Two rare-earth-based quaternary chalcogenides EuCdGeQ4 (Q = S, Se) with strong second-harmonic generation

Abstract

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.