18 research outputs found
Spin-induced symmetry breaking in orbitally ordered NiCr_2O_4 and CuCr_2O_4
At room temperature, the normal oxide spinels NiCr_2O_4 and CuCr_2O_4 are
tetragonally distorted and crystallize in the I4_1/amd space group due to
cooperative Jahn-Teller ordering driven by the orbital degeneracy of
tetrahedral Ni () and Cu (). Upon cooling, these
compounds undergo magnetic ordering transitions; interactions being somewhat
frustrated for NiCr_2O_4 but not for CuCr_2O_4. We employ variable-temperature
high-resolution synchrotron X-ray powder diffraction to establish that at the
magnetic ordering temperatures there are further structural changes, which
result in both compounds distorting to an orthorhombic structure consistent
with the Fddd space group. NiCr_2O_4 exhibits additional distortion, likely
within the same space group, at a yet-lower transition temperature of = 30
K. The tetragonal to orthorhombic structural transition in these compounds
appears to primarily involve changes in NiO_4 and CuO_4 tetrahedra
Interplay of material thermodynamics and surface reaction rate on the kinetics of thermochemical hydrogen production
Interplay of material thermodynamics and surface reaction rate on the kinetics of thermochemical hydrogen production
Production of chemical fuels using solar energy has been a field of intense research recently, and two-step thermochemical cycling of reactive oxides has emerged as a promising route. In this process, the oxide of interest is cyclically exposed to an inert gas, which induces (partial) reduction of the oxide at a high temperature, and to an oxidizing gas of either H_2O or CO_2 at the same or lower temperature, which reoxidizes the oxide, releasing H_2 or CO. Thermochemical cycling of porous ceria was performed here under realistic conditions to identify the limiting factor for hydrogen production rates. The material, with 88% porosity and moderate specific surface area, was reduced at 1500 °C under inert gas with 10 ppm residual O_2, then reoxidized with H_2O under flow of 600 sccm g^(−1) of 20% H_2O in Ar to produce H_2. The fuel production process transitions from one controlled by surface reaction kinetics at temperatures below ∼1000 °C to one controlled by the rate at which the reactant gas is supplied at temperatures above ∼1100 °C. The reduction of ceria, when heated from 800 to 1500 °C, is observed to be gas limited at a temperature ramp rate of 50 °C min^(−1) at a flow of 1000 sccm g^(−1) of 10 ppm O_2 in Ar. Consistent with these observations, application of Rh catalyst particles improves the oxidation rate at low temperatures, but provides no benefit at high temperatures for either oxidation or reduction. The implications of these results for solar thermochemical reactors are discussed
Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4
We examine the evolution of magnetic properties in the normal spinel oxides
Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The
end-member compounds of the solid solution series have been studied in some
detail because of their very interesting magnetic behavior. MgCr2O4 is a highly
frustrated system that undergoes a first order structural transition at its
antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room
temperature as a result of Jahn-Teller active tetrahedral Cu^2+ and undergoes a
magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic
Mg^2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4
dramatically affects magnetic behavior. In the composition range 0 < x < 0.3,
the compounds are antiferromagnetic. A sharp peak observed at 12.5K in the heat
capacity of MgCr2O4 corresponding to a magnetically driven first order
structural transition is suppressed even for small x suggesting glassy
disorder. Uncompensated magnetism - with open magnetization loops - develops
for samples in the x range 0.43 < x < 1. Multiple magnetic ordering
temperatures and large coercive fields emerge in the intermediate composition
range 0.43 < x < 0.47. The Neel temperature increases with increasing x across
the series while the value of the Curie-Weiss Theta decreases. A magnetic
temperature-composition phase diagram of the solid solution series is
presented
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Magnetostructural and magnetodielectric coupling in spinel oxides
Spinels oxides are of great interest functionally as multiferroic, battery, and magnetic materials as well as fundamentally because they exhibit novel spin, structural, and orbital ground states. Competing interactions are at the heart of novel functional behavior in spinels. Here, we explore the intricate landscape of spin, lattice, and orbital interactions in magnetic spinels by employing variable-temperature high-resolution synchrotron x-ray powder diffraction, total neutron scattering, magnetic susceptibility, dielectric, and heat capacity measurements. We show that the onset of long-range magnetic interactions often gives rise to lattice distortions. We present the complete crystallographic descriptions of the ground state structures of several spinels, thereby paving the way for accurate modeling and design of structure-property relationships in these materials. We also report the emergence