Drastic enhancement of magnetic critical temperature and amorphization in topological magnet EuSn₂P₂ under pressure

The study addresses how hydrostatic pressure tunes the crystal structure, magnetic order, and electronic topology in the intrinsic magnetic topological material EuSn₂P₂. Intrinsic magnetic topological systems combine nontrivial band topology with magnetic order and can host exotic quantum phenomena (quantum Hall effect, axion electrodynamics, Majorana states). EuSn₂P₂ is a layered rhombohedral compound (R3m) with type-A antiferromagnetic order below 30 K and has been identified as a magnetic topological system. Pressure is a clean parameter to alter interatomic distances and electronic interactions, potentially inducing emergent phases. The research aims to determine how pressure affects EuSn₂P₂’s structure (including possible amorphization), magnetic ordering temperature and orientation, and Eu valence state, and to connect these to changes in bonding and topological properties.

The paper situates EuSn₂P₂ within the broader class of intrinsic magnetic topological materials, which are narrow-gap systems exhibiting nontrivial band topology and long-range magnetic order. Prior studies have focused on MnBi₂Te₆-related materials, with limited availability of other candidates. Recent work has identified or proposed Eu-based compounds as intrinsic topological semimetals, and EuSn₂P₂ specifically exhibits type-A AFM order below 30 K and a rhombohedral R3m structure analogous to A₂B₃ topological insulators. Pressure has previously been shown to suppress antiferromagnetism and induce superconductivity in correlated systems (heavy fermions, iron-based superconductors) and to trigger structural transitions including pressure-induced amorphization (PIA) in various materials (ice, AlPO₄, SnI₄, VO₂, EuIn₂As₂). In EuIn₂As₂, pressure enhanced intraplane exchange and led to amorphization with loss of magnetic order in the amorphous phase. These studies motivate examining how pressure modifies magnetism, structure, and topology in EuSn₂P₂.

- Sample synthesis: Single crystals of EuSn₂P₂ grown by Sn-flux method as previously reported. - High-pressure X-ray diffraction (XRD): Conducted at 13BM-C (PX2) beamline, APS, ANL with λ = 0.434 Å and 15×15 µm beam. Powdered single crystal loaded into diamond anvil cells (DACs). Two runs: - Run 1: BX90 DAC with 500 µm culets; helium as pressure-transmitting medium up to 23.3 GPa; pressure measured by ruby fluorescence. - Run 2: Symmetric DAC with 300 µm culets, cBN seats; neon as pressure medium; initial pressure by ruby during gas loading, subsequent pressures from in situ Au equation of state at sample position. Rhenium gaskets used. 2D images integrated with DIOPTAS; Rietveld refinements in GSAS‑II. Equation of state fit by third-order Birch–Murnaghan. - Synchrotron Mössbauer spectroscopy (SMS, ¹⁵¹Eu): Performed at APS 3ID in 24-bunch mode (153 ns bunch spacing). Time-domain nuclear forward scattering used to probe magnetic order and isomer shift. Data analyzed using CONUSS, modeling magnetic hyperfine field (H_hf), quadrupole splitting, and sample thickness. Reference Eu₂O₃ placed downstream to determine absolute isomer shift. Measurements across temperature at multiple pressures, including up to 42.7 GPa for magnetic orientation determination. - Partial fluorescence-yield X-ray absorption spectroscopy (PFY-XAS): Eu L₃-edge (2p₃/₂→5d) measured up to 47 GPa to determine Eu valence evolution; identification of Eu²⁺ and Eu³⁺ features separated by ~8 eV. - Molecular orbital (MO) calculations: Constructed HOMO/LUMO orbital visualizations to assess bonding evolution with pressure (ambient vs 23.3 GPa), indicating changes in Eu–Sn, Eu–P, and Sn–Sn interactions. - Electronic band structure calculations: GGA+U (U = 6 eV) with spin–orbit coupling used to compute surface states and spin texture under pressure, considering in-plane vs c-axis spin orientations to assess possible topological changes.

