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

Abstract

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 H 2 O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO 4 is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the longrange structure. Here, we describe the role of the Tl + 6s 2 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. Firstprinciples density functional calculations clearly show the contribution of the Tl + 6s 2 in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO 4 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.

Cation and Lone Pair Order–Disorder in the Polymorphic Mixed Metal Bismuth Scheelite Bi3FeMo2O12

“…The observed contraction in the unit cell volume during the transformation from the tetragonal to monoclinic structures is believed to be a consequence of increased coherence in the arrangement of the Bi 6s lone pair electrons. A similar effect is observed in TlReO 4 and in the lead fluorapatite Pb 5 (VO 4 ) 3 F, where the unit cell volume of the lone pair ordered phases is noticeably smaller than that of the disordered phases. , Extrapolation of the equivalent volume of the monoclinic phase (red dashed lines in Figure c) suggests that a similar effect is observed here. The observed contraction in the cell at this stage 2 is anisotropic, as shown in Figure S9 of the Supporting Information.…”

“…A similar effect is observed in TlReO 4 and in the lead fluorapatite Pb 5 (VO 4 ) 3 F, where the unit cell volume of the lone pair ordered phases is noticeably smaller than that of the disordered phases. 5,49 Extrapolation of the equivalent volume of the monoclinic phase (red dashed lines in Figure 8c) suggests that a similar effect is observed here. The observed contraction in the cell at this stage 2 is anisotropic, as shown in Figure S9 of the Supporting Information.…”

“…This monoclinic structure also has two fully occupied and crystallographically distinct Bi sites, in which Bi 6s electrons exhort a stereochemical influence with different orientations. The transition between the two structures results in a reduction in the unit cell volume, analogous to that seen in TlReO 4 and Pb 5 (VO 4 ) 3 F, where the unit cell volume of the lone pair-ordered phases is noticeably smaller than that of the disordered phases. This is a consequence of the well-known distortion theorem …”

“…This phenomenon has been called emphanisis and has been observed in systems containing cations with lone pair electrons. For example, emphanisis is associated with the thermoelectric properties of the binary lead chalcogenides and is believed to be responsible for the unusual re-entrant behavior of TlReO 4 . , Lone pair-induced emphanisis has also been observed in CsSnBr 3 …”

Tetrahedral Displacive Disorder in the Scheelite-Type Oxide RbReO4

“…For example, both CsReO 4 and CsTcO 4 exhibit an orthorhombic structure with the space group Pnma at room temperature. This structure is sometimes referred to as the pseudo-scheelite structure to highlight the similarities of the structural motifs and is favored as the size of the A -site cation increases. , Although TlTcO 4 also adopts the orthorhombic pseudo-scheelite structure at room temperature, TlReO 4 adopts a unique monoclinic structure with the space group P 2 1 / c at room temperature. − Bastide proposed that it was possible to predict the tendency for a given AB O 4 oxide to form the ideal scheelite structure based on the value of the ratio of r A / r O and r B / r O where r i is the IR of the eight-coordinate A -type; six-coordinate B -type; and O anion . Based on this, RbReO 4 is predicted to be on the edge of the stability of the I 4 1 / a structure, noting that Bastide did not consider the I 4 1 / amd structure observed for RbReO 4 …”