As predicted and more: modulated channel occupation in YZn5+x

Abstract

Like many complex intermetallic phases, the crystal structures of REZn5+xcompounds (RE = lanthanide or Group 3 element) based on the EuMg5type have gradually unfolded. The original reports described a complex hexagonal structure with an unusual combination of tetrahedrally close-packed regions and open spaces, as well as observations of superstructure reflections. More recently, we reinvestigated the structure of YZn5, reclassifying it as the EuMg5+x-type compound YZn5+x(x ≃ 0.2), in which disordered channels run alongcthrough the spaces formerly considered open. In addition, DFT-chemical pressure (DFT-CP) analysis of ordered models of YZn5+xhighlighted paths for communication between neighboring channels setting the stage for superstructure formation. Herein, the experimental elucidation of this effect is presented with the synthesis and structure determination of a modulated form of YZn5+x. By slow-cooling samples of YZn5+xfrom the annealing temperature, crystals were obtained that exhibit satellite reflections with the modulation wavevectorq= {1\over 3}a*+ {1\over 3}b*+ 0.3041c*. Structure solution and refinement using a (3+1)D model in superspace groupP31c({1\over 3}\,\!{1\over 3}σ3)00sreveals incommensurate order in the structure's channels. Here, two Zn sites associated with the channels are present, each with discontinuous atomic domains that are slanted in thex3x4plane. Their slanting corresponds to adjustments along thecaxis for the presence or absence of close neighbors along that axis, while the occupation patterns of neighboring channels are shifted by {1\over 3} of the modulation period. These features follow earlier predictions from CP analysis, highlighting how this approach can be used predictively in search of new phenomena.