Investigation of the First-Order Phase Transition in the Co1−xMgxMoO4Solid Solution and Discussion of the Associated Thermochromic Behavior

Tilting and Distortion in Rutile-Related Mixed Metal Ternary Uranium Oxides: A Structural, Spectroscopic, and Theoretical Investigation

A systematic investigation examining the origins of structural distortions in rutile-related ternary uranium AUO₄ oxides using a combination of high-resolution structural and spectroscopic measurements supported by ab initio calculations is presented. The structures of β-CdUO₄, MnUO₄, CoUO₄, and MgUO₄ are determined at high precision by using a combination of neutron powder diffraction (NPD) and synchrotron X-ray powder diffraction (S-XRD) or single crystal X-ray diffraction. The structure of β-CdUO₄ is best described by space group Cmmm whereas MnUO₄, CoUO₄, and MgUO₄ are described by the lower symmetry Ibmm space group and are isostructural with the previously reported β-NiUO₄ [Murphy et al. Inorg. Chem. 2018, 57, 13847]. X-ray absorption spectroscopy (XAS) analysis shows all five oxides contain hexavalent uranium. The difference in space group can be understood on the basis of size mismatch between the A²⁺ and U⁶⁺ cations whereby unsatisfactory matching results in structural distortions manifested through tilting of the AO₆ polyhedra, leading to a change in symmetry from Cmmm to Ibmm. Such tilts are absent in the Cmmm structure. Heating the Ibmm AUO₄ oxides results in reduction of the tilt angle. This is demonstrated for MnUO₄ where in situ S-XRD measurements reveal a second-order phase transition to Cmmm near T = 200 °C. Based on the extrapolation of variable temperature in situ S-XRD data, CoUO₄ is predicted to undergo a continuous phase transition to Cmmm at ∼1475 °C. Comparison of the measured and computed data highlights inadequacies in the DFT+U approach, and the conducted analysis should guide future improvements in computational methods. The results of this investigation are discussed in the context of the wider AUO₄ family of oxides.

CoMoO4/bamboo charcoal hybrid material for high-energy-density and high cycling stability supercapacitors

Here we report a supercapacitor with high energy density and high cycling stability using low-cost and environmentally friendly CoMoO4/bamboo charcoal (BC) hybrid materials as the cathode. The hybrid materials were fabricated via a one-pot solvothermal reaction followed by an annealing process. The optimized CoMoO4/BC hybrid material has a specific surface area of 74.4 m2 g-1, being 1.7-fold higher than that of the CoMoO4 precursor. The hybrid electrode shows a high specific capacitance of 422.3 F g-1 at 0.5 A g-1 and 304.8 F g-1 at 50 A g-1. The as-assembled CoMoO4/BC||activated carbon supercapacitor exhibits a high energy density of 56.7 W h kg-1 and 18.3 W h kg-1 at a power density of 785 W kg-1 and 40 000 W kg-1, respectively. Furthermore, it also shows excellent long-term cycling stability. Subjected to 40 000 cycles of charge-discharge test at a current density of 50 A g-1, there is only about 10% capacitance loss (occurring only during the first 5000 cycles). This excellent electrochemical performance is ascribed to the covalent C-Mo and C-O bonds formed between CoMoO4 and BC as well as the porous feature of the hybrid material, which provide highways for electron transfer and ion transportation within the electrodes and at the electrode-electrolyte interface.

Geometric considerations of the monoclinic–rutile structural transition in VO2

Geometrical and experimental examinations of VO₂ show how hysteretic phase transition phenomena across the MIT can be driven by positive crystal energy effects of increasing unit cell volume.

Probing Co- and Fe-doped LaMO3 (M = Ga, Al) perovskites as thermal sensors

The synthesis of a Co-doped or Fe-doped La(Ga,Al)O₃ perovskite via the Pechini process aimed to achieve a color change induced by temperature and associated with spin crossover (SCO). In Fe-doped samples, iron was shown to be in the high-spin state, whereas SCO from the low-spin to the high-spin configuration was detected in Co-doped compounds when the temperature increased. Fe-doped compounds clearly adopted the high-spin configuration even down to 4 K on the basis of Mössbauer spectroscopic analysis. The original SCO phenomenon in the Co-doped compounds LaGa_1-xCo_xO₃ (0 < x < 0.1) was evidenced and discussed on the basis of in situ X-ray diffraction analysis and UV-vis spectroscopy. This SCO is progressive as a function of temperature and occurs over a broad range of temperatures between roughly 300 °C and 600 °C. The determination of a crystal field strength of about 2 eV and a Racah parameter B of about 500 cm^-1 for Co³⁺ (3d⁶) ions show that these values allow the occurrence of SCO. Hence, this study shows the possibility of using LaGa_1-xCo_xO₃ compounds as thermal sensors at low Co contents (x = 0.02). The competition between steric and electronic effects in LaGaO₃ in which Co³⁺ is stabilized in the LS state shows that electronic effects with the creation of M-O covalent bonds are predominant and contribute to the stabilization of a high crystal field around Co³⁺ (LS) although its ionic radius is smaller in comparison with that of Ga³⁺.

