Structural and electrical characteristics of Gd³⁺ and Dy³⁺ based bismuth layer structured ferroelectric ceramics

Highlights

• Bi₂GdZrVO₉ and Bi₂DyZrVO₉ were synthesized by the mixed oxide reaction method and crystallized in the orthorhombic system.

• Impedance analysis of both the samples substantiates the role of only grain effect towards the total polarization.

• The ${{\epsilon }^{\prime }}_r$of BGdZrV and BDyZrV are 557 and 307 respectively as compare to that of 170 for Bi₃TiNbO₉ at 1 MHz and 525°C.

• Jonscher's power law favors the OLPT mechanism in BGdZrV and the CBH mechanism in the BDyZrV conduction process.

Abstract

Two new compounds Bi₂GdZrVO₉ and Bi₂DyZrVO₉ of the Aurivillius family having two pseudo perovskite units per cell are processed by the solid-state reaction method. The effect of the lanthanides at the A-site and a combination of transition elements at the B-site of the bismuth layer structured compounds are discussed in detail. The Nyquist plots confirm the contribution of only grain to the polarization mechanism, and thus the depression angle was obtained to estimate its deviation from the Debye behavior. The activation energy estimated for the relaxation process, conduction process, and the grain resistance suggests the involvement of ionization of oxygen vacancy in these processes. The dielectric properties of both the samples were studied in detail and found to have better values as compared to those of the other members of the bismuth layer family. The conductivity analysis using Jonscher's power law is employed to ascertain the conduction mechanism in the samples.

Introduction

Bismuth layer structured ferroelectric compounds (BLSF) of Aurivillius family, with the chemical formula unit (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)^2-, are endowed with many application-oriented properties like exceptional switching speed, poor leakage current density, low operating voltage, etc. [[1], [2], [3], [4], [5]]. Owing to their superior ferroelectric properties, these compounds have found applications in memory devices, high-temperature sensors, aerospace engineering, nuclear power industries, etc, and have captivated the attention of many research groups [6]. In the compound n number of pseudo perovskite layers (A_n−1B_nO_3n+1)^2- are stacked between two bismuth oxide layers. The pseudo perovskite layer and the bismuth oxide layer can be considered to be stacked alternatively along the c axis [7,8]. The number of pseudo perovskite layers, n in the compound can vary from 1 to 10. The compound with n = 2 is more popular from the point of synthesis and application potentiality. In Bi₃TiNbO₉, the most referred BLSF material, the bismuth at the A site of the pseudo-perovskite unit with its lone-pair causes a large structural distortion, and thus the compound suffers from fatigue [[16], [17], [18]]. Further, bismuth being volatile at high temperature also tends to create both Bi and O vacancies during synthesis leading to an increase in the leakage current. These drawbacks are putting constraints on the application potentiality of the compound [9]. Thus efforts are channelized towards developing new compounds in the family through substitution of appropriate elements at the A and B sites of the pseudo perovskite layer and improving the synthesis process to minimize the mentioned deficiencies [[10], [11], [12]]. While for the A-site occupancy, the ions considered generally pertain to Ca²⁺, Bi³⁺, ions of rare earth elements, etc, those at B sites accommodate Ti⁴⁺, Nb⁵⁺, V⁵⁺, Zr⁴⁺, etc [[13], [14], [15]]. Such wide availability of the ions for occupation at the A and B sites opens up opportunities to synthesize a wide spectrum of compounds, find the application potentiality of each of the compounds, and compare their properties to find the best. The occupancy of lanthanides at the A-site is considered to be the most excellent option to stabilize the oxygen vacancies [[19], [20], [21], [22], [23]].

Continuing with the trend to develop the BLSF compound with n = 2 which exhibits enriched properties among the family, we were inspired to synthesize two compounds namely, Bi₂GdZrVO₉ and Bi₂DyZrVO₉ by the cost-effective solid-state reaction method. We have not come across any report on the substitution of the two lanthanide ions namely Gd³⁺ and Dy³⁺ in the BLSF compounds. Here, we have chosen the lanthanides (Gd and Dy) at the A-site and the combination of two transition elements of different oxidation numbers (Zr⁴⁺ and V⁵⁺) at the B site to maintain the charge balance. The Gd–O and the Dy–O bond with an electronegativity difference of 2.24 and 2.21 between the constituent elements of the bond are expected to be much more ionic than that of the Bi–O bond with an electronegativity difference of 1.42. Further, the bond dissociation energy of the Gd–O and the Dy–O bond at 716 kJ/mol and 611 kJ/mol is far higher than the Bi–O bond with bond dissociation energy of 343 kJ/mol. The stronger lanthanoid-oxygen ionic bonds with higher bond dissociation energy as compared to the Bi–O bond are expected to decrease the oxygen vacancy in the compounds and hence decrease the leakage current. At the B-site, the transition metal elements Zr⁴⁺ and V⁵⁺ with no d electrons are favored to have ferroelectric properties. Zr⁴⁺ is resistant to corrosion and makes the sample suitable for application in high-temperature parts of jet engines, as a heating fragment in space vehicles, etc [24,25]. It is worth pointing here that the Zr–O bond is more ionic and stronger with an electro-negativity difference of 2.11 and bond dissociation energy of 760 kJ/mol as compared to the Ti–O bond with an electronegativity difference of 1.90 and bond dissociation energy of 662 kJ/mol. Hence, the Zr⁴⁺ ion at the B site of the pseudo perovskite layer is expected to give more stability as compared to that of Ti⁴⁺ based compounds with the B site substitution. Further, V⁵⁺ ion being the smallest ion (0.54 Å) among all the +5 oxidation state ions, provide larger rattling space inside the BO₆ octahedron, and is likely to promote polarization and cause an increase in the dielectric constant of the sample [26,27]. Here, in this paper, we have discussed the fabrication of these compounds and compared their structural, dielectric, impedance, and conductivity properties in different conditions.