Ruddlesden-Popper Oxyfluorides La2Ni1-xCuxO3F2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

Ruddlesden-Popper oxyfluorides La2Ni1-xCuxO3F2 (0 ≤ x ≤ 1) were obtained by topochemical reaction of oxide precursors La2Ni1-xCuxO4, prepared by citrate-based soft chemistry synthesis, with polyvinylidene fluoride (PVDF) as the fluorine source. Systematic changes of the crystal structure in the oxide as well as the oxyfluoride substitution series were investigated. For 0.2 ≤ x ≤ 0.9, the oxyfluorides adopt the monoclinic (C2/c) structural distortion previously solved for the x = 0.8 compound based on neutron powder diffraction data, whereas the sample with a lower Cu content of x = 0.1 crystallizes in the orthorhombic (Cccm) structure variant of La2NiO3F2. The orthorhombic-to-monoclinic structural transition was found to be the result of an additional tilt component of the Jahn-Teller elongated CuO4F2 octahedra. The structural transitions were additionally studied by DFT calculations, confirming the monoclinic space group symmetry. The "channel-like" anionic ordering of the endmembers La2NiO3F2 and La2CuO3F2 was checked by 19F MAS NMR experiments and was found to persist throughout the entire substitution series.

A Living Topochemical Ring-Opening Polymerization of Achiral Amino Acid N-Carboxy-Anhydrides in Single Crystals

A living topochemical ring-opening polymerization (ROP) of achiral amino-acid N-carboxyanhydrides (NCAs) is reported. Single crystals of the NCAs of α-aminoisobutyric acid (Aib) and 1-aminocyclohexanecarboxylic acid (ACHC) were grown, allowing a ring-opening polymerization macroscopically induced by amines. The single crystals could be polymerized at temperatures from 25-50 °C after physically contacting the amine-based initiator with the crystals. Topochemical polymerization of the crystals was proven by MALDI-ToF MS and XRD, generating polymers with chain lengths of up to 40 units and a complete affixation of the initiating amine at the polymer's head. Due to the proper alignment of the reacting groups in the crystal, longer polymer chains with improved purities can be reached, as chain-transfer is reduced as compared to solution polymerization. Simple purification of the polymers can be achieved by separation of the unreacted NCA via dispersion in acetonitrile. Overall, this method enables the preparation of polymers with higher chain length and purities at mild conditions, finally demonstrating a crystal-based ring opening polymerization.

Influence of Disorder on the Bad Metal Behavior in Polar Amalgams

The two new ternary amalgams K_1-xRb_xHg₁₁ [x = 0.472(7)] and Cs_3-xCa_xHg₂₀ [x = 0.20(3)] represent two different examples of how to create ternary compounds from binaries by statistical atom substitution. K_1-xRb_xHg₁₁ is a Vegard-type mixed crystal of the isostructural binaries KHg₁₁ and RbHg₁₁ [cubic, BaHg₁₁ structure type, space group Pm3̅m, a = 9.69143(3) Å, Rietveld refinement], whereas Cs_3-xCa_xHg₂₀ is a substitution variant of the Rb₃Hg₂₀ structure type [cubic, space group Pm3̅n, a = 10.89553(14) Å, Rietveld refinement] for which a fully substituted isostructural binary Ca phase is unknown. In K_1-xRb_xHg₁₁, the valence electron concentration (VEC) is not changed by the substitution, whereas in Cs_3-xCa_xHg₂₀, the VEC increases with the Ca content. Amalgams of electropositive metals form polar metal bonds and show "bad metal" properties. By thermal analysis, magnetic susceptibility and resistivity measurements, and density functional theory calculations of the electronic structures, we investigate the effect of the structural disorder introduced by creating mixed-atom occupation on the physical properties of the two new polar amalgam systems.

