Crystal structure and thermodynamic properties of dipotassium diiron(III) hexatitanium oxide

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Crystal structure and thermodynamic properties of dipotassium diiron(III) hexatitanium oxide A. V. Knyazev • N. G. Chernorukov • I. A. Letyanina • Yu. A. Zakharova • I. V. Ladenkov

Received: 24 April 2012 / Accepted: 13 July 2012 / Published online: 9 September 2012  Akade´miai Kiado´, Budapest, Hungary 2012

Abstract In the present work, the temperature dependence of heat capacity of dipotassium diiron(III) hexatitanium oxide has been measured for the first time in the range from 10 to 300 K by means of precision adiabatic vacuum calorimetry. The experimental data were used to calculate standard thermodynamic functions, namely the heat capacity Cp ðTÞ, enthalpy H  ðTÞ  H  ð0Þ, entropy S ðTÞ  S ð0Þ; and Gibbs function G ðTÞ  H  ð0Þ for the range from T ? 0 to 300 K. The structure of K2Fe2Ti6O16 is refined by the Rietveld ˚, method: space group I4/m, Z = 1, a = 10.1344(2) A 3 ˚ ˚ c = 2.97567(4) A, V = 305.618(7) A . The high-temperature X-ray diffraction was used for the determination of coefficients of thermal expansion. Keywords Dipotassium diiron(III) hexatitanium oxide  Adiabatic vacuum calorimetry  Heat capacity  Thermodynamic functions  X-ray diffraction

Introduction Materials with the hollandite structure have been studied for a range of applications including their use as adsorbents [1],

Electronic supplementary material The online version of this article (doi:10.1007/s10973-012-2606-x) contains supplementary material, which is available to authorized users. A. V. Knyazev (&)  N. G. Chernorukov  I. A. Letyanina  Yu. A. Zakharova  I. V. Ladenkov Nizhny Novgorod State University, Gagarin Prospekt 23/2, 603950 Nizhny Novgorod, Russia e-mail: [email protected]

as radioactive waste form materials [2], as ionic conductors [3], as inorganic pigments [4], and as catalysts [5]. The ideal hollandite structure has tetragonal symmetry (space group I4/m) and stoichiometry Mkx(AzyTi8-y)O16 where Mk is a large, low valent cation (e.g., Na?, K?, Rb?, Cs?, Ba2?) and Az is a smaller cation that can adopt octahedral coordination (e.g., Mn2?, Al3?, Ti3?, Fe3?). The basis of the hollandite structure is the square tunnel enclosed by columns of two edge-sharing octahedra which in turn share corners to form two-by-two tunnels running parallel to the short axis of the structure. The titanium and A-type cations are at the center of the edge-sharing (A/Ti)O6 octahedra with the eight-coordinate M-type cations in the channels. The archetypal hollandite structure is tetragonal; however, many hollandites are actually monoclinic. The monoclinic distortion occurs when the tunnel ions are unable to support the octahedral tunnel walls and these collapse into the tunnel. In this collapse, the octahedral tunnel walls behave more or less as a rigid framework with the corner linkages acting as hinges. As a general rule, hollandites with large M ions and small A ions are tetragonal, whereas those with small M ions and large A ions are monoclinic at room temperature. It should be noted that rM/rA/Ti = 2.08 corresponds to the