Suppression of the valence transition in solution-grown single crystals of Eu$_2$Pt$_6$Al$_{15}$

TL;DR

This study reports a new polymorph of Eu$_2$Pt$_6$Al$_{15}$ grown by solution methods that suppresses the valence transition seen in arc-melted samples, enabling Eu$^{2+}$ magnetic ordering at 14 K.

Contribution

The paper introduces a solution-grown polymorph of Eu$_2$Pt$_6$Al$_{15}$ with suppressed valence transition, highlighting the influence of crystal stacking and thermal treatment on valence behavior.

Findings

Solution-grown crystals do not exhibit the valence transition seen in arc-melted samples.

Eu$^{2+}$ moments order antiferromagnetically at around 14 K in the new polymorph.

Different thermal treatments do not significantly alter the intrinsic properties of the solution-grown crystals.

Abstract

The study of Eu intermetallic compounds has allowed the exploration of valence fluctuations and transitions in 4f electron systems. Recently, a Eu$_2$Pt$_6$Al$_{15}$ phase synthesized by arc-melting followed by a thermal treatment was reported [M. Radzieowski \textit{et al.}, J Am Chem Soc 140(28), 8950-8957 (2018)], which undergoes a transition upon cooling below 45~K that was interpreted as a valence transition from Eu$^{2+}$ to Eu$^{3+}$. In this paper, we present the discovery of another polymorph of Eu$_2$Pt$_6$Al$_{15}$ obtained by high temperature solution growth, which presents different physical properties than the arc-melted polycrystalline sample. Despite the similarities in crystal structure and chemical composition, the Eu valence transition is almost fully suppressed in the solution-grown crystals, allowing the moments associated with the Eu$^{2+}$ state to order antiferromagnetically at around 14~K. A detailed analysis of the crystal structure using single crystal X-ray diffraction reveals that, although the solution grown crystals are built from the same constituent layers as the arc-melted samples, these layers present a different stacking. The effect of different thermal treatments is also studied. Different anneal procedures did not result in significant changes in the intrinsic properties, and only by arc-melting and quenching the crystals we were able to convert them into the previously reported polymorph.