Probing the effect of Zn²⁺ on the local structure, dielectric and magnetic properties of La₂CuMnO₆ by solid state synthesis

Highlights

• Structural, vibrational, spectroscopy, dielectric, and magnetic studies of novel ceramic La₂Cu_0.5Zn_0.5MnO₆ was carried out.

• An investigation of disordering parameters revealed steric, orbital, and Jahn-Teller distortion in La₂Cu_1-xZn_xMnO₆ ceramics.

• O K-edge, Mn K-edge analysis was carried out using SXAS and XNES spectroscopy.

• Below room temperature, frequency-dependent relaxor characteristic was observed and studied by C-W law.

• DC-magnetic behavior of the samples exhibited canted-antiferromagnetic nature.

Abstract

The comparable ionic-radius cation substitution provides a route to investigate the origin of the physical properties of ceramics on the local structure and electronic charge state background. In analogy to our previous work [19] of La₂CuMnO₆ (LCM) prepared via solid-state route, here we are reporting the effect of Zn substitution at the Cu site of LCM. The detailed structural analysis confirmed the Jahn-Teller (JT) distortion in LCM and Zn²⁺ substituted LCM (i.e., LCZM) ceramics. Raman analysis further confirmed the JT distortion in these oxides. O K-edge and Mn K-edge XANES spectra confirmed the mixed hybridization states for Mn. A frequency-dependent relaxor state was observed in the dielectric study of LCM and LCZM. The dielectric responses of these systems are prominently driven by electron-phonon coupling. Their magnetic studies showed positive-Curie temperature (T_C) and non-zero coercivity, indicating the ceramics' canted-antiferromagnetic nature.

Introduction

Over the last few decades, materials scientists have worked vigorously to develop multifunctional materials, reducing power consumption and giving additional functionality for the upcoming electronic age [1], [2], [3], [4], [5], [6], [7], [8], [9]. In this context, double perovskite (DP) compounds have attracted tremendous attention, which are represented as A₂B′B″O₆ and A′A″B₂O₆, where A′ and A″ sites are usually adopted by rare earth or alkaline earth elements and B′ and B″ sites mostly adopted by transition metal (TM) elements. However, very few reports showed the local structural analysis of DPs in which row-one TMs occupied both B-sites. In this context, 6s-3d-based DPs showed excellent physical properties. For example, Lanthanum-based DP compounds like La₂FeCoO₆, La₂NiMnO₆, and La₂CoMnO₆ showed FM structure [10], [11], [12], [13], [14], and La₂CuMnO₆ showed AFM structure [15]. The half-metallicity and thermophysical properties of La₂CuMnO₆ have been studied (theoretically) extensively by Mir et al. [15]. They have proposed a significant contribution of Mn⁴⁺- and Cu²⁺-ions to the magnetic response of these ceramics. Mtougui et al. studied the magnetic properties of La₂CuMnO₆ [16] using Monte Carlo simulation and observed a soft magnetic behavior with T_C = 70 K. Palakkal et al. [17] reported orthorhombic crystal symmetry with effective magnetic moments of 4.2 μ_B for La₂CuMnO₆ ceramics. In another report, Cu-substituted La₂NiMnO₆ showed FM transition near room temperature [18]. Singh et al. reported the structural, morphological, and high-temperature dielectric properties of La₂CuMnO₆ (LCM) and La₂ZnMnO₆ (LZM) ceramics [19], [20], where they have shown Debye and non-Debye dielectric transition for the ceramics. In these reports, various properties of LCM are studied, but the origin of these properties and the underline physics are missing. The oxidation state of copper in LCM decides the extent of the JT distortion, which affects the physical properties of the ceramics by affecting their crystal-lattice structure, super-exchange angles, and charge exchange interaction between Cu-O-Mn and Mn-O-Mn ions. Therefore, to understand the role of Cu in the physical properties of LCM, we substitute Cu with some comparable ionic radii element, which also has the same oxidation state. Hence, Zn is used to substitute it. As, Zn has +2 oxidation state (Cu⁺²) and ionic radii ∼ 0.88 Å (Cu∼ 0.87 Å).

The present work highlights the effect of Cu²⁺-substitution by Zn²⁺ on the structural, electronic, dielectric, and magnetic properties of the LCM. We have prepared single-phase Zn modified La₂Cu_1−xZn_xMnO₆ (x = 0.0, 0.5 and 1.0) ceramics. The non-rigidity in BO₆-octahedra and JT distortion produced an unusual trend in the dc magnetic response of these compounds. Also, the extent of the JT distortion influenced the low-temperature dielectric behavior of these oxides. We also discussed frequency-dependent relaxor dielectric features corresponding to uneven grain size distribution and electron-phonon coupling of these oxides. These ceramics appear promising candidates for fabricating electronic and magnetic devices such as energy storage, radio frequency filters, spintronic, and memory devices.