Patrick M. Woodward

Recent studies have revealed metal-to-metal and intervalence-charge transfer transitions in cation ordered 6H perovskites with Cs3M+M2+M3+Cl9 stoichiometry. The structure and properties of two new members of this family, Cs3NaMnVCl9 and Cs3NaMnCrCl9, are reported here. In both compounds Na+ ions occupy sites in octahedra that share only corners, electronically isolating face-sharing M2Cl9 bioctahedral clusters. Analysis of synchrotron X-ray and neutron powder diffraction patterns reveals no sign of long-range ordering of the Mn2+ and V3+/Cr3+ ions, suggesting a random orientation of bimetallic Mn2+M3+Cl94– clusters. Magnetometry studies reveal paramagnetic behavior in both compounds, with negative Weiss temperatures and effective moments that closely align with expected values based on spin-only moments. No sign of long-range magnetic order is observed in either compound down to at least 2 K. The optical absorption spectra feature d-to-d transitions, ligand-to-metal charge transfer (LMCT), and metal-to-metal charge transfer (MMCT) transitions. Cs3NaMnVCl9 exhibits a Mn2+ to V3+ MMCT transition at ∼1390 nm (∼0.9 eV), whereas the comparable MMCT transition in Cs3NaMnCrCl9 falls at a much higher energy and cannot be resolved due to overlap with the LMCT transition. The striking difference in MMCT energy can be attributed to differences in the electron configuration of V3+ (t2g2eg0) and Cr3+ (t2g3eg0).

The structures of seven new hybrid cadmium thiocyanate halides are reported. The inclusion of terminal halide ions act as molecular “scissors” breaking up the three-dimensional structure of Cd(SCN) ₂ into one-dimensional chains or ribbons.

Despite numerous reports of an insulating ferromagnetic state in epitaxial LaCoO₃ thin films, no consensus has been reached on the details of ferromagnetism in these films. To better understand the origins of magnetic order in such films, stoichiometric LaCoO₃ films have been deposited on SrTiO₃(001) and LaAlO₃(001) substrates using off-axis sputtering. This technique allows growth to occur in conditions that minimize deviations from the ideal stoichiometry. SQUID magnetometry shows that ferromagnetism is stabilized only in films grown under tensile strain on SrTiO₃. The magnetic properties of these films (T _C ≈ 70 K, M _sat ≈ 0.3 μ_B/Co, and H _C ≈ 5 kOe) are essentially independent of thickness, consistent with nearly uniform magnetization. At room temperature, strain induced by the SrTiO₃ substrate breaks the rhombohedral symmetry of the bulk structure, leading to a ^– a ^– c ⁰ octahedral tilting and an anisotropic distortion of the Co-centered octahedra. Low-temperature (T = 36 K) X-ray absorption spectroscopy reveals that tensile strain inherent to the SrTiO₃ substrate stabilizes a substantial fraction of high- or intermediate-spin Co³⁺ ions, facilitating magnetic order, whereas films grown on LaAlO₃ are made up nearly entirely of low-spin Co³⁺ ions.

To investigate the scope of ferroelectric behavior in La-substituted BiFeO3 films, LaxBi1−xFeO3 epitaxial films were synthesized using off-axis co-sputtering on SrTiO3(001) and DyScO3(110) substrates with a SrRuO3 bottom electrode layer. A digital-doping deposition method was used to enable precise control and continuous tuning of La concentration in high-quality LaxBi1−xFeO3 films across a wide range of x = 0.05–0.60, which was systematically investigated using piezoresponse force microscopy. Robust and reversible out-of-plane ferroelectric switching has been observed up to x = 0.35, while films with x ≥ 0.37 exhibit no measurable ferroelectric behavior, indicating a sharp ferroelectric-to-paraelectric phase transition between x = 0.35 and 0.37. This represents the highest reported La concentration in LaxBi1−xFeO3 films that retains ferroelectric ordering, highlighting opportunities to engineer ferroelectric and multiferroic properties in complex oxide heterostructures.

Organic-inorganic hybrid thiocyanates include a variety of compositions and structure types. To develop a better understanding of the interactions that control the crystal structure in this family of materials, six hybrid thiocyanate halide compounds with the general formula A₂Cd(SCN)₂X₂ (A = CH₃NH₃⁺, CH₃CH₂NH₃⁺, CH₃(CH₂)₂NH₃⁺, CH₃(CH₂)₃NH₃⁺; X = Cl^-, Br^-) have been synthesized. Single crystal X-ray diffraction shows that five of the six compounds crystallize with triclinic P1̅ symmetry, the lone exception being (CH₃(CH₂)₃NH₃)₂Cd(SCN)₂Cl₂ which adopts P2₁/c symmetry. Thermal analysis reveals decomposition temperatures between 180 and 210 °C. The intermediate hard-soft acidic nature of Cd²⁺ favors bridging thiocyanate groups and lowers the dimensionality compared to their Pb²⁺ and Sn²⁺ analogues, which adopt two-dimensional crystal structures with bridging halide ions. The presence of bridging thiocyanate groups lead to one-dimensional chains of [Cd(SCN)₂X^₂]²⁺ octahedra. Hydrogen bonding interactions between terminal halide ions and the amine headgroup of the organic cation dictate the crystal packing and play a key role in determining the thermal stability of these compounds. Diffuse reflectance measurements reveal large band gaps, ranging from 3.7 to 4.4 eV, and electronic structure calculations reveal narrow bands, with dispersions on the order of 0.5 eV. These attributes are likely to be characteristic of compounds with bridging thiocyanate linkers.

