NMR Study of AgInTe₂ at Normal and High Pressures

Abstract

The ternary semiconductor AgInTe2 is a thermoelectric material with a chalcopyrite-type structure, which is believed to transform 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, 115In and 125Te NMR analyses are used to study AgInTe2 for ambient conditions and pressures up to 5 GPa. Magnetic field-dependent and magic angle spinning (MAS) experiments of 125Te 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 115In, as well as the chemical shift distribution in agreement with the crystal structure. 115In NMR is further governed by a small quadrupolar interaction (ν Q ≈ 90 kHz) and shows an orders of magnitude faster nuclear relaxation in comparison to that of 125Te. At a pressure of about 3GPa, the 115In quadrupole interaction increases sharply to about 2400 kHz, indicating a phase transition to a structure with a well-defined though noncubic local symmetry, while the 115In 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.