Search

International audience; We report bulk dilatometry and diffraction data through the T MIT = 427 K metal-insulator transition of the n = 3 member of the V n O 2n-1 Magnéli series. Besides VO 2 , V 3 O 5 is the only other vanadium oxide with a potentially useful MIT transition above room temperature. A narrow (ΔT = 10 K) abrupt negative thermal expansion of α L =-21.4 x 10-6 (dilatometry) and α V =-213 x 10-6 K-1 (crystallographic) is observed. We argue that the combination of the MIT along with the simultaneous vanadium charge ordering are responsible for such large values. The low temperature magnetic properties are also clarified and neutron diffraction measurements show a k = [½ ½ 0] magnetic structure at 1.5 K. DFT calculations of the exchange interactions support the low dimensionality and allow modelling the magnetic susceptibility.

Mixed-valent transition-metal compounds display complex structural, electronic and magnetic properties, which often intricately coexist. Here, we report the new ternary oxide GaV

The first triple perovskite with Mn in A- and 1 : 2 B-site order Mn

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.

International audience; 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.

International audience; 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.

The first triple perovskite with Mn in A- and 1 : 2 B-site order Mn3MnNb2O9, prepared using high pressure phase transformation of the magnetodielectric Mn4MnNb2O9, 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.

International audience; Mn11Ta4O21 is presented as the first Hexagonal A-site Manganite. Based on simple rules, the structure is compatible with a 14Hlayer (cchchch)2 stacking sequence; related to BaVO3 and BaCrO3 high pressure polymorphs. The A-site over-stoichiometry 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.

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.