Experimental heat capacities, excess entropies, and magnetic properties of bulk and nano Fe3O4-Co3O4 and Fe3O4-Mn3O4 spinel solid solutions

An Unexpected Decrease in Vibrational Entropy of Multicomponent Rutile Oxides

High-entropy oxides (HEOs), featuring infinite chemical composition and exceptional physicochemical properties, are attracting much attention. The configurational entropy caused by a component disorder of HEOs is popularly believed to be the main driving force for thermal stability, while the role of vibrational entropy in the thermodynamic landscape has been neglected. In this study, we systematically investigated the vibrational entropy of multicomponent rutile oxides (including Fe0.5Ta0.5O2, Fe0.333Ti0.333Ta0.333O2, Fe0.25Ti0.25Ta0.25Sn0.25O2, and Fe0.21Ti0.21Ta0.21Sn0.21Ge0.16O2) by precise heat capacity measurements. It is found that vibrational entropy gradually decreases with increasing component disorder, beyond what one could expect from an equilibrium thermodynamics perspective. Moreover, all multicomponent rutile oxides exhibit a positive excess vibrational entropy at 298.15 K. Upon examinations of configuration disorder, size mismatch, phase transition, and polyhedral distortions, we demonstrate that the excess vibrational entropy plays a pivotal role in lowering the crystallization temperature of multicomponent rutile oxides. These findings represent the first experimental confirmation of the role of lattice vibrations in the thermodynamic landscape of rutile HEOs. In particular, vibrational entropy could serve as a novel descriptor to guide the predictive design of multicomponent oxide materials.

Vacancy-Induced Temperature-Dependent Thermal and Magnetic Properties of Holmium-Substituted Bismuth Ferrite Nanoparticle Compacts

Multiferroics have gained widespread acceptance for room-temperature applications such as in spintronics, ferroelectric random access memory, and transistors because of their intrinsic magnetic and ferroelectric coupling. However, a comprehensive study, establishing a correlation between the magnetic and thermal transport properties of multiferroics, is still missing from the literature. To fill the void, this work reports the temperature-dependent thermal and magnetic properties of holmium-substituted bismuth ferrite (BiFeO₃) and their dependencies on oxygen vacancies and structural modifications. Two distinct magnetic transitions on temperature-dependent magnetic and heat capacity responses are identified. Experimental analysis suggests that the excess of oxygen vacancies shifts the magnetic transition temperature by ∼64 K. The holmium substitution-induced structural modification increases BiFeO₃ heat capacity by 30% up to the antiferromagnetic phase transition temperature. Furthermore, an unsaturated heat capacity even at temperatures as high as 850 K is observed and is ascribed to anharmonicity and partial densification of the nanoparticles used during heat capacity measurements. The room-temperature thermal conductivity of BiFeO₃ is ∼0.33 ± 0.11 W m^-1 K^-1 and remains unchanged at high temperatures due to defect scattering from porosities.

Highly dispersed copper cobalt oxide nanoclusters decorated carbon nitride with efficient heterogeneous interfaces for enhanced H2 evolution

Photocatalytic reaction refers to a sophisticated heterogeneous catalyzing process. Exploring the interfacial reaction of catalysts will provide insights into efficient artificial photosynthetic system and promote its design. In this study, highly dispersed bimetallic CuCo₂O₄ nanoclusters decorated g-C₃N₄ heterojunction photocatalyst was produced by in-situ deposition of 0D CuCo₂O₄ spinel on the 2D g-C₃N₄ surface. Compared with CuO or Co₃O₄ modified g-C₃N₄, the optimal composite exhibits a significantly higher H₂ evolution rate of 4187.6 μmol∙g_cat^-1∙h^-1 with an apparent quantum yield (AQY) of 4.57% under the irradiation of monochromatic light (400 ± 7.5 nm) in the absence of noble metal. As suggested from the results of the photoelectrochemistry characterizations and NH₃-temperature programmed desorption (NH₃-TPD) analysis, CuCo₂O₄/g-C₃N₄ exhibited faster HER kinetics and considerable surface acidity sites, and it facilitated triethanolamine (TEOA) chemisorption and H₂ evolution, further highlighting the merits of such mixed-metal compounds. Moreover, the transfer pathway of charge carriers between CuCo₂O₄ and g-C₃N₄ heterogeneous interface was demonstrated by photo-degradation of RhB and selective photo-deposition Pt nanoparticles.