Structural stability, optical and dielectric properties of the (Ba1/5Pb1/5Sr1/5RE1/5K1/5)TiO3 high-entropy ceramic

Abstract

A high-entropy matrix with highly polarizable elements sharing a rare-earth element at the same crystallographic site was designed using the chemical formula Ba1/5Pb1/5Sr1/5RE1/5K1/5TiO3 (BPSREKTO), where rare-earth (RE) = La, Nb, Sm, Gd, Dy, Ho, Y, and Lu. Single-phase stability was observed only in the BPSREKTO with RE = La, Nd, and Sm high-entropy compounds. The crystal structure, optical properties, and ferroelectric nature of the single-phase ceramic compounds were investigated. Elemental and structural analyses revealed that all the cations were homogeneously distributed in a global centrosymmetric cubic structure (S.G. Pm¯3m). Optical absorption showed that the RE = Nd compound is more photoactive in the 200–1000 nm wavelength range, unlike the RE = La, Sm high-entropy compounds. The introduction of RE elements in high-entropy ceramic (HEC) systems affects the indirect bandgap of BPSREKTO with RE = La, Nd, and Sm. It was also found that cationic disorder increases the Urbach energy, leading to a decrease in the indirect energy bandgap in the HEC compound compared to the homologue BaTiO3/SrTiO3 single-phase. The dielectric spectra show a broad peak in the dielectric constant and dielectric loss, which are shifted in temperatures with increasing frequencies due to a relaxor ferroelectric transition typical of the diffuse phase transitions. This relaxor behavior was unexpected, because the global crystal structure was centrosymmetric, implying an increase in the number of polar nanoregions (PNRs). These PNRs coexisting with non-polar regions (NPRs) were observed using piezo-force microscopy. Furthermore, the slim polarization loop confirmed the relaxor behavior of BPSREKTO with RE = La, Nd, and Sm. These ferroelectric features make these RE-modified HEC materials good candidates for high-energy storage applications.

1. Introduction

There has been extensive research activity around high-entropy ceramic (HEC) compounds since Rost et al. [1] synthesized entropy-stabilized rock-salt ceramic compounds for the first time in 2015. Two years later, the high-entropy approach was extended toward more complex ceramics [2–4]. This approximation has opened a new research area for finding, enhancing, and improving the physical properties of known ceramic matrices. Furthermore, the design of simple phases is based on thermodynamic principles. In solid-state physics, the solubility and stability of simple phases from their liquid phase depend on the Gibbs free energy (ΔG). Two important parameters govern the formation kinetics of the existing phases: the enthalpy of formation ΔHmix and entropy of configuration ΔSconf. It has been found that minimizing ΔHmix and increasing ΔSconf in equimolar ratios at specific sites in the crystal lattice leads to simple phases in alloys [5]. This situation in ceramics can become more complicated because of the nature of interactions between metallic and non-metallic elements, even when there is a wide composition range of single-phase solid solutions at high temperatures [6].

A significant advantage that can be achieved by increasing the configurational entropy is the possibility of introducing cations that solubilize very poorly under conventional doping strategies, resulting in unexpected properties and fascinating functional performance in multicomponent material systems.

In particular, the ABO3 perovskite structure contains a twelve-fold coordinated A cation sublattice and a six-fold-coordinated B cation sublattice shared with the oxygen anion sublattice. Thus, owing to their distinct and highly tunable chemistry, which allows for individual or simultaneous tailoring, it is possible to attain unique physical properties using the high-entropy approach. A significant number of studies have recently focused on perovskite structures with a Goldschmit tolerance factor (t) of approximately 1, especially, scaling towards high-entropy dielectric/ferroelectric compounds. First, to explore the stability of entropic phases in perovskite structures, Jiang et al. [3] and Tang et al. [7] substituted various cations at the B-site of the perovskite structure and found a large number of combinations where the single-phase stability was driven by the Goldschmit tolerance factor as well as the valency difference of the cations involved. Phases with tetragonal and cubic structures are stable under normal conditions in a large combination of compounds at the A- or B-sites of the perovskite crystal structure [8–10]. Regardless of their overall crystalline structure, in most of these compounds, the dielectric behaviors indicate relaxor ferroelectric behaviors at room temperature with excellent dielectric constant (ε′) and dielectric loss (tanδ). More recently, rare-earth (RE) elements have been dissolved in high-entropy perovskites with t ≈ 1 and formula Sr0.25Ca0.25Ba0.25RE0.25TiO3 with RE = Nd, Sm, G