Structural modulation in K2V3O8

To elucidate the phase transition at 115 K in the fresnoite-type compound K2V3O8, we undertook temperature-dependent neutron powder diffraction and single-crystal X-ray diffraction (XRD). For structure refinements in the nominal space group P4bm, the most dramatic change is evidenced by the a cell edge, which initially expands on cooling, then abruptly begins to contract at 115 K. The c cell edge contracts monotonically. The atomic displacement parameters (ADPs) also deviate from their expected temperature dependence at 115 K, where the oxygen atoms in the vanadium oxide plane exhibit an increase in apparent positional disorder. Similar changes in lattice parameters and ADPs are observed from the single-crystal XRD refinements. Below 115 K, weak superlattice reflections are clearly evident in XRD patterns recorded by a CCD detector, and these extra reflections can be indexed with the wave vector ±1/3〈110〉*+1/2c*. Possible space groups for the modulated structure are P42bc and P4nc.