Charge Density Wave in the New Polymorphs of RE2Ru3Ge5 (RE = Pr, Sm, Dy).

A new polymorph of the RE2Ru3Ge5 (RE = Pr, Sm, Dy) compounds has been grown as single crystals via an indium flux. These compounds crystallize in tetragonal space group P4/mnc with the Sc2Fe3Si5-type structure, having lattice parameters a = 11.020(2) Šand c = 5.853(1) Šfor RE = Pr, a = 10.982(2) Šand c = 5.777(1) Šfor RE = Sm, and a = 10.927(2) Šand c = 5.697(1) Šfor RE = Dy. These materials exhibit a structural transition at low temperature, which is attributed to an apparent charge density wave (CDW). Both the high-temperature average crystal structure and the low-temperature incommensurately modulated crystal structure (for Sm2Ru3Ge5 as a representative) have been solved. The charge density wave order is manifested by periodic distortions of the one-dimensional zigzag Ge chains. From X-ray diffraction, charge transport (electrical resistivity, Hall effect, magnetoresistance), magnetic measurements, and heat capacity, the ordering temperatures (TCDW) observed in the Pr and Sm analogues are ∼200 and ∼175 K, respectively. The charge transport measurement results indicate an electronic state transition happening simultaneously with the CDW transition. X-ray absorption near-edge spectroscopy (XANES) and electronic band structure results are also reported.

Mixed-Valent NaCu4Se3: A Two-Dimensional Metal.

The new ternary copper selenide NaCu4Se3 crystallizes in the RbCd4As3 structure type with the trigonal space group R3̅m and lattice constants a = 4.0316(4) Å and c = 31.438(8) Å. Its structure is built from two-dimensional slabs of (2)/∞[Cu4Se3] separated by Na(+) cations. The compound is formally mixed-valent with Se(2-)/Se(-) atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm(-1) at room temperature and a thermopower of ∼10 µV K(-1). Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10(21) cm(-3) at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the (2)/∞[Cu4Se3] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism.

La(1-x)Bi(1+x)S3 (x ≈ 0.08): An n-Type Semiconductor.

The new bismuth chalcogenide La(0.92)Bi(1.08)S3 crystallizes in the monoclinic space group C2/m with a = 28.0447(19) Å, b = 4.0722(2) Å, c = 14.7350(9) Å, and ß = 118.493(5)°. The structure of La(0.92)Bi(1.08)S3 is built of NaCl-type Bi2S5 blocks and BiS4 and LaS5 infinitely long chains, forming a compact three-dimensional framework with parallel tunnels. Optical spectroscopy and resistivity measurements reveal a semiconducting behavior with a band gap of ∼1 eV and activation energy for transport of 0.36(1) eV. Thermopower measurements suggest the majority carriers of La(0.92)Bi(1.08)S3 are electrons. Heat capacity measurements indicate no phase transitions from 2 to 300 K. Band structure calculations at the density functional theory level confirm the semiconducting nature and the indirect gap of La(0.92)Bi(1.08)S3.

NaCu6Se4: a layered compound with mixed valency and metallic properties.

A new ternary compound NaCu6Se4 was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group R3̅m with a = 4.0465(3) Å and c = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of (1)/∞[Cu6Se4] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 10(3) S cm(-1) at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4-11 µV K(-1) from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 10(21) cm(-3) and a hole mobility of ∼8.75 cm(2) V(-1) s(-1) at 300 K. NaCu6Se4 exhibits temperature-independent Pauli paramagnetism.

NaBa2Cu3S5: a doped p-type degenerate semiconductor.

Mixed S(2-/)S(1-) oxidation states have been discovered in the new quaternary compound NaBa2Cu3S5. Synthesized from the reaction of Cu in a molten alkali metal/polysulfide flux, the compound crystallizes in monoclinic space group C2/m with a = 16.5363(7) Å, b = 5.5374(5) Å, c = 10.3717(10) Å, ß = 98.535(8)°. The Na(+) Ba2(+2) [Cu(+)3S3](3-)S2(2-) crystal structure contains layers of edge sharing CuS4 tetrahedra and sheets of S2(2-) dimers. These layers are separated by mixed Ba/Na cation layers. The conductivity of the single crystals of NaBa2Cu3S5 is ∼450 S cm(-1) at room temperature, and increasing conductivity with decreasing temperature is observed, indicating metallic behavior despite the optical band gap of 0.45 eV. A small positive thermopower (45-55 µV K(-1) from 300 K to 500 K) and Hall effect measurements also confirm p-type conductivity with carrier concentration at 200 K of ∼1.6 × 10(21) cm(-3) and a hole mobility of ∼2 cm(2) V(-1) s(-1). NaBa2Cu3S5 exhibits temperature-independent Pauli paramagnetism.

Unprecedented robust antiferromagnetism in fluorinated hexagonal perovskites.

The diversification of antiferromagnetic (AFM) oxides with high Néel temperature is of fundamental as well as technical interest if one considers the need for robust AFM in the field of spin-tronics (exchange bias, multiferroics, etc.). Within the broad series of so-called hexagonal perovskites (HP), the existence of face-sharing octahedral units drastically lowers the strength of magnetic exchanges as compared to corner-sharing octahedral edifices. Here, we show that the partial introduction of F(-) in several Fe-based HP types leads to a drastic increase of the AFM ordering close to the highest values reported in iron oxides (T(N) ≈ 700 K). Our experimental results are supported by ab initio calculations. The T(N) increase is explained by the structural effect of the aliovalent F(-) for O(2-) substitution occurring in preferred anionic positions: it leads to local changes of the Fe-O-Fe connectivity and to chemical reduction into predominant Fe(3+), both responsible for drastic magnetic changes.

High dilution of anionic vacancies in Sr(0.8)Ba(0.2)Fe(O,F)(~2.5).

The (Ba,Sr)FeO(3-δ) system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)cubic compounds (generally δ < 0.27, Fe(3/4+)). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (δ ≈ 0.5, Fe(3+)) within the room-temperature stable tetragonal (pseudocubic) Sr(0.8)Ba(0.2)FeF(~0.1)(O,F)(~2.5.) The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/□ constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr(0.8)Ba(0.2)FeO(~2.7) shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe(3+) of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe(3+) cations (M(Fe) ≈ 2.64(1) µB at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.

Engineering Kitaev exchange in stacked iridate layers: impact of inter-layer species on in-plane magnetism.

Novel functionalities may be achieved in oxide electronics by appropriate stacking of planar oxide layers of different metallic species, MO p and M'O q . The simplest mechanism allowing the tailoring of the electronic states and physical properties of such heterostructures is of electrostatic nature-charge imbalance between the M and M' cations. Here we clarify the effect of interlayer electrostatics on the anisotropic Kitaev exchange in H3LiIr2O6, a recently proposed realization of the Kitaev spin liquid. By quantum chemical calculations, we show that the precise position of H+ cations between magnetically active [LiIr2O6]3- honeycomb-like layers has a strong impact on the magnitude of Kitaev interactions. In particular, it is found that stacking with straight interlayer O-H-O links is detrimental to in-plane Kitaev exchange since coordination by a single H-ion of the O ligand implies an axial Coulomb potential at the O site and unfavorable polarization of the O 2p orbitals mediating the Ir-Ir interactions. Our results therefore provide valuable guidelines for the rational design of Kitaev quantum magnets, indicating unprecedented Kitaev interactions of ≈40 meV if the linear interlayer linkage is removed.