Understanding Complex Interplay among Different Instabilities in Multiferroic BiMn7O12 Using 57Fe Probe Mössbauer Spectroscopy.

Here, we report the results of a Mössbauer study on hyperfine electrical and magnetic interactions in quadruple perovskite BiMn7O12 doped with 57Fe probes. Measurements were performed in the temperature range of 10 K < T < 670 K, wherein BiMn6.9657Fe0.04O12 undergoes a cascade of structural (T1 ≈ 590 K, T2 ≈ 442 K, and T3 ≈ 240 K) and magnetic (TN1 ≈ 57 K, TN2 ≈ 50 K, and TN3 ≈ 24 K) phase transitions. The analysis of the electric field gradient (EFG) parameters, including the dipole contribution from Bi3+ ions, confirmed the presence of the local dipole moments pBi, which are randomly oriented in the paraelectric cubic phase (T > T1). The unusual behavior of the parameters of hyperfine interactions between T1 and T2 was attributed to the dynamic Jahn-Teller effect that leads to the softening of the orbital mode of Mn3+ ions. The parameters of the hyperfine interactions of 57Fe in the phases with non-zero spontaneous electrical polarization (Ps), including the P1 ↔ Im transition at T3, were analyzed. On the basis of the structural data and the quadrupole splitting Δ(T) derived from the 57Fe Mössbauer spectra, the algorithm, based on the Born effective charge model, is proposed to describe Ps(T) dependence. The Ps(T) dependence around the Im ↔ I2/m phase transition at T2 is analyzed using the effective field approach. Possible reasons for the complex relaxation behavior of the spectra in the magnetically ordered states (T < TN1) are also discussed.

Layered van der Waals Chalcogenides FeAl2Se4, MnAl2S4, and MnAl2Se4: Atomically Thin Triangular Arrangement of Transition-Metal Atoms.

Layered van der Waals (vdW) chalcogenides of 3d transition metals are a rich source of two-dimensional (2D) nanomaterials, in which atomically thin layers with the terminating chalcogen atoms exhibit promising functionality for novel spintronic devices. Here, we report on the synthesis, crystal growth, and magnetic properties of FeAl2Se4, MnAl2S4, and MnAl2Se4 ternary chalcogenides. Crystal structures are probed by powder X-ray diffraction, Mössbauer spectroscopy, and high-resolution transmission electron microscopy. We improve the structural models of FeAl2Se4 and MnAl2S4 and show that isostructural MnAl2S4 and MnAl2Se4 crystallize in the centrosymmetric R3̅̅m space group. In the crystal structure, transition metal and Al atoms mutually occupy the octahedral and tetrahedral voids of four close-packing chalcogen layers terminated by vdW gaps. The transition-metal atoms form a triangular arrangement inside the close-packing layers. As a result, FeAl2Se4 and MnAl2S4 show no long-range magnetic order in the studied temperature range. In the paramagnetic state, Fe and Mn possess effective magnetic moments of 4.99(2) and 5.405(6) µB, respectively. Furthermore, FeAl2Se4 enters a frozen spin-disordered state below 12 K.

Endohedral Cluster Superconductors in the Mo-Ga-Sn System Explored by the Joint Flux Technique.

Endohedral Ga cluster compounds feature nontrivial superconducting states including the two-gap superconductivity similar in nature to MgB2. We use the joint flux synthetic technique to introduce Sn into the Ga matrix and tune the valence electron count in the two new endohedral cluster superconductors Mo8Ga41-xSnx and Mo4Ga21-x-δSnx with critical temperatures of Tc = 8.7 and 5.85 K, respectively. While the former compound is a derivative of the previously known Mo8Ga41 superconductor, where Sn atoms are enclosed inside the Sn@Ga6 octahedral clusters, the latter is a new architecture built upon Mo@Ga9Sn clusters, Ga@Ga12 cuboctahedra, and Sn4 squares. We show that this novel Mo4Ga21-x-δSnx superconductor features strong electron-phonon coupling with the large ratio of 2Δ(0)/(kBTc) = 4.1 similar to that of the Mo8Ga41 superconductor with the closely related crystal structure.

