Giant Magnetoresistance in the Half‐Metallic Double‐Perovskite Ferrimagnet Mn2FeReO6

The first transition‐metal‐only double perovskite compound, Mn²⁺₂Fe³⁺Re⁵⁺O₆, with 17 unpaired d electrons displays ferrimagnetic ordering up to 520 K and a giant positive magnetoresistance of up to 220 % at 5 K and 8 T. These properties result from the ferrimagnetically coupled Fe and Re sublattice and are affected by a two‐to‐one magnetic‐structure transition of the Mn sublattice when a magnetic field is applied. Theoretical calculations indicate that the half‐metallic state can be mainly attributed to the spin polarization of the Fe and Re sites.     

Double Double Cation Order in the High‐Pressure Perovskites MnRMnSbO6

Cation ordering in ABO₃ perovskites adds to their chemical variety and can lead to properties such as ferrimagnetism and magnetoresistance in Sr₂FeMoO₆. Through high‐pressure and high‐temperature synthesis, a new type of “double double perovskite” structure has been discovered in the family MnRMnSbO₆ (R=La, Pr, Nd, Sm). This tetragonal structure has a 1:1 order of cations on both A and B sites, with A‐site Mn²⁺ and R³⁺ cations ordered in columns and Mn²⁺ and Sb⁵⁺ having rock salt order on the B sites. The MnRMnSbO₆ double double perovskites are ferrimagnetic at low temperatures with additional spin‐reorientation transitions. The ordering direction of ferrimagnetic Mn spins in MnNdMnSbO₆ changes from parallel to [001] below T_C=76 K to perpendicular below the reorientation transition at 42 K at which Nd moments also order. Smaller rare earths lead to conventional monoclinic double perovskites (MnR)MnSbO₆ for Eu and Gd.     

Direct Synthesis of Chromium Perovskite Oxyhydride with a High Magnetic‐Transition Temperature

We report a novel oxyhydride SrCrO₂H directly synthesized by a high‐pressure high‐temperature method. Powder neutron and synchrotron X‐ray diffraction revealed that this compound adopts the ideal cubic perovskite structure with O^2?/H^? disorder. Surprisingly, despite the non‐bonding nature between Cr 3d t_2g orbitals and the H 1s orbital, it exhibits G‐type spin ordering at T_N≈380 K, which is higher than that of RCrO₃ (R=rare earth) and any chromium oxides. The enhanced T_N in SrCrO₂H with four Cr‐O‐Cr bonds in comparison with RCr³⁺O₃ with six Cr‐O‐Cr bonds is reasonably explained by the tolerance factor. The present result offers an effective strategy to tune octahedral tilting in perovskites and to improve physical and chemical properties through mixed anion chemistry.     

Structural,electronic, and magnetic properties of double perovskite Pb2FeReO6 thin films with (001) orientation and three possible terminations

The structural, electronic, and magnetic properties of the three possible terminated (001)‐oriented thin films of the double perovskite Pb₂FeReO₆ compound, 9‐L FeReO₄ terminated, 10‐L FeReO₄+PbO terminated, and 11‐L PbO terminated, have been studied by using the first‐principles calculations. An outward relaxation is observed for several layers near surface, and the relaxed fractional rumpling s of the PbO layer is larger than that of the adjacent FeReO₄ layer, and both have a decrease tend from surface layer to inner layer. The O atom is closer to the adjacent Re atom than to the adjacent Fe atom. Except for Fe or Re transition‐metal (TM) atoms on the film's symmetrical central layer, the two axial TM‐O bond lengths are not equal in the survived FeO₆ or ReO₆ octahedra. The maintained HM‐FM character ensures the three possible terminated (001)‐oriented thin films of the double perovskite Pb₂FeReO₆ compound a potential application in magnetoresistive and spintronics devices. An FM coupling is obtained within each Fe or Re TM sublattice, whereas two TM sublattices are coupled AFM. The delocalized distribution of the spin/magnetic charge densities around the Fe atom on the first FeReO₄ layer leads to a smaller magnetic moment. The remained two Re 5d² electrons are mainly located on the down‐spin t_2g (d_xy, d_yz, and d_zx) orbitals.     

