Isostructural Phase Transition in β-YbV4O8

Advanced Nano Materials Laboratory, National Institute for Materials Science* Departamento Fisica Materia Condensada, Universidad del Pais Vasco, Spain**
â—‹Yasushi Kanke* Andrzej Grzechnik** Karen Friese**

The α- and β-phases of MV4O8 (M=Y3+, Yb3+) are composed of essentially identical V4O8 octahedral frameworks and differ only in the arrangement of the trivalent cations [1,2]. As a consequence, the one-dimensional disorder and the co-existence of the two polytypic modifications is frequently observed [3,4]. Magnetic susceptibility [4] and specific heat measurements show an anomaly for β-YV4O8 at approximately 190 K. Due to magnetic impurities, the corresponding measurements could not be carried out on β-YbV4O8.
The diffraction intensities of β -YbV4O8 have been measured on a single crystal in the temperature range from 290 K to 100 K at the Single Crystal Diffraction Beamline at the Institute for Synchrotron Radiation, Anka, Karlsruhe, Germany. The space group symmetry (space group A21/d11 with lattice parameters a=9.030(5), 21.44(3), c=5.752(2) at 290K, [1]) does not change in the whole temperature range, yet significant discontinuities in the lattice parameters indicate an isostructural phase transition between 200 and 175 K. Atomic coordinates also show significant and discontinuous changes, which can most probably be attributed to a different tilting of the octahedral framework before and after the phase transition. Another striking change concerns the anisotropic displacement parameters of part of the oxygen atoms, as the longest axes of the displacement ellipsoids clearly change their orientation.

[1] Y. Kanke, K. Kato (1997), Chem. Mater. 9, 141-147.
[2] K. Kato, Y. Kanke, K. Friese (1997), Z. Krist. 212, 110-114.
[3] K. Friese, O. Jarchow, K. Kato, Y. Kanke (1997), Z. Krist. 212, 859-863.
[4] M. Onoda, A.-C. Dhaussy, Y. Kanke (2003), Acta. Cryst. B59, 429-438.