The Role of Semilabile Oxygen Atoms for Intercalation Chemistry of the Metal-Ion Battery Polyanion Cathodes.

Using the orthorhombic layered Na2FePO4F cathode material as a model system we identify the bonding of the alkali metal cations to the semilabile oxygen atoms as an important factor affecting electrochemical activity of alkali cations in polyanion structures. The semilabile oxygens, bonded to the P and alkali cations, but not included into the FeO4F2 octahedra, experience severe undercoordination upon alkali cation deintercalation, causing an energy penalty for removing the alkali cations located in the proximity of such semilabile oxygens. Desodiation of Na2FePO4F proceeds through a two-phase mechanism in the Na-ion cell with a formation of an intermediate monoclinic Na1.55FePO4F phase with coupled Na/vacancy and Fe2+/Fe3+ charge ordering at 50% state of charge. In contrast, desodiation of Na2FePO4F in the Li-ion cell demonstrates a sloping charge profile suggesting a solid solution mechanism without formation of a charge-ordered intermediate phase. A combination of a comprehensive crystallographic study and extensive DFT-based calculations demonstrates that the difference in electrochemical behavior of the alkali cation positions is largely related to the different number of the nearest neighbor semilabile oxygen atoms, influencing their desodiation potential and accessibility for the Na/Li chemical exchange, triggering coupled alkali cation-vacancy ordering and Fe2+/Fe3+ charge ordering, as well as switching between the "solid solution" and "two-phase" charging mechanistic regimes.

Spin-Driven Multiferroic Properties of PbMn7O12 Perovskite.

We synthesize PbMn7O12 perovskite under high-pressure (6 GPa) and high-temperature (1373 K) conditions and investigate its structural, magnetic, dielectric, and ferroelectric properties. We find that PbMn7O12 exhibits rich physical properties from interplay among charge, orbital, and spin degrees of freedom and rich structural properties. PbMn7O12 crystallizes in space group R3̅ near room temperature and shows a structural phase transition at TCO = 397 K to a cubic structure in space group Im3̅; the Im3̅-to-R3̅ transition is associated with charge ordering. Below TOO = 294 K, a structural modulation transition associated with orbital ordering takes place. There are two magnetic transitions with Néel temperatures of TN1 = 83 K and TN2 = 77 K and probably a lock-in transition at TN3 = 43 K (on cooling). There is huge hysteresis on specific heat (between ∼37 and 65 K at 0 Oe), dielectric constant (between ∼20 and 70 K at 0 Oe), and dc and ac magnetic susceptibilities around the lock-in transition. Sharp dielectric constant, dielectric loss, and pyroelectric current anomalies are observed at TN2, indicating that electric polarization is developed at this magnetic transition, and PbMn7O12 perovskite is a spin-driven multiferroic. Polarization of PbMn7O12 is measured to be ∼4 µC/m(2). Field-induced transitions are detected at ∼63 and ∼170 kOe at 1.6-2 K; similar high-magnetic field properties are also found for CdMn7O12, CaMn7O12, and SrMn7O12. PbMn7O12 exhibits a quite small magnetodielectric effect, reaching approximately -1.3 to -1.7% at 10 K and 90 kOe.

High-pressure synthesis, crystal structures, and properties of CdMn7O12 and SrMn7O12 perovskites.