Contrasting Magnetic Behaviors in Rhodium‐ and Ruthenium‐Doped Cubic Perovskite SrFeO3: Nearly Ferromagnetic Metal versus Spin‐Glass Insulator

The effects of Rh and Ru doping for SrFeO₃, a helimagnetic metal with a cubic perovskite structure, are studied by magnetic and resistivity measurements. Although SrRhO₃ is a paramagnetic metal and SrRuO₃ is a ferromagnetic one, the Rh doping induces a nearly ferromagnetic metallic state, whereas the Ru doping induces a spin‐glass insulating state. Mössbauer measurements evidence a marked difference between SrFe_0.8Rh_0.2O₃ and SrFe_0.8Ru_0.2O₃ in the formal valences of Fe, which are estimated to be 4+ and 3.75+, respectively. The contrasting magnetic behaviors of Rh‐ and Ru‐doped SrFeO₃ are discussed in terms of the subtle balance between the double‐exchange ferromagnetism and the superexchange antiferromagnetism.     

Fused Perovskite Tunnel Structures in Ba5Fe6+xS4+xO8 (0.44≤x≤0.55) with x‐Dependent Two‐Stage Magnetizations

Ba₅Fe_6+xS_4+xO₈ was synthesized through a solid‐state reaction, and pure powders of nominal compositions x=0.44–0.55 were obtained after being rinsed with water. The crystal structures (P4/mmm, a=10.13, c=4.03 Å) and sample purities were investigated by powder synchrotron X‐ray diffraction and were found to be composed of a tunnel lattice (Ba₅Fe₆S₄O₈), built from fused perovskite units and the tunnel filling (Fe_xS_x). The variable composition, that is, the tunnel filling (x), causes partially occupied sites as well as crystallographic split positions. Ba₅Fe_6+xS_4+xO₈ (x=0.525) is semiconducting and all investigated compositions exhibit magnetic ground states that could be described as either semi‐spin‐glass‐like (x>0.5) or canted antiferromagnetic (x<0.5). The spin‐glass in x=0.525 exhibits magnetic relaxations that are affected by ageing.     

Ferromagnetism Induced by Substitution of the Iron(IV) Ion by an Unusual High‐Valence Nickel(IV) Ion in Antiferromagnetic SrFeO3

Novel cubic perovskites SrFe_1?xNi_xO₃ (0≤x≤0.5) with unusual high‐valence iron(IV) and nickel(IV) ions were obtained by high‐pressure and high‐temperature synthesis. Substantial magnetic moments of Ni^IV, which is intrinsically nonmagnetic with a nominal d⁶ electron configuration, were induced by the large magnetic moments of Fe^IV through orbital hybridization with oxygen. As a result, ferromagnetism with the transition temperature (T_c) above room temperature could be induced.     

Rattling in the Quadruple Perovskite CuCu3V4O12

Of particular interest is a peculiar motion of guest atoms or ions confined to nanospace in cage compounds, called rattling. While rattling provides unexplored physical properties through the guest–host interactions, it has only been observed in a very limited class of materials. Herein, we introduce an A‐site‐ordered quadruple perovskite, CuCu₃V₄O₁₂, as a new family of cage compounds. This novel AA′₃B₄O₁₂‐type perovskite has been obtained by a high‐pressure synthesis technique and structurally characterized to have cubic Im$\bar 3$ symmetry with an ionic model of Cu²⁺Cu²⁺₃V⁴⁺₄O₁₂. The thermal displacement parameter of the A‐site Cu²⁺ ion is as large as U_iso≈0.045 Ų at 300 K, indicating its large‐amplitude thermal oscillations in the oversized icosahedral cages. Remarkably, the presence of localized phonon modes associated with rattling of the A‐site Cu²⁺ ion manifests itself in the low‐temperature specific heat data. This work sheds new light on the structure–property relations in perovskites.     

