  Large unit cell copper-chalcogenide based minerals with high crystalline
anharmonicity have a potential for thermoelectric applications owing to their
inherent poor lattice thermal conductivity. Here, the softening of
copper-selenium bonding and hence crystal framework plays an important role in
superionic conduction and thermal conductivity. We have studied Cu26Nb2Sn6Se32,
Cu26Nb2Sn6Se31.5 and Cu26Nb2Sn6Se30Te2 minerals with a strategically tailored
anionic disorders. These compounds have p-type degenerate behavior with carrier
concentration ranging between 1020 cm-3 at 300 K, high power factor and low
lattice thermal conductivity at 640 K. The existence of two low frequency Raman
active optical modes associated with soft Cu and Se atoms, three localized
Einstein modes in specific heat, suggest high scattering between acoustic and
optical branches with very short phonon lifetime less than 1 ps. The excess
vibrational density of states at low energies with compressed and flat optical
branches strongly hinders the heat transport in these crystalline mineral.
Comparatively, Cu26Nb2Sn6Se30Te2 is a promising thermoelectric material because
of high crystalline anharmonicity and softening of Cu-Se framework due to
heavier tellurium atom.