- Structure under pressure: Rhombohedral structure (R3m) stable to 33 GPa; transition to an amorphous phase at 36 GPa. Amorphous phase persists to at least 62 GPa. This contrasts with EuSn₂As₂ (rhombohedral-to-monoclinic). - Equation of state (crystalline phase): Third-order Birch–Murnaghan fit yields B₀ = 58(2) GPa, B′₀ = 3.6(1), V₀ = 375.1(9) ų. Eu–Sn and Eu–P interatomic distances decrease monotonically with pressure. - Magnetism: Magnetic ordering temperature T₀ increases monotonically with pressure from ~30 K at ambient to 130 K at 41.2 GPa (>4× enhancement). SMS shows Eu spins remain in-plane (hyperfine field orientation in ab-plane) up to 42.7 GPa. Magnetic order persists into the amorphous phase (observed at 42.7 GPa). - Hyperfine/isomer shift: Time-domain SMS with Eu₂O₃ reference reveals isomer shift δ shifts from −10.30(1) mm/s (1.0 GPa, 300 K) toward −5.39(6) mm/s (41.2 GPa, 160 K). Representative values: −9.98(1) mm/s at 3.7 GPa (100 K), −8.25(1) at 16.0 GPa (105 K), −6.67(4) at 20.7 GPa (110 K), −6.26(1) at 28.9 GPa (130 K). Caution: δ changes include compression effects beyond 4f occupancy. - Valence evolution: PFY-XAS indicates Eu remains mostly divalent up to ~19.8–20 GPa. Above ~20 GPa, a Eu³⁺-related peak ~8 eV above Eu²⁺ emerges and grows, indicating transition to intermediate valence at higher pressures (to 47 GPa). Combining PFY-XAS and δ suggests intermediate valence above 20 GPa. - Bonding and RKKY: MO calculations show pressure strengthens metallic Eu–Sn bonding, weakens Sn–Sn covalency, and shortens Eu–P distance (likely 3p localization on P). Enhanced Eu–Sn interaction strengthens RKKY exchange, accounting for increased T₀; intralayer exchange dominates due to shorter in-plane Eu–Eu distances. - Topology considerations: GGA+U+SOC indicates decreasing in-plane lattice constants can influence topological states if spins remain in-plane; small canting toward c-axis could qualitatively change topological properties. Experimentally, spins remain in-plane in measured range, so any topological change would stem from lattice parameter changes. - Phase diagram: dT₀/dP shows a slope change above ~20 GPa, coincident with onset of Eu valence evolution toward Eu³⁺. Magnetic order persists in the amorphous state (distinct from EuIn₂As₂ where amorphization suppresses magnetic order).

The results demonstrate that pressure substantially enhances magnetic exchange via the RKKY mechanism in EuSn₂P₂ by strengthening Eu–Sn metallic bonding and reducing interatomic separations, thereby elevating T₀ from 30 K to 130 K. Despite a pressure-induced crystalline-to-amorphous transition at 36 GPa, long-range magnetic order persists, highlighting robust exchange interactions, especially within the layers where Eu–Eu spacing is smallest. The Eu valence remains largely 2+ up to ~20 GPa and evolves to an intermediate valence at higher pressures; the change in dT₀/dP near this threshold suggests coupling between valence fluctuations and exchange interactions. Band structure calculations indicate that topological properties are sensitive to magnetic spin orientation; since in-plane spins persist under pressure, any topological modifications are expected to arise from compressed lattice parameters rather than spin canting. Together, these findings link enhanced RKKY interactions, bonding evolution, valence changes, and structural disorder, offering insights into controlling magnetism and topology via pressure.

This study establishes EuSn₂P₂ as a platform where hydrostatic pressure drives a rhombohedral-to-amorphous transition and dramatically enhances the magnetic ordering temperature via strengthened RKKY exchange associated with increased Eu–Sn bonding. Eu spins remain in-plane to at least 42.7 GPa, magnetic order survives in the amorphous phase, and Eu adopts an intermediate valence above ~20 GPa. Calculations suggest that while lattice compression can influence topological states, spin canting would have a larger impact, which was not observed experimentally. Future work should (i) perform detailed electronic structure calculations under pressure to quantify Eu valence and its dynamics, (ii) investigate the microscopic mechanism of pressure-induced amorphization, (iii) explore possible superconductivity and magnetism–topology interplay at higher pressures and lower temperatures, and (iv) directly probe surface/topological states under pressure and potential spin canting effects.

- The precise mechanism of pressure-induced amorphization remains unresolved; further experimental and computational studies (elastic stability, charge/orbital considerations) are needed. - Mean Eu valence under pressure could not be quantified accurately from PFY-XAS due to diminishing peak intensities; comprehensive electronic calculations are required. Isomer shift changes partly reflect compression effects unrelated to 4f occupancy. - A minor oxide impurity contributed features in some SMS spectra (e.g., 16.4 GPa at 96 K), though modeled; this may introduce small uncertainties. - Complete topological characterization under pressure is indirect; calculations indicate sensitivity to spin canting, but experimental confirmation of potential canting-induced topological transitions was not obtained. - Some author affiliations for specific contributors (¹⁰, ¹¹) were not provided in the available text; experimental conditions were split between hydrostatic and quasihydrostatic media, though runs agreed reasonably well.