Pires M, Lacerda LCT, Ramalho TC, Dias A, Pereira MC. Electrocatalytic performance of different cobalt molybdate structures for water oxidation in alkaline media

Among the CoMoO₄ polymorphs, the α-phase exhibits the highest performance for the oxygen evolution reaction.

Size-dependent optical and thermochromic properties of Sm3Fe5O12

Reversible thermochromic inorganic materials of Sm₃Fe₅O₁₂with different particle sizes have been synthesized by a conventional high temperature solid state reaction method (2.51 μm) and sol–gel method (0.16 μm).

In Situ Encapsulation of Ultrasmall CuO Quantum Dots with Controlled Band-Gap and Reversible Thermochromism

Silica encapsulated ultrasmall CuO quantum dots (QDs; CuO@SiO2) were synthesized by reverse microemulsion. The CuO QDs with sizes ranging from 2.0 to 1.0 nm with corresponding band gaps of 1.4 to 2.6 eV were prepared simply by varying the concentration of the Cu(2+) precursor. The samples were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and UV-vis spectroscopy. The CuO@SiO2 composite displayed reversible thermochromism which resulted from the strong electron-phonon coupling of ultrasmall CuO in the confined space of SiO2 and the enhanced band-gap shift in the visible light region depending on temperature. Besides, the as synthesized CuO@SiO2 was found to be highly stable for reversible thermochromism due to the micropore structure of silica matrix and local confinement of the QDs.

CoMoO4/CuMo0.9W0.1O4 mixture as an efficient piezochromic sensor to detect temperature/pressure shock parameters

A mixture of two piezochromic compounds can be used as a universal shock detector, i.e., to determine the shock pressure without knowing a priori the temperature at which the shock occurred. For this purpose, both piezochromic compounds must exhibit different temperature influences in their transition-pressure values. This demonstration uses two piezochromic compounds (CoMoO4 and CuMoO4-type oxides) that exhibit a first-order phase transition between their two allotropic forms associated with a drastic color change. The colorimetric coordinates of the mixture indicate the pressure and temperature of a shock.

Understanding the relationships between structural features and optical/magnetic properties when designing Fe(1-x)Mg(x)MoO4 as piezochromic compounds

Fe1-xMgxMoO4 compounds with x = 0, 0.25, 0.5, 0.75, and 1.0 were obtained after annealing under inert gas at T = 700 °C. All of the compounds exhibit a pressure-induced and/or temperature-induced phase transition between the two polymorphs adopted by AMoO4 compounds (A = Mn, Fe, Co, and Ni). For the FeMoO4 compound, for both the α and the β allotropic forms, the structural features have been correlated to the magnetic properties, the Mössbauer signals, and the optical absorption properties to gain a better understanding of the phenomena at the origin of the piezo(thermo)chromic behavior. The different contributions of the Mössbauer signals were attributed to the different Fe(2+) ions or Fe(3+) ions from the structural data (Wyckoff positions, bond distances and angles) and were quantified. Furthermore, the low Fe(3+) concentration (9 and 4 mol %, respectively, in the α and the β allotropic forms) was also quantified based on the magnetic susceptibility measurements. The net increase in the Fe(3+) quantity in the α-form in comparison to the β-form, which is associated with the occurrence of Fe-Mo charge transfer, is at the origin of the important divergence of coloration of the two forms. To design new piezo(thermo)chromic oxides and to control the pressure (temperature) of this first-order phase transition, FeMoO4-MgMoO4 solid solutions were synthesized. The optical contrast between the two allotropic forms was increased due to magnesium incorporation, and the phase transition (β → α) pressure increased steadily with the Mg content. A new generation of nontoxic and chemically stable piezochromic compounds that are sensible to various pressures was proposed.

Sub-micrometric β-CoMoO4 rods: optical and piezochromic properties

Sub-micrometric β-CoMoO4 rods have been obtained after thermal treatment of CoMoO4·H2O previously prepared by the precipitation method. The color of the sub-micrometric particles has been investigated through diffuse reflectance in the VIS-IR range and the spectra have been compared with those corresponding to samples with larger isotropic particles. The O(2-)→Mo(6+) charge transfer band shifts to the UV region with decreasing the particle size, revealing a more covalent bond. This can be justified under the consideration of a higher proportion of surface-atoms in sub-micrometric particles that present lower coordination number. The piezochromic behavior of the sub-micrometric β-CoMoO4 rods has been investigated. As the sub-micrometric size of the particles stabilizes the low-coordination phase (β-phase), the transition pressure to the α-phase is higher in comparison with that corresponding to larger particles. In addition, it was not possible to obtain a 100% transformation of the β-phase in the sample with sub-micrometric particles. A similar temperature influence on the transition pressure regardless of particle size has also been observed.