Electric field gradients at 111In/111Cd probe atoms on A-sites in 211-MAX phases

The method of perturbed angular correlation (PAC) was applied to selected MAX phases with 211 stoichiometry. Radioactive 111In ions were implanted in order to measure the electric field gradients (EFG) in the key compounds Ti2InC and Zr2InC to determine the strength and symmetry of the EFG at the In-site. Further PAC studies in the In-free MAX phases Ti2AlN, Nb2AlC, Nb2AsC and Cr2GeC were performed to confirm that the In probes occupy the A-site as well. The strength of the EFG, with a quadrupole coupling constant νQ between 250 and 300 MHz in these phases, is quite similar to the ones found in Ti2InC with νQ = 292(1) MHz and in Zr2InC with νQ = 344(1) MHz, respectively. Different annealing behavior was observed whereas in all cases a linear decrease of νQ with increasing measuring temperatures was found. The experimental results are also in excellent agreement with those predicted by ab initio calculations using the APW+lo method implemented in the WIEN2k code. This study shows in an exceptional manner that 111In → 111Cd atoms are suitable probes to investigate the local surrounding at the A-site in 211-MAX phases. (paper)