Hexagonal EuPt6Al16: Structure & Magnetism

• The paper reports synthesis and single-crystal X-ray diffraction revealing precise hexagonal lattice parameters and unusual partial site occupancies.
• A low residual resistivity ratio (<1.5) indicates temperature-independent electron scattering due to significant crystallographic disorder.
• Magnetization data confirms Eu2+ ions exhibit long-range antiferromagnetic order below 3 K with metamagnetic transitions near 12 kOe.

The hexagonal structure type EuPt$_6$Al$_{16}$ represents a previously unreported intermetallic phase discovered through the synthesis and comprehensive characterization of single crystalline Eu$_{0.8}$Pt$_6$Al$_{15.8}$. Distinguished by its unique crystallographic disorder and magnetic order, this compound expands the known taxonomy of rare-earth aluminum platinides and offers a model system to interrogate the interplay between crystal structure, electronic transport, and antiferromagnetic ordering.

1. Crystallographic Structure

EuPt$_6$Al$_{16}$ crystallizes in a hexagonal symmetry, specifically in the non-centrosymmetric space group P$\overline{6}$2m. The refined unit cell parameters measured by single-crystal X-ray diffraction are $a = 14.2254(6)$ Å, $c = 8.7363(4)$ Å, with a unit cell volume of 1531.04(12) Å$^3$ and $Z=4$ formula units per cell. The formula is best represented as Eu$_{1-x}$Pt$_6$Al$_{16-y}$, though the experimentally determined composition for the synthesized crystals is Eu$_{0.795(7)}$Pt$_6$Al$_{15.81(6)}$.

The crystal structure admits two independent Eu sites: the 2c Wyckoff position displays full Eu occupancy, while the 2e Wyckoff site is partially occupied, exhibiting a vacancy rate near $41\%$ ($59(1)\%$ occupancy). Additionally, a specific aluminum position (Al10) is also found to be only partially occupied. This dual-site disorder for Eu and Al is atypical among rare-earth intermetallics, imparting substantial atomic-scale randomness.

Key features of the atomic arrangement are illustrated in the structural figures of the cited work, where fully and partially occupied Eu atoms are differentiated, and distinct polyhedra formed by Pt and Al atoms articulate the connectivity underlying the hexagonal motif.

Parameter	EuPt$_6$Al$_{16}$	Notes
Space Group	P$\overline{6}$2m	Hexagonal symmetry
$a$ (Å)	14.2254(6)	From SCXRD refinement
$c$ (Å)	8.7363(4)	From SCXRD refinement
Eu Sites (occupancy)	2c (100%), 2e ($\sim$59%)	Full and partial occupation, respectively

The manifest vacancies on both Eu and Al sites constitute a foundation for enhanced disorder, directly impacting electronic and magnetic subsystems.

2. Electronic Transport Properties

Electrical transport in EuPt$_6$Al$_{16}$ is characterized by pronounced temperature-independent scattering, as deduced from AC resistance measurements performed along both the crystallographic $a$- and $c$-axes. The normalized resistance, $R(T)/R_{300K}$, reveals a low residual resistivity ratio (RRR$<1.5$), indicative of non-negligible static disorder limiting the conduction electron mean free path across the full temperature range studied.

Notably, the resistance does not present a clear signature at the magnetic ordering temperature $T_N$, underscoring the dominance of defect-driven scattering relative to magnetic ordering phenomena. This behavior is plausibly linked to the partial occupancy on Eu and Al sites, which introduces a high density of random potential centers for electron scattering.

3. Magnetic Properties and Antiferromagnetic Order

The compound's magnetic properties are governed by the Eu ions, each adopting a divalent state (Eu$^{2+}$, $4f^7$, $J=7/2$), as confirmed by both susceptibility and magnetization data. Magnetization measurements exhibit that the Eu$^{2+}$ moments undergo long-range antiferromagnetic (AFM) ordering below $T_N = 2.8(2)$ K.

Detailed examination of the temperature-dependent magnetic susceptibility, $\chi(T)$, shows a pronounced downturn below $3$ K along the $a$ and $b^*$ axes, whereas only a subtle kink is realized along $c$. Field-dependent magnetization at $2$ K confirms the AFM ground state, with a linear $M(H)$ curve at low fields and magnetic saturation consistent with $\sim7$ $\mu_B$ per Eu$^{2+}$ ion (after considering the stoichiometry deficit), with a weak metamagnetic transition at approximately $12$ kOe observed in-plane.

Curie–Weiss fits to the paramagnetic regime using

$$\chi^{-1} = \frac{\mu_\text{eff}^2 N_A}{3k_B}(T-\theta_\text{CW})$$

yield an effective moment $\mu_\text{eff} = 8.0(1)$ $\mu_B$ (consistent with $7.94$ $\mu_B$ for free Eu$^{2+}$) and a Weiss temperature $\theta_\text{CW} = -5.0(1)$ K, confirming the dominant AFM interactions.

4. Experimental Techniques

The synthesis of EuPt$_6$Al$_{16}$ was achieved via high-temperature solution growth from a Eu–Pt–Al melt with a substantial excess of aluminum, necessitating the pre-melting of Al and Pt to mitigate violent reactions during crystal growth. Resulting crystals were analyzed for composition and structure using a suite of techniques:

• Single-crystal X-ray diffraction (SCXRD): Delivered precise lattice parameters, space group identification, and site occupancy refinement.
• Powder X-ray diffraction (PXRD) and Rietveld refinement: Used to confirm phase purity and validate the structural model from single crystals.
• Energy-dispersive X-ray spectroscopy (EDS): Provided confirmation of the bulk chemical composition consistent with diffraction results.
• SQUID magnetometry: Enabled detailed temperature- and field-dependent magnetization characterization.
• AC resistance measurements (Physical Property Measurement System, PPMS): Profoundly characterized electronic transport properties.

5. Structural Disorder and Its Physical Consequences

The presence of partially occupied Eu and Al sites establishes a regime of structural disorder rarely found in classical rare-earth intermetallics. This disorder manifests as anomalously elevated temperature-independent residual resistivity and significant potential for random-fluctuation-driven phenomena in electronic, magnetic, and possibly lattice properties.

A plausible implication is that the interplay of disorder with antiferromagnetic order may yield unconventional magnetic excitation spectra or novel topological defects, particularly as the disorder disturbs long-range Eu–Eu exchange connectivity. While direct evidence for such behaviors requires further neutron or μSR studies, the compound serves as a platform for such future exploration.

6. Implications for Materials Design and Research Directions

The identification and characterization of EuPt$_6$Al$_{16}$ enrich the phenomenology of rare-earth aluminum platinides, specifically by providing an archetype with robust AFM order and notable crystallographic disorder. Immediate applications are in fundamental studies of magnetic ordering and quantum criticality in disordered systems. The material’s properties—low $T_N$, moderate electronic scattering, and variable site occupancy—may be leveraged in the targeted synthesis of compounds for quantum magnetism, spintronic devices, or magnetocaloric applications.

Continued investigation of EuPt$_6$Al$_{16}$ and cognate hexagonal phases could clarify the relationship between site disorder, electron scattering, and the stability of collective magnetic phenomena, informing materials design strategies that control such parameters for desired functionalities.