Diffused phase transition in fine-grained bismuth vanadate ceramics
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Diffused phase transition in fine-grained bismuth vanadate ceramics K. Shantha and K.B.R. Varmaa) Materials Research Centre, Indian Institute of Science, Bangalore-560 012, India (Received 18 November 1998; accepted 13 September 1999)
Nanocrystalline powders of ferroelectric bismuth vanadate, Bi4V2O11 (n-BiV), with crystallite size less than 50 nm, were obtained by mechanical milling of a stoichiometric mixture of bismuth oxide and vanadium pentoxide. The n-BiV powders on sintering yielded high-density, fine-grained ceramics with improved dielectric and polar characteristics. Dielectric studies on samples obtained from milled powders indicated that the ferroelectric-to-paraelectric phase transition temperature is strongly frequency dependent. The Curie–Weiss law is found to be valid only at a temperature away from the transition temperature, confirming the diffused nature of the transition, which is attributed to the presence of compositional inhomogeneity, because of partial reduction of vanadium.
I. INTRODUCTION
Bismuth vanadate, Bi4V2O11 (BiV), is an important member of the Aurivillius family of bismuth-based layered-structured oxides, consisting of (Bi2O2)2+ layers interleaved with perovskite-like sheets of (VO3.5䡺0.5)2−.1,2 BiV exhibits two polymorphs ␣ →  (720 K) and  → ␥ (840 K).3 The most attractive feature of this compound is its strong polar response4,5 and high ionic mobility,6 which are generally incompatible in most ferroelectrics. It finds a wide variety of applications in catalysts,7 gas sensors,8 and solid-state electrolytes,9 as electrode materials for lithium rechargeable batteries10 and pyroelectric detectors.4 It is known that the dielectric and polar properties and the nature of phase transitions in ceramics are very sensitive to their microstructure.11,12 As a result, studies concerning the effects of grain size on the dielectric and polar properties of ferroelectric materials are interesting from both scientific and technological points of view. In recent years, there has been an increasing interest in the field of processing of ceramic components from ultrafine-grained powders which exhibit improved sinterability and hence yield fine-grained ceramics with improved electrical properties.13–16 Hence, we have fabricated fine-grained ceramics from nanopowders of BiV (n-BiV) and studied their dielectric and polar properties. Some of the interesting observations that were made during our investigations are reported in this paper. The main chemical complexity associated with BiV is related to the ability of V5+ to be easily reduced to V4+,
a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 14, No. 12, Dec 1999
during thermal treatment or when the oxygen partial pressure is modified. This leads to the formation of mixed valence phases of the form (Bi2O2)2 (V4+ xV5+ 2−x) O7−x, where x varies between 0 and 0.33, i.e., all phases between Bi4V2O11 and Bi4V2O10.66 can exist.17 So it is very difficult t
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