Pressure-Induced Collapse Transition in BaTi2Pn2O (Pn = As, Sb) with an Unusual Pn-Pn Bond Elongation.

Making and breaking bonds in a solid-state compound greatly influences physical properties. A well-known playground for such bonding manipulation is the ThCr2Si2-type structure AT2X2, allowing a collapse transition where a X-X dimer forms by a chemical substitution or external stimuli. Here, we report a pressure-induced collapse transition in the structurally related BaTi2Pn2O (Pn = As, Sb) at a transition pressure Pc of ∼15 GPa. The Pn-Pn bond formation is related with Pn-p band filling, which is controlled by charge transfer from the Ti-3d band. At Pc, the Sb-Sb distance in BaTi2Sb2O shrinks due to bond formation, but interestingly, the Sb-Sb expands with increasing pressure above Pc. This expansion, which was not reported in ThCr2Si2-type compounds, may arise from heteroleptic coordination geometry around titanium, where a compression of the Ti-O bond plays a role. Electrical resistivity measurements of BaTi2Sb2O up to 55 GPa revealed an increasing trend of the superconducting transition temperature with pressure. This study presents structure motifs that allow flexible bonding manipulation and property control with heteroleptic coordination geometry.

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 Vdπ-Opπ-Vdπ overlap. Remarkably, dispersion along c is limited by the orthogonal Vdπ-H1s-Vdπ 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.

Pressure-Driven Spin Crossover Involving Polyhedral Transformation in Layered Perovskite Cobalt Oxyfluoride.

We report a novel pressure-driven spin crossover in layered cobalt oxyfluoride Sr2CoO3F with a distorted CoO5 square pyramid loosely bound with a fluoride ion. Upon increasing pressure, the spin state of the Co(III) cation gradually changes from a high spin state (S = 2) to a low spin state (S = 0) accompanied by a anomalously large volume contraction (bulk modulus, 76.8(5) GPa). The spin state change occurs on the CoO5 pyramid in a wide pressure range, but the concomitant gradual shrinkage of the Co-F bond length with pressure gives rise to a polyhedral transformation to the CoO5F octahedron without a structural phase transition, leading to the full conversion to the LS state at 12 GPa. The present results provide new effective strategy to fine-tune electronic properties of mixed anion systems by controlling the covalency in metal-ligand bonds under pressure.

Rattling in the Quadruple Perovskite CuCu3 V4 O12.

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, CuCu3 V4 O12 , as a new family of cage compounds. This novel AA'3 B4 O12 -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 Cu(2+) Cu(2+) 3 V(4+) 4 O12 . The thermal displacement parameter of the A-site Cu(2+) ion is as large as Uiso ≈0.045 Å(2) 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 Cu(2+) ion manifests itself in the low-temperature specific heat data. This work sheds new light on the structure-property relations in perovskites.

Pressure-induced superconductivity in the iron-based ladder material BaFe2S3.

All the iron-based superconductors identified so far share a square lattice composed of Fe atoms as a common feature, despite having different crystal structures. In copper-based materials, the superconducting phase emerges not only in square-lattice structures but also in ladder structures. Yet iron-based superconductors without a square-lattice motif have not been found, despite being actively sought out. Here, we report the discovery of pressure-induced superconductivity in the iron-based spin-ladder material BaFe2S3, a Mott insulator with striped-type magnetic ordering below ∼120 K. On the application of pressure this compound exhibits a metal-insulator transition at about 11 GPa, followed by the appearance of superconductivity below Tc = 14 K, right after the onset of the metallic phase. Our findings indicate that iron-based ladder compounds represent promising material platforms, in particular for studying the fundamentals of iron-based superconductivity.

Charge-order melting in charge-disproportionated perovskite CeCu3Fe4O12.

A novel quadruple perovskite oxide CeCu3Fe4O12 has been synthesized under high-pressure and high-temperature conditions of 15 GPa and 1473 K. (57)Fe Mössbauer spectroscopy displays a charge disproportionation transition of 4Fe(3.5+) → 3Fe(3+) + Fe(5+) below ∼270 K, whereas hard X-ray photoemission and soft X-ray absorption spectroscopy measurements confirm that the Ce and Cu valences are retained at approximately +4 and +2, respectively, over the entire temperature range measured. Electron and X-ray diffraction studies reveal that the body-centered cubic symmetry (space group Im3̅, No. 204) is retained at temperatures as low as 100 K, indicating the absence of any types of charge-ordering in the charge-disproportionated CeCu3Fe4O12 phase. The magnetic susceptibility and neutron powder diffraction data illustrate that the antiferromagnetic ordering of Fe ions is predominant in the charge-disproportionated CeCu3Fe4O12 phase. These findings suggest that CeCu3Fe4O12 undergoes a new type of electronic phase in the ACu3Fe4O12 series and that the melting of the charge-ordering in CeCu3Fe4O12 is caused by the substantial decrease in the Fe valence and the resulting large deviation from the ideal abundance ratio of Fe(3+):Fe(5+) = 1:1 for rock-salt-type charge-ordering.

