Eu(II)‐Based Quaternary Chalcogenides with Noncentrosymmetric Structures Stabilized by Site Disorder of <i>d</i>¹⁰ Metal Cations

Quaternary metal‐chalcogenides combining rare‐earth cations with late transition metal cations are attracting growing attention for their optical properties, such as for solar energy conversion or second harmonic generation. Synthetic explorations of the II 3 ‐ I 2 ‐ IV 2 ‐ Ch 8 family ( II = Eu; I = Cu or Ag; IV = Si, Ch = S or Se) have yielded Eu 3 Ag 2 Si 2 S 8 ( 1 ) and Eu 3 Cu 1.08(1) Si 2.42(1) Se 8 ( 2 ). Their structures have been characterized by X‐ray diffraction to form in the noncentrosymmetric space group I 3 d and to exhibit two distinct types of mixed‐site occupancies, for the Ag(I) cations in 1 and mixed Cu(I)/Si(IV) cations in 2 . In both, the cation disorder occurs to achieve charge balancing with the chalcogenide anions. A high yield of 1 can be achievedwith optical measurements showing indirect and direct band transitions of ≈2.2(1) and ≈2.4(1) eV, respectively. Its second harmonic generation response is found to be relatively strong, approximately 0.9 × AgGaS 2 , confirming its noncentrosymmetric structure. Band structure calculations reveal the valence and conduction band edges stem predominantly from the filled Ag(I)/Cu(I)‐based states and empty Si(IV)‐based states, respectively, with additional contributions from the chalcogenide anions. Calculation results also show that cation disorder facilitates a reduction in the antibonding interactions between the Ag(I)/Cu(I) d ‐based and chalcogenide p ‐based states.