Pressure-Induced Structural, Magnetic, and Transport Transitions in the Two-Legged Ladder Sr3Fe2O5

Exceptionally robust magnetism and structure of SrFeO[Formula: see text] above 100 GPa

We report the exceptional structural and magnetic stability of SrFeO[Formula: see text] under pressure by X-Ray Magnetic Circular Dichroism (XMCD) and X-ray Diffraction (XRD) up to the Mbar range. The XMCD data confirm the onset of ferromagnetism above 30 GPa and its stability up to 102 GPa while XRD shows that SrFeO[Formula: see text] structure remains unchanged from 30 GPa up to 111 GPa without any sign of structural transition. Our results demonstrate the robustness of Fe properties under extreme conditions in the square planar environment.

Sequential Pressure-Induced B1-B2 Transitions in the Anion-Ordered Oxyhydride Ba2YHO3

We present a detailed experimental and computational investigation of the influence of pressure on the mixed-anion oxyhydride phase Ba₂YHO₃, which has recently been shown to support hydride conductivity. The unique feature of this layered perovskite is that the oxide and hydride anions are segregated into distinct regions of the unit cell, in contrast to the disordered arrangement in closely related Ba₂ScHO₃. Density functional theory (DFT) calculations reveal that the application of pressure drives two sequential B1–B2 transitions in the interlayer regions from rock salt to CsCl-type ordering, one in the hydride-rich layer at approximately 10 GPa and another in the oxide-rich layer at 35–40 GPa. To verify the theoretical predictions, we experimentally observe the structural transition at 10 GPa using high-pressure X-ray diffraction (XRD), but the details of the structure cannot be solved due to peak broadening of the XRD patterns. We use DFT to explore the structural impact of pressure on the atomic scale and show how the pressure-dependent properties can be understood in terms of simple electrostatic engineering.

We investigate a sequence of pressure-induced phase transitions in Ba₂YHO₃, a perovskite oxyhydride with a unique layered anion ordering. Density functional theory and X-ray diffraction together provide a detailed and informative picture of the changes to the crystal structure across the pressure range. This work provides new insights into nonuniform structural flexibility in 2D materials, which can aid targeted materials design in other chemical systems.

Hydride-Reduced Eu2SrFe2O6: A T-to-T' Conversion Enabling Fe2+ in Square-Planar Coordination

Low-temperature reaction of A-site-ordered layered perovskite Eu₂SrFe₂O₇ (T structure) with CaH₂ induces a shift in the Eu₂O₂ slabs to form Eu₂SrFe₂O₆ with a T' structure (I4/mmm space group) in which only the Fe cation is reduced. Contrary to the previously reported T' structures with Jahn-Teller-active d⁹ cations (Cu²⁺ and Ni⁺), stabilization of Eu₂SrFe₂O₆ with the Fe²⁺ (d⁶) cation reflects the stability of the FeO₄ square-planar unit. The stability of T'-type Eu₂SrFe₂O₆ over a T-type polymorph is confirmed by density functional theory calculations, revealing the d_z² occupancy for the T' structure. Eu₂SrFe₂O₆ has a bilayer magnetic framework with an Fe-O-Fe superexchange J_∥ and an Fe-Fe direct exchange J_⊥ (where J_∥ > J_⊥), which broadly explains the observed T_N of 390-404 K. Interestingly, the magnetic moments of Eu₂SrFe₂O₆ lie in the ab plane, in contrast to the structurally similar Sr₃Fe₂O₄Cl₂ having an out-of-plane spin alignment.

Pressure-Induced Transitions in the 1-Dimensional Vanadium Oxyhydrides Sr2VO3H and Sr3V2O5H2, and Comparison to 2-Dimensional SrVO2H

High-pressure X-ray diffraction measurements on the layered oxyhydrides Sr₂VO₃H and Sr₃V₂O₅H₂ reveal that both compounds undergo a pressure-induced rock-salt to CsCl (B1-B2) structural transition, similar to those observed in binary compounds (oxides, halides, chalcogenides, etc.). This structural transition, observed at 43 and 45 GPa in Sr₂VO₃H and Sr₃V₂O₅H₂, respectively, relieves almost all of the accumulated strain on the infinite V-O-V ladders, such that the V-O bond lengths are almost identical at 0 and 50 GPa but are substantially compressed at intermediate pressures. The resistances of both materials with 1-dimensional VO ladders decrease with increasing pressure, but unlike SrVO₂H that contains 2-dimensional VO₂ sheets, they remain insulating even at the highest accessible pressures. The reduction in dimensionality from planar to linear VO networks reduces the dispersion of the V-O π bands that define the band gap and leads to insulating behavior at all measured pressures.

