Giant caloric effects in charge-spin-lattice coupled transition-metal oxides

The caloric effects of solids can provide us with highly efficient and environmentally friendly energy systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Typical solid caloric effects are magnetocaloric, electrocaloric, and barocaloric effects induced respectively by magnetic fields, electric fields, and pressure, and materials showing large caloric responses through the effects attract lots of recent attention. In this perspective article, novel transition-metal oxides that show giant caloric effects are highlighted. The compounds are NdCu₃Fe₄O₁₂ and BiCu₃Cr₄O₁₂ containing unusually high valence states of transition-metal ions and show charge transitions to relieve the electronic instabilities. The charge, spin, and lattice degrees of freedom in the compounds are strongly correlated and the primarily induced charge transitions cause unusual first-order magnetic phase transitions that provide significant latent heat. Importantly, the large latent heat can be utilized through the caloric effects in multiple ways. The details of the giant caloric effects in the charge–spin–lattice coupled transition-metal oxides are summarized and the mechanism of the effects is discussed

Thermal properties and phase transition behaviors of possible caloric materials Bi₀.₉₅Ln₀.₀₅NiO₃

Thermal properties and phase transition behaviors of possible caloric materials Bi₀.₉₅Ln₀.₀₅NiO₃ (Ln = La, Nd, Sm, Eu, Gd, Dy), which show intersite charge transfer between Bi and Ni ions, were investigated. Although a few of the compounds showed large latent heats at the intersite-charge-transfer transition temperatures, the values are not comparable to that observed in the giant caloric effect compound NdCu₃Fe₄O₁₂. In the present Bi₀.₉₅Ln₀.₀₅NiO₃, contrary to our expectation, the magnetic transitions of Ni²⁺ spins are not induced by the intersite-charge-transfer transitions and the magnetic entropy changes do not contribute to the latent heat produced by the intersite-charge-transfer transitions. To obtain giant caloric effects, materials for which the “intrinsic” magnetic transition temperatures are much higher than the charge-transfer-transition temperatures may be needed

Beneficial effects of nasal high flow oxygen therapy after weaning from non-invasive ventilation: A prospective observational study

It remains unknown whether application of nasal high flow (NHF) is effective after liberation from non-invasive ventilation (NIV). This study was aimed at investigating the effect of NHF in patients ready for weaning from NIV. With institutional ethic committee approval, patients receiving NIV due to hypoxemic respiratory failure for more than 24 hours were enrolled. After passing the weaning criteria with continuous positive airway pressure (CPAP) mode [fraction of inspiratory oxygen (FIO2) ≦0.5, positive end expiratory pressure (PEEP) 4 cmH2O], patients received NHF (Flow 50 L/min, FIO2 ≦0.5) immediately after liberation from NIV. Before the initiation of the study, eight sequential patients who received oxygen via face mask after NIV treatment, served as the historical control. Respiratory parameters [partial pressure of arterial oxygen (PaO2) to FIO2 ratio (P/F ratio), respiratory rate (RR)] 1 hour after liberation from NIV were evaluated with those during NIV as the primary outcome. The frequency of rescue NIV therapy, intubation, and respiratory failure were also recorded. Nine eligible patients received NHF therapy after liberation from NIV. P/F ratio and RR did not change significantly compared with those during NIV (231 ± 43.6 versus 250.7 ± 34.2 mmHg, 20.8 ± 2.3 versus 21 ± 1.6 /min), while P/F ratio decreased significantly in the historical control group (194.3 ± 20.1 versus 255.9 ± 58.1 mmHg, p=0.013). Rescue NIV therapy, intubation, and respiratory failure never occurred in the NFH group, although two patients received NIV rescue therapy, of whom one was intubated in the historical control. NHF after liberation from NIV might be effective in patients recovering from hypoxemic respiratory failure

Giant multiple caloric effects in charge transition ferrimagnet

磁場と圧力でマルチに冷却可能な酸化物新材料 --フェリ磁性電荷転移酸化物におけるマルチ熱量効果の実証--. 京都大学プレスリリース. 2021-06-22.Caloric effects of solids can provide us with innovative refrigeration systems more efficient and environment-friendly than the widely-used conventional vapor-compression cooling systems. Exploring novel caloric materials is challenging but critically important in developing future technologies. Here we discovered that the quadruple perovskite structure ferrimagnet BiCu₃Cr₄O₁₂ shows large multiple caloric effects at the first-order charge transition occurring around 190 K. Large latent heat and the corresponding isothermal entropy change, 28.2 J K⁻¹ kg⁻¹, can be utilized by applying both magnetic fields (a magnetocaloric effect) and pressure (a barocaloric effect). Adiabatic temperature changes reach 3.9 K for the 50 kOe magnetic field and 4.8 K for the 4.9 kbar pressure, and thus highly efficient thermal controls are achieved in multiple ways

Oxygen Release and Incorporation Behaviors in BaFeO₃ Polymorphs with Unusually High-Valence Fe⁴⁺

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 °C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions

Oxygen Release and Incorporation Behaviors in BaFeO₃ Polymorphs with Unusually High-Valence Fe⁴⁺

Fully oxygenated perovskite BaFeO3 containing unusually high-valence Fe4+ shows three crystal polymorphs with the same chemical composition. The 3C-type BaFeO3 has a simple cubic perovskite structure consisting of corner-sharing FeO6 octahedra, while the 6H- and 12R-type BaFeO3 have hexagonal perovskite structures consisting of both corner-sharing and face-sharing FeO6 octahedra. The compounds readily release oxygen into the air to reduce the high-valence state of the Fe ions, but the oxygen release behaviors strongly depend on the crystal structure. The 3C-type BaFeO3 releases oxygen topotactically from the corner-shared sites of the FeO6 octahedra at a temperature as low as 130 °C. In contrast, the 6H- and 12R-type BaFeO3 preferentially release oxygen from the face-shared sites above 320 and 460 °C, respectively, although they include the corner-shared sites in the crystal structures. The resultant oxygen-deficient 3C-type BaFeO2.5 does not incorporate back oxygen in air, whereas the 12R-type hexagonal structure shows completely reversible oxygen release and incorporation in air. Once the 12R-type structure is established, unusually high-valence states such as Fe4+ can be stabilized without extreme conditions