Density functional theory (DFT) investigation on the structure and photocatalysis properties of double-perovskite Gd1-x Ca x BaCo2O5+δ (0 ≤ x ≤ 0.4)

Limits of Solid Solution and Evolution of Crystal Morphology in (La1-xREx)FeO3 Perovskites by Low Temperature Hydrothermal Crystallization

The crystallization of a new series of A-site substituted lanthanum ferrite materials (La_1-xRE_x)FeO₃ was explored by the hydrothermal method at 240 °C, for rare earth (RE) = Nd, Sm, Gd, Ho, Er, Yb, and Y, with 0 ≤ x ≤ 1. The effect of elemental substitution on the morphological, structural, and magnetic properties of the materials was studied using high-resolution powder X-ray diffraction, energy dispersive spectroscopy (EDS) on the scanning electron microscope, Raman spectroscopy, and SQUID magnetometry. If the radius of the La³⁺ and the substituent ions is similar, such as for Nd³⁺, Sm³⁺, and Gd³⁺, homogeneous solid solutions are formed, with the orthorhombic GdFeO₃-type structure, and a continuous evolution of Raman spectra with composition and distinct magnetic behavior from the end members. When the radius difference between substituents and La³⁺ is large, such as for Ho³⁺, Er³⁺, Yb³⁺, and Y³⁺, then instead of forming solid solutions, crystallization in separate phases is found. However, low levels of element mixing are found and intergrowths of segregated regions give composite particles. In this case, the Raman spectra and magnetic behavior are characteristic of mixtures of phases, while EDS shows distinctive elemental segregation. A-site replacement induces an evolution in the crystallite shape with an increasing amount of substituent ions and this is most evident for RE = Y from cube-shaped crystals seen for LaFeO₃ to multipodal crystals for (La_1-xY_x)FeO₃, providing evidence for a phase-separation-driven evolution of morphology.

The effect of Sr substitution on the structural and physical properties of manganite perovskites Ca1-xSrxMnO3-δ (0 ≤ x ≤ 1)

Calcium manganite (CaMnO_3-δ) has been extensively utilized in many applications due to its unique physical and chemical properties. In this study, the effect of Sr-substitution at the Ca-site on the structural, magnetic, electronic and electrical properties of CaMnO₃ manganite perovskites is investigated in detail. The perovskite compounds Ca_1-xSr_xMnO_3-δ (x = 0, 0.25, 0.5, 0.75 and 1) were synthesized through the sol-gel method at 1200 °C. From the patterns of X-ray diffraction, it was observed that all of the synthesized compounds show a pure perovskite phase at room temperature. The refinement results of the perovskite series suggest that a structural transformation from an orthorhombic (Pnma) to a hexagonal (P6₃/mmc) system occurred for 0.50 < x ≤ 0.75. We note however that the sample with the composition x = 0.50 showed a phase mixture of orthorhombic (Pnma) and hexagonal (P6₃/mmc). Based on DFT calculations, we have demonstrated the energetic stability of all compounds by negative formation energy and confirmed the semiconductor behavior by the presence of a band gap. The change in the band gap value with the Sr content suggests the potential tuning of the electronic behavior of CaMnO₃-SrMnO₃ solid solution. Furthermore, as the temperature increases from 300 to 1000 K, the electrical resistivity exhibits a reduction while the Seebeck coefficient (S) shows an augmentation. The negative values of S indicated the n-type-semiconductor nature of all compounds. The obtained values of the activation energy from thermal evolution of resistivity suggested that the electrical transport behavior of all the compounds followed the mechanism of small polaron hopping. Power factor is greatly affected by the Sr amount and reached a maximum value at x = 0.50. Overall, introducing Sr into the CaMnO_3-δ matrix improved the power factor and reduced electrical resistivity. According to the obtained results, the studied manganite perovskites could be proposed as suitable materials for photocatalytic and thermoelectric applications.