Novel structural transformation in K3ReP2S8 thiophosphates originating from the rare-earth (Re) cation sizes induced local coordination asymmetry

Abstract

Noncentrosymmetric (NCS) structure is a prerequisite condition for nonlinear optical (NLO) crystals and searching for feasible design strategy on the discovery of new NCS compounds has become extremely important. For that reason, new series of rare-earth (Re) based NCS K3ReP2S8 (Re = Y, Ho, Er; P21 space group) and CS K3ReP2S8 (Re = Pr, Sm, Gd; P21/c) thiophosphates were successfully prepared and this interesting CS to NCS structural remodeling shows the interior relationship with cation-size effect of Re metals. Systematic structural analysis indicates that several of varying influence factors including Re's ionic radii, average Re–S bond lengths and distortion degrees of ReS8 units induce the born of local coordination asymmetry between ReS8 and PS4 units, which facilitates the transformation from CS to NCS structures in K3ReP2S8 system. Moreover, excellent performances including strong NLO responses (1.1–1.4 times that of commercial AgGaS2) and required phase-matching behavior in P21–K3ReP2S8 were determined. Note that P21–K3ReP2S8 can be viewed as first NLO examples in the quaternary rare-earth thiophosphates. Moreover, NCS K3ReP2S8 also satisfy the well-balance between large NLO effects (1.1–1.4 × AgGaS2) and high laser damage thresholds (2.5–7.0 × AgGaS2). Theoretical study demonstrates that synergetic contributions of ReS8 and PS4 units guarantee the intrinsic origin of NLO response and ReS8 units provide the major NLO contribution. Therefore, this work not only affords one effective pathway to design new NCS materials through breaking the local symmetry in CS structures based on the size-directing effect of Re cations, but also verifies that bridging ReSn units into thiophosphates is beneficial to the huge enhancement of intrinsic NLO performances.