W0265

The title compound has the fresnoite-type structure, which is of interest because of unusual low temperature magnetic properties. K2V3O8 is a two-dimensional S = 1/2 antiferromagnet (TN ~ 4K) that exhibits field-induced spin reorientation [1] and enhancement of heat transport [2]. At 110K infrared, permittivity, and heat capacity measurements [3] indicate a structural distortion occurs, which may involve a change in the coordination geometry of the magnetic V+4 ion. To better understand the details of the structural changes occurring below110K and how they might impact the interpretation of the magnetic field induced changes at lower temperature, we undertook neutron powder diffraction measurements at 5, 10, 15, 20, 30, 40, 70, 100, 110, 130, 170, 200, 230, 270, and 295K on the time-of-flight instrument Vega at the high energy accelerator research organization (KEK). For refinements made in the nominal space group P4bm, the most dramatic change in the structure is evidenced by the a cell edge, which expands initially on cooling, then abruptly begins to contract at 110K. The c cell edge contracts monotonically. The atomic displacement parameters also deviate from the expected temperature dependence at 110K, where the oxygen atoms in the vanadium oxide plane exhibit an increase in positional disorder. The overall positional disorder of the apical oxygen of the magnetic V+4 ion is also greater at all temperatures than the other oxygen atoms. Weak supercell reflections appear in the powder diffraction patterns below 40K. Bond length changes as a function of temperature indicate that the V+4O5 ion rotates as the oxygen coordination polyhedra of the K atom collapses. These and other structural changes are being used to construct a model for the low temperature structure, and relate it to the changes in the transport and dynamical properties.