Pressure‐Induced Intersite BiM (M=Ru, Ir) Valence Transitions in Hexagonal Perovskites

Pressure‐induced charge transfer from Bi to Ir/Ru is observed in the hexagonal perovskites Ba_3+nBiM_2+nO_9+3n (n=0,1; M=Ir,Ru). These compounds show first‐order, circa 1 % volume contractions at room temperature above 5 GPa, which are due to the large reduction in the effective ionic radius of Bi when the 6s shell is emptied on oxidation, compared to the relatively negligible effect of reduction on the radii of Ir or Ru. They are the first such transitions involving 4d and 5d compounds, and they double the total number of cases known. Ab initio calculations suggest that magnetic interactions through very short (ca. 2.6 Å) M? M bonds contribute to the finely balanced nature of their electronic states.     

Polarization Rotation in the Monoclinic Perovskite BiCo(1-x) Fe(x) O(3)

The monoclinic perovskite BiCo(1-x) Fe(x) O(3) (x≈0.7) undergoes a second-order structural transition from tetragonal to monoclinic, which is accompanied by a rotation of the polarization vector from the [001] to [111] directions of a pseudo cubic cell. The crystal structure, determined by electron diffraction and powder synchrotron X-ray diffraction, was the same as that of Pb(Ti(1-x) Zr(x) )O(3) at the morphotropic phase boundary.     

Chitosan‐Assisted Crystallization and Film Forming of Perovskite Crystals through Biomineralization

Biomimetic mineralization is a powerful approach for the synthesis of advanced composite materials with hierarchical organization and controlled structure. Herein, chitosan was introduced into a perovskite precursor solution as a biopolymer additive to control the crystallization and to improve the morphology and film‐forming properties of a perovskite film by way of biomineralization. The biopolymer additive was able to control the size and morphology of the perovskite crystals and helped to form smooth films. The mechanism of chitosan‐mediated nucleation and growth of the perovskite crystals was explored. As a possible application, the chitosan–perovskite composite film was introduced into a planar heterojunction solar cell and increased power conversion efficiency relative to that observed for the pristine perovskite film was achieved. The biomimetic mineralization method proposed in this study provides an alternative way of preparing perovskite crystals with well‐controlled morphology and properties and extends the applications of perovskite crystals in photoelectronic fields, including planar‐heterojunction solar cells.     

Temperature‐Induced Intersite Charge Transfer Involving Cr ions in A‐Site‐Ordered Perovskites ACu3Cr4O12 (A=La and Y)

Changes in the valence state of transition‐metal ions in oxides drastically modify the chemical and physical properties of the compounds. Intersite charge transfer (ISCT), which involves simultaneous changes in the valence states of two valence‐variable transition‐metal cations at different crystallographic sites, further expands opportunities to show multifunctional properties. To explore new ISCT materials, we focus on A‐site‐ordered perovskite‐structure oxides with the chemical formula AA′₃B₄O₁₂, which contain different transition‐metal cations at the square‐planar A′ and octahedral B sites. We have obtained new A‐site‐ordered perovskites LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ by synthesis under high‐pressure and high‐temperature conditions and found that they showed temperature‐induced ISCT between A′‐site Cu and B‐site Cr ions. The compounds are the first examples of those, in which Cr ions are involved in temperature‐induced ISCT. In contrast to the previously reported ISCT compounds, LaCu₃Cr₄O₁₂ and YCu₃Cr₄O₁₂ showed positive‐thermal‐expansion‐like volume changes at the ISCT transition.     

Rare‐Earth‐Metal Nitridophosphates through High‐Pressure Metathesis

Developing a synthetic method to target an broad spectrum of unknown phases can lead to fascinating discoveries. The preparation of the first rare‐earth‐metal nitridophosphate LiNdP₄N₈ is reported. High‐pressure solid‐state metathesis between LiPN₂ and NdF₃ was employed to yield a highly crystalline product. The in situ formed LiF is believed to act both as the thermodynamic driving force and as a flux to aiding single‐crystal formation in dimensions suitable for crystal structure analysis. Magnetic properties stemming from Nd³⁺ ions were measured by SQUID magnetometry. LiNdP₄N₈ serves as a model system for the exploration of rare‐earth‐metal nitridophosphates that may even be expanded to transition metals. High‐pressure metathesis enables the systematic study of these uncharted regions of nitride‐based materials with unprecedented properties.