Suppression of structural instability in LaOBiS Se by Se substitution.

Isovalent substitution of S by Se in LaOBiS Se has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS structural instability, we have studied the local structure of LaOBiS Se ([Formula: see text]) using temperature dependent Bi L-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS sub-lattice is largely distorted in LaOBiS (x  =  0.0), with two in-plane Bi-S1 distances separated by  ∼0.4 Å instead LaOBiSSe (x  =  1.0) showing much smaller local disorder with two in-plane Bi-Se distances in the plane being separated by  ∼0.2 Å. Temperature dependent study shows that the two Bi-S1 distances are characterized by different bond strength in LaOBiS (x  =  0.0) while it is similar for the Bi-Se distances in LaOBiSSe (x  =  1.0). The out of plane Bi-S2 bond is harder in LaOBiSSe indicating that the structural instability of BiS layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS Se is an important factor to describe differing electronic and thermal transport of the two compounds.