We synthesize CdMn7O12 and SrMn7-xFexO12 (x = 0, 0.08, and 0.5) perovskites under high pressure (6 GPa) and high temperature (1373-1573 K) conditions and investigate their structural, magnetic, dielectric, and ferroelectric properties. CdMn7O12 and SrMn7O12 are isostructural with CaMn7O12: space group R3̅ (No. 148), Z = 3, and lattice parameters a = 10.45508(2) Å and c = 6.33131(1) Å for CdMn7O12 and a = 10.49807(1) Å and c = 6.37985(1) Å for SrMn7O12 at 295 K. There is a structural phase transition at 493 K in CdMn7O12 and at 404 K in SrMn7O12 to a cubic structure (space group Im3̅), associated with charge ordering as found by the structural analysis and Mössbauer spectroscopy. SrMn6.5Fe0.5O12 crystallizes in space group Im3̅ at 295 K with a = 7.40766(2) Å and exhibits spin-glass magnetic properties below 34 K. There are two magnetic transitions in CdMn7O12 with the Néel temperatures TN2 = 33 K and TN1 = 88 K, and in SrMn7O12 with TN2 = 63 K and TN1 = 87 K. A field-induced transition is found in CdMn7O12 from about 65 kOe, and TN2 = 58 K at 90 kOe. No dielectric anomalies are found at TN1 and TN2 at 0 Oe in both compound, but CdMn7O12 exhibits small anomalies at TN1 and TN2 at 90 kOe. In pyroelectric current measurements, we observe large and broad peaks around magnetic phase transition temperatures in CdMn7O12, SrMn7O12, and SrMn6.5Fe0.5O12; we assign those peaks to extrinsic effects and compare our results with previously reported results on CaMn7O12. We also discuss general tendencies of the AMn7O12 perovskite family (A = Cd, Ca, Sr, and Pb).

Sc2NiMnO6: A Double-Perovskite with a Magnetodielectric Response Driven by Multiple Magnetic Orders.

Perovskite materials provide a large variety of interesting physical properties and applications. Here, we report on unique properties of a fully ordered magnetodielectric double-perovskite, Sc2NiMnO6 (space group P21/n, a = 4.99860 Å, b = 5.35281 Å, c = 7.34496 Å, and ß = 90.7915°), exhibiting sequential magnetic transitions at T1 = 35 K and T2 = 17 K. The transition at T1 corresponds to a single-k antiferromagnetic phase with propagation vector k1 = (1/2, 0, 1/2), while the second transition at T2 corresponds to a 2-k magnetic structure with propagation vectors k1 = (1/2, 0, 1/2) and k2 = (0, 1/2, 1/2). Symmetry analysis suggests that the two ordering wave vectors are independent, and calculations imply that k1 is associated with the Mn sublattice and k2 with the Ni sublattice, suggesting that Mn-Ni coupling is very small or absent. A magnetodielectric anomaly at T2 likely arises from an antiferroelectric ordering that results from the exchange-striction between the two magnetic sublattices belonging to k1 and k2. The behavior of Sc2NiMnO6 demonstrates 3d double-perovskites with small A-site cations as a promising avenue in which to search for magnetoelectric materials.

Structure and cation distribution in perovskites with small cations at the A site: the case of ScCoO3.

We synthesize ScCoO3 perovskite and its solid solutions, ScCo1-x Fe x O3 and ScCo1-x Cr x O3, under high pressure (6 GPa) and high temperature (1570 K) conditions. We find noticeable shifts from the stoichiometric compositions, expressed as (Sc1-xMx )MO3 with x = 0.05-0.11 and M = Co, (Co, Fe) and (Co, Cr). The crystal structure of (Sc0.95Co0.05)CoO3 is refined using synchrotron x-ray powder diffraction data: space group Pnma (No. 62), Z = 4 and lattice parameters a = 5.26766(1) Å, b = 7.14027(2) Å and c = 4.92231(1) Å. (Sc0.95Co0.05)CoO3 crystallizes in the GdFeO3-type structure similar to other members of the perovskite cobaltite family, ACoO3 (A3+ = Y and Pr-Lu). There is evidence that (Sc0.95Co0.05)CoO3 has non-magnetic low-spin Co3+ ions at the B site and paramagnetic high-spin Co3+ ions at the A site. In the iron-doped samples (Sc1-xMx )MO3 with M = (Co, Fe), Fe3+ ions have a strong preference to occupy the A site of such perovskites at small doping levels.

Perovskite-structure TlMnO3: a new manganite with new properties.

