NMR Study of AgInTe2 at Normal and High Pressures

The ternary semiconductor AgInTe2is a thermoelectric materialwith a chalcopyrite-type structure, which is believed to transform into a rocksalt-type structure under high pressure. Nuclear magnetic resonance (NMR) isconsidered to provide unique insight into material properties on interatomiclength scales, especially in the context of structural phase transitions. Here,115Inand125Te NMR analyses are used to study AgInTe2for ambient conditions andpressures up to 5 GPa. Magneticfield-dependent and magic angle spinning(MAS) experiments of125Te prove strongly enhanced internuclear couplings, aswell as a distribution of isotropic chemical shifts, suggesting a certain degree ofcation disorder. The indirect nuclear coupling is smaller for115In, as well as thechemical shift distribution in agreement with the crystal structure.115In NMR isfurther governed by a small quadrupolar interaction (nu Q approximate to 90 kHz) and showsan orders of magnitude faster nuclear relaxation in comparison to that of125Te.At a pressure of about 3GPa, the115In quadrupole interaction increases sharply to about 2400 kHz, indicating a phase transition to astructure with a well-defined though noncubic local symmetry, while the115In shift suggests no significant changes of the electronicstructure. The NMR signal is lost above about 5 GPa (at least up to about 10 GPa). However, upon releasing the pressure, a signal isrecovered that points to the reported metastable ambient pressure phase with a high degree of disorder.