2D to 3D Magnetism in Synthetic Micas.

Fe-based mica minerals usually display two opposing magnetic ground states, either they behave as spin-glasses or as layered ferrimagnets. No definite reason has been proposed as an explanation for this duality. This conundrum is unraveled by comparing the synthetic micas KFe3[MGe3]O10X2 with M═Fe and Ga, X═OH- and F-. Neutron diffraction demonstrates a 2D to 3D magnetic transition in KFe3[FeGe3]O10(OH)2 while just hints or no order at all are observed for the fluorides with M═Fe and Ga respectively. The 3D transition is triggered by the presence of iron in the intralayer tetrahedra. DFT+U calculations show that the magnetic exchange couplings between the previously believed solely magnetic octahedral layers would otherwise be frustrated without this intralayer iron.

Antipolar 2D Metallicity with Tunable Valence Wx+ (5 ≤ x ≤ 5.6) in the Layered Monophosphate Tungsten Bronzes [Ba(PO4)2]WmO3m-3.

The newly discovered series of layered monophosphate tungsten bronzes (L-MPTB) [Ba(PO4)2]WmO3m-3 consist of m-layer-thick slabs of WO6 octahedra separated by barium-phosphate spacers. They display a 2D metallic behavior confined in the central part of the perovskite slabs. Here, we report the missing m = 2 member of this series, containing the rather uncommon W5+ oxidation state. We have analyzed its structure-property relationships in relation to the other members of the L-MPTB family. In particular, we have determined its crystal structure by means of single-crystal X-ray and electron diffraction and investigated its physical properties from resistivity, Seebeck-coefficient and heat-capacity measurements combined with first-principles calculations. All the L-MPTB compounds show metallic behavior down to 1.8 K without any clear charge-density-wave (CDW) order. The m = 2 member, however, displays an increased influence of the spacer that translates into anisotropic negative thermal expansion, reversed thermopower and reversed crystal-field splitting of the tungsten t2g orbitals. Our analysis of the full [Ba(PO4)2]WmO3m-3 series reveals a systematic and significant W off-centering in their octahedral coordination. We identify the resulting anti-polar character of these W displacements as the crucial aspect behind the 2D metallicity of these systems: It leads to the presence of bound charges whose screening determines the distribution of mobile charges, tending to accumulate at the center of the [WmO3-m] block. We argue that this mechanism is analogous to enhanced conductivity observed for charged domain walls in ferroelectrics, thus providing a general design rule to promote 2D metallicity in layered systems.

Ferromagnetic Mn3+ Sawtooth Chains in A2(Mn2O)(SeO3)3 (A = K, Rb) Quaternary Selenites.

Two quaternary manganese selenites, A2(Mn2O)(SeO3)3 (A = K, Rb), have been synthesized by hydrothermal reactions. They both crystallize in a complex triclinic (P-1) structure built of Jahn-Teller (JT) distorted Mn3+O4+2 octahedra, connected into nearly isosceles [Mn3O14] triangles, themselves arranged into so-called "sawtooth (ST) chains". The K and Rb compounds show subtle variations in the orientations of the MnO4 planes inside the elementary triangles. The ST chains are structurally and magnetically isolated by SeO3 groups and alkali cations. In the ST chains, predominant ferromagnetic interactions were calculated and verified experimentally, which finally order antiferromagnetically between the chains around TN ≈ 22 K. The spin exchanges calculated by DFT + U and fitted by Monte Carlo simulations allow for the quantification of an effective "overall" model. The specific role of the µ3-O bridge on the ferromagnetic (FM) exchanges is discussed, together with spin reorientations observed in the ordered state. Magnetocrystalline anisotropy along the [110] direction stabilized by ∼50 meV per Mn by spin-orbit coupling (SOC) was found by DFT + U + SOC.

CoVO3 High-Pressure Polymorphs: To Order or Not to Order?