Cuprate Oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 with "Channel-like" Anion Ordering

Highly fluorinated cuprate Ruddlesden-Popper oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were obtained by topochemical reaction between poly(vinylidene fluoride) (PVDF) and the corresponding oxides La₂Cu_0.8Ni_0.2O₄ and La₂CuO₄ prepared by citrate-based soft chemistry synthesis. The crystal structures of both oxyfluorides were investigated by powder diffraction techniques. The structure of La₂Cu_0.8Ni_0.2O₃F₂ was solved based on combined neutron and X-ray powder diffraction. It crystallizes in a new monoclinic distorted version [C2/c a = 13.1880(3) Å, b = 5.7244(1) Å, c = 5.6007(1) Å, and β = 90.85(1)°] of the anionic ordered structure lately reported for La₂NiO₃F₂. For La₂CuO₃F₂, an even less symmetrical triclinic structure was derived from X-ray powder diffraction data [P1̅ a = 5.6180(5) Å, b = 5.7316(6) Å, c = 7.1978(9) Å, α = 113.32(1)°, β = 90.89(9)°, and γ = 90.16(11)°]. For both compounds, an additional tilt component of the partially Jahn-Teller elongated (Cu,Ni)O₄F₂ octahedra was found as the origin for the lowered symmetry. The formation reaction of La₂CuO₃F₂ was studied by in situ XRD measurements. In these investigations, two new reaction intermediates were identified. The magnetic properties of both oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were characterized by field- and temperature-dependent measurements. An antiferromagnetic ordering with T_N = 240 K was found for La₂Cu_0.8Ni_0.2O₃F₂. In La₂CuO₃F₂, additional weak ferrimagnetism was observed, resulting in a pronounced hysteresis but a weak saturation moment, which was attributed to result from a canted antiferromagnetic spin arrangement.

Anisotropic properties of Ba1−xSrxFe12O19 single crystals

Single crystals of Ba1−xSrxFe12O19 with x = 0, 0.25, 0.5, 0.75, and 1 were grown from a Na2CO3 flux.

Structure, Magnetism, and Thermal Stability of La2NiO2.5F3: A Ruddlesden-Popper Oxyfluoride Crystallizing in Space Group P42/nnm

We report on the new Ruddlesden-Popper (RP) oxyfluoride La₂NiO_2.5F₃ containing an unprecedented high amount of fluorine and Ni²⁺. This oxyfluoride was prepared by topochemical low-temperature fluorination of La₂NiO₄, which was obtained by a soft chemistry synthesis, with poly(vinylidene difluoride) (PVDF) as fluorinating agent. La₂NiO_2.5F₃ is the first n = 1 RP compound crystallizing in the tetragonal space group P4₂/nnm (a = 5.7297(6) Å and c = 13.0106(2) Å). The crystal structure shows a unique tilting scheme of the NiO₄F₂ octahedra that has so far been only theoretically predicted. Combined neutron and X-ray powder diffraction experiments together with bond-valence-sum and DFT+U calculations reveal an unusual anion ordering with fluoride being located on the apical anion sites of the NiO₄F₂ octahedra. Excess fluorine ions were found to populate two of the four interstitial anion sites in an ordered fashion. A third interstitial anion position is occupied by oxygen ions while the fourth site remains unoccupied. This hitherto unobserved ordering scenario in RP oxyfluorides promotes a strong layerwise alternating tilting of the NiO₄F₂ octahedra. Magnetic measurements show strong antiferromagnetic interactions with a high Néel temperature of about 225 K and a pronounced ZFC/FC splitting most likely as the result of a small ferromagnetic moment arising from spin canting. The electronic structure was characterized by DFT and UV-vis spectroscopy, and a strong increase of E_g was found compared to La₂NiO₄ (3.4 eV vs 1.3 eV).

