High-temperature properties of titanium-substituted yttrium niobate
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FOCUS ISSUE
THERMODYNAMICS OF COMPLEX SOLIDS
High-temperature properties of titanium-substituted yttrium niobate Aleksandra Mielewczyk-Gry n1,a)
, Piotr Winiarz1, Sebastian Wachowski1, Maria Gazda1
1
Department of Solid State Physics, Faculty of Applied Physics and Mathematics, Gda nsk University of Technology, Gda nsk 80-233, Poland Address all correspondence to this author. e-mail: [email protected]
a)
Received: 28 March 2019; accepted: 6 May 2019
The defect fluorite titanium-doped yttrium niobate samples Y3Nb1−xTixO7−d have been synthesized and investigated by the means of high-temperature X-ray diffraction, dilatometry, and thermogravimetry. Thermal expansion coefficients (TECs) as well as chemical expansion coefficients for material with 5, 10, and 15 mol% of titanium were determined. All investigated samples exhibit chemical contraction caused by Ti doping. The values of TECs obtained by two different methods show similar results, which suggests the isotropy of the polycrystalline ceramic. Thermogravimetric studies have shown that all of the compositions exhibit a mass increase upon being exposed to a humid air atmosphere. The total proton concentration calculated on the basis of these results was in the range of 0.1 mol%. Moreover, titanium content influences chemical expansion coefficient, water uptake, and protonic defects concentration, whereas it does not significantly affect the values of TECs.
Introduction Ceramics based on the rare-earth compounds have extensive potential and unique features and have been applied in numerous uses such as the optical, magnetic, electrical, catalytic, chemical, and structural materials [1]. Yttrium niobate Y3NbO7 crystallizes in the defect fluorite (CaF2) cubic structure (space group no. 225, Fm3m) and is one of many niobates widely investigated for various phenomena observed in these materials, e.g., luminescence [2, 3] or ionic transport [4]. The fluorite unit cell of MO2 oxides contains M4þ 4 O8 . If the four tetravalent metal ions are replaced by three trivalent ions (RE) and one pentavalent M51 ion, one oxygen vacancy per unit cell is formed. As a result of the substantial differences in ionic radii between the RE31 and M51 ions, cation ordering occurs on the metal sites and the oxygen-vacancy orders on the anion sites [5]. It has been shown that Y3NbO7 is an oxygen ion conductor in a wide range of oxygen partial pressure [6] but also exhibits proton conductivity [7, 8] remaining stable at high temperature under both oxidizing and reducing atmospheres [9]. Additionally, at low oxygen partial pressures, the n-type conductivity is observed, giving a significant contribution to total conductivity [10]. Materials exhibiting ion conductivity are currently the subject of wide research activity, driven by their technological
ª Materials Research Society 2019
applications within solid oxide fuel cells (SOFCs), oxygen separation membranes, and gas sensors. The previous studies have shown that in the case of most rare-earth niobates the ionic conductivity occurs, both
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