Rare-earth Transition Metal Oxides Research Articles

Magnetic and intrinsic dielectric properties of multiferroic GdFe1-xNixO3 (0.0 ≤ x ≤ 0.2) perovskites

Rare-earth transition-metal oxides with a perovskite crystal structure are at the forefront of the quest for new multifunctional materials, focusing on rare-earth orthoferrites ReFeO3. GdFeO3 is a multiferroic material that exhibits antiferromagnetic behavior, weak ferromagnetism, and ferroelectricity. The multiferroism of GdFeO3 can be tailored by substituting Fe with Ni. Recently, it was reported that the polycrystalline GdFe1-xNixO3 (x = 0, 0.1, 0.2) samples are materials with high dielectric constant at radio frequencies and room temperature, showing an increase in dielectric constant by introducing Ni3+ ions at Fe sites and reaching values in the order of ε'∼103 to 104, enabling their application as capacitors and miniaturized microwave resonators. In this paper, we examined the impact of Ni-substitution on the magnetic and intrinsic dielectric properties of GdFeO3 perovskite by analyzing the active-polar phonons through infrared reflectance spectroscopy and showed that its high dielectric constant has an extrinsic origin.

Superexchange antiferromagnetism and cryogenic magnetocaloric effect in scheelite-type GdTi0.5Mo0.5O4 compound

Scheelite-type LnLiF4 compounds have become promising cryogenic magnetic refrigerants due to their giant low-field magnetocaloric effect (MCE). This work presents an example of a scheelite-type rare-earth transition metal oxide GdTi0.5Mo0.5O4. The magnetic interactions, magnetic phase transition (MPT) and MCE of GdTi0.5Mo0.5O4 compound were investigated through magnetization measurements and a mean-field approach. A mean-field model was used to elucidate the role of primarily superexchange antiferromagnetic (AFM) coupling on the magnetocaloric performance in this compound. From the analysis of the magnetization dependence, an isotropic AFM exchange coupling with energy scale εex = JnnS2 ≈ 1.7 K was estimated. The magnetic susceptibility reveals the occurrence of MPT from paramagnetic (PM) to AFM at the Néel temperature (TN ≈ 1.3 K). GdTi0.5Mo0.5O4 exhibits a large magnetic entropy change up to 43.3 J/(kg·K) at 1.3 K for μ0ΔH = 0−7 T, which makes it a cryogenic magnetic refrigerant with potential applications. Furthermore, a mean-field model with superexchange AFM above 3 K can well predict the MCE observed in GdTi0.5Mo0.5O4.

Structural distortions in rare-earth transition-metal oxide perovskites under high pressure

Owing to their structural complexity and wide range of possible chemical combinations, perovskite oxides exhibit many technically important physical properties. Pressure is a thermodynamic parameter which is useful for tuning physical properties; however, the response of the complex crystal structure to high pressure has not been thoroughly studied and rationalized. This study focuses on in situ high-pressure x-ray diffraction of the orthorhombic perovskite oxides ${A}^{3+}{B}^{3+}{\mathrm{O}}_{3}$, commonly found for the rare-earth transition-metal oxides of the $RM{\mathrm{O}}_{3}$ formula. Each of the four families of $RM{\mathrm{O}}_{3}$ ($M=\mathrm{Ti}$, Cr, Mn, Fe) perovskites in this study all crystallize in the same orthorhombic perovskite structure with the Pbnm space group. The lanthanide contraction in these materials leads to varying degrees of orthorhombic distortions that are primarily associated with octahedral site rotations. The pressure-induced change of the lattice parameters demonstrates an evolution from a suppression to an enlargement of the orthorhombic distortion for substitution of the rare-earth element from $R=\mathrm{La}$ to Lu in $RM{\mathrm{O}}_{3}$ perovskites. This unusual crossover of the lattice parameters' dependence on pressure contradict the results from first-principles calculation but can be rationalized by the intrinsic distortion of the perovskite structure.

