Timothy J. Boyle

Qualitative model for the fatigue‐free behavior of SrBi2Ta2O9

SrBi2Ta2O9 (SBT) thin films are known to exhibit no polarization fatigue with electric field cycling. However, we have discovered that optical illumination combined with a bias voltage near the switching threshold can result in significant (≳90%) suppression of the switchable polarization of SBT thin film capacitors. A similar effect has also been reported for Pb(ZrxTi1−x)O3 (PZT) capacitors. However, it is found that electric field cycling of the optically fatigued SBT capacitors results in near‐complete recovery of the suppressed polarization. In contrast, electric field cycling of optically fatigued PZT capacitors does not result in any polarization recovery. These results suggest that optical fatigue in both SBT and PZT capacitors results from pinning of domain walls due to trapping of the photogenerated carriers at domain boundaries, whereas the recovery exhibited by SBT thin films indicates that the domain walls are more weakly pinned in SBT than in PZT thin films. Consequently, the fatigue‐free beh...

Formation of SrBi 2 Ta 2 O 9 : Part I. Synthesis and characterization of a novel “sol-gel” solution for production of ferroelectric SrBi 2 Ta 2 O 9 thin films

We have developed a simple and rapid method for the synthesis of a precursor solution used in the production of SBT powders and thin films of the layered-perovskite phase SrBi{sub 2}Ta{sub 2}O{sub 9} (SBT). Precursor solution preparation takes less than 30 min and involves the generation of two solutions: (a) Bi(O{sub 2}CMe){sub 3} dissolved in pyridine and (b) Ta(OCH{sub 2}Me){sub 5} added to Sr(O{sub 2}CMe){sub 2} and then solubilized by HO{sub 2}CMe. After stirring separately for 10 min, these solutions are combined, stirred for an additional 10 min, and used without any further modifications. The individual solutions and ternary mixture were studied using a variety of analytical techniques. Films of the layered-perovskite phase were formed at temperatures as low as 700{degree}C. Ferroelectric testing of SBT films, fired at 750{degree}C, reveals standard hysteresis loops with no fatigure for up to 4{times}10{sup 9} cycles. {copyright} {ital 1996 Materials Research Society.}

FORMATION OF SRBI2TA2O9 : PART II. EVIDENCE OF A BISMUTH-DEFICIENT PYROCHLORE PHASE

A bismuth-deficient pyrochlore phase has been observed in both powder and film samples fired at 775{degree}C. The estimated stoichiometry of this pyrochlore (based on calculated diffraction patterns) was Sr{sub 0.2}(Sr{sub 0.5}Bi{sub 0.7})Ta{sub 2}O{sub 6.75}. This bismuth-deficient pyrochlore phase may be considered deleterious to the formation of the SrBi{sub 2}Ta{sub 2}O{sub 9} {open_quote}{open_quote}SBT{close_quote}{close_quote} ferroelectric compound since a significant presence of this pyrochlore compound implies a large deviation from the desired cation ratios. Additionally, films prepared on platinized silicon substrates indicate the SBT phase formation may be encouraged by the substrate; there appears to be some 00{ital l} preferential orientation for stoichiometric SBT thin films. {copyright} {ital 1996 Materials Research Society.}

Growth and morphology of cadmium chalcogenides: the synthesis of nanorods, tetrapods, and spheres from CdO and Cd(O2CCH3)2

In this work, we investigated the controlled growth of nanocrystalline CdE (E = S, Se, and Te) via the pyrolysis of CdO and Cd(O2CCH3)2 precursors, at the specific Cd to E mole ratio of 0.67 to 1. The experimental results reveal that while the growth of CdS produces only a spherical morphology, CdSe and CdTe exhibit rod-like and tetrapod-like morphologies of temporally controllable aspect ratios. Over a 7200 s time period, CdS spheres grew from 4 nm (15 s aliquot) to 5 nm, CdSe nanorods grew from dimensions of 10.8 × 3.6 nm (15 s aliquot) to 25.7 × 11.2 nm, and CdTe tetrapods with arms 15 × 3.5 nm (15 s aliquot) grew into a polydisperse mixture of spheres, rods, and tetrapods on the order of 20 to 80 nm. Interestingly, long tracks of self-assembled CdSe nanorods (3.5 × 24 nm) of over one micron in length were observed. The temporal growth for each nanocrystalline material was monitored by UV-VIS spectroscopy, transmission electron spectroscopy, and further characterized by powder X-ray diffraction. This study has elucidated the vastly different morphologies available for CdS, CdSe, and CdTe during the first 7200 s after injection of the desired chalcogenide.

