NMR study of AgInTe$_2$ at normal and high pressure

The ternary semiconductor AgInTe$_2$ is a thermoelectric material with chalcopyrite-type structure that transforms reversibly into a rocksalt-type structure under high pressure. Nuclear magnetic resonance (NMR) is considered to provide unique insight into material properties on interatomic length scales, especially in the context of structural phase transitions. Here, $^{115}$In and $^{125}$Te NMR is used to study AgInTe$_2$ for ambient conditions and pressures up to 5 GPa. Magnetic field dependent and magic angle spinning (MAS) experiments of $^{125}$Te prove strongly enhanced internuclear couplings, as well as a distribution of isotropic chemical shifts suggesting a certain degree of cation disorder. The indirect nuclear coupling is smaller for $^{115}$In, as well as the chemical shift distribution in agreement with the crystal structure. The $^{115}$In NMR is further governed by a small quadrupolar interaction ($\nu_\mathrm{Q} \approx$ 90 kHz) and shows an orders of magnitude faster nuclear relaxation in comparison to that of $^{125}$Te. At a pressure of about 3 GPa, the $^{115}$In quadrupole interaction increases sharply to about 2400 kHz, indicating a phase transition to a structure with a well defined, though non-cubic local symmetry, while the $^{115}$In shift suggests no significant changes of the electronic structure. The NMR signal is lost above about 5 GPa (at least up to about 10 GPa). However, upon releasing the pressure a signal is recovered that points to the reported metastable ambient pressure phase with a high degree of disorder.