Formation of Perovskite and Pyrochlore Phases during Mechanochemical Synthesis of Lead Ferroniobate
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ation of Perovskite and Pyrochlore Phases during Mechanochemical Synthesis of Lead Ferroniobate A. A. Guseva, *, I. P. Raevskib, and V. P. Isupova aInstitute
of Solid State Chemistry and Mechanochemistry, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630128 Russia b Research Institute of Physics and Faculty of Physics, Southern Federal University, Rostov-on-Don, 344090 Russia *e-mail: [email protected] Received June 21, 2019; revised March 19, 2020; accepted April 8, 2020
Abstract—We have studied the effect of mechanical activation in lead ferroniobate synthesis on the formation of perovskite and pyrochlore phases during both mechanochemical synthesis and subsequent firing. It has been shown that, during mechanical activation, the first to form is the perovskite structure. During subsequent sintering of the material, the pyrochlore structure begins to form as well. As the firing temperature is raised to 650–750°C, the perovskite structure is again formed. We provide explanation for this process. Keywords: mechanical activation, mechanochemical synthesis, lead ferroniobate, crystal structure, perovskite, pyrochlore, sintering, X-ray diffraction DOI: 10.1134/S0020168520090083
INTRODUCTION The effect of mechanical activation and subsequent firing on phase composition was studied using lead ferroniobate synthesis as an example. Lead ferroniobate, Pb(Fe0.5Nb0.5)O3 (PFN), is a multiferroic material, comprising an electrical and a magnetic subsystem. Multiferroics have attracted a great deal of attention owing to the possibility of producing memory devices combining recording by an electric field and reading by a magnetic field [1, 2]. In addition, PFN is a promising basis for producing ceramic capacitors, piezoelectric and pyroelectric materials, and positive temperature coefficient resistors [3–8]. PFN and PFN-based materials should have the perovskite structure, and the formation of impurity phases with the pyrochlore structure during the synthesis process leads to degradation of their performance characteristics [6, 9]. PFN structure formation during the classic solidstate “thermal” synthesis (firing of a starting mixture in a furnace) begins with the formation of a pyrochlore-like structure. As the firing temperature is raised, a perovskite phase begins to form and gradually becomes predominant [9]. However, for many compounds it is impossible to bring this solid-state reaction to completion and obtain required perovskites free of pyrochlore phase impurities [9, 10]. In the case of mechanochemical synthesis, the structure formation process occurs in a somewhat dif-
ferent way [11–16]. During the mechanical activation process, the first to form is the perovskite structure. Under “mild,” low-energy activation conditions, at a relatively low ball velocity, characteristic of vibratory and ball mills, this may not occur [12, 15]. In such mills, the acceleration of balls is considerably lower and their interaction with the material being milled is considerably weaker than those in a centrifugal planetary mi
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