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 VO2, 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 3d1 honeycomb lattice system. Here, we perform an in-depth examination of the V–V dimerization (formation of V–V direct bond) in ilmenite-type MgVO3, which is a 3d1 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 3d1 honeycomb lattice system.

The nature of chemical bonds determines the electronic and magnetic properties of compounds. A metal–metal bonding (V–V dimer) and its effect on the magnetism of ilmenite-type CoVO3 were studied. Polycrystalline CoVO3 samples were synthesized using a high-pressure synthesis method. Crystal structure refinement revealed that V–V dimers exist at temperatures below 550 K in the vanadium layers. Co2+ in CoVO3 exhibits an S = 3/2 state, whereas a J eff = 1/2 state was reported in ilmenite-type CoTiO3. The existence of V–V dimers reduces the structural symmetry (from R to P ), which can change the magnetic ground state.

We successfully obtained high-quality single-domain crystals of ilmenite-type MnVO3 under high-pressure and high-temperature conditions (4 GPa and 1173 K for 1–8 h). The maximum crystal size was ∼0.5 mm. Single crystals were grown by solid-state recrystallization near the perovskite–ilmenite phase boundary under high-pressure conditions. Crystal growth occurred without fluxes or water, unlike conventional single crystal growth by high-pressure synthesis methods. Crystal structure refinement revealed that V–V dimers, produced by direct chemical bonding between vanadium ions, existed below 550 K at ambient pressure. The V–V dimerization was accompanied by a structural phase transition from rhombohedral to triclinic ilmenite-type structures. The obtained single crystals can facilitate precise analyses using resonant inelastic X-ray scattering and angle-resolved photoemission spectroscopy, which can reveal the mechanism underlying the phenomenon of V–V dimerization.

A significant negative thermal expansion (NTE) in an ilmenite-type MgVO3 ceramic is reported in this paper. MgVO3 undergoes V–V dimerization at 500–550 K accompanied by a structural phase transition from the triclinic (low-temperature) to rhombohedral (high-temperature) phase. This transition induces a large anisotropic crystal structure deformation and results in a large NTE in the ceramic sample, which is caused by the microstructural effect. The NTE was observed over a temperature range of 450–630 K, and the total volume shrinkage reached approximately −0.7%. This study demonstrates that controlling the chemical bonding is a promising method for the creation of NTE materials.