We synthesize a new member of the AMnO3 perovskite manganite family (where A is a trivalent cation)--thallium manganite, TlMnO3--under high-pressure (6 GPa) and high-temperature (1500 K) conditions and show that the structural and magnetic properties are distinct from those of all other AMnO3 manganites. The crystal structure of TlMnO3 is solved and refined using single-crystal X-ray diffraction data. We obtain a triclinically distorted structure with space group P1̅ (No. 2), Z = 4, and lattice parameters a = 5.4248(2) Å, b = 7.9403(2) Å, c = 5.28650(10) Å, α = 87.8200(10)°, ß = 86.9440(10)°, and γ = 89.3130(10)° at 293 K. There are four crystallographic Mn sites in TlMnO3 forming two groups based on the degree of their Jahn-Teller distortions. Physical properties of insulating TlMnO3 are investigated with Mössbauer spectroscopy and resistivity, specific heat, and magnetization measurements. The orbital ordering, which persists to the decomposition temperature of 820 K, suggests A-type antiferromagnetic ordering with the ferromagnetic planes along the [-101] direction, consistent with the measured collinear antiferromagnetism below the Néel temperature of 92 K. Hybrid density functional calculations are consistent with the experimentally identified structure, insulating ground state, and suggested magnetism, and show that the low symmetry originates from the strongly Jahn-Teller distorted Mn(3+) ions combined with the strong covalency of the Tl(3+)-O bonds.

Bulk ferromagnetism in cleavable van der Waals telluride NbFeTe2.

A van der Waals telluride, NbFeTe2, has been synthesized using chemical vapor transport reactions. The optimized synthetic conditions yield high-quality single crystals with a novel monoclinic crystal structure. Monoclinic NbFeTe2 demonstrates a (100) cleavage plane, bulk ferromagnetism below 87 K, and a metallic ground state-the necessary prerequisites for needed spintronics technologies.

Controlled Reduction of Sn4+ in the Complex Iodide Cs2SnI6 with Metallic Gallium.

Metal gallium as a low-melting solid was applied in a mixture with elemental iodine to substitute tin(IV) in a promising light-harvesting phase of Cs2SnI6 by a reactive sintering method. The reducing power of gallium was applied to influence the optoelectronic properties of the Cs2SnI6 phase via partial reduction of tin(IV) and, very likely, substitute partially Sn4+ by Ga3+. The reduction of Sn4+ to Sn2+ in the Cs2SnI6 phase contributes to the switching from p-type conductivity to n-type, thereby improving the total concentration and mobility of negative-charge carriers. The phase composition of the samples obtained was studied by X-ray diffraction (XRD) and 119Sn Mössbauer spectroscopy (MS). It is shown that the excess of metal gallium in a reaction melt leads to the two-phase product containing Cs2SnI6 with Sn4+ and ß-CsSnI3 with Sn2+. UV-visible absorption spectroscopy shows a high absorption coefficient of the composite material.

High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Herein, we demonstrate for the first time compact ferrite ceramics with giant coercivity. The materials are manufactured via sintering single-domain Sr0.67Ca0.33Fe8Al4O19 particles synthesized by a citrate-nitrate auto-combustion method. The obtained ceramics show coercivities up to 22.5 kOe and natural ferromagnetic resonance frequencies (NFMR) in a sub-THz range of 160-282 GHz. At a maximum density of 95%, the sample displays coercivity of 18.5 kOe, which is the highest value among dense ferrite materials reported so far. In addition, we report an unusual blueshift of the NFMR frequency from 160 to 200 GHz, which occurs during material sintering.

Correction: High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance.

Correction for 'High-coercivity hexaferrite ceramics featuring sub-terahertz ferromagnetic resonance' by Evgeny A. Gorbachev et al., Mater. Horiz., 2022, 9, 1264-1272, DOI: https://doi.org/10.1039/D1MH01797G.

New Fe-based layered telluride Fe3-δAs1-yTe2: synthesis, crystal structure and physical properties.

