Spontaneous lattice distortion in HoB4


The rare earth tetraboride HoB4 was studied in detail by using X-ray powder diffraction in the temperature range of 3.5 − 8 K to investigate the crystal structure of this compound below its Neel temperature TN1 = 7.1 K. The system undergoes a first order transition of magnetic nature at TN2 = 5.7 K, yet the crystal structure is already distorted in between these two magnetic transitions. In fact, crystalline unit cell changes from the tetragonal symmetry with P4/mbm space group to a monoclinic symmetry. The ground state of this tetraboride compound at 3 K found to be best described by the space group P21/b. Furthermore, large bifurcation in unit cell volume between the warming and cooling cycle was observed, if the data are treated with tetragonal crystal symmetry - providing an additional support that the crystal structure is distorted indeed.