Sol-gel processing of PZT thin films: A review of the state-of-the-art and process optimization strategies

Abstract Sol-gel processing has been widely employed for the fabrication of lead zirconate titanate (PZT) thin films. To successfully optimize thin film material properties for different applications, we must develop a fundamental understanding of the processing-property relationships inherent in the sol-gel fabrication process. In the asprepared state, sol-gel thin films are amorphous, have large organic contents, and can possess significant porosity. The preparation of thin films with acceptable properties requires that we effectively control densification and crystallization of the as-deposited film. We have determined that this “structural evolution” into the dense, crystalline ceramic phase may be affected by many aspects of the preparation process. In this paper we review the effects that tailored solution precursor characteristics and heat-treatment ramp rate can have on structural evolution. Specific examples are presented for process variation effects on the densification of thin films in the mod...

6Li, 7Li nuclear magnetic resonance investigation of lithium coordination in binary phosphate glasses

{sup 6}Li and {sup 7}Li solid state magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the local coordination environment of lithium in a series of xLi{sub 2}O {center_dot} (1-x)P{sub 2}O{sub 5} glasses, where 0.05 {le} x {le} 0.55. Both the {sup 6}Li and {sup 7}Li show chemical shift variations with changes in the Li{sub 2}O concentration, but the observed {sup 6}Li NMR chemical shifts closely approximate the true isotropic chemical shift and can provide a measure of the lithium bonding environment. The {sup 6}Li NMR results indicate that in this series of lithium phosphate glasses the Li atoms have an average coordination between four and five. The results for the metaphosphate glass agree with the coordination number and range of chemical shifts observed for crystalline LiPO{sub 3}. An increase in the {sup 6}Li NMR chemical shift with increasing Li{sub 2}O content was observed for the entire concentration range investigated, correlating with increased cross-linking of the phosphate tetrahedral network by O-Li-O bridges. The {sup 6}Li chemical shifts were also observed to vary monotonically through the anomalous glass transition temperature (T{sub g}) minimum. This continuous chemical shift variation shows that abrupt changes in the Li coordination environment do not occur as the Li{sub 2}O concentration is increased, and such abrupt changes can not be used to explain the T{sub g} minimum.

Phase formation and characterization of the SrBi2Ta2O9 layered-perovskite ferroelectric

Abstract The Sr-Bi-Ta-O system is of interest for thin-film non-volatile ferroelectric memories. A better understanding of the process by which the perovskite phase forms can provide insight for improved processing of this ferroelectric compound. We have prepared thin-films by a chemical method using Sr-acetate, Bi-acetate and Ta-ethoxide; cation ratios were ∼1:2:2 for Sr, Bi, and Ta, respectively. Results of in-situ crystallization studies using High-Temperature Grazing-Incidence X-ray Diffraction (HTGIXRD) have demonstrated that a fluorite structure, forming in the ∼600 – 700 °C range, acts an intermediate phase prior to the crystallization of the perovskite. Additional samples with cation ratios of –1:0.8:2 were also investigated. Results for samples prepared with the 0.8 Bi content indicated that a pyrochlore phase forms which contains a substantial deficiency in Bi compared to the composition of the perovskite phase. The structures of the pyrochlore and fluorite phases and their relation to the forma...

