Single crystal growth and properties of Au- and Ge-substituted EuPd$_2$Si$_2$

TL;DR

This work reports Czochralski-grown single crystals of EuPd$_2$Si$_2$ with Au and Ge substitutions to probe valence fluctuations and magnetic ordering under chemical pressure. Au incorporation is limited and leads to lattice expansion and side-phase formation, with no evidence for a first-order valence transition or intrinsic AFM order of the target Eu(Pd$_{1-x}$Au$_x$)$_2$Si$_2$ phase up to $x_{ m nom}=0.2$, while the valence crossover temperature $T^{\prime}_v$ shifts with composition. In contrast, Ge substitution near $x_{ m nom}=0.2$ places the system at the edge of valence crossover and reveals pronounced compositional inhomogeneity, including the discovery of a quaternary side phase EuPd$_{1.42}$Si$_{1.27}$Ge$_{0.31}$ that orders AFM at $T_N=17$ K. The results highlight extreme sensitivity of Eu-based valence phenomena to tiny compositional variations and demonstrate the necessity of micro-focused chemical mapping (μ-XRF) to interpret magnetic and thermodynamic measurements, with implications for exploring critical elasticity near valence transitions.

Abstract

We report on the single crystal growth of Eu(Pd$_{1-x}$Au$_x$)$_2$Si$_2$, $0< x\leq 0.2$, from a levitating Eu-rich melt using the Czochralski method. Our structural analysis of the samples confirms the ThCr$_2$Si$_2$-type structure as well as an increase of the room temperature $a$ and $c$ lattice parameters with increasing $x$. Chemical analysis reveals that, depending on the Au concentration, only about 25-35\% of the amount of Au available in the initial melt is incorporated into the crystal structure, resulting in a decreasing substitution level for increasing $x$. Through Au substitution, chemical pressure is applied and large changes in valence crossover temperatures are already observed for low substitution levels $x$. In contrast to previous studies, we do not find any signs of a first-order transition in samples with $x_{\rm nom}=0.1$ or AFM order for higher $x$. Furthermore, we observe the formation of quarternary side phases for a higher amount of Au in the melt. In addition, cubic-mm-sized single crystals of EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ with $x_{\rm nom}=0.2$ were grown. The analysis of the X-ray fluorescence revealed that the crystals exhibit a slight variation in the Ge content. Such tiny compositional changes can cause changes in the sample properties concerning variations of the crossover temperature or changes of the type of the transition from crossover to magnetic order. Furthermore, we report on a new orthorhombic phase EuPd$_{1.42}$Si$_{1.27}$Ge$_{0.31}$ that orders antiferromagnetically below $17\,\rm K$.

Figures (17)

• Figure 1: Czochralski growth from a levitating melt. Inset: Result of the Czochralski growth process of a sample EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$ with a nominal Ge concentration of $x = 0.20$ on a mm-grid. S marks the seed crystal, A the area where the target phase crystallizes, and B the part where inclusions of a secondary phase appear. In part C, the side phase EuPd$_{1.42}$Si$_{1.27}$Ge$_{0.31}$ occurs exclusively.
• Figure 2: Eu(Pd$_{1-x}$Au$_x$)$_2$Si$_2$, $x_{\rm nom}=0.1$ (a) SEM image of an oriented cut and polished single crystal. The red cross marks the location where the Laue diffraction pattern presented in (b) was recorded. The sharp spots in the diffraction pattern indicate the high crystallinity of the grown sample.
• Figure 3: Temperature dependence of the lattice parameters of Eu(Pd$_{1-x}$Au$_x$)$_2$Si$_2$, $x_{\rm nom}=0.1$. Corresponding PXRD data are shown in Fig. \ref{['TT-PXRD_Au-subst_EuPd2Si2']}.
• Figure 4: Eu(Pd$_{1-x}$Au$_x$)$_2$Si$_2$ (a) Temperature dependence of the heat capacity, $x=0$ data were taken from Ref. Peters2023. The upper inset shows the time dependence of a long heat pulse for $x=0.1$. The lower inset shows an enlarged view of the data below 70 K. $T_m^1$, and $T_m^2$ mark the temperatures where magnetic transitions of a side phase appear. (b) Temperature dependence of the magnetic susceptibility in $\mu_0H=0.1\,\rm T$. The data for $x=0$ were taken from Kliemt2022a and $x=0.15$ (gray) from Segre1982. Arrows mark the crossover temperatures $T^\prime_v$ for the samples with the different Au concentrations.
• Figure 5: EuPd$_2$(Si$_{1-x}$Ge$_x$)$_2$, $x_{\rm nom}=0.2$ with cleaved surface; (a) The EDLM image shows one large area of the same violet color indicating the presence of one single crystal grain in part A. (b) Laue image of the crystallographic $[001]$ direction recorded in part A (white cross). The pulling direction of the crystal was along $[110]$. (c) The $\mu$-XRF spectroscopy image shows the elemental distributions of Eu and Si. In part A, the homogeneous blue-violet color indicates a constant Eu-Si ratio while in part B Eu-rich phases (red-violet) occur.