Combination of structural and microstructural effects in the multiferroic response of Nd and Ti co-doped BiFeO3 bulk ceramics

Abstract

Different dopant strategies are currently under investigation in order to overcome the many problems that limit the commercial viability of BiFeO 3-based ceramic devices. Neodymium substitution onto the A site of the perovskite lattice provokes significant changes in the crystal structure of the parent material which can derive in enhanced multiferroic properties, but the conductivity in the bulk system is still too high. Titanium doping on the other hand generates a distinctive micro-nanostructure in the consolidated ceramics which can largely increase the dc resistivity of the whole material. A combination of these two effects is here attempted in a co-doping approach which evidences that the microstructural effect caused upon Ti-doping, provoking a reduction of the leakage currents, eventually allows the co-doped material to capitalize on the unique piezoelectric and magnetic properties structurally triggered by the Nddoping.

Nanostructure stabilization by low-temperature dopant pinning in multiferroic BiFeO3-based thin films produced by aqueous chemical solution deposition

“…As described, in the bulk samples the formulated titanium ions segregate to the grain boundaries as the temperature rises, until they form a highly resistive Ti-rich interconnected skeleton that neatly reduces the conductivity of the material. 4,5 In the films, on the contrary, this titanium stays confined in the perovskite structure, no such skeleton of Ti-enriched boundaries is ever formed, and as a direct consequence the conductivity is not sufficiently restrained: at room temperature, some leakages are still present in the films, which initially hinders the exploitation of their ferroelectric potential. This impediment however disappears when the ferroelectric response of the films is measured at liquid nitrogen temperatures.…”

“…3a), but in the BNFTO sample it goes together with the entrance of a new maximum around 2y = 47.51 which must be ascribed to the orthorhombic Pbam phase of BiFeO 3 . 5 This different scenario is nevertheless not surprising and relates to the specific formulation used for the two co-doped compositions. On one hand, the BSFTO sample was formulated with 12 mol% of samarium, which places this composition right on the edge of a theoretical morphotropic phase boundary (MPB); 33 this is found to render an enhancement of the electromechanical (piezoelectric) properties of the material and, actually, in the particular case of Sm-modified BiFeO 3 ceramics it has been observed that such enhancement could be quite significant in the vicinity of the MPB (i.e., with no need to be inside the MPB itself).…”

“…2 In recent years, research has been conducted around this window, and recently our team came to satisfactory (functional) results upon doping the parent BiFeO 3 structure with RE 3+ (rare-earth) and Ti 4+ . [3][4][5] Specifically, we were able to obtain bulk samples through a sequential two-step process comprising a calcination stage of synthesis and a subsequent sintering treatment, in a strict range of heating conditions which do not allow synthesis temperatures lower than 800 1C and sintering temperatures higher than 900 1C. Within these temperature limits, several solid-state diffusion processes are successively activated, which provoke first the incorporation of titanium into the perovskite lattice during the synthesis episode, and then its posterior segregation to the grain boundaries during the subsequent recrystallization stage of sintering.…”

“…In this way titanium delineates the microstructure of the bulk specimen, generating an interconnected skeleton of highly-resistive Ti-rich grain boundaries that controls the macroscopic conductivity of the material and allows for improved exploitation of its ferroelectric characteristics. 4 This, combined with the structure-related effect that the rare-earth produces on both the electric and the magnetic properties of the parent compound, [5][6][7][8][9] eventually leads to a promising multiferroic behaviour with potential application in practical devices. Now the question is, is this also accessible in a thin film configuration?…”

“…30 These specific compositions were selected on the basis of different reports in the specialized literature which catalogue them as ideal from the point of view of the multiferroic properties. 5,33 The starting products used for the synthesis were bismuth(III) citrate (…”