the Bi-terminated BiOBr {001} facets may tend to yield NH instead of N H .