Understanding the Re-entrant Phase Transition in a Non-magnetic Scheelite

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H₂O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO₄ is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl⁺ 6s² lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl⁺ 6s² in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO₄ occur due to changes in the correlation length of the Tl⁺ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl⁺ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.