Structural and electronic properties of the perovskite-type ferrimagnetic semiconductor CaCo0.5Zr0.5O3
Description
The purpose of this work is to report the structural, magnetic, electrical and electronic properties of the perovskite-type semiconductor CaCo0.5Zr0.5O3. For the synthesis of the material, the standard solid-state reaction method was employed, followed by an analysis of its crystal structure by Rietveld refinement applied to X-ray diffraction patterns, which revealed the crystallisation of the compound in an orthorhombic structure belonging to the Pcmn space group. Through morphological and compositional analyses, it was determined that the surface of the samples evidenced mean grain size of 1.47 ± 0.01 µm, as well as the occurrence of minimal levels of impurities. The curves of magnetic susceptibility as a function of temperature and magnetisation as a function of external magnetic field showed a weakly ferrimagnetic response of the material, with Weiss temperature =-72 K and effective magnetic moment 1.78 B, suggesting that Co4+ does not present crystalline field splitting and the spins adopt low spin states. The electrical resistivity exhibits behaviour and order of magnitude (106 .m) characteristic of a semiconductor feature and current-voltage curves with a typical thermistor-type semiconductor response. Density of states and band structure analysis indicate that the material exhibits ferromagnetic semiconductor-like behaviour at low temperatures, with a mean band gap of 0.89 eV and an effective magnetic moment of 2.0 µB.
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Samples production: Samples of the perovskite-type material CaCo0.5Zr0.5O3 were synthesized by using the standard solid-state reaction method. For the synthesis, precursor oxides — CaO (99.9%), CoO (98.0%), and ZrO2 (99.0%) — were employed. These precursors were measured in stoichiometric proportions aligned with the chemical formula of the target material, ensuring the production of 1.5 g of the precursor oxide mixture. Subsequently, the mixture was homogenized in an agate mortar for two hours, employing 70% ethyl alcohol as a facilitator. The precursor oxide mixture was then compressed at 13.8 MPa for two minutes to form tablets with an 8 mm diameter. These tablets were subjected to a sequential heat treatment process: initially at 850 °C for 16 hours, followed by 900 °C for 16 hours, and finally at 1150 °C for 36 hours to encourage grain growth and ion accommodation within the structure. Calculations: The theoretical methodology employed in this study has been detailed in previous scientific literature [13–20]. The key issues relevant to this theoretical research are briefly outlined below. Total energy calculations were performed using Density Functional Theory (DFT) with the Projector Augmented-Wave (PAW) method approach [25,26], as implemented in the VASP software [27]. Specifically, the Generalised Gradient Approximation (GGA-U) was applied to evaluate the exchange energy and correlation, adopting the Perdew and Wang functions (GGA-PW91) [28]. In addition, a correction of Hubbard's potential was incorporated (GGA+U), setting U potential at 3.32 for the d orbitals of cobalt. The valence electrons considered in the calculations were as follows: 10 for calcium (3s²3p⁶4s²), 9 for cobalt (3d⁷4s²), 12 for zirconium (4s²4p⁶4d²5s²), and 6 for oxygen (2s²2p⁴). A cutoff energy of 520 eV was determined for the PAW potentials, enabling the attainment of high convergence in the total energies, with discrepancies less than 0.001 eV per atom. The Brillouin zone sampling was conducted using the Monkhorst–Pack method, with a 7x5x7 k-point mesh [29], which was found to be sufficient for obtaining highly convergent energies, with variances less than 1 meV per atom. For the calculation of the partial occupancies of the electronic states near the Fermi level, the smearing technique proposed by Methfessel-Paxton was applied [30], utilizing a smearing parameter with a width of 0.05 eV. These parameters collectively ensured that the calculations achieved convergence in the total energy to within 1 meV. In addition, the lattice parameters and ion positions were optimised until the forces acting on the ions were minimised to less than 30 meV/Å.