On the incorporation of iron into hexagonal barium titanate: II. Magnetic moment, electron paramagnetic resonance (EPR) and optical transmission

Systematic measurements of the magnetic moment in dependence on temperature and magnetic field of hexagonal 6H-BaTiO3 + 0.04 BaO + x/2 Fe2O3 (0.005 ⩽ x ⩽ 0.05) ceramics were performed to study the influence of Fe ions on the magnetic properties. While the samples show Curie-Weiss paramagnetism for Fe concentrations ⩽1.0 mol%, antiferromagnetic interactions become manifest for 2.0 and 5.0 mol% iron. With increasing Fe content the antiferromagnetic interaction, which is assumed to be caused by a superexchange mechanism [Formula: see text], becomes stronger. At external magnetic fields smaller than 1 T a further, ferromagnetic interaction between Fe3+ ions is detected below 200 K. The interactions between Fe3+ ions in the samples with 2.0 and 5.0 mol% iron are also manifest in the EPR spectra by numerous lines with low intensity. Q-band EPR investigations of 5.0 mol% Fe doped single crystals confirm the existence of only one type of Fe3+-VO associates in the samples.

On the incorporation of iron into hexagonal barium titanate: I. electron paramagnetic resonance (EPR) study

Electron paramagnetic resonance (EPR) investigations of BaTiO₃  +  0.04 BaO  +  x/2 Fe₂O₃ (0.007  ⩽  x  ⩽  0.05) ceramics and BaTi_0.98Fe_0.02O₃ single crystals were performed to study the incorporation of Fe ions in the hexagonal 6H-BaTiO₃ lattice and their defect properties. The samples were characterized by x-ray diffraction and wavelength-dispersive x-ray electron probe microanalysis. EPR spectra were recorded both in X- and Q-bands at room temperature. Angle-dependent single crystal EPR investigations and simulations of the ceramic powder EPR spectra revealed three different centers, which can be attributed to Fe³⁺ ions incorporated on crystallographically different Ti sites. Only one of them was already known before. Two spectra with axial symmetry belong to isolated Fe³⁺ ions incorporated at Ti(1) sites (exclusively corner-sharing oxygen octahedra) and Ti(2) sites (face-sharing octahedra). The difference of their spectral parameters arises from the different trigonal distortions of the two types of octahedra. The third spectrum has orthorhombic symmetry and is caused by Fe³⁺ centers associated with a nearest-neighbor charge-compensating oxygen vacancy. A model for the location of this associate is proposed.

Yttrium Iron Garnet Thin Films with Very Low Damping Obtained by Recrystallization of Amorphous Material

We have investigated recrystallization of amorphous Yttrium Iron Garnet (YIG) by annealing in oxygen atmosphere. Our findings show that well below the melting temperature the material transforms into a fully epitaxial layer with exceptional quality, both structural and magnetic. In ferromagnetic resonance (FMR) ultra low damping and extremely narrow linewidth can be observed. For a 56 nm thick layer a damping constant of α = (6.15 ± 1.50) · 10(-5) is found and the linewidth at 9.6 GHz is as small as 1.30 ± 0.05 Oe which are the lowest values for PLD grown thin films reported so far. Even for a 20 nm thick layer a damping constant of α = (7.35 ± 1.40) · 10(-5) is found which is the lowest value for ultrathin films published so far. The FMR linewidth in this case is 3.49 ± 0.10 Oe at 9.6 GHz. Our results not only present a method of depositing thin film YIG of unprecedented quality but also open up new options for the fabrication of thin film complex oxides or even other crystalline materials.