Materials properties are determined by their compositions and structures. In ABO3 oxides different cation orderings lead to mainly perovskite- or corundum like derivatives with exciting physical properties. Sometimes, a material can be stabilized in more than one structural modification, providing a unique opportunity to explore structure-properties relationship. Here, CoVO3 obtained in both ilmenite-(CoVO3 -I) and LiNbO3 -type (CoVO3 -II) polymorphs at moderate (8-12 GPa) and high pressures (22 GPa), respectively are presented. Their distinctive cation distributions affect drastically the magnetic properties as CoVO3 -II shows a cluster-glass behavior while CoVO3 -I hosts a honeycomb zigzag magnetic structure in the cobalt network. First principles calculations show that the influence of vanadium is crucial for CoVO3 -I, although it is previously considered as non-magnetic in a dimerized spin-singlet state. Contrarily, CoVO3 -II shows two independent interpenetrating antiferromagnetic Co- and ferromagnetic V-hcp sublattices, which intrinsically frustrate any possible magnetic order. CoVO3 -II is also remarkable as the first oxide crystallizing with the LiNbO3 -type structure where both metals contain free d electrons. CoVO3 polymorphs pinpoint therefore as well to a much broader phase field of high-pressure A-site Cobaltites.

Multivalued Memory via Freezing of Super-Hard Magnetic Domains in a Quasi 2D-Magnet.

The design of high-density non-volatile memories is a long-standing dream, limited by conventional storage "0" or "1" bits. An alternative paradigm exists in which regions within candidate materials can be magnetized to intermediate values between the saturation limits. In principle, this paves the way to multivalued bits, vastly increasing storage density. Single-molecule magnets, are good examples offering transitions between intramolecular quantum levels, but require ultra-low temperatures and limited relaxation time between magnetization states. It is showed here that the quasi 2D-Ising compound BaFe2 (PO4 )2 overcomes these limitations. The combination of giant magneto-crystalline anisotropy, strong ferromagnetic exchange, and strong intrinsic pinning creates remarkably narrow magnetic domain walls, collectively freezing under Tf ≈15 K. This results in a transition from a soft to a super-hard magnet (coercive force > 14 T). Any magnetization can then be printed and robustly protected from external fields with an energy barrier >9T at 2 K.

Layered Monophosphate Tungsten Bronzes [Ba(PO4 )2 ]Wm O3m-3 : 2D Metals with Locked Charge-Density-Wave Instabilities.

Phosphate tungsten and molybenum bronzes represent an outstanding class of materials displaying textbook examples of charge-density-wave (CDW) physics among other fundamental properties. Here we report on the existence of a novel structural branch with the general formula [Ba(PO4 )2 ][Wm O3m-3 ] (m=3, 4 and 5) denominated 'layered monophosphate tungsten bronzes' (L-MPTB). It results from thick [Ba(PO4 )2 ]4- spacer layers disrupting the cationic metal-oxide 2D units and enforcing an overall trigonal structure. Their symmetries are preserved down to 1.8 K and the compounds show metallic behaviour with no clear anomaly as a function of temperature. However, their electronic structure displays the characteristic Fermi surface of previous bronzes derived from 5d W states with hidden nesting properties. By analogy with previous bronzes, such a Fermi surface should result into CDW order. Evidence of CDW order was only indirectly observed in the low-temperature specific heat, giving an exotic context at the crossover between stable 2D metals and CDW order.

Multiple dimensionalities in A2M3(SO4)4 (A = Rb, Cs; M = Co, Ni) analogues.

New representatives of the A2M3(SO4)4 (A = Rb and Cs, M = Co, Ni) family were found, inspired by the discovery and characterization of itelmenite, a mineral of composition Na2CuMg2(SO4)4. Four new compounds were obtained by high-temperature solid-state reactions in air. All new compounds were structurally characterized by single-crystal and powder X-ray diffraction. Rb2Ni3(SO4)4 and Rb2Co3(SO4)4 crystallize in the monoclinic space group P21/c, Cs2Ni3(SO4)4 in P21/n whereas Rb2Co3(SO4)4 crystallizes in the orthorhombic space group P212121. In order to determine the temperature of crystallization of the new phases DTA and TG were performed for the mixtures of the precursors. Several synthesis strategies were tested and discussed. The investigation of the reactivity upon heating highlights the stability of the precursors before they collapse, explaining the difficulties to get pure powder samples.

