Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation

Abstract

According to phase diagrams based on x-ray measurements, In70-Pt30 has the cubic Sn₇Ir₃ crystal structure (D8_f, cI40) but the alloys In70-Ni30 and In70-Pd30 have been variously reported to have either a cubic gamma-brass (D8_1 − 3, cI52) or the Sn₇Ir₃ structures. In this study, hyperfine interaction measurements are applied as an alternate method to identify phases. Perturbed angular correlation (PAC) measurements were made of characteristic nuclear quadrupole interactions of ¹¹¹In/Cd probe atoms, and demonstrated a common, characteristic “signature” of the Sn₇Ir₃ structure in all three alloys. The Sn₇Ir₃ structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and point symmetries indicate that the electric-field gradients at both sites should be axially symmetric. Measured perturbation functions for all three alloys exhibited two axially symmetric quadrupole interaction signals having the expected 3:4 ratio of amplitudes, as expected for the structure. Furthermore, ratios of the two quadrupole interaction frequencies in each alloy were characteristically large, with frequencies for probe atoms on In(3) sites roughly five times greater than on In(4) sites. Taken together, these observations confirm that all three phases have the Sn₇Ir₃ structure. Quadrupole interaction frequencies are also reported for isostructural alloys of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed in measurements near room temperature in all six phases, indicating excellent atomic ordering at the stoichiometric 70:30 compositions.