Defect properties of cobalt-doped hexagonal barium titanate ceramics

X-ray diffraction (XRD) patterns, electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) and the magnetic susceptibility of BaTiO3 + 0.04 BaO + x/2 Co2O3 (0.001 ⩽ x ⩽ 0.02) ceramics were studied to investigate the incorporation of Co ions in the BaTiO3 lattice and their valence states as well as the development of the hexagonal phase (6H modification) in dependence on doping level x and sintering temperature Ts. At Ts = 1400 °C the 6H modification begins to occur at a nominal Co concentration x of about 0.001 and for x > 0.005 the samples are completely hexagonal at room temperature. Two different EPR spectra were observed in the 6H modification of BaTiO3, which were both assigned to paramagnetic Co(2+) ions located at the two crystallographically non-equivalent Ti sites in 6H-BaTiO3. The EPR g tensor values as well as the molar paramagnetic susceptibility, measured in the temperature range 5 K-300 K at a magnetic field of 9 T, were analyzed in the framework of the ligand field theory using the program CONCORD. The combination of EPR and magnetic measurements reveals that in air-sintered 6H BaTiO3, the incorporated Co occurs as a mixture of paramagnetic Co(2+) and diamagnetic Co(3+) ions, whereas in samples annealed in reducing atmosphere the majority of Co is in the divalent state. The occurrence of Co(4+) can be excluded for all investigated samples. The sample color caused by Co(2+) and Co(3+) ions is beige/light yellow and dark grey/black, respectively. The majority of the Co(2+) ions substitutes Ti in the exclusively corner-sharing oxygen octahedra possessing nearly cubic symmetry. The corresponding ligand field parameter [Formula: see text] amounts to about -28 000 cm(-1) (Wybourne notation, 10Dq ≈ 20 000 cm(-1)). In the reduced samples nearly 5% of the detected Co(2+) ions occupy the Ti site in the face-sharing oxygen octahedra, which are significantly trigonally distorted. The negative sign of the obtained ligand field parameter [Formula: see text] ≈ -7300 cm(-1) reflects a compression of this octahedron in direction of the hexagonal c-axis.

Synthesis, structure and properties of [Co(NCS)2(4-(4-chlorobenzyl)pyridine)2]n, that shows slow magnetic relaxations and a metamagnetic transition

[Co(NCS)₂(4-(4-chlorobenzyl)pyridine)₂]_n shows a metamagnetic behavior and slow relaxations of magnetization indicative of a single chain magnet behavior.

Lithiated sulfoxides: α-sulfinyl functionalized carbanions

Lithiated sulfoxides of the type [Li₂{CRR′S(O)Ar}₂(TMEDA)₂], their molecular structures, and diastereoselective C–C bond formation reactions with benzaldehyde and benzophenone are presented.

Valence properties of Cu and Ru in titanium-substituted LnCu3Ru4O12+δ (Ln = La, Pr, Nd) investigated by XANES and TGA

In La_yCu₃Ru_xTi_4−xO_12+δ ruthenium remains as Ru⁴⁺, while the Cu valence decreases from +2 to +1.6 with increasing x.

Electronic transitions and dielectric function tensor of a YMnO3 single crystal in the NIR-VUV spectral range

The dielectric function tensor elements of an YMnO₃ single crystal yield M0 critical point like band-band absorption with discrete spectrally localized Lorentzian transitions within the band gaps.

Enhanced dielectric properties of sol–gel-BaTiO3/P(VDF-HFP) composite films without surface functionalization

BaTiO₃–P(VDF-HFP) composite films without surface functionalization show a tenfold increase of the relative permittivity compared to the pure polymer.

Suppression of Ru (S = 1) spin dimerization in La2RuO5 by Ti substitution

La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru(1-y)Ti(y)O5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.

Pressure-dependent structural and electronic properties of quasi-one-dimensional (TMTTF)2PF6

We have performed detailed x-ray investigations of the quasi-one-dimensional organic conductor (TMTTF)(2)PF(6) at room temperature and hydrostatic pressures up to 27 kbar. Based on the pressure-dependent crystal structure, the electronic band structure was calculated by density functional theory (DFT). Our systematic study provides important information on the coupling among the organic molecules but also to the anions. We discuss the consequences for the electronic properties and compare them with optical investigations under pressure. The increasing plasma frequency observed perpendicular to the stacks corresponds to a widening of the bands for the b-direction. Around 20 kbar a dimensional crossover occurs from a one-dimensional Mott insulator to a two-dimensional metal.

