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

Abstract

The Bi 3 FeMo 2 O 12 system is examined as a rare example of a transition-metal oxide which, upon heating, undergoes a symmetry lowering and a 2:1 ordering of the transition-metal cations. The compound was synthesized in the tetragonal scheelite structure (S.G. #88: I4 1 /a) by a sol−gel method and converted into the monoclinic polymorph (S.G. #15: C2/c) by calcination above 500 °C. The structure of both polymorphs was analyzed using a combination of X-ray and neutron diffraction data, and the temperature-dependent phase transition between these was investigated in situ using variable-temperature neutron powder diffraction and scanning transmission electron microscopy. The results show that the structural phase transition takes place at low temperatures (∼500 °C) and is first order in nature, as evident from the coexistence of both structures. The transition from tetragonal to monoclinic results in the reduction of the equivalent unit cell volume. The role of the Bi 3+ 6s lone pairs in the temperature-driven phase transition has been studied using neutron pair distribution function analysis. Local structure analysis via neutron total scattering revealed the Bi 3+ 6s lone pairs to be stereochemically active in both structures, with short correlation lengths in the tetragonal structure and long correlation lengths in the monoclinic structure, leading to facile phase conversion and to a more efficient packing density with highly correlated lone pairs in the monoclinic structure. Magnetization isotherms of the tetragonal structure collected at 1.8 K exhibit ferromagnetic behavior, suggesting that the interplay between the observed short-range monoclinic order, defects, and surface-to-bulk effects alters the magnetic interaction, leading to short-range ferromagnetic interactions, which is highly unexpected given the low-temperature antiferromagnetic order observed in the monoclinic structure.

Tetrahedra Rotational and Displacive Disorder in the Scheelite-Type Oxide CsReO4

“…This results in a range of Tl−O bond distances in TlReO 4 and distortions of the O−W−O bond angles of the WO 4 tetrahedra in PbWO 4 . 22,35,36 Rotational disorder of the BO 4 tetrahedra has also been observed in ABO 4 (A 2+ = Ca, Sr, Ba; B 6+ = Mo, W) scheelites, 37,38 as well as NaRE(BO 4 ) 2 (RE 3+ = rare earth lanthanoids La−Yb, Y; B 6+ = Mo, W) double scheelites. 39 It should be noted that this rotational disorder was more prevalent in materials containing lighter B-site elements, such as Mo 6+ compared to the heavier W 6+ analogues.…”

“…55,56 Our work has highlighted the importance of understanding the local interactions within ABO 4 materials for a greater understanding of the structure− property relationships of various photocatalysts, selective oxidation catalysts, and potential nuclear waste forms. 22,35 Additionally, these features may impact the itinerant nature of magnetic materials, such as ABO 4 (B = Ru, Os), where the bulk magnetic properties are influenced by the BO 4 tetrahedra correlations. 8,31 Relationships such as these are of great significance given the vast array of applications scheelite-type structures are suited for, and additional local structure studies are required to identify additional ways of fine-tuning this local scale disorder.…”

“…When stereochemically active electron lone pairs are present on the A-site cation (such as the 6s 2 lone pairs of Tl + , Pb 2+ , and Bi 3+ ), hybridization between the ns 2 or nd 0 valence orbitals of the A- or B-site cations with the 2p orbitals of the oxygen anions causes distortions in the AO 8 polyhedra and BO 4 tetrahedra. This results in a range of Tl–O bond distances in TlReO 4 and distortions of the O–W–O bond angles of the WO 4 tetrahedra in PbWO 4 . ,, Rotational disorder of the BO 4 tetrahedra has also been observed in ABO 4 (A 2+ = Ca, Sr, Ba; B 6+ = Mo, W) scheelites, , as well as NaRE­(BO 4 ) 2 (RE 3+ = rare earth lanthanoids La–Yb, Y; B 6+ = Mo, W) double scheelites . It should be noted that this rotational disorder was more prevalent in materials containing lighter B-site elements, such as Mo 6+ compared to the heavier W 6+ analogues .…”

Seeing the Unseen: The Structural Influence of the Lone Pair Electrons in PbWO4

“…The XPDF G(r) for PbWO 4 and CaWO 4 were calculated using the PDFgetX3 software from the xPDFsuite with a Q max of 20.0 Å −1 . 32,33 Neutron total scattering data were collected at 100, 115, and 300 K using the Nanoscale Ordered MAterials Diffractometer (NOMAD) at the Spallation Neutron Source (SNS) located at Oak Ridge National Laboratory (ORNL). 38 PbWO 4 was loaded into a 3 mm quartz capillary and run during mail-in operations, whereas CaWO 4 was loaded into a 6 mm vanadium can and run during regular operations.…”

“…However, in both of these examples, neutron total scattering methods showed the local symmetry to be lower than the average tetragonal structure as a consequence of uncorrelated displacements associated with the 6s 2 lone pairs. 31,32 These two oxides both display unusual emphaniticlike behavior in their long-range structure, whereby they undergo symmetry lowering upon warming. 33 The observation of "unseen" lone pair ordering in the tetragonal polymorphs of TlReO 4 and Bi 3 FeMo 2 O 12 was made possible using neutron pair distribution function (NPDF) analysis.…”