Polycrystalline samples of Ba₃Cr₂ReO₉ and Ba₂CrReO₆ were synthesized using solid-state methods and characterized using laboratory X-ray powder diffraction (XRPD), magnetometry, specific heat, and variable temperature neutron powder diffraction (NPD). Ba₃Cr₂ReO₉ and Ba₂CrReO₆ both crystallize in the 6H hexagonal structure with space group symmetry P6₃/mmc. The transition metal ions in Ba₃Cr₂ReO₉ are ordered with Re⁶⁺ ions preferentially occupying the octahedra that share corners, and Cr³⁺ ions preferentially occupying sites in the face-sharing bioctahedra. Though there is a small amount of antisite mixing between the two ions. Ba₂CrReO₆ adopts a very similar crystal structure with a substantially higher fraction of Re⁵⁺ in the face-sharing bioctahedra. Both Ba₃Cr₂ReO₉ and Ba₂CrReO₆ exhibit long-range antiferromagnetic ordering. The magnetic structures feature antiferromagnetic Re-O-Cr superexchange coupling through shared octahedral corners and antiferromagnetic Cr-Cr (or Cr-Re) direct exchange interactions across shared octahedral faces. The refined Cr³⁺ magnetic moments are typical of values seen in other oxides, suggesting that overlap of Cr 3d orbitals is not sufficient to form metal-metal bonds that lead to molecular orbitals that span the Cr₂O₉ bioctahedron.

The hexagonal perovskites Cs₃NaFe₂Cl₉ and Cs₃NaMnFeCl₉ have been synthesized and investigated. Both compounds adopt the 6H perovskite structure with P6₃/mmc symmetry. This structure consists of dimers of face-sharing octahedra arranged on the vertices of a triangular network. The transition metal ions occupy sites in these octahedra, leading to Fe₂Cl₉^4- and FeMnCl₉^4- bioctahedra, respectively. The bioctahedral clusters are sandwiched by layers of corner-sharing octahedra occupied by Na⁺ cations. Diffuse reflectance spectroscopy reveals optical transitions that arise from metal-to-metal charge transfer (Cs₃NaMnFeCl₉) and intervalence charge transfer (Cs₃NaFe₂Cl₉) excitations. In Cs₃NaFe₂Cl₉, magnetic susceptibility measurements reveal local ferromagnetic coupling (θ_CW = 16.7 K), mediated by the rapid exchange of an electron between the iron sites within each dimer. In contrast, the magnetic coupling between Fe³⁺ and Mn²⁺ in Cs₃NaMnFeCl₉ is antiferromagnetic (θ_CW = -41.4 K). At 100 K, the Mössbauer spectrum is dominated by a single type of iron that corresponds to Fe^2.5+, signaling electron exchange between iron sites that is faster than the Mössbauer time scale (∼100 ns). Upon further cooling, the Fe^2.5+ signal gives way to a 1:1 ratio of Fe²⁺ and Fe³⁺, as the thermally activated hopping slows down.

Three new hexagonal perovskites with Cs3M+M2+RhCl9 (M+ = Na+, Ag+; M2+ = Mn2+, Fe2+) stoichiometry have been synthesized from solution precipitation reactions. These air-stable compounds crystallize as triply cation-ordered variants of the 6H perovskite structure. This structure contains octahedra that share a common face to form M2+RhCl9 4- dimers that are arranged on a two-dimensional triangular network. M+ cations reside in octahedral holes located between dimer layers and connect M2+RhCl9 4- dimers through corner-sharing linkages. The cation sites in the corner-sharing layers are fully occupied by Na+/Ag+, except for Cs3AgMnRhCl9, where a small amount of Ag+/Mn2+ antisite disorder is observed. The M2+RhCl9 4- dimers adopt an ordered configuration that lowers the symmetry from P63/mmc to P63 mc. Optical absorption in the near ultraviolet (UV) and visible regions is dominated by Rh d-to-d transitions and metal-to-metal charge transfer transitions. Variable temperature magnetic susceptibility measurements show no sign of long-range magnetic order down to 2 K. Curie-Weiss fits reveal relatively weak antiferromagnetic interactions between M2+ ions, with Weiss constants that range from -2.3 to -5.4 K. The strength of the antiferromagnetic interactions between M2+ ions increases as the covalency of the M2+-Cl bond increases. The ordering of three different cations in the 6H perovskite structure diminishes magnetic coupling between layers, leading to a new family of two-dimensional triangular lattice antiferromagnets.