The systematic tuning of crystal lattice parameters to achieve improved kinematic compatibility between different phases is a broadly effective strategy for improving the reversibility, and lowering the hysteresis, of solid–solid phase transformations. Kinematic compatibility refers to the fitting together of the phases. Here an apparently paradoxical example is presented in which tuning to near perfect kinematic compatibility results in an unusually high degree of irreversibility. Specifically, when cooling the kinematically compatible ceramic (Zr/Hf)O2(YNb)O4 through its tetragonal-to-monoclinic phase transformation, the polycrystal slowly and steadily falls apart at its grain boundaries (a process which is termed weeping) or even explosively disintegrates. If instead the lattice parameters are tuned to satisfy a stronger ‘equidistance’ condition, the resulting material exhibits reversible behavior with low hysteresis. These results show that a diversity of behaviors—from reversible at one extreme to explosive at the other—is possible in a chemically homogeneous ceramic system by manipulating conditions of compatibility in unexpected ways. These concepts could prove critical in the current search for a shape-memory oxide ceramic (1).

(1) 1. H. Gu, J. Rohmer, J. Jetter, A. Lotnyk, L. Kienle, E. Quandt, R. D. James, Nature 599 (2021), 416.