Ferroelectric and Dielectric Properties of Strontium Bismuth Niobate Vanadates
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Strontium bismuth niobate vanadates, SrBi2(Vx Nb1−x)2O9 (with 0 艋 x 艋 0.1), were prepared by reaction sintering of powder mixtures of constituent oxides. With partial substitution of niobium by vanadium cations (up to 10 at.%), the single-phase layered perovskite structure was preserved, and the sintering temperature of the system was significantly lowered (∼200 °C). The incorporation of vanadium into the layered perovskite structure resulted in a shift of the Curie point to higher temperatures from 435 to 457 °C, with 10 at.% vanadium doping, and an increase in dielectric constant from ∼700 to ∼1100, with 10 at.% vanadium doping, at their respective Curie points. The remanent polarization increased from ∼2.4 to ∼8 C/cm2, while the coercive field decreased from ∼63 to ∼45 kV/cm with 10 at.% V5+ doping.
I. INTRODUCTION
Ferroelectrics are excellent candidates for the applications in data storage in digital memory systems, in addition to many other important applications such as piezoelectrics, pyroelectrics, and electro-optics in sensors, actuators, and microelectromechanical systems (MEMS).1–5 Random-access memories (RAMs) based on semiconductor integrated technology have been a great success; however, these semiconductor RAMs can retain information only when power is on. A serious drawback is that when power is interrupted, all information is lost (volatile memory). Furthermore, these RAMs are very sensitive to radiation and this is detrimental for military and space applications. Among ferroelectric materials, Pb(Zr,Ti)O3 (PZT) is one of the most popular materials attracting many investigators for many years. PZT has an isotropic perovskite crystal structure with a high remanent polarization (30–50 C/cm2).6–8 Unfortunately, PZT films tend to degrade most of the initial amount of switching charge (so-called “fatigue”) after 106–108 cycles of full polarization switching.9 To be competitive with electrically erasable read-only memories (EEPROM), ferroelectric memories (FeRAMs), which are also called nonvolatile random-access memories (NvRAMs), must be improved to withstand at least 1012 erase/rewrite operations or they must have qualitatively different nondestructive read operations.3 Recently, bismuth oxide layered perovskite materials, such as SrBi2Nb2O9 (SBN), SrBi2Ta2O9
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Address all correspondence to the author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 7, Jul 2000
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(SBT), and SrBi2(Nb,Ta)2O9 (SBTN), for FeRAM applications have attracted an increasing attention in the research community, because they are fatigue-free and lead-free and possess ferroelectric properties independent of film thickness.10,11 Layered perovskite ferroelectrics, however, suffer from two drawbacks: a relatively low remanent polarization and a high processing temperature.12 Recently, efforts have been made to enhance the properties of layered perovskite ferroelectrics by the addition or substitution of alternative cations. For example, partial substitution of
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