Michael W. Lufaso

Materials showing negative thermal expansion (NTE) are technologically important and a few transition-metal oxides with the perovskite structure show large NTE-like volume changes due to intermetallic charge transfer. Here we show that the observed volume changes are primarily caused by the size effect of the constituent ions in the oxides. Not only the NTE-like behaviors but also the positive volume change due to the intermetallic charge transfer can be qualitatively reproduced with a simple ionic crystal model by using the SPuDS software program. The results demonstrate that materials showing NTE-like behavior due to intermetallic charge transfer can be strategically designed.

Particles of formula Rb_0.24Co[Fe(CN)₆]_0.74@K_0.10Co[Cr(CN)₆]_0.70·nH₂O with a light-responsive rubidium cobalt hexacyanoferrate (RbCoFe) core and a magnetic potassium cobalt hexacyanochromate (KCoCr) shell have been prepared and exhibit light-induced changes in the magnetization of the normally light-insensitive KCoCr shell, a new property resulting from the synergy between the core and shell of a coordination polymer heterostructure. A single batch of 135 ± 12 nm RbCoFe particles are used as seeds to generate three different [email protected] samples, with KCoCr shell thicknesses of approximately 11, 23 and 37 nm, to probe the influence of the shell thickness over the particles’ morphology and structural and magnetic properties. Synchrotron powder X-ray diffraction reveals that structural changes in the shell accompany the charge transfer induced spin transition (CTIST) of the core, giving direct evidence that the photomagnetic response of the shell is magnetomechanical in origin. The depth to which the KCoCr shell contributes to changes in magnetization is estimated to be approximately 24 nm when using a model that assumes a constant magnetic response of the core within the series of particles. In turn, the presence of the shell changes the nature of the CTIST of the core. As opposed to the usually observed first order transition exhibiting hysteresis, the CTIST becomes continuous in the [email protected] particles.

We present an electron diffraction study of three sillenites, Bi12SiO20, Bi25FeO39, and Bi25InO39 synthesized using the solid-state method. We explore a hypothesis, inspired by optical studies in the literature, that suggests that trivalent sillenites have additional disorder not present in the tetravalent compounds. Electron diffraction patterns of Bi25FeO39 and Bi25InO39 show streaks that confirm deviations from the ideal sillenite structure. Multi-slice simulations of electron-diffraction patterns are presented for different perturbations to the sillenite structure - partial substitution of the M site by Bi3+, random and ordered oxygen-vacancies, and a frozen-phonon model. Although comparison of experimental data to simulations cannot be conclusive, we consider the streaks as evidence of short-range ordered oxygen-vacancies.

A crossover from the A-site-ordered double-perovskite structure with Im3̅ cubic symmetry to the simple-perovskite structure with Pnma orthorhombic symmetry is found in LnMn₃V₄O₁₂ (Ln = La, Nd, Gd, Y, Lu) synthesized under high-pressure conditions. Relatively large Ln³⁺ ions (La³⁺, Nd³⁺, and Gd³⁺) induce the a⁺a⁺a⁺ in-phase cooperative tilting of the VO₆ octahedra, resulting in the A-site-ordered double-perovskite structure with chemical composition Ln³⁺Mn²⁺₃V^3.75+₄O₁₂. Compounds with small Ln³⁺ ions like Y³⁺ and Lu³⁺, on the other hand, crystallize with the Pnma simple-perovskite structure with chemical composition (Ln³⁺_1/4Mn²⁺_3/4)V^3.75+O₃, where the Ln³⁺ and Mn²⁺ ions are disordered at the A site. The random distribution of the small A-site cation induces the a^–b⁺a^– tilting distortion of the VO₆ octahedra. The observed phase crossover is well explained by the structural stability calculation based on the bond-valence-sum model, and the most stable crystal structure gives the smallest unit-cell volume. This A-site-cation size-dependent phase transition between the A-site-ordered double-perovskite and A-site-disordered simple-perovskite structures in LnMn₃V₄O₁₂ is thus a result of the structural stability due to the cooperative tilting of the VO₆ octahedra. The Mn²⁺ ions at the A′(A) site contribute local magnetic moments, whereas the V^3.75+ ions at the B site play a role in metallic conduction. The observed magnetic behaviors are consistent with the order–disorder distribution of the Mn²⁺ ions at the A site, antiferromagnetism in the A-site-ordered double perovskites, and magnetic spin glass in the A-site-disordered simple perovskites.