Diploma - Max Planck Institute for Solid State Research

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4 EuRh 2 Si 2 – semi-localized electrons
4.1 Overview – properties and classification
The ternary compound EuRh 2 Si 2 crystallizes in the tretragonal body-centered ThCr 2 Si 2
structure. The respective lattice parameters and Wyckoff positions are given in tab. 4.1
(experimental and calculated relaxed 1 parameters).
Surprisingly, the differences between
the computionally relaxed parameters and the experimental ones are rather small
(an indication of the well-chosen basis set in FPLO and keeping the 4f occupation fixed
a suitable approximation – at least for total energy). The crystal structure is depicted
in fig. 4.5a, whereas the tretagonal unit cell is bordered by dotted lines. It is evident,
that this is a layered structure with respect to the [001] direction.
The main transport properties and the phase diagram of EuRh 2 Si 2 have already
been explored [3], hence only a short review is given here. Since similar ternary compounds
show for example Heavy-Fermion behaviour (YbRh 2 Si 2 in [5]), superconductivity
(CeRh 2 Si 2 in [6]), mixed-valent (EuPd 2 Si 2 in [37, 38]) or spin-density wave behaviour
(EuFe 2 As 2 in [4]) one expects also interesting electronic properties and magnetic behaviour
in EuRh 2 Si 2 . Belonging to the stable divalent europium materials it reveals
an antiferromagnetic (AF) ordered phase of the localized 4f 7 moments below 25 K, the
exact configuration of which is unknown.
It is supposed to be a ferromagnetic coupling
in the Eu plane and an AF order between the respective layers [3]. The energy
gain of magnetic order is small and thus yet a small pertubation induces a different
magnetic order or partially non-magnetic contribution to the ground state. Therefore
1 In the case of the lattice constants the computational relaxation has been performed by a self-written
implementation of the gradient method minimizing the total energy. Plotting the energy functional
for a deviation of 15% from the experimental lattice parameters shows a smooth energy surface.
The Wyckoff positions were force-minimized afterwards by the implemented procedure in FPLO. In
both cases, the total error of the computation has been smaller than the experimental one and the
former has been rounded to the accuracy of the latter.
Table 4.1: (a) lattice constants and (b) Wyckoff positions: experimental [36] and calculated
relaxed parameters (fplo 9.07.41, LDA, 4f 7 unpolarized open core)
(a) lattice constants
exp. [Å] relaxed [Å]
a x 4.107 4.089
a y 4.107 4.089
a z 10.25 10.244
(b) Wyckoff positions
exp.
relaxed
x y z x y z
Eu 0 0 0 0 0 0
Rh 0 0.5 0.25 0 0.5 0.25
Si 0 0 0.375 0 0 0.375