Inversion in Mg_1–xNi_xAl₂O₄ Spinel: New Insight into Local Structure

Abstract

A wide variety of compositions adopt the isometric spinel structure (ABO), in which the atomic-scale ordering is conventionally described according to only three structural degrees of freedom. One, the inversion parameter, is traditionally defined as the degree of cation exchange between the A- and B-sites. This exchange, a measure of intrinsic disorder, is fundamental to understanding the variation in the physical properties of different spinel compositions. Based on neutron total scattering experiments, we have determined that the local structure of MgNiAlO spinel cannot be understood as simply being due to cation disorder. Rather, cation inversion creates a local tetragonal symmetry that extends over sub-nanometer domains. Consequently, the simple spinel structure is more complicated than previously thought, as more than three parameters are needed to fully describe the structure. This new insight provides a framework by which the behavior of spinel can be more accurately modeled under the extreme environments important for many geophysics and energy-related applications, including prediction of deep seismic activity and immobilization of nuclear waste in oxides.

Oxidative Dehydrogenation of n‐Octane over Niobium‐Doped NiAl2O4: An Example of Beneficial Coking in Catalysis over Spinel

“…NiAl 2 O 4 had been used for the partial oxidation of methane, methane (steam or dry) reforming, oxidative coupling of methane, diesel steam reforming, tar reforming, and tetradecane reforming . NiAl 2 O 4 is known to have an inverse spinel structure, and the degree of this inversion can be varied by modifying the preparation technique and the temperature of the synthesis . Furthermore, migration of nickel between the two sub‐lattices (changing the degree of inversion) or to the NiAl 2 O 4 surface is well established and seems to be highly dependent on catalytic reaction conditions …”

“…Beside, few reports exist that discuss the effect of foreign‐metal doping in the lattice of these binary metal oxides and their physicochemical characteristics . Therefore, this study focuses on the modification of the physical and chemical properties of this structure upon coke deposition.…”

Empirical models for layered A‐site ordering in perovskite titanates

“…In fact, electron diffraction shows that the structure consists of nanodomains of order (short‐range order); thus, even short‐range order can be quantified with this model. Indeed, such regions of short‐range order may be a more common phenomenon than is widely realized, as similar regions have also been found within long‐range disordered spinel and pyrochlore structures. In both the NLT and LLT systems, the degree of vacancy ordering increases with x while the degree of cation ordering decreases …”

Unprecedented lattice volume expansion on doping stereochemically active Pb2+ into uniaxially strained structure of CaBa1−xPbxZn2Ga2O7

“…These observations suggest that neither the CaO 6 nor (Zn/Ga)O 4 polyhedra extracted from the neutron PDF data can be well described by the average structure model, especially in the low r range. This is a strong indication of the local structural 24,25 , which were also probed by the neutron total scattering technique. Here, the refined nanoscale structures for CaBa 1−x Pb x Zn 2 Ga 2 O 7 (x = 0 and 1) ( Supplementary Fig.…”