Pd and octahedra do not get along: Square planar [PdS4] units in non-centrosymmetric La6PdSi2S14

Abstract

Non-linear optical (NLO) materials require a balance of high second-harmonic generation (SHG) signal and laser damage threshold (LDT), as well as phase matchable behavior. Herein, we report a new member of the (RE)6(TM)x(Tt)2Q14 family of compounds, La6PdSi2S14, which, unlike all other reported TM analogues crystallizing in hexagonal P63 space group, crystallizes in the non-centrosymmetric monoclinic P21 space group. The crystal structure contains chains of edge-sharing distorted square planar [PdS4] units. The square-planar coordination of Pd in La6PdSi2S14 exhibits remarkable NLO properties with high SHG (3.7 × AgGaS2) and LDT (3 × AgGaS2) values as well as phase matchability. This shows the promise of novel materials with distorted structural motifs for enhanced NLO properties. Upon formation of bimetallic chiral sulfides containing both Cu and Pd, Cu occupies the opposite faces of the octahedra forming [CuS3] units while Pd can be stabilized in the center of PdS6 octahedra in the hexagonal P63 crystal structure of La6Pd0.5CuSi2S14. This suggests that it is possible to form mixed metal systems which could further enhance NLO properties by incorporation of additional structural distortions.