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Blue/turquoise crystals of Sr 2 Cr II (PO 4 ) 2 with prismatic shape and edge-length of up to 1 mm were obtained by a vapor-phase moderated solid-state reaction at 1273 K in sealed silica tubes. Its crystal structure was solved and refined from a triply twinned ("trilling") crystal [ Pbca (no. 61), Z = 12, a = 10.7064(6) Å, b = 9.2730(5) Å, c = 21.2720(7) Å, R 1 = 0.038]. Sr 2 Cr(PO 4 ) 2 belongs to the small family of inorganic solids containing divalent chromium, where the rare Cr 2+ ions are stabilized by the inductive effect of the phosphate groups. As expected from its d 4 ( S = 2) electronic configuration, the Jahn-Teller effect (JT) is prominent, leading for the two independent Cr 2+ ions to square-pyramidal Cr(1)O 4+1 and square-planar Cr(2)O 4 coordination within a 3D chromium phosphate network [Cr II 2 (PO 4 ) 4 ] 8 . Topologically, the Cr(1) and Cr(2) cations are arranged in separate alternating layers stacked along the c axis. In their respective layers, Cr(1) shows a gapped 2D topology and only weak interaction with the adjacent Cr(2) layers. However, below T N1 ∼11.3 K, Cr(1) orders antiferromagnetically into a noncollinear structure, leaving nearly paramagnetic Cr(2) idle spins, strongly frustrated by the Cr(1) moments of the next layers. On further cooling, below T N2 ∼3.6 K, the ordering of Cr(2) occurs via an additional magnetic irreducible representation, which splits the Cr(1) into Cr(1)a and Cr(1)b orbits, thus lifting the frustration on Cr(2). The corresponding P2 1 ca.29.99 magnetic space group forces a crystal symmetry lowering, plausibly signed by a change of the magnetostrictive coefficient from positive to negative below T N2 . The optical transitions observed for the JT d 4 ions are in good agreement with our crystal picture from the DFT calculations. A detailed analysis within the angular overlap model explains the surprisingly different d orbital splitting by the ligand field for the chromophores Cr(1)O 4+1 and Cr(2)O 4 .

Barium-cobaltate-based perovskite (BaCoO 3-δ ) and barium-cobaltate-based nanocomposites have been intensively studied in energy storage and conversion devices mainly due to flexible oxygen stoichiometry and tunable nonprecious transition metal oxidation states. Although a rich and complex family of structural polymorphs has already been reported for these perovskites in the literature, the potential structural evolution that may occur during the oxygen reduction reaction and the oxygen evolution reaction has not been investigated so far. In this study, we synthesized and characterized the lowest Co-oxidation state possible in the compound, BaCoO 2 , which exhibits a quartz-derived, trigonal structure with a helicoidally corner-sharing, CoO 4 -tetrahedral-framework as already proposed by Spitsbergen et al. Oxygen can reversibly be inserted in such a crystal structure to form BaCoO 3-δ , i.e., with 0 ≤ δ ≤ 1, based on the results of an in situ coupled thermogravimetric - neutron diffraction study and which presents therefore giant oxygen capacity storage due to the extreme tunability of the electronic configuration of the cobalt cations which defines the fundamental origins of the materials performance. The reversible conversion of BaCoO 2 to BaCoO 3-δ associated with a similar electronic conductivity above 900 K permits to clarify the high potential of BaCoO 3-δ -based energy storage and conversion devices.

Mixed anion oxyhalides with the formula Ca 2 MnO 3 X (X = Cl, Br) are synthesized using solid-state reaction methods. These two materials crystallize in a novel structure type due to the small ionic radius of Ca and the strong Jahn-Teller effect of Mn 3+ . The resulting structure (space group Cmcm ) contains one-dimensional chains of MnO 4 square planes, with an angle of ∼120° between neighboring planes. At low temperatures, the two materials adopt magnetic arrangements, with ferromagnetic chains coupled antiferromagnetically. On applying a magnetic field, both materials experience spin-flop transitions.

The layered-ruthenate family of materials possess an intricate interplay of structural, electronic and magnetic degrees of freedom that yields a plethora of delicately balanced ground states. This is exemplified by Ca 3 Ru 2 O 7 , which hosts a coupled transition in which the lattice parameters jump, the Fermi surface partially gaps and the spins undergo a 90 ∘ in-plane reorientation. Here, we show how the transition is driven by a lattice strain that tunes the electronic bandwidth. We apply uniaxial stress to single crystals of Ca 3 Ru 2 O 7 , using neutron and resonant x-ray scattering to simultaneously probe the structural and magnetic responses. These measurements demonstrate that the transition can be driven by externally induced strain, stimulating the development of a theoretical model in which an internal strain is generated self-consistently to lower the electronic energy. We understand the strain to act by modifying tilts and rotations of the RuO 6 octahedra, which directly influences the nearest-neighbour hopping. Our results offer a blueprint for uncovering the driving force behind coupled phase transitions, as well as a route to controlling them., (© 2023. Springer Nature Limited.)

BaCoX 2 O 7 (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 BaCoAs 2 O 7 and BaCoP 2 O 7 , 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.