Pressure Effects on Quadrupolar and Magnetic Ordering in HoB₂C₂ Observed by Single-Crystal Neutron Diffraction

High-pressure single-crystal neutron diffraction experiments were carried out on a tetragonal HoB2C2 where the antiferroquadrupolar (AFQ) order coexists with the antiferromagnetic (AFM) order competitively. The transition temperature T-N increases monotonically with increasing pressure up to 9.8 GPa, whereas T-Q decreases and vanishes at approximately 4 GPa as pressure increases, indicating that applying pressure enhances the AFM interaction and simultaneously suppresses the AFQ order. We presume that the volume shrinkage by applying pressure constrains the local atomic displacement of (B C) layers and leads to the suppression of the AFQ order. We additionally found pressure-induced phases where the ordered state was considered to be antiferromagnetic.