  The motivation of this work is our recent observation of the robustness of
superconductivity in a High-entropy (HE) superconductor
Ag0.2In0.2Sn0.2Pb0.2Bi0.2Te (CsCl-type) to external pressure. The
superconducting transition temperature (Tc) of Ag0.2In0.2Sn0.2Pb0.2Bi0.2Te is
almost constant with pressure, described as robustness of superconductivity to
pressure, whereas the PbTe with zero configurational entropy of mixing exhibits
a clear decrease in Tc with pressure. Here, we investigated the atomic
displacement parameters (Uiso), the atomic-vibration characteristics, and the
electronic states of metal tellurides (MTe) with various configurational
entropy of mixing (DSmix) at the M site. The Uiso for the M site is clearly
increased by M-site alloying with DSmix > 1.1R, which is the evidence of local
disorder introduced by the increase in DSmix via the solution of three or more
M elements. The revealed vibrational density of states (DOS) shows a remarkable
broadening with DSmix > 1.1R, which indicates glassy characteristics of atomic
vibration in HE MTe with a NaCl-type structure (low-pressure phase). On the
electronic states of the CsCl-type (high-pressure) phases, where the robustness
of Tc is observed, blurry electronic band structure appears with increasing
DSmix, which indicates the evolution of blurry (glassy) electronic states in HE
MTe with the CsCl-type structure. The estimated electronic DOS at Fermi energy
cannot explain the changes in Tc for HE MTe when assuming conventional
electron-phonon superconductivity, but the conventional explanation seems to
work for PbTe. Therefore, the pairing mechanisms in MTe with DSmix > 1.1R are
affected by glassy phonon and/or blurry electronic states in MTe, and the
robustness of superconductivity would be originating from unique
electron-phonon coupling.
