The V 1−x Mo x O 2 phase diagram has high structural and electronic complexity that is driven by strong, short-range correlations that compete with the long-range rutile crystal structure. The substitution regime near 50% Mo occupancy is no exception, but there has so far been no significant progress in determining the actual structure. Reported here is a combined study using single crystal X-ray diffraction, powder X-ray diffraction, and representational analysis to examine both the local and crystallographically averaged atomic structures simultaneously near x = 0.50. Between about x = 0.50 and 0.60, the average structure of V 1−x Mo x O 2 is the parent rutile phase, but the local symmetry is broken by atomic displacements that are best described using the orthorhombic subgroup Fmmm . This model is locally similar to the two-dimensionally ordered 2D-M2 phase recently reported in the compositional range 0.19 ≤ x ≤ 0.30, except the correlation length is much shorter in the 2D plane, and longer in the frustrated one, making it more isotropic. This work also extends the 2D-M2 phase regime up to x = 0.43, and suggests that the local- Fmmm phase observed here can be seen as the end result of the continued suppression of the 2D-M2 phase through enhanced geometric frustration between the intrinsic order parameters. This suggests that other doped-rutile phases with elusive structures may also be dominated by similar short-range correlations that are hidden in the diffuse scattering.
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Epitaxial intergrowths and local oxide relaxations in natural bixbyite Fe 2−x Mn x O 3
The scattering pattern of a crystal obeys the symmetry of the crystal structure through the corresponding Laue group. This is usually also true for the diffuse scattering, containing information about disorder, but here a case is reported where the diffuse scattering is of lower symmetry than the parent crystal structure. The mineral bixbyite has been studied by X-ray and neutron scattering techniques since 1928 with some of the most recent studies characterizing the low-temperature transition to a magnetically disordered spin-glass state. However, bixbyite also exhibits structural disorder, and here single-crystal X-ray and neutron scattering is used to characterize the different modes of disorder present. One-dimensional rods of diffuse scattering are observed in the cubic mineral bixbyite, which break the expected symmetry of the scattering pattern. It is shown that this scattering arises from epitaxial intergrowths of the related mineral, braunite. The presence of this disorder mode is found to be directly observable as well-defined residuals in the average structure refined against the Bragg diffraction. An additional three-dimensional diffuse scattering component is observed in neutron scattering data, which is shown to originate from the substitutional disorder on the Fe/Mn sites. This occupational disorder gives rise to local relaxations of the oxide sublattice, and the pattern of oxide displacements can be rationalized based on crystal-field theory. The combined use of neutron and X-ray single-crystal scattering techniques highlights their great complementarity. In particular, the large sample requirements for neutron scattering experiments prove to be an obstacle in solving the intergrowth disorder due to several growth orientations, whereas for X-ray scattering the one-dimensional nature of the intergrowth disorder renders solving this a more tractable task. On the other hand, the oxide relaxations cannot be resolved using X-rays due to the low Mn/Fe contrast. By combining the two approaches both types of disorder have been characterized.
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- Award ID(s):
- 1834750
- NSF-PAR ID:
- 10392716
- Date Published:
- Journal Name:
- IUCrJ
- Volume:
- 9
- Issue:
- 4
- ISSN:
- 2052-2525
- Page Range / eLocation ID:
- 523 to 532
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1107/S2052252522006315
