Stabilization of Bi Vibration along C-Axis in BiS2-based Layered Compounds La(O,F)Bi(S,Se)2 by Se Substitution

BiCh _2 -based layered compounds have been extensively studied as potential thermoelectric and unconventional superconducting materials. For both functionalities, in-plane chemical pressure effects improve their thermoelectric or superconducting properties. In this study, we investigate the effects of in-plane chemical pressure on atomic vibrations of Bi by analyzing lattice specific heat measured at T = 1.9–300 K with multiple Debye and Einstein models for thermoelectric LaOBi(S,Se) _2 and superconducting LaO _0.5 F _0.5 Bi(S,Se) _2 . We reveal that in-plane chemical pressure enhances the oscillator number of the Einstein mode corresponding to large-amplitude Bi vibration along the c -axis in both the systems.