Anomalous bulk modulus in vanadate spinels

All single-valent oxide spinels are insulators. The relatively small activation energy in the temperature dependence of resistivity in vanadate spinels led to the speculation that the spinels are near the crossover from localized to itinerant electronic behavior, and the crossover could be achieved under pressure. We have performed a number of experiments and calculations aimed at obtaining information regarding structural changes under high pressure for the whole series of vanadate spinels, as well as transport and magnetic properties under pressure for MgV2O4. We have also studied the crystal structure under pressure of wide-gap insulators ACr2O4 (A=Mg, Mn, Fe, Zn) for comparison. Moreover, the relationship between the bulk modulus and the cell volume of AV2O4 (A=Mg, Mn, Fe, Co, Zn) has been simulated by a density functional theory calculation. The proximity of AV2O4 spinels to the electronic state crossover under high pressure has been tested by three criteria: (1) a predicted critical V-V bond length, (2) the observation of a sign change in the pressure dependence of Néel temperature, and (3) measurement of a reduced bulk modulus. The obtained results indicate that, although the crossover from localized to itinerant π bonding V-3d electrons in the AV2O4 spinels is approached by reducing under pressure the V-V separation R, the critical separation Rc is not reached by 20 GPa in CoV2O4, which has the smallest V-V separation in the AV2O4 (A=Mg, Mn, Fe, Co, Zn) spinels.