Binary compounds R-M (R – rare earth and M – 3d transition metal) are interesting due to interplay between the lanthanide 4f and 5d electrons and the transition-metal 3d electrons. It has already brought many application perspectives as hard permanent magnets, magnetocaloric cooling, sensors, magnetostrictive devices etc. The very important aspect is an understanding to the behavior of this interplay which is not fully understood yet. One example of not fully understood behavior are structure changes in RCo2 at temperature of magnetic transitions. HoCo2 belongs to the family of RCo2 compounds which crystallize in the cubic MgCu2–type Laves phase (space group Fd-3m). HoCo2 single crystal orders magnetically at TC = 83 K. Together with the ordering of the Ho magnetic moments at TC also a metamagnetic transition occurs where the induced Co magnetic moments appear and orient antiparallel to the Ho ones [1]. The magnetic structure below TC is ferrimagnetic with the easy axis along the crystallographic [100] direction [2,3]. At temperatures below this point a tetragonal distortion (I41/amd space group) is present. The transition at TC is first order type, similar to some other RCo2 compounds – DyCo2 and ErCo2 [4,5].
Among other RCo2’s the HoCo2 compound (together with NdCo2) is an exception while it possess another magnetic transition at the spin reorientation temperature TSR = 14 K [2]. At TSR the easy direction of magnetization changes from [100] to [110] crystallographic direction upon cooling [2,3] and another crystallographic distortion occurs at this temperature. The crystallographic structure below TSR is not clearly determined so far. The statements of the orthorhombic distortion below TSR in various references are in agreement [2,6]. Nevertheless, the determination of a space group of the orthorhombic structure was not convincingly stated so far. According to the authors of the Ref [6] the fits of the X-ray powder diffraction pattern provide nearly identical quality when refined using either the Fddd or the Imma space group [6]. The calculations of total energy prefer Fddd space group which has a slightly smaller energy than the Imma space group [6].
We were successful in growing of the HoCo2 single crystal and subjected it to low temperature X-ray powder- and single-crystal diffraction study. We faced to the same problem with X-ray powder diffraction pattern of HoCo2 to distinguish space groups Fddd and Imma as mentioned above. We have in addition performed the measurement of selected reflections of the single crystal at low temperatures around TSR in order to determine the space group more reliably. The most significant observed splitting of the 555 reflection corresponds to the Imma space group. The fact that for Fddd space group the reflection 555 does not split allow us to exclude this possibility and state clearly that the orthorhombic Imma space group is adopted in HoCo2 below TSR.