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.

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.

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.

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 T_N ≈ 21.5 K in the Ni²⁺ case. For comparison the Mn²⁺ compound develops a nearly collinear spin (canted) ordering below T_N ≈ 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/K_b ≈ 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 E_g 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 (T_N1 = 58 K) that reorientates (42 K) and further splits (T_N2 = 23.5 K) creating a coexisting helix. Increasing Co content within the Mn_3-xCo_xTeO₆ 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.

High pressure exploration in the Li-Ln-V-O system

Using in situ high pressure Raman spectroscopy, two structural changes were observed in a sample of the composition LiLa5O5(VO4)2. Taking this into account and by combining different conditions, three new compounds were further obtained from high pressure-high temperature synthesis. Their crystal structure description was done using the antiphase approach, which implies the presence of oxygen-centered [OLn4] building units, where Ln is La for (1) β-LiLa5O5(VO4)2 and (2) β-LiLa2O2(VO4) or Nd for (3) LiNd5O5(VO4)2 compounds. (1) crystallizes in the triclinic space group P1[combining macron] with unit cell parameters of a = 5.8167(15) Å, b = 12.2954(28) Å, c = 18.7221(69) Å, α = 102.03(2)°, β = 98.76(2)°, and γ = 103.54(2)°; a 3D structure was deduced from the ambient pressure polymorph. (2) also crystallizes in P1[combining macron] with a = 5.8144(7) Å, b = 5.8167(7) Å, c = 8.5272(1) Å, α = 98.184(7)°, β = 100.662(7)° and γ = 92.579(7)°. It shows a 2D structure with [La2O2]2+ layers surrounded by [LiO4] and [VO4] tetrahedra sharing corners and edges. (3) exhibits a 3D architecture isotypic with AP-LiLa5O5(VO4)2. The crucial role of high pressure in such types of synthesis and materials is also discussed.

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 T_N1 = 88 K and T_N2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below T_N1 and T_N2, respectively. A complementary description with 14H-(hhccccc)₂ sequence of only Mn octahedra provides a direct comparison with BaMnO_3-δ hexagonal perovskites and naturally explains the AFM order. Below T_N2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

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

Magnetic frustration and bandgap engineering in the high pressure Mn₂MnTeO₆ perovskite.

Ferri- and ferro-magnetism in CaMnMReO6 double double perovskites of late transition metals M = Co and Ni

Doubly ordered perovskites are reported for CaMnMReO₆ (M = Co, Ni). CaMnNiReO₆ is a remarkable example of a ferromagnetic oxide with four distinct spin sublattices all collinearly ordered below T_C = 152 K.

Structural percolation in the PbM(1-x)M(x)'O(3) (M, M' =Ti, Cr, and V) perovskites

Structural properties and the influence of d electrons' insertion in PbTiO(3) have been determined in the study of PbM(1-x)M(x)'O(3) (M, M' = Ti, Cr, and V) solid solutions by means of X-ray diffraction, high-resolution transmission electron microscopy, magnetization measurements, and strain mapping analysis. PbTi(1-x)V(x)O(3) is the only system that preserves the same space group (P4mm) for all x, whereas PbTi(1-x)Cr(x)O(3) and PbV(1-x)Cr(x)O(3) change to cubic (Pm ̅3m) at x = 0.30 and 0.4, respectively. These values have been related with the percolation threshold for a cubic net (P(c) = 0.31). The microscopy study coincides with the X-ray diffraction determination, and neither supercell nor short-range order maxima are observed. However, for x ≥ 0.7 in PbTi(1-x)Cr(x)O(3) the presence of modulated zones is observed in both the electron diffraction pattern as well as high-resolution transmission electron micrographs, as is typical for PbCrO(3). (1) Furthermore, the tetragonal region in PbV(1-x)Cr(x)O(3) suffers a great stress because of the contrast of [Cr-O(6)] octahedra and [V-O(5)] square-based pyramids end members basic units.