Existence of Ti²⁺ States on the Surface of Heavily Reduced SrTiO₃ Nanocubes

Existence of Ti²⁺ States on the Surface of Heavily Reduced SrTiO₃ Nanocube

Strongly Exchange Coupled Core|Shell Nanoparticles with High Magnetic Anisotropy: A Strategy toward Rare-Earth-Free Permanent Magnets

Antiferromagnetic­(AFM)|ferrimagnetic­(FiM) core|shell (CS) nanoparticles (NPs) of formula Co_0.3Fe_0.7O|Co_0.6Fe_2.4O₄ with mean diameter from 6 to 18 nm have been synthesized through a one-pot thermal decomposition process. The CS structure has been generated by topotaxial oxidation of the core region, leading to the formation of a highly monodisperse single inverted AFM|FiM CS system with variable AFM-core diameter and constant FiM-shell thickness (∼2 nm). The sharp interface, the high structural matching between both phases, and the good crystallinity of the AFM material have been structurally demonstrated and are corroborated by the robust exchange-coupling between AFM and FiM phases, which gives rise to one among the largest exchange bias (<i>H</i> _E) values ever reported for CS NPs (8.6 kOe) and to a strongly enhanced coercive field (<i>H</i> _C). In addition, the investigation of the magnetic properties as a function of the AFM-core size (<i>d</i> _AFM), revealed a nonmonotonous trend of both <i>H</i> _C and <i>H</i> _E, which display a maximum value for <i>d</i> _AFM = 5 nm (19.3 and 8.6 kOe, respectively). These properties induce a huge improvement of the capability of storing energy of the material, a result which suggests that the combination of highly anisotropic AFM|FiM materials can be an efficient strategy toward the realization of novel rare-earth-free permanent magnets

Na_2/7Gd_4/7MoO₄: a Modulated Scheelite-Type Structure and Conductivity Properties

Scheelite-type compounds with the general formula (A1,A2)_<i>n</i>[(B1,B2)­O₄]_<i>m</i> (²/₃ ≤ <i>n</i>/<i>m</i> ≤ ³/₂) are the subject of large interest owing to their stability, relatively simple preparation, and optical properties. The creation of cation vacancies (□) in the scheelite-type framework and the ordering of A cations and vacancies can be a new factor in controlling the scheelite-type structure and properties. For a long time, cation-deficient Nd³⁺:M_2/7Gd_4/7□_1/7MoO₄ (M = Li, Na) compounds were considered as potential lasers with diode pumping. They have a defect scheelite-type 3D structure (space group <i>I</i>4₁/<i>a</i>) with a random distribution of Li⁺(Na⁺), Gd³⁺, and vacancies in the crystal. A Na_2/7Gd_4/7MoO₄ single crystal with scheelite-type structure has been grown by the Czochralski method. Transmission electron microscopy revealed that Na_2/7Gd_4/7MoO₄ has a (3 + 2)­D incommensurately modulated structure. The (3 + 2)­D incommensurately modulated scheelite-type cation-deficient structure of Na_2/7Gd_4/7MoO₄ [super space group <i>I</i>4̅ (α–β0,βα0)­00] has been solved from single-crystal diffraction data. The solution of the (3 + 2)­D incommensurately modulated structure revealed the partially disordered distribution of vacancies and Na and Gd cations. High-temperature conductivity measurements performed along the [100] and [001] orientation of the single crystal revealed that the conductivity of Na_2/7Gd_4/7MoO₄ at <i>T</i> = 973 K equals σ = 1.13 × 10^–5 Ω^–1 cm^–1

Na_2/7Gd_4/7MoO₄: a Modulated Scheelite-Type Structure and Conductivity Properties

Scheelite-type compounds with the general formula (A1,A2)_<i>n</i>[(B1,B2)­O₄]_<i>m</i> (²/₃ ≤ <i>n</i>/<i>m</i> ≤ ³/₂) are the subject of large interest owing to their stability, relatively simple preparation, and optical properties. The creation of cation vacancies (□) in the scheelite-type framework and the ordering of A cations and vacancies can be a new factor in controlling the scheelite-type structure and properties. For a long time, cation-deficient Nd³⁺:M_2/7Gd_4/7□_1/7MoO₄ (M = Li, Na) compounds were considered as potential lasers with diode pumping. They have a defect scheelite-type 3D structure (space group <i>I</i>4₁/<i>a</i>) with a random distribution of Li⁺(Na⁺), Gd³⁺, and vacancies in the crystal. A Na_2/7Gd_4/7MoO₄ single crystal with scheelite-type structure has been grown by the Czochralski method. Transmission electron microscopy revealed that Na_2/7Gd_4/7MoO₄ has a (3 + 2)­D incommensurately modulated structure. The (3 + 2)­D incommensurately modulated scheelite-type cation-deficient structure of Na_2/7Gd_4/7MoO₄ [super space group <i>I</i>4̅ (α–β0,βα0)­00] has been solved from single-crystal diffraction data. The solution of the (3 + 2)­D incommensurately modulated structure revealed the partially disordered distribution of vacancies and Na and Gd cations. High-temperature conductivity measurements performed along the [100] and [001] orientation of the single crystal revealed that the conductivity of Na_2/7Gd_4/7MoO₄ at <i>T</i> = 973 K equals σ = 1.13 × 10^–5 Ω^–1 cm^–1

Structural and Magnetic Phase Transitions in the A_<i>n</i>B_<i>n</i>O_{3<i>n</i>–2} Anion-Deficient Perovskites Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆

Novel anion-deficient perovskite-based ferrites Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ were synthesized by solid-state reaction in air. Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ belong to the perovskite-based A_<i>n</i>B_<i>n</i>O_{3<i>n</i>–2} homologous series with <i>n</i> = 5 and 6, respectively, with a unit cell related to the perovskite subcell <i>a</i>_p as <i>a</i>_p√2 × <i>a</i>_p × <i>na</i>_p√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]_p(1̅01)_p crystallographic shear (CS) planes. The CS operation results in (1̅01)_p-shaped perovskite blocks with a thickness of (<i>n</i> – 2) FeO₆ octahedra connected to each other through double chains of edge-sharing FeO₅ distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above <i>T</i> ≈ 750 K for Pb₂Ba₂BiFe₅O₁₃ and <i>T</i> ≈ 400 K for Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb₂Ba₂BiFe₅O₁₃ and Pb_1.5Ba_2.5Bi₂Fe₆O₁₆ undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe–Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all <i>n</i> = 4, 5, 6 A_<i>n</i>Fe_<i>n</i>O_{3<i>n</i>–2} (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623–632 K