A new ternary telluride, Fe3-δAs1-yTe2, was synthesized from elements at 600 °C. It crystallizes in the hexagonal P63/mmc space group with the unit cell parameters a = 3.85091(9) Å and c = 17.1367(4) Å for δ = 0.3 and y = 0.04. Its layered crystal structure contains partially occupied intralayer and interlayer Fe positions, which give rise to significant nonstoichiometry: Fe3-δAs1-yTe2 was found to possess the homogeneity range of 0.25 < δ < 0.45 and y = 0.04. Regions of local vacancy ordering alternate with regions of randomly distributed vacancies, so that the ordering of Fe atoms and vacancies is not complete in the average structure. Clear evidence of the magnetic phase transition is obtained by thermodynamic measurements, Mössbauer spectroscopy, and neutron powder diffraction. Magnetic susceptibility measurements reveal weak ferromagnetism below TC = 123 K with a net moment of MS∼ 0.1µB/Fe at T = 2 K. This transition is confirmed by differential scanning calorimetry. Additionally, neutron powder diffraction indicates the onset of a complex AFM-like magnetic ordering below 100 K.

Effect of Transition Metal Substitution on the Structure and Properties of a Clathrate-Like Compound Eu7Cu44As23.

A series of substitutional solid solutions-Eu7Cu44-xTxAs23 (T = Fe, Co, Ni)-based on a recently discovered clathrate-like compound (Eu7Cu44As23) were synthesized from the elements at 800 °C. Almost up to 50% of Cu can be substituted by Ni, resulting in a linear decrease of the cubic unit cell parameter from a = 16.6707(1) Å for the ternary compound to a = 16.3719(1) Å for the sample with the nominal composition Eu7Cu24Ni20As23. In contrast, Co and Fe can only substitute less than 20% of Cu. Crystal structures of six samples of different composition were refined from powder diffraction data. Despite very small differences in scattering powers of Cu, Ni, Co, and Fe, we were able to propose a reasonable model of dopant distribution over copper sites based on the trends in interatomic distances as well as on Mössbauer spectra for the iron-substituted compound Eu7Cu36Fe8As23. Ni doping increases the Curie temperature to 25 K with respect to the parent compound, which is ferromagnetically ordered below 17.5 K, whereas Fe doping suppresses the ferromagnetic ordering in the Eu sublattice.

High-pressure synthesis, crystal structure and magnetic properties of TlCrO3 perovskite.

TlMO(3) perovskites (M(3+) = transition metals) are exceptional members of trivalent perovskite families because of the strong covalency of Tl(3+)-O bonds. Here we report on the synthesis, crystal structure and properties of TlCrO(3) investigated by Mössbauer spectroscopy, specific heat, dc/ac magnetization and dielectric measurements. TlCrO(3) perovskite is prepared under high pressure (6 GPa) and high temperature (1500 K) conditions. The crystal structure of TlCrO(3) is refined using synchrotron X-ray powder diffraction data: space group Pnma (no. 62), Z = 4 and lattice parameters a = 5.40318(1) Å, b = 7.64699(1) Å and c = 5.30196(1) Å at 293 K. No structural phase transitions are found between 5 and 300 K. TlCrO(3) crystallizes in the GdFeO(3)-type structure similar to other members of the perovskite chromite family, ACrO(3) (A(3+) = Sc, In, Y and La-Lu). The unit cell volume and Cr-O-Cr bond angles of TlCrO(3) are close to those of DyCrO(3); however, the Néel temperature of TlCrO(3) (TN≈ 89 K) is much smaller than that of DyCrO(3) and close to that of InCrO(3). Isothermal magnetization studies show that TlCrO(3) is a fully compensated antiferromagnet similar to ScCrO(3) and InCrO(3), but different from RCrO(3) (R(3+) = Y and La-Lu). Ac and dc magnetization measurements with a fine step of 0.2 K reveal the existence of two Néel temperatures with very close values at T(N2) = 87.0 K and T(N1) = 89.3 K. Magnetic anomalies near T(N2 )are suppressed by static magnetic fields and by 5% iron doping.