Single crystalline EuPd$2$Si$2$: Exploring the effect of Ge-doping on the valence transition

The study of collective phenomena raising from enhanced coupling between electrons and phonons is focussed on materials which exhibit phase transitions involving both, electronic and lattice-degrees of freedom. One system providing such a strongly coupled phase transition is EuPd$_2$Si$_2$ of the ThCr$_2$Si$_2$ structural type. This compound shows a temperature induced valence transition of europium between the energetically vicinal valence states Eu$^{2+}$ and Eu$^{3+}$ at about 170 K [1] which is connected to a strong change in the a lattice parameter. First reports on the synthesis of single crystals came up only recently [2], but a deep investigation of the valence transition in this compound is still missing, especially the location of the system in the generalized phase diagram is under investigation. We aim to tune the system by negative chemical pressure towards a possible second order critical endpoint in this phase diagram via substitution of Si by Ge.
We applied the Czochralski method using a levitating melt under enhanced Ar pressure for the successful growth of mm-sized single crystals of EuPd$_2$Si$_2$. Recently, this procedure was developed and successfully applied to the single crystal growth of the ferromagnetic quantum-critical compound YbNi$_4$P$_2$ [3]. In this contribution, we will present the characterization of EuPd2Si2 crystals as well as of the Ge substituted system EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ for different substitution levels x. The combination of careful chemical and structural analysis with magnetization, thermodynamic and transport measurements shows that EuPd$_2$Si$_2$ grows in a homogenity range and that the physical properties are very sensitive to minor changes in the composition of the sample.

[1] E. V. Sampathkumaran et al., Journal of Physics C 14, L237 (1981).
[2] Y. Onuki et al., Philosophical Magazine 97, 3399 (2017).
[3] K. Kliemt, C. Krellner, J. Cryst. Growth 449, 129 (2016).