NMR and specific heat study of atomic dynamics and spin-orbit behavior in Cu$_{\mathrm{2-x}}$Ag$_{\mathrm{y}}$Te

We report studies of Cu$_{\mathrm{2}}$Te and Cu$_{\mathrm{2-x}}$Ag$_{\mathrm{y}}$Te, promising candidates for thermoelectric and photovoltaic applications. Cu and Te NMR show that above a well-defined 200 K onset, Cu$_{\mathrm{2}}$Te exhibits Cu-ion hopping, leading to the higher-temperature superionic motion. In Cu$_{\mathrm{1.98}}$Ag$_{\mathrm{0.2}}$Te the onset increases to 250 K. In the low-temperature static phase the properties are nearly identical. Aside from Korringa terms there are large diamagnetic contributions for all nuclei, comparable to those for other systems with very large spin-orbit and/or inverted band configurations. Thus the system may be a topologically interesting system like the similar phase Ag$_{\mathrm{2}}$Te. Results will be compared to DFT calculations of NMR shifts. The low-temperature spectra also indicate two distinct local environments for Cu sites, one corresponding to high symmetry such as characterizes the high-temperature cubic phase, and one with much more asymmetry. In addition, specific heat results are consistent with about 50{\%} of the Cu ions being weakly bound on Einstein-oscillator sites. We tentatively connect these results to reported local inhomogeneity due to vacancy condensation in similar systems.