The role of π-blocking hydride ligands in a pressure-induced insulator-to-metal phase transition in SrVO2H

Transition-metal oxyhydrides are of considerable current interest due to the unique features of the hydride anion, most notably the absence of valence p orbitals. This feature distinguishes hydrides from all other anions, and gives rise to unprecedented properties in this new class of materials. Here we show via a high-pressure study of anion-ordered strontium vanadium oxyhydride SrVO₂H that H⁻ is extraordinarily compressible, and that pressure drives a transition from a Mott insulator to a metal at ~ 50 GPa. Density functional theory suggests that the band gap in the insulating state is reduced by pressure as a result of increased dispersion in the ab-plane due to enhanced V_dπ-O_pπ-V_dπ overlap. Remarkably, dispersion along c is limited by the orthogonal V_dπ-H_1s-V_dπ arrangement despite the greater c-axis compressibility, suggesting that the hydride anions act as π-blockers. The wider family of oxyhydrides may therefore give access to dimensionally reduced structures with novel electronic properties.

Incorporating hydride anions into transition metal oxides can dramatically affect their structural and electronic properties. Here the authors reveal a pressure-induced insulator-to-metal transition in SrVO₂H and show that the compressibility of hydride anions without π-symmetry valence orbitals causes them to act as π-blockers.

Impact of Lanthanoid Substitution on the Structural and Physical Properties of an Infinite-Layer Iron Oxide

The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO₂ was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and ⁵⁷Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH₂. While the introduction of the smaller Ln³⁺ ion reduces the a axis as expected, we found an unusual expansion of the c axis as well as the volume. Rietveld refinements along with pair distribution function analysis revealed the incorporation of oxygen atoms between FeO₂ layers with a charge-compensated composition of (Sr_1-xLn_x)FeO_2+x/2, which accounts for the failed electron doping to the FeO₂ layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO₂ sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (Sr_0.75Ho_0.25)FeO_2.125 suggest a possible stabilization of an intermediate spin state in comparison with SrFeO₂, revealing a certain contribution of the in-plane Fe-O distance to the pressure-induced transition.

First-principles study of the electronic and magnetic properties of the spin-ladder iron oxide Sr3Fe2O5

The electronic and magnetic properties in the novel spin-ladder iron oxide Sr₃Fe₂O₅, containing the unusual square-planar coordination around high-spin Fe²⁺ cations, are investigated via first principles calculations.

Pressure induced structural and spin state transitions in Sr3Fe2O5

The effect of pressure on the structural, electronic and magnetic properties of the two-legged spin ladder structure Sr₃Fe₂O₅ was investigated, using density functional theory within the generalized gradient approximation (GGA) + U method.

Pressure-induced transition from an antiferromagnet to a ferrimagnet for Mn(II)(TCNE)[C4(CN)8]1/2 (TCNE = tetracyanoethylene)

Mn(II)(TCNE)[C(4)(CN)(8)](1/2) (TCNE = tetracyanoethylene) exhibits a reversible pressure-induced piezomagnetic transition from a low magnetization antiferromagnetic state to a high magnetization ferrimagnetic state above 0.50 ± 0.15 kbar. In the ferrimagnetic state, the critical temperature, T(c), increases with increasing hydrostatic pressure and is ~97 K at 12.6 kbar, the magnetization increases by 3 orders of magnitude (1000-fold), and the material becomes a hard magnet with a significant remnant magnetization.

(Sr(1-x)Ba(x))FeO2 (0.4 ≤ x ≤ 1): a new oxygen-deficient perovskite structure

Topochemical reduction of (layered) perovskite iron oxides with metal hydrides has so far yielded stoichiometric compositions with ordered oxygen defects with iron solely in FeO(4) square planar coordination. Using this method, we have successfully obtained a new oxygen-deficient perovskite, (Sr(1-x)Ba(x))FeO(2) (0.4 ≤ x ≤ 1.0), revealing that square planar coordination can coexist with other 3-6-fold coordination geometries. This BaFeO(2) structure is analogous to the LaNiO(2.5) structure in that one-dimensional octahedral chains are linked by planar units, but differs in that one of the octahedral chains contains a significant amount of oxygen vacancies and that all the iron ions are exclusively divalent in the high-spin state. Mössbauer spectroscopy demonstrates, despite the presence of partial oxygen occupations and structural disorders, that the planar-coordinate Fe(2+) ions are bonded highly covalently, which accounts for the formation of the unique structure. At the same time, a rigid 3D Fe-O-Fe framework contributes to structural stabilization. Powder neutron diffraction measurements revealed a G-type magnetic order with a drastic decrease of the Néel temperature compared to that of SrFeO(2), presumably due to the effect of oxygen disorder/defects. We also performed La substitution at the Ba site and found that the oxygen vacancies act as a flexible sink to accommodate heterovalent doping without changing the Fe oxidation and spin state, demonstrating the robustness of this new structure against cation substitution.