of magnetodielectric coupling in the magnetostructural phases of some of the studied spinels. We begin by examining spin-lattice coupling in the Jahn-Teller active systems NiCr2O4 and CuCr2O4. Orbital ordering yields a cubic to tetragonal lattice distortion in these materials above their magnetic ordering temperatures, however, we find that magnetic ordering also drives structural distortions in these spinels through exchange striction. We provide the first orthorhombic structural descriptions of NiCr2O4 and CuCr2O4. Our observation of strong spin-lattice coupling in NiCr2O4 and CuCr2O4 inspired the study of magnetodielectric coupling in these spinels. Magnetocapacitance measurements of NiCr2O4 reveal multiferroic behavior and new magnetostructural distortions below the Néel temperature. This observation illustrates the sensitivity of dielectric measurements to magnetostructural transitions in spinel materials. Finally, in the examination of NiCr2O4 we show that magnetodielectric coupling is well described by Ginzburg-Landau theory. In addition to exchange striction, geometric frustration couples spin interactions to the lattice of the spinels MgCr2O4 and ZnCr2O4. Novel spin ground states that are important for memory and quantum computing applications are predicted to exist in these spinels. However, their structural and spin ground states are not well understood. We find that tetragonal and orthorhombic phases coexist in antiferromagnetic MgCr2O4 and ZnCr2O4. The structural deformations in these materials lift spin degeneracy by primarily distorting the pyrochlore Cr sublattice. In subsequent studies, we probe the effect of adding dilute spins on the non-magnetic cation sites of MgCr2O4 and ZnCr2O4. Substitution of Co2+ cations in Zn1-xCoxCr2O4 completely suppress the spin-Jahn-Teller distortion of ZnCr2O4 while, Cu2+ substitutions in Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 induce Jahn-Teller distortions at temperatures above their magnetic ordering temperatures. The Jahn-Teller distortions of Mg1-xCuxCr2O4 and Zn1-xCuxCr2O4 do not lift spin degeneracy, therefore magnetic ordering is still suppressed down to low temperatures. We show that only more than 20% magnetic A substituents can lift spin degeneracy in MgCr2O4 and ZnCr2O4.We have also examined the magnetostructural phase transition of the spinel Mn3O4. We show that Mn3O4 undergoes a magnetostructural phase transition from tetragonal I41/amd symmetry to a phase coexistence regime consisting of tetragonal I41/amd and orthorhombic Fddd symmetries. Phase coexistence in Mn3O4 is mediated by strain due to a significant lattice mismatch between the low temperature orthorhombic phase and the high temperature tetragonal phase. We propose that strain could be used to control the structure and properties of Mn3O4. Our investigations of spin-driven lattice distortions in spinel oxides illustrate that structural phase coexistence is prevalent for spinels with Néel temperatures below 50 K
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Magnetodielectric coupling in the ilmenites MTiO3 ( M=Co , Ni)
Antiferromagnetic Spin Correlations Between Corner-Shared [FeO<sub>5</sub>]<sup>7–</sup> and [FeO<sub>6</sub>]<sup>9–</sup> Units, in the Novel Iron-Based Compound: BaYFeO<sub>4</sub>
A novel
quaternary compound in the Ba–Y–Fe-O phase diagram was
synthesized by solid-state reaction and its crystal structure was
characterized using powder X-ray diffraction. The crystal structure
of BaYFeO<sub>4</sub> consists of a unique arrangement of Fe<sup>3+</sup> magnetic ions, which is based on alternate corner-shared units of
[FeO<sub>5</sub>]<sup>7–</sup> square pyramids and [FeO<sub>6</sub>]<sup>9–</sup> octahedra. This results in the formation
of stairwise channels of FeO polyhedra along the <i>b</i> crystallographic axis. The structure is described in an orthorhombic
crystal system in the space group <i>Pnma</i> with lattice
parameters <i>a</i> = 13.14455(1) Ã…, <i>b</i> = 5.694960(5) Ã…, and <i>c</i> = 10.247630(9) Ã….
The temperature-dependent magnetic susceptibility data reveal two
antiferromagnetic (AFM) transitions at 33 and 48 K. An upturn in the
magnetic susceptibility data above these transitions is observed,
which does not reach its maximum even at 390 K. The field-dependent
magnetization data at both 2 and 300 K show a nearly linear dependence
and do not exhibit significant hysteresis. Heat capacity measurements
between 2 and 200 K reveal only a broad anomaly without any indication
of long-range ordering. The latter data set is not in good agreement
with the magnetic susceptibility data, which makes it difficult to
exactly determine the magnetic ground state of BaYFeO<sub>4</sub>.
Accordingly, a temperature-dependent neutron diffraction study is
in order, which will enable resolving this issue. The theoretical
study of the relative strengths of magnetic exchange interactions
along various possible pathways, using extended Hückel spin
dimer analysis, shows that only interactions between square pyramidal
and octahedral centers are significant, and among them, the intrachannel
correlations are stronger than interchannel interactions. This is
the first physical property study in such a magnetic ion substructure