Crystal structure and optical properties of the new 8O polytype of Ca2Ta2O7

Single crystals of the new 8O modification of Ca2Ta2O7 were grown by optical floating zone melting and their structure was solved by X-ray diffraction. 8O Ca2Ta2O7 crystallizes in space group C2221 with a = 7.3690(2) Å, b = 12.7296(3) Å, c = 48.263(1) Å. The structure is related to the weberite structure type and can be described as a stacking of 8 basic building units per unit cell. Each building unit consists of one Ta3Ca and one Ca3Ta layer with a cubic close packed arrangement. Transmission measurements in the UV-Vis and IR region reveal a high transparency in a wide energy range from 0.2–3.5 eV. Dielectric measurements yield a high ɛ′ value of 60 at room temperature.

Orbital ordering and spin-ladder formation in La2RuO5

The semiconductor-semiconductor transition of La2RuO5 is studied by means of augmented spherical wave electronic structure calculations as based on density-functional theory and the local density approximation. This transition has lately been reported to lead to orbital ordering and a quenching of the local spin magnetic moment. Our results hint towards an orbital ordering scenario which, markedly different from the previously proposed scheme, preserves the local S=1 moment at the Ru sites in the low-temperature phase. The unusual magnetic behavior is interpreted by the formation of spin ladders, which result from the structural changes occurring at the transition and are characterized by antiferromagnetic coupling along the rungs.

High- and low-temperature La2RuO5by powder neutron diffraction

The structure of dilanthanum ruthenium pentoxide was solved by powder neutron diffraction at room temperature and 1.5 K. High-temperature La2RuO5 crystallizes in the monoclinic space group P2(1)/c. Upon cooling, the sample undergoes a phase transition to the triclinic low-temperature form (space group P-1). This transition leads to pronounced changes in the Ru-O-Ru bond distances, resulting in a dimerization of the ruthenium ions.

Structural and Magnetic Properties of Hexagonal Perovskites La1.2Sr2.7MO7.33 (M = Ru, Ir) Containing Peroxide Ions

The structures of the new compound La(1.2)Sr(2.7)IrO(7.33) and the recently discovered La(1.2)Sr(2.7)RuO(7.33) have been solved using a combination of X-ray and neutron diffraction. Both compounds crystallize in the trigonal space group Rm and consist of isolated MO6 (M = Ru, Ir) octahedra, which are arranged in well-defined hexagonal perovskite slabs. These slabs are separated by (Sr2O(1+delta)) layers containing both O2- and (O2)2- ions. The composition can therefore be written as La(1.2)Sr(2.7)MO(7-delta)(O2)delta with delta = 0.33. Results of the magnetic susceptibility and XANES measurements show that the transition metal cations are in a pentavalent state. While in La(1.2)Sr(2.7)RuO(7.33) an antiferromagnetic interaction between the Ru5+ ions is found, La(1.2)Sr(2.7)IrO(7.33) shows a very small temperature-independent paramagnetism down to 1.8 K due to the strong spin-orbit coupling characteristic for the 5d element iridium.

Powder neutron diffraction of SrNbO2N at room temperature and 1.5 K

The structure of strontium niobium dioxygen nitride, SrNbO2N, has been solved by powder neutron diffraction at room temperature and 1.5 K. SrNbO2N crystallizes in the tetragonal space group I4/mcm, with a = 5.7056 (4) and c = 8.1002 (9) A at room temperature, and a = 5.6938 (4) and c = 8.0974 (8) A at 1.5 K. The crystal structure is derived from the cubic perovskite archetype by a slight rotation of the Nb(O,N)6 octahedra with respect to the tetragonal axis. A partially ordered distribution of oxygen and nitrogen on the anionic sites was found.

Two Ti-doped distrontium ruthenium tetraoxides: Sr2Ru0.93Ti0.07O4 and Sr2Ru0.81Ti0.19O4

Crystals of titanium-doped distrontium ruthenium tetraoxide, Sr(2)Ru(1-x)Ti(x)O(4), with x = 0.07 and 0.19, were grown by floating-zone melting, and their structures were solved using single-crystal X-ray diffraction. Increasing Ti content leads to a distinctive systematic variation of cell parameters and interatomic distances with respect to the undoped material.