Supercooled Jahn-Teller ice

When the spins on the frustrated pyrochlore lattice obey the celebrated 2-in-2-out ice rule, they stay in a correlated disordered phase and break the third law of thermodynamics. Similarly, if the atomic ions on the pyrochlore lattice move in and outward of the tetrahedra, they may obey a constraint resembling the ice rule. We discover that a model for pyrochlore molybdates ${A}_{2}{\mathrm{Mo}}_{2}{\mathrm{O}}_{7}$ ($A=\mathrm{Y}$, Dy, Tb) exhibits a ``supercooled ice'' state of the displacement degrees of freedom of ${\mathrm{Mo}}^{4+}$ ions, when we take account of the Jahn-Teller (JT) effect. The JT effect occurs when the lattice distortions reduce the symmetry of the local crystal field, resulting in the orbital-energy splitting that causes the local energy gain. Unlike the standard JT effect that leads to periodic long-range ordering, the displacements of ${\mathrm{Mo}}^{4+}$ ions are disordered following the icelike rule. We microscopically derive a model that describes this situation by having the 2nd- and 3rd-neighbor interactions between in-out lattice displacements comparably as strong as the nearest-neighbor interactions of standard ice. There, the well-known nearly flat energy landscape of the ice state is altered to a metastable highly quasidegenerate icelike liquid state coexisting with a crystalline-like ground state. Our Monte Carlo simulations show that this liquid remains remarkably stable down to low temperatures by avoiding the putative first-order transition. The relaxation in the supercooled JT ice state exhibits glassy dynamics with a plateau structure. They fit the feature of a ``good glass former'' very often found in molecular liquids but that has never been observed in material solids. The high glass-forming ability of the interacting lattice degrees of freedom will play a key role in the spin-glass transition of the material.