Cation Dimerization in a 3d Honeycomb Lattice System.

In one-dimensional systems with partially filled valence bands, simultaneous changes occur in the electronic states and crystal structures. This is known as the Peierls transition. The Peierls transition (cation dimerization) in VO, which has a quasi-one-dimensional structure, is well-known, and its mechanism has been extensively discussed. Honeycomb lattices exhibit the Peierls instability owing to their low dimensionality. However, cation dimerization is rare in the 3d honeycomb lattice system. Here, we perform an in-depth examination of the V-V dimerization (formation of V-V direct bond) in ilmenite-type MgVO, which is a 3d honeycomb lattice system. A ladderlike pattern was observed in the V-V dimers through synchrotron X-ray experiments at temperatures below 500 K. This dimerization was accompanied by a magnetic-to-nonmagnetic transition. Moreover, a valence bond liquid phase may exist at 500-600 K. Our results reveal the behavior of the valence electrons in the 3d honeycomb lattice system.