We present neutron diffraction results for YbIn${\mathrm{Cu}}_4$, and Rietveld refinements of the structure. The ground state has the same structure (C15B) as the high temperature state, so that the first-order phase transition at $T_s=40\mathrm{}$ K is indeed isomorphic, i.e., the lattice constant changes without a change of crystal symmetry. The site disorder and the diffraction linewidths decrease systematically on going from polycrystal samples with two transitions (at 40 and 70 K) to polycrystals with a single (40 K) transition to flux-grown single crystals with a sharp transition at 40 K. We argue that in site-disordered samples, the effect of doping Yb onto the In site is to increase $T_s$ and cause the transition to become continuous.

• Received 1 April 1996

DOI:https://doi.org/10.1103/PhysRevB.54.6011