  The metal to insulator transition (MIT) in Mott-Hubbard systems is one of the
most important discoveries in condensed matter physics, and results in abrupt
orbital transitions from the insulating to metallic phases by elevating
temperature across a critical point (TMIT). Although the MIT was previously
expected to be mainly driven by the orbital Coulomb repulsion energy, the
entropy contribution to the orbital free energy that also determines the
relative stability of the metallic and insulating phases was largely
overlooked. Herein, we demonstrate an orbital-entropy dominated reversible
electronic phase transition in the metastable perovskite family of correlated
rare-earth nicklates (ReNiO3), in addition to their previously known MIT driven
by orbital Coulomb energies. In reverse to MIT, the resistivity of ReNiO3
abruptly increases by 2-3 orders by elevating T across another critical point
(TR-MIT) below TMIT, and such transition is named as reverse-metal to insulator
transition (R-MIT). Combining the afterwards exponentially decreasing
resistivity in the insulating phase of ReNiO3 at further temperature elevation,
a distinguished delta-temperatural transport character is established, which is
potentially applicable for locking the working temperatures range for electric
devices. The TR-MIT is shown to be enhanced via reducing the compositional
complexity and size of Re or imparting bi-axial compressive strains, and
meanwhile the transition sharpness of delta-temperatural transport is reduced.
Our discovery indicates that temperature range for a thermodynamically stable
insulating phase of ReNiO3 is in between of TR-MIT and TMIT, while a new
conductive phase with high orbital entropy is formed by further descending
temperature below TR-MIT.
