Morphotropy, isomorphism, and polymorphism of Ln 2 M 2 O 7 -based ( Ln = La-Lu, Y, Sc; M = Ti, Zr, Hf, Sn) oxides
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Morphotropy, Isomorphism, and Polymorphism of Ln2M2O7Based (Ln = La–Lu, Y, Sc; M = Ti, Zr, Hf, Sn) Oxides A. V. Shlyakhtina Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow 119991, Russia email: [email protected], [email protected] Received June 1, 2011
Abstract—Structural studies of compounds of variable composition and measurements of their conductivity have made it possible to identify new oxygenionconducting rareearth pyrochlores, Ln2Ti2O7 (Ln = Dy–Lu) and Ln2Hf2O7 (Ln = Eu, Gd), with intrinsic hightemperature oxygen ion conductivity (up to 1.4 × 10–2 S/cm at 800°C). Twenty six systems have been studied, and more than 50 phases based on the Ln2M2O7 (Ln= La–Lu; M = Ti, Zr, Hf) oxides have been synthesized and shown to be potential oxygen ion conductors. The morphotropy and polymorphism of the Ln2M2O7 (Ln = La–Lu; M = Ti, Zr, Hf) rareearth pyrochlores have been analyzed in detail for the first time. Thermodynamic and kinetic (growthrelated) phase transitions have been classified with application to the pyrochlore family. DOI: 10.1134/S1063774513020259
INTRODUCTION In the 1980s, the Ln2Zr2O7 (Ln = Nd, Sm, Gd) rareearth zirconates with the pyrochlore structure were first shown to undergo a pyrochlore–defect fluo rite (order–disorder) phase transition [1], resulting in rather high oxygen ion conductivity of their hightem perature phase [2], comparable to the conductivity of 9 mol % Y2O3stabilized ZrO2. The Ln2Hf2O7 (Ln = Nd, Sm–Gd) hafnates and R2Ti2O7 (R = Y, Gd–Lu) titanates with the pyrochlore structure have been stud ied in much less detail. Also, there is insufficient, frag mentary information about the phase transitions and the associated hightemperature oxygen ion conduc tivity of the rareearth pyrochlores. This paper sum marizes and systematizes the results of many years of a targeted search for pyrochlorestructure solid electro lytes in the Ln2O3–MO2 (Ln = Sm–Lu; M = Ti, Zr, Hf) systems, which involved structural characteriza tion of rareearth pyrochlores synthesized at different temperatures and measurements of their electrical conductivity. Efforts in this direction have enabled the preparation of new pyrochlorestructure oxygen ion conductors, stable in the range 700 to 900°С and potentially attractive for practical applications as elec trolytes and anodes of solid oxide fuel cells (SOFCs). Studies of a wide range of compounds of variable com position have made it possible to classify the thermo dynamic and kinetic (growthrelated) phase transi tions of the Ln2M2O7 (Ln = La–Lu, M = Ti, Zr, Hf) mixed oxides and understand distinctive features of the morphotropy and polymorphism of these com pounds.
1. MORPHOTROPY IN RAREEARTH TITANATE, ZIRCONATE, AND HAFNATE SERIES Morphotropy is an abrupt change in crystal struc ture in a systematic series of chemical compounds, with no changes in the quantitative relationship between their structural units [3]. The morphotropic phase transitions in series of rareearth
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