Suppression of structural instability in LaOBiS_2-xSe_x by Se substitution

Isovalent substitution of S by Se in LaOBiS_2-xSe_x has a substantial effect on its electronic structure and thermoelectric properties. To investigate the possible role of BiS₂ structural instability, we have studied the local structure of LaOBiS_2-xSe_x () using temperature dependent Bi L₃-edge extended x-ray absorption fine structure measurements. The results reveal that the local structure of the two compounds is significantly different. The BiS₂ sub-lattice is largely distorted in LaOBiS₂ (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 LaOBiS₂ (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 BiS₂ layer has large effect on the out-of-plane atomic correlations. The results suggest that the local structure of LaOBiS_2-xSe_x is an important factor to describe differing electronic and thermal transport of the two compounds.