A comparative study of influence of isovalent and aliovalent A-site substitutions on the structural, magnetic, and dielectric properties of Sr2CrMoO6 double perovskites

Abstract

We report on a comparative study of the structural, magnetic, and dielectric and properties of isovalent and aliovalent A-site substituted Sr_2-xA_xCrMoO₆ (A = Ca, x = 0.0, 0.1, 0.3, 0.5, 0.7, 0.8, 1.0; A = K, La only x = 0.5) double perovskites, which were synthesized via sol-gel process. X-ray diffraction patterns and Rietveld refinements demonstrated that all the Sr_2-xA_xCrMoO₆ powders crystallized in a cubic crystal structure with space group of Fm $\bar{3}$ m. SEM images revealed that the Sr_2-xA_xCrMoO₆ powders exhibited a spherical morphology. Their chemical compositions were determined by quantitative energy dispersive X-ray spectroscopy, which were close to the nominal values. FTIR spectra analyses verified the presence of (Cr, Mo)O₆ octahedra in these powders. XPS spectra validated the existence of Sr²⁺, K⁺, Ca²⁺, La³⁺, and Cr³⁺ ions in the powders, and oxygen existed in the forms of lattice oxygen and absorbed oxygen respectively, whereas Mo element had a mixed chemical valence state of Mo⁴⁺ and Mo⁶⁺. The Sr₂CrMoO₆ (SCMO) powders had a saturation magnetization (M_S) of 0.016 μ_B/f.u. at 2 K, magnetic Curie temperature (T_C) of 337 K, and irreversibility temperature (T_irr) of 329 K where the zero-field cooled (ZFC) and field-cooled (FC) magnetization (M_ZFC and M_FC, respectively) curves became bifurcated. A spin-glass behavior was observed at temperature (T_B) of 81 K. Both T_irr and T_B shifted towards low temperature with increasing the external magnetic field. In addition, the M_ZFC and M_FC curves became almost overlapped under the external field of 50 kOe, and the spin-glass behavior disappeared. The M-H data demonstrated that both isovalent and aliovalent A-site substitutions could improve the magnetic properties and B-site ordering degree (η) of the SCMO powders. However, isovalent Ca-substitution exhibited more significant enhanced effect on the magnetic properties in comparison with the aliovalent A-site substitutions either by K- or La-substitution. Among the isovalent Ca-substituted Sr_2-xCa_xCrMoO₆ (x = 0.1–1.0) powders, Sr_1.2Ca_0.8CrMoO₆ sample had a maximum M_S value of 0.64 μ_B/f.u. at 2 K, over 30 times enhancement of that of the pristine SCMO powders. The Sr_2-xA_xCrMoO₆ ceramics exhibit a strong frequency dependent dielectric behavior, which can be well interpreted by Maxwell-Wagner relaxation model. The anomalous dielectric relaxor behavior of the SCMO ceramics observed at 260 K was ascribed to the jumping of the electrons trapped in a shallower energy level created by oxygen vacancies. The dielectric constant of the isovalent Ca-substituted Sr_1.5Ca_0.5CrMoO₆ samples was 30 times larger than that of the pristine SCMO samples. Our present results demonstrate that isovalent Ca-substitution at A-site could improve the magnetic and dielectric properties of the sol-gel derived SCMO double perovskite oxides, which have potential applications in the fields of spintronic devices and dielectric devices.