Aging Characteristics of a Hybrid Sol-gel Pb(Zr, Ti)O 3 Precursor Solution

The {open_quotes}aging{close_quotes} characteristics of an acetic acid/methanol solvent-based lead zirconate titanate (PZT) precursor solution, prepared by the Inverted Mixing Order (IMO) process, have been studied for an extended period of time. The changes in film properties were characterized using optical microscopy, optical scattering, and ferroelectric testing. Films generated from the IMO process exhibit an increase in thickness as a function of solution age due to chemical {open_quotes}aging{close_quotes} (esterification) of the precursor solution. This increased thickness results in a decrease in the microstructural uniformity, which affects the electrical and optical properties. In order to understand and eventually control this phenomenon, we have quantified the {open_quotes}aging{close_quotes} of this solution using a variety of analytical methods, including {sup 1}H NMR spectroscopy, pH measurements, and Fourier transform infrared (FTIR) spectroscopy. It is of note that we have discovered a method which circumvents this {open_quotes}aging{close_quotes} problem by removal of the volatile material, forming an IMO powder which can be redissolved to produce high quality PZT thin films whenever desired. {copyright} {ital 1997 Materials Research Society.}

New coordination environments for yttrium formed in situ by heterometallic bridging: Crystal structures of (C5H4SiMe3)Y[(μ-OCMe3)(μ-Me)AlMe2]2 and (Me3SiCH2)Y[(μ-CH2)2 SiMe2][(μ-OR)Li(THF)2]2

Abstract The bimetallic yttrium complexes [(C 5 R 5 )Y(μ-OCMe 3 )(OCMe 3 )] 2 react with aluminum and lithium organometallic reagents to give complexes containing new combinations of ligands held together by heterometallic bridging atoms. [(C 5 H 4 SiMe 3 )Y(μ-OCMe 3 )(OCMe 3 ) (OCMe 3 )] 2 , 1 , reacts with AlMe 3 to form the mixed-metal mixed-ligand complex (C 5 H 4 SiMe 3 )Y(μ-OCMe 3 )(μ-Me)AlMe 2 ] 2 , 2 , which contains a formally seven-coordinate yttrium linked to two four-coordinate aluminum atoms by bridging methyl and tert-butoxide ligands. Complex 2 contains two four-membered YOAlM e rings. [(C 9 H 7 )Y(μ-OR)(OR)] 2 , 3 , reacts with LiCH 2 SiMe 3 to give a mixture of products from which (Me 3 SiCH 2 )Y[(μ-CH 2 ) 2 SiMe 2 ][(μ-OR)Li(THF) 2 ] 2 , 4 , can be isolated by crystallization from hexane/THF. Complex 4 contains a pentacoordinate yttrium atom ligated by a terminal CH 2 SiMe 3 group two bridging tert-butoxide ligands, and the two bridging methylene moieties of a (μ-CH 2 ) 2 SiMe 2 group. Each lithium atom is coordinated to two molecules of THF and the alkoxide and methylene bridges. The bridging groups combine to make three fused four-membered metallacyclic rings, two YOLiC rings and one YCSiC ring. The structures of 2 and 4 were determined by single crystal X-ray diffraction studies.

Synthesis, characterization, and structural study of sterically hindered magnesium alkoxide and siloxide compounds

Abstract A series of sterically hindered Mg(OR) 2 , [OR=OC(CH 3 )(C 6 H 5 ) 2 (DPE), and OSi(C 6 H 5 ) 3 (TPS)] were synthesized from the reaction of Mg(Bu) 2 with 2 HOR in a variety of solvents. The resultant products were crystallographically characterized as Mg 3 (μ 3 -DPE)(μ-DPE) 3 (DPE) 2 ( 1 ), Mg[(μ-TPS) 2 Mg(TPS)] 2 ( 2 ), [Mg(μ-DPE)(DPE)(solv)] 2 (solv=THF, 3 ; py, 4 ), and [Mg(TPS) 2 (solv)] 2 (solv=THF, 5 ; py, 6 ). For 1 , the three metal centers are in a triangular arrangement and are bound by a single μ 3 -DPE, three μ-DPE, and two terminal DPE ligands. Two of the three Mg cations are tetrahedrally (Td) coordinated while the remaining metal center adopts a trigonal pyramidal geometry. In contrast, 2 displays a more linear arrangement of the Mg centers supported by four μ-TPS and two terminal TPS ligands. Two of the Mg cations adopt trigonal planar arrangements while the remaining central Mg exhibits a distorted Td geometry. For the dinuclear complexes ( 3 – 6 ), each metal possesses a Td geometry satisfied by two μ-OR, one terminal OR, and one coordinated solvent. 1 H NMR characterization indicates that 1 is likely fluxional in solution while 2 – 5 remain intact in solution and 6 is disrupted by pyridine to yielding a monomer.