Morphotropism in fumarolic mineral-related anhydrous sulfates: novel representatives in A+2M2+(SO4)2 and A+2M2+2(SO4)3 series.

The discovery of numerous endemic anhydrous sulfate minerals in fumaroles of the Tolbachik volcano (Kamchatka, Russia) has revived interest in the whole family of anhydrous sulfates. Herein is reported the crystal structure of Cs2Cu(SO4)2 which adds important data on the `final' contributor with the largest A+ cation to the A2[Cu(SO4)2] morphotropic series (A = Na, K, Rb, Cs), the `initial' structurally characterized representative of this family being saranchinaite Na2Cu(SO4)2. With increasing ionic radius of the alkali metal cation(s), embedded in the [Cu(SO4)2]2- framework, symmetry-breaking transformations occur. Cs2Cu(SO4)2, which is here designated as the ϵ-phase, has a layered structure. Cs2Co2(SO4)3 is a new representative of another morphotropic series of the orthorhombic A2[M2+2(SO4)3] family, being also the first anhydrous Cs-Co sulfate. Structural relationships in A+2M2+(SO4)2 and A+2M2+2(SO4)3 morphotropic series are discussed in detail.

Compressibility of structural modulation waves in the chain compounds BaCoX2O7 (X = As, P): a powder study.

BaCoX2O7 (X = As, P) are built on magnetic 1D units in which strong aperiodic undulations originate from incommensurate structural modulations with large atomic displacive amplitudes perpendicular to the chain directions, resulting in very unique multiferroic properties. High-pressure structural and vibrational properties of both compounds have been investigated by synchrotron X-ray powder diffraction and Raman spectroscopy at room temperature and combined with density functional calculations. A structural phase transition is observed at 1.8 GPa and 6.8 GPa in BaCoAs2O7 and BaCoP2O7, respectively. Sharp jumps are observed in their unit-cell volumes and in Raman mode frequencies, thus confirming the first-order nature of their phase transition. These transitions involve the disappearance of the modulation from the ambient-pressure polymorph with clear spectroscopic fingerprints, such as reduction of the number of Raman modes and change of shape on some peaks. The relation between the evolution of the Raman modes along with the structure are presented and supported by density functional theory structural relaxations.

From (S = 1) Spin Hexamer to Spin Tetradecamer by CuO Interstitials in A2Cu3O(CuO)x(SO4)3 (A = alkali).

(Na,K)2Cu3O(SO4)3 compounds form structural chains of Cu6 hexameric units with nominal S = 1 spins due to the interplay between inner strong antiferromagnetic and ferromagnetic exchanges. We show here that the lattice relaxation after the replacement of alkali by larger Rb and Cs ones is accompanied by the insertion of neutral CuO species into (Rb,Cs)2Cu3O(CuO)x(SO4)3 phases. Structurally, interstitial CuO links the next two Cu6 units in longer Cu14 tetradecameric ones. For A = Cs (x = 0.5), the cationic ordering is perfect inside a double-cell superstructure. Magnetically, the original Cu14 units consist of frustrated fragments of an S = 1/2 spin ladder, with ferromagnetic rung-like but antiferromagnetic leg-like and next-nearest neighbor couplings. It returns S = 1 Cu14 spin clusters, effective around 100 K. Our density functional theory calculations and susceptibility fits also show that at low temperatures they interact in two-dimensional lattices, despite the existence of short inter-Cu-Cu distances between the next two clusters along pseudo-one-dimensional chains.

Hybrid electrons in the trimerized GaV4O8.

Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV4O8, a structural sibling of skyrmion-hosting lacunar spinels. GaV4O8 contains a vanadium trimer and an original spin-orbital-charge texture that forms upon the structural phase transition at TS = 68 K followed by the magnetic transition at TN = 35 K. The texture arises from the coexistence of orbital molecules on the vanadium trimers and localized electrons on the remaining vanadium atoms. Such hybrid electrons create opportunities for novel types of spin, charge, and orbital order in mixed-valent transition-metal compounds.

Mn3MnNb2O9: high-pressure triple perovskite with 1 : 2 B-site order and modulated spins.

The first triple perovskite with Mn in A- and 1 : 2 B-site order Mn3MnNb2O9, prepared using high pressure phase transformation of the magnetodielectric Mn4Nb2O9, is reported herein. It has a complex magnetic behaviour with a transition from a collinear AFM into an evolving incommensurate spin density wave (SDW) further stabilised into a lock-in structure dictated by the B-site order.

Design, Hemiysnthesis, crystal structure and anticancer activity of 1, 2, 3-triazoles derivatives of totarol.

A new series of diverse triazoles linked to the hydroxyl group of totarol were synthesized using click chemistry approach. The structures of these compounds were elucidated by HRMS, IR and NMR spectroscopy. The structure of compound 3 g was also confirmed by x-ray single crystal diffraction. The cytotoxicity of these compounds was evaluated by the MTT method against four cancer cell lines, including fibrosarcoma HT-1080, lung carcinoma A-549 and breast adenocarcinoma (MDA-MB-231 and MCF-7), and the results indicated that all compounds showed weak to moderate activities against all cancer cell lines with IC50 values ranging from 14.44 to 46.25 µM. On the basis of our research the structure-activity relationships (SAR) of these compounds were discussed. This work provides some important hints for further structural modification of totarol towards developing novel and highly effective anticancer drugs respectively. It is interesting to note that compound 3 g indicated a very significant cytotoxicity against HT-1080 and A-549 cell lines. The molecular docking showed that compound 3 g activated the caspase-3 and inhibited tubulin by forming stable protein-ligand complexes.

Cycloidal Magnetic Order Promoted by Labile Mixed Anionic Paths in M2(SeO3)F2 (M = Mn2+, Ni2+).

Two M2(SeO3)F2 fluoro-selenites (M = Mn2+, Ni2+) have been synthesized using optimized hydrothermal reactions. Their 3D framework consists of 1D-[MO2F2]4-chains of edge-sharing octahedra with a rare topology of alternating O-O and F-F µ2 bridges. The interchain corner-sharing connections are assisted by the mixed O vs F anionic nature and develop a complex set of M-X-M superexchanges as calculated by LDA+U. Their interplay induces prominent in-chain antiferromagnetic frustration, while the interchain exchanges are responsible for the cycloidal magnetic structure observed below TN ≈ 21.5 K in the Ni2+ case. For comparison the Mn2+ compound develops a nearly collinear spin (canted) ordering below TN ≈ 26 K with ferromagnetic chain units.

S = 1/2 Chain in BiVO3F: Spin Dimers versus Photoanodic Properties.

BiVO3F was prepared, characterized, and identified as a unique example of bismuth vanadyl oxyhalide with paramagnetic V4+ centers. Its crystal structure shows 1D magnetic units with rare alternation of edge-sharing O-O and F-F µ2 bridges along the octahedral chains. Structural pairing across the O2 edges induces antiferromagnetic spin dimers (S = 0) with J/Kb ≈ 300 K, ∼15 times greater than the exchange across the F2 bridges, within a non-ordered magnetic ground state. Despite multiple compositional, structural, and electronic analogies with the BiVO4 scheelite compound, one of the most promising photoanodes for solar water splitting, the photoactivity of BiVO3F is relatively modest, partially due to this electronic pairing benefitting fast electron-hole recombination. Similar to monoclinic VO2, the V4+ spin dimerization deters the singlet → triplet electronic photoexcitation, but results in potential carrier lifetime benefits. The reduction of the bandgap from an Eg of ∼2.4 eV to ∼1.7 eV after incorporation of d1 cations in BiVO4 makes BiVO3F an inspiring compound for local modifications toward an enhanced photoactive material. The direct d → d transition provides a significant enhancement of the visible light capture range and opens a prospective route for the chemical design of performant photoanodes with a mixed anionic sublattice.

