Two rare-earth-based quaternary chalcogenides EuCdGeQ (Q = S, Se) with strong second-harmonic generation.

Two new rare-earth-based chalcogenides EuCdGeQ4 (Q = S, Se) have been designed and constructed by using Eu2+ and the classical NLO-active SBUs of [CdQ4] and [GeQ4]. They crystallize in a non-centrosymmetric Ama2 (no. 40) space group. Benefiting from the synergistic effects of [GeQ4] and highly distorted [CdQ4] tetrahedra, both compounds possess type-I phase-matching behaviour and large powder second harmonic generation (SHG) effects at 2.09 μm (2.6 and 3.8 × AgGaS2 for sulfide and selenide), as well as large direct band gaps (2.5 eV and 2.25 eV). Besides, they melt congruently at relatively low temperatures (997 °C for EuCdGeS4 and 882 °C for EuCdGSe4), which is suitable for bulk crystal growth by the Bridgman method. In addition, their electronic structures and some optical coefficients are calculated by first-principles.