Point-Defect Chemistry on the Polarization Behavior of Niobium Doped Bismuth Titanate
Total Page：16
File Type：pdf, Size：1020Kb
J. Mex. Chem. Soc. 2017, 61(4), 317-325 Article © 2017, Sociedad Química de México ISSN 1870-249X Point-Defect Chemistry on the Polarization Behavior of Niobium Doped Bismuth Titanate F. Ambriz-Vargas,1* R. Zamorano-Ulloa,3 A. Romero-Serrano,2 J. Ortiz-Landeros,2 J. Crespo-Villegas,2 D. Ramírez-Rosales3 and C. Gómez-Yáñez2 1 Centre Énergie, Matériaux et Télécommunications, INRS, 1650 Lionel-boulet, Varennes, Québec J3X1S2, Canada 2 Departamento de Ingeniería en Metalurgia y Materiales, ESIQIE, Instituto Politécnico Nacional, UPALM, Zacatenco, 07738, DF, México. 3 Departamento de Física, ESFM; Instituto Politécnico Nacional, UPALM, Zacatenco, 07738, DF, México. Corresponding Author: *Phone: +1.514.228.6978 E-mail: [email protected] Postal address: 1650, Boulevard Lio- nel Boulet, Varennes, Québec, J3X 1S2, CANADA. Received February 17th, 2017; Accepted July 20th, 2017 Abstract: The present work shows the defect chemistry at room tem- Resumen: En el presente trabajo se estudia la química de defectos perature of Bi4Ti3O12, emphasizing the effect of point defects on the del Bi4Ti3O12 a temperatura ambiente, haciendo énfasis en el efecto ferroelectric properties. Electrical measurements of conductivity, di- de los defectos puntuales sobre las propiedades ferroelectricas. Me- electric permittivity and dielectric loss as well as structural character- diciones de conductividad eléctrica, permitividad dieléctrica, perdi- ization and Electron Spin Resonance (ESR) were used to deduce the da dieléctrica, caracterización estructural y resonancia de spin existence of different point defects. Pure and Niobium doped bismuth electrónico fueron utilizadas para demostrar la existencia de diferen- titanate ceramic were prepared by a conventional solid state reaction tes defectos puntuales. Titanato de Bismuto puro y dopado con nio- technique. Rietveld refinement analysis suggested that niobium atoms bio fueron sintetizados a partir del método convencional “Reacción occupy the titanium lattice sites and the presence of bismuth vacan- del estado sólido”. El análisis de refinamiento de Rietveld reveló la cies. Electron Spin Resonance measurements showed signals that are formación de vacancias de bismuto al igual que la formación de áto- associated to iron impurities. The present communication supports the mos de niobio en los sitios atómicos del titanio, mientras que las models of compensation mechanisms dominated by free electrons and mediciones de resonancia de spin electrónico revelaron señales aso- bismuth vacancies. ciadas a impurezas de hierro. El presente comunicado soporta meca- Keywords: Ceramics; Dielectric properties; Point defects; Electrical nismos de compensación dominados por la presencia de electrones properties. libres y vacancias de bismuto. Palabras clave: Cerámicos; Propiedades dieléctricas, Defectos pun- tuales; Propiedades eléctricas. 1. Introduction The crystal structure of Bi4Ti3O12 can be described by ti- tanium atoms surrounded by six oxygen atoms ([TiO6] clus- In recent years ferroelectric random access memories (FeRAMs) ters) in an octahedral configuration. These octahedrons are have attracted considerable attention to be applied as a nonvola- tilted into the orthorhombic structure. The [TiO6] clusters tile technology, because they combine the DRAM benefits such are sandwiched between the bismuth-oxide layers, while bis- as small cell size (22 F2, “F” stands for feature size), low voltage muth atoms are located in the corners of pseudo-perovskite of operation and fast read/write access time (10ns/10ns), with units surrounding the [TiO6] octahedral clusters as shown in nonvolatile data storage [1-4]. One of the designs considers the Fig. 1 [13-14]. use of a ferroelectric material in the capacitor of a FeRAM cell The origin of the polarization properties in Bi4Ti3O12 is at- [5-7]. Taking advantage of the polarization properties of the tributed to the tilting of the TiO6 octahedron cell from the c-axis ferroelectric material, the positive and negative remnant polar- and a-displacement of the bismuth atoms in the perovskite layer ization states could be codified as “0” and “1” digital states in [15-16]. a binary code [4, 8-10]. However, the reading process requires Recent studies have reported that the substitution of Ti4+ 5+ a polarizing-depolarizing external action which implies fatigue ions by Nb ions in the crystal structure of Bi4Ti3O12, results in problems [3, 7, 11]. Bi4Ti3O12 has shown to be a good ferroelec- the improvement of the polarization properties [17-19]. Since tric material for this application, the drawback of the pure com- Nb5+ ions donate more electrons to the crystal structure than pound is the relatively high leakage