Control of bond-strain-induced electronic phase transitions in iron perovskites.

Unusual electronic phase transitions in the A-site ordered perovskites LnCu3Fe4O12 (Ln: trivalent lanthanide ion) are investigated. All LnCu3Fe4O12 compounds are in identical valence states of Ln(3+)Cu(2+)3Fe(3.75+)4O12 at high temperature. LnCu3Fe4O12 with larger Ln ions (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb) show an intersite charge transfer transition (3Cu(2+) + 4Fe(3.75+) → 3Cu(3+) + 4Fe(3+)) in which the transition temperature decreases from 360 to 240 K with decreasing Ln ion size. In contrast, LnCu3Fe4O12 with smaller Ln ions (Ln = Dy, Ho, Er, Tm Yb, Lu) transform into a charge-disproportionated (8Fe(3.75+) → 5Fe(3+) + 3Fe(5+)) and charge-ordered phase below ∼250-260 K. The former series exhibits metal-to-insulator, antiferromagnetic, and isostructural volume expansion transitions simultaneously with intersite charge transfer. The latter shows metal-to-semiconductor, ferrimagnetic, and structural phase transitions simultaneously with charge disproportionation. Bond valence calculation reveals that the metal-oxygen bond strains in these compounds are classified into two types: overbonding or compression stress (underbonding or tensile stress) in the Ln-O (Fe-O) bond is dominant in the former series, while the opposite stresses or bond strains are found in the latter. Intersite charge transfer transition temperatures are strongly dependent upon the global instability indices that represent the structural instability calculated from the bond valence sum, whereas the charge disproportionation occurs at almost identical temperatures, regardless of the magnitude of structural instability. These findings provide a new aspect of the structure-property relationship in transition metal oxides and enable precise control of electronic states by bond strains.

BaFeO3: a ferromagnetic iron oxide.

Magnetic attraction: The cubic perovskite BaFeO(3) (see picture, Ba blue, Fe brown, O white), which is obtained by a low-temperature reaction using ozone as an oxidant, exhibits ferromagnetism with a fairly large moment of 3.5 µ(B) per Fe ion above a small critical field of approximately 0.3 T. This specific ferromagnetism is attributed to the enhancement of O→Fe charge transfer that arises from deepening of the Fe(4+) d levels.

B1-to-B2 structural transitions in rock salt intergrowth structures.

The rock salt (B1) structure of binary oxides or chalcogenides transforms to the CsCl (B2) structure under high pressure, with critical pressures P(s) depending on the cation to anion size ratio (R(c)/R(a)). We investigated structural changes of A(2)MO(3) (A = Sr, Ca; M = Cu, Pd) comprising alternate 7-fold B1 AO blocks and corner-shared MO(2) square-planar chains under pressure. All of the examined compounds exhibit a structural transition at P(s) = 29-41 GPa involving a change in the A-site geometry to an 8-fold B2 coordination. This observation demonstrates, together with the high pressure study on the structurally related Sr(3)Fe(2)O(5), that the B1-to-B2 transition generally occurs in these intergrowth structures. An empirical relation of P(s) and the R(c)/R(a) ratio for the binary system holds well for the intergrowth structure also, which means that P(s) is predominantly determined by the rock salt blocks. However, a large deviation from the relation is found in LaSrNiO(3.4), where oxygen atoms partially occupy the apical site of the MO(4) square plane. We predict furthermore the occurrence of the same structural transition for Ruddlesden-Popper-type layered perovskite oxides (AO)(AMO(3))(n), under higher pressures. For investigating the effect on the physical properties, an electrical resistivity of Sr(2)CuO(3) is studied.

Pressure-induced structural, magnetic, and transport transitions in the two-legged ladder Sr3Fe2O5.

The layered compound SrFeO(2) with an FeO(4) square-planar motif exhibits an unprecedented pressure-induced spin state transition (S = 2 to 1), together with an insulator-to-metal (I-M) and an antiferromagnetic-to-ferromagnetic (AFM-FM) transition. In this work, we have studied the pressure effect on the structural, magnetic, and transport properties of the structurally related two-legged spin ladder Sr(3)Fe(2)O(5). When pressure was applied, this material first exhibited a structural transition from Immm to Ammm at P(s) = 30 ± 2 GPa. This transition involves a phase shift of the ladder blocks from (1/2,1/2,1/2) to (0,1/2,1/2), by which a rock-salt type SrO block with a 7-fold coordination around Sr changes into a CsCl-type block with 8-fold coordination, allowing a significant reduction of volume. However, the S = 2 antiferromagnetic state stays the same. Next, a spin state transition from S = 2 to S = 1, along with an AFM-FM transition, was observed at P(c) = 34 ± 2 GPa, similar to that of SrFeO(2). A sign of an I-M transition was also observed at pressure around P(c). These results suggest a generality of the spin state transition in square planar coordinated S = 2 irons of n-legged ladder series Sr(n+1)Fe(n)O(2n+1) (n = 1, 2, 3, ...). It appears that the structural transition independently occurs without respect to other transitions. The necessary conditions for a structural transition of this type and possible candidate materials are discussed.