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

TL;DR

The study demonstrates that a solution-grown polymorph of Eu$_2$Pt$_6$Al$_{15}$, despite identical composition to the arc-melted phase, exhibits a suppressed valence transition and develops antiferromagnetic order at $T_N \approx 14$ K due to a different stacking sequence of the same structural layers. Using high-temperature solution growth, SCXRD, PXRD, Mössbauer spectroscopy, and magnetic/resistivity measurements, the authors show that the SG phase forms a tripled $c$-axis superstructure with an $...ABA'CDC'...$ stacking, contrasting the AM $...AB...$ stacking. The Eu valence remains largely Eu$^{2+}$-like in the SG phase down to low temperatures, with only a small fraction increasing toward Eu$^{3+}$, which permits magnetic ordering. This work highlights crystal stacking as a nonthermal tuning parameter for valence fluctuations in Eu intermetallics and suggests synthesis routes to access and control competing valence and magnetic states in related systems.

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.

Figures (19)

• Figure 1: Schematic phase diagram for different Eu compounds as a function of temperature, $T$, and a non-thermal tuning parameter, $g$, such as pressure Honda2016Honda2018Huyan2023 or chemical composition Segre1982Seiro2011Wada1999. The solid black line represents the second-order antiferromagnetic transition, the dashed black line represents the first order the valence transition, which can reach a critical endpoint, represented by the solid black circle, beyond which the valence change becomes continuous.
• Figure 2: (a) Powder X-ray diffraction pattern of the Eu$_{2}$Pt$_{6}$Al$_{15}$ solution-grown phase (black), the best fit obtained by Rietveld refinement (red), the residues (green), and the peak positions (blue). A photograph of two crystals is also shown. (b) Powder X-ray diffraction pattern of the Eu$_{2}$Pt$_{6}$Al$_{15}$ phase obtained after arc-melting the SG crystals (black), the best fit obtained by Rietveld refinement (red), the residues (green), and the peak positions (blue). A photograph a piece of the arc-melted button is also shown; note the faceting on the arc-melted piece.
• Figure 3: Main panel: Resistance measured upon warming normalized to its value at 250 K of the Eu$_2$Pt$_6$Al$_{15}$ phase obtained by solution growth with the current applied perpendicular (blue) and parallel (red) to $c$. Inset: enlarged scale in order to indicate the position of the resistance maxima (vertical black line), corresponding to $T_N$. The uncertainty (indicated by the gray region) is determined by the difference between the maxima for both current directions.
• Figure 4: (a) Temperature dependent ZFC and FC magnetization by the applied field of 10 kOe of the solution-grown Eu$_2$Pt$_6$Al$_{15}$ phase with the field applied perpendicular (blue) and parallel (red) to $c$, as well as for the polycrystalline average (magenta, see main text for details). The inset shows the polycrystalline average in an enlarged scale, with the transition temperature indicates with a black line and the uncertainty with a gray rectangle. (b) Polycrystalline average of the inverse susceptibility as a function of temperature for the solution-grown Eu$_2$Pt$_6$Al$_{15}$ phase (magenta); the linear fit is plotted with a cyan line.
• Figure 5: Magnetization of Eu$_{2}$Pt$_{6}$Al$_{15}$ as a function of the applied field, oriented perpendicular (blue) and parallel (red) to $c$, measured at 2 K (main panel) and at 30 K (inset).