Dielectrical performance of high-k yttrium copper titanate thin films for electronic applications

The increasing constraints in the miniaturization of modern electronic devices is driving the search for new high-k dielectric materials. Rare-earth transition metal oxides are very interesting because of the large values of dielectric constant observed in bulk samples. Here, we report on a comparison among the dielectric properties of yttrium copper titanate (YCTO) thin films and those of commonly used dielectrics such as SiO2 and MgO, grown in similar device structures. The YCTO permittivity was found to depend strongly on the oxygen pressure during deposition and can reach values even higher than those reported in bulk YCTO with good performances in terms of losses.

Electric and magnetic polarizabilities of hexagonalLn2CuTiO6(Ln=Y, Dy, Ho, Er, and Yb)

We investigated the rare-earth transition metal oxide series, Ln2CuTiO6 (Ln=Y, Dy, Ho, Er and Yb), crystallizing in the hexagonal structure with non-centrosymmetric P63cm space group for possible occurrences of multiferroic properties. Our results show that while these compounds, except Ln=Y, exhibit a low temperature antiferromagnetic transition due to the ordering of the rare-earth moments, the expected ferroelectric transition is frustrated by the large size difference between Cu and Ti at the B-site. Interestingly, this leads these compounds to attain a rare and unique combination of desirable paraelectric properties with high dielectric constants, low losses and weak temperature and frequency dependencies. First-principles calculations establish these exceptional properties result from a combination of two effects. A significant difference in the MO5 polyhedral sizes for M = Cu and M = Ti suppress the expected co-operative tilt pattern of these polyhedra, required for the ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against any temperature variation. Changes in the temperature stability of the dielectric constant amongst different members of this series are shown to arise from changes in relative contributions from soft polar modes.

Magnetic properties and structural transitions of orthorhombic fluorite-related compounds Ln3MO 7 ( Ln=rare earths, M=transition metals)

Magnetic properties and structural transitions of ternary rare-earth transition-metal oxides Ln 3 MO 7 ( Ln=rare earths, M=transition metals) were investigated. In this study, we prepared a series of molybdates Ln 3MoO 7 ( Ln=La–Gd). They crystallize in an orthorhombic superstructure of cubic fluorite with space group P2 12 12 1, in which Ln 3+ ions occupy two different crystallographic sites (the 8-coordinated and 7-coordinated sites). All of these compounds show a phase transition from the space group P2 12 12 1 to Pnma in the temperature range between 370 and 710 K. Their magnetic properties were characterized by magnetic susceptibility measurements from 1.8 to 400 K and specific heat measurements from 0.4 to 400 K. Gd 3MoO 7 shows an antiferromagnetic transition at 1.9 K. Measurements of the specific heat for Sm 3MoO 7 and the analysis of the magnetic specific heat indicate a “two-step” antiferromagnetic transition due to the ordering of Sm magnetic moments in different crystallographic sites, i.e., with decreasing temperature, the antiferromagnetic ordering of the 7-coordinated Sm ions occur at 2.5 K, and then the 8-coordinated Sm ions order at 0.8 K. The results of Ln 3MoO 7 were compared with the magnetic properties and structural transitions of Ln 3 MO 7 ( M=Nb, Ru, Sb, Ta, Re, Os, or Ir).

Development of the rare earth-transition metal oxide catalysts active surface for industrial gas detoxication

The catalytic study of Cr-Ln and Cr-Zr oxide systems in the reaction of simultaneous NO x and CO degradation supported with the data on their red-ox and structural properties ( together with the study of Mn-Ln, Fe-Ln, Ni-Ln and Cu-Ln oxide samples ) elucidates the development of active surface for the transition metal-promoted rare earth oxide catalysts. The evidence for reactive (NO) 2 2− complex formation on Cr-Zr sample implies the possibility of that complex participation in NO reduction route.

EXAFS-Studies of amorphous rare-earth transition metal oxides

Amorphous rare-earth transition metal oxides show a short range order, which is dependent on the preparative method. With EXAFS-spectroscopy, coordination numbers and interatomic distances of the first oxygen shells of the cations as a function of the cation stoichiometry and the preparation temperature were determined. From EXAFS-spectroscopy a discrimation between octahedral and tetrahedral coordination of the transition metals was possible.