Complex magnetism in Ni3TeO6-type Co3TeO6 and high-pressure polymorphs of Mn3-xCoxTeO6 solid solutions.

New Ni3TeO6-type (NTO) and double perovskite (DPv) polymorphs of Co3TeO6 are synthesised at pressures of 15 GPa. A complex elliptic helical magnetic order is observed in the NTO polymorph (TN1 = 58 K) that reorientates (42 K) and further splits (TN2 = 23.5 K) creating a coexisting helix. Increasing Co content within the Mn3-xCoxTeO6 system changes the dominant DPv phase to NTO structural type and drastically modifies the magnetic behaviour. DPv Co3TeO6 is the first A-site double cobaltite.

Magnetic Structures of Mn11Ta4O21 and Interpretation as an Hexagonal A-site Manganite.

Mn11Ta4O21 is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)2 stacking sequence that is related to BaVO3 and BaCrO3 high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at TN1 = 88 K and TN2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below TN1 and TN2, respectively. A complementary description with 14H-(hhccccc)2 sequence of only Mn octahedra provides a direct comparison with BaMnO3-δ hexagonal perovskites and naturally explains the AFM order. Below TN2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

Original Oxo-Centered Frameworks in Bi3(VO4)O3 and Bi3.5(AsO4)(OH)0.5O3.5 by Supercritical Steam.

Two new bismuth compounds, oxovanadate Bi3(VO4)O3 and oxoarsenate Bi3.5(AsO4)(OH)0.5O3.5, were prepared using supercritical hydrothermal pressure. Dealing with the anionic sublattice, both crystal structures are built on anti-oxo-OBi4/OBi3 or -OBi4/OBi5 units connected together in infinite corrugated 2D layers surrounded by isolated XO4 (X = V or As) tetrahedra. These edifices complete a series initiated by the recent Bi3(PO4)O3 prepared under similar conditions. With the latter being assigned to the "simplest" bioxophosphate in terms of structural complexity, this aspect was investigated among the other compounds in their chemical ternaries. These phases are suggested to be high-pressure polymorphs, not possible to tackle when working at ambient pressure and temperature conditions.

Oxysulfide Ba5(VO2S2)2(S2)2 Combining Disulfide Channels and Mixed-Anion Tetrahedra and Its Third-Harmonic-Generation Properties.

Mixed-anion compounds are among the most promising systems to design functional materials with enhanced properties. In particular, heteroleptic environments around transition metals allow tuning of the polarity or band-gap engineering for instance. We present the original oxysulfide Ba5(VO2S2)2(S2)2, the fifth member in the quaternary system Ba-V-S-O. It exhibits the mixed-anion building units V5+O2S2 and isolated disulfide pairs (S2)2-. The structure is solved by combining single-crystal and powder X-ray diffraction and transmission electron microscopy. First-principles calculations were combined in order to highlight the anion roles. In particular, our density functional theory study shows that the 3p states of the disulfide pairs dictate the band gap. In this study, we point out anionic tools for band-gap engineering that can be useful for the design of phases for numerous applications. Finally, third harmonic generation (THG) was measured and compared to the large THG observed for Cu2O, which reveals the potential for nonlinear-optical properties that should be further investigated.

Magnetic frustration in the high-pressure Mn2MnTeO6 (Mn3TeO6-II) double perovskite.

A new double perovskite Mn2MnTeO6 has been obtained by high pressure phase transformation of a corundum-related precursor. It is antiferromagnetic below 36 K and develops a magnetic structure with magnetic moments of 4.8 µB and 3.8 µB for Mn2+ at the A and B sites respectively. This new polymorph accounts for a recently reported decrease in the bandgap of Mn3TeO6 under pressure that may lead to useful light-harvesting properties.