current. Lanthanum doping Ti4+ ions (Nb5+ ion is an aliovalent dopant), the electrical neu- has shown to solve this problem [12]. trality is disrupted, so spontaneous defect generation occurs 318 J. Mex. Chem. Soc. 2017, 61(3) F. Ambriz-Vargas et al. TiO2 (Merck, 99% purity), Bi2O3 (Sigma-Aldrich, 99.9%) and Nb2O5 (Sigma-Aldrich, 99.9%) were used as starting reagents. All the powders were weighted and mixed in a polyethylene container with de-ionized water and zirconia balls (ZrO2). To homogenize the mixture, the container was rotated for 12 h and then heated in an oven at 80 °C for 18 h to remove moisture. Once dried, the powders were placed in a platinum crucible which was protected by an alumina plate inside of the furnace (Carbolite RHF 17/3E). The powder mixture was calcined at 1100°C (heating rate = 10 °C/min) in two steps. The first heat- ing step was at 750°C during 2h to promote the formation of Bi4Ti3O12 and intermediate bismuth titanate phases. The second heating step was at 1100°C during 1h to obtain Bi4Ti3O12 single phase powders [24]. In every step, the weight of the samples, before and after the thermal treatments was recorded in order to ensure that there was not a weight loss. The solid solution formation can be written as: (3) Fig. 1. Schematic representation of the crystal structure of Bi4Ti3O12. From the above equation it can be observed that the ox- idant character of the chamber (Pt crucible covered with alu- to restore neutrality [20]. Using the Kröger-Vink notation, the mina plate) increases when the dopant concentration increases. simplest compensations are: The calcined powders were analyzed for the presence of phases using X-ray diffractometer (D8-focus Brucker diffrac- (1) tometer) equipped with Cu Kα radiation. Bi4Ti3O12 powders were uniaxially pressed into discs (115 MPa) and sintered in air at 1100 °C for 1 h [17]. The lattice parameters and the unit (2) cell volume were determined by the Rietveld method using the software “Topas-academic version 4.1” [25-26]. Titanium vacancies (VBi ) could be included in Eq. (2) how- The above mentioned synthesis treatment was designed ever, it is known that titanium vacancy formation requires a to accomplish the complete dissolution of the highest niobium high amount of energy [21]. Thus, it is possible that Nb doping concentration (2 at. %) into Bi4Ti3O12. Such determination was compensation could be dominated by conduction electrons (e ) carried out by varying the dwell time of the furnace and mea- and bismuth vacancies (VBi ). suring the lattice parameters. When the complete dissolution Eqs. (1) and (2) suggest the simplest compensation mech′- is accomplished the lattice parameters do not change anymore anisms assuming that the only possible defects are holes (h●), at longer dwell times. Moreover, sintered samples were mir- free electrons (e ), Bismuth vacancies (VBi ), Titanium vacan- ror polished and the surface was observed through optical mi- cies (VTi ) and oxygen vacancies (VO ). However it is clear, croscopy. Any secondary phase is commonly observed as small that the number ′of the different mechanisms of compensation spots on the surface, but in this case, using the processing con- depend not only on the concentration of Nb5+ ions but also on ditions described above only one phase was observed. the presence of unavoidable impurities in the precursors and The microstructure analysis was carried by scanning elec- other possible defect complexes in the lattice [22-23]. tron microscopy (SEM, JEOL JSM-6701F) on polished and The aim of this work is to contrast electrical properties, chemically etched surfaces. Based on literature, the chemical overall the polarization ones, with Eqs. (1) and (2) and to seek etching essays were carried out with HF/Nh4F/H2O (molar ratio for other point defects using Electron Spin Resonance (ESR). 2:1.2:3, at 50ᵒC during 180 seconds) [27]. The average grain To simplify variables the addition of lanthanum was deliberately size was determined using the micrographs and the linear inter- avoided. It is well known that point defects have a strong influ- cept technique [24, 28]. ence on the polarization properties of a material, thus it is crucial To conduct the electrical characterization, Au-Pd elec- to have a clear model about the defect chemistry of Bi4Ti3O12. trodes were deposited by sputtering technique on each side of the discs. Copper wires were cold soldered onto the Au-Pd pads using silver paste. The entire device was covered with a high 2. Materials and methods resistivity epoxy resin (Epolyglas MPT-M3, Mexico) [29]. The room-temperature electrical conductivity, dielectric constant Bi4Ti3O12 ceramic samples both pure and doped with Nb were and dielectric loss factor were measured using LCR Meter Es- fabricated by a conventional solid-phase reaction technique.