XY-like Incommensurate Magnetic Order in Ce$_2$SnS$_5$

TL;DR

Ce2SnS5 is shown to host XY-like antiferromagnetic order driven by a highly anisotropic crystal field in a distorted TTP coordination around Ce3+. Through two-stage CVT crystal growth, orientation-dependent magnetization, heat capacity, and powder neutron diffraction, the study uncovers an incommensurate magnetic state just below $T_N = 2.4$ K that locks into a commensurate, two-$q$ structure with propagation vectors $oldsymbol{q}=( frac{1}{3},0,0)$ and $oldsymbol{q}=(0,0,0)$, with moments confined to the $ab$-plane. The ground-state Kramers doublet (Γ7) yields strong in-plane anisotropy, with low-temperature g-factors $g_ extparallel o 1.2$ and $g_ extperp o 2.5$, indicating XY-like behavior. These results establish Ce2SnS5 as a new material platform to study the interplay between crystal-field–driven anisotropy and incommensurate-to-commensurate magnetic order in a 3D XY-like system. The combination of synthesis, detailed magnetic/thermodynamic measurements, and neutron diffraction provides a foundation for future single-crystal studies and critical-exponent analysis.

Abstract

We report the synthesis of single crystals of Ce$_2$SnS$_5$ through a two-stage chemical vapor transport method. The Ce$_2$SnS$_5$ system is a member of the orthorhombic $Pbam$ (No. 55) space group and realizes a distorted trigonal tricapped prism (TTP) crystal field around each cerium site. We characterized the sample through orientation-dependent magnetization and heat capacity measurements to probe the magnetic anisotropy in the system characteristic of XY-like anisotropic Heisenberg model behavior. Ce$_2$SnS$_5$ furthermore enters a zero-field ordered phase under $T_N =$ 2.4 K; powder neutron diffraction measurements reveal incommensurate magnetic order near $T_N$. The system then locks into a commensurate, two-$q$ magnetic structure below approximately 1.2 K. This commensurate structure belongs to the Shubnikov group $Pb'a'm'$ (MSG 55.359) and realizes the propagation vectors $\vec{q} = (1/3,0,0)$ and $\vec{q} = (0,0,0)$.

Figures (13)

• Figure 1: (a) Ce2SnS5 single crystal prepared for a heat capacity measurement with the $[001]$ direction indicated by the blue arrow. (b) Ce2SnS5 crystal structure showing the cerium in green, the sulfur in yellow, and the tin in purple. The coordination polyhedron around each cerium site is shown on the right with the $C_3$ symmetry axis of the undistorted polyhedron labeled and the sulfur atoms forming the polyhedron drawn in blue (the dark blue atoms lie in the plane of the page whereas the light blue atoms lie in a plane behind that of the page). Crystal structure drawn using VESTA VESTA. (c) Powder XRD pattern (obtained using copper $K-\alpha$ radiation) of material obtained at the end of the second synthesis phase. The $(hkl)$ indices of the Ce2SnS5 peaks are labeled.
• Figure 2: (a) Magnetic susceptibility of crystal A taken in $0.1 \, \text{T}$ of applied magnetic field with the field aligned perpendicular to the c-axis (black) and parallel to the c-axis (blue). The inset shows the low temperature magnetic susceptibility with the magnetic transition at $T_N = 2.4 \, \text{K}$ marked by the vertical dashed line. (b) Inverse magnetic susceptibility from the same measurement with a Curie-Weiss fit between $300-400 \, \text{K}$ (dashed, red curve).
• Figure 3: Magnetization of crystal A taken as a function of applied magnetic field perpendicular to the c-axis (a) and parallel to the c-axis (b). The magnetization was measured at several temperatures both below and above $T_N = 2.4 \, \text{K}$ and is presented in units of $\mu_B$ per cerium atom.
• Figure 4: Heat capacity of crystal B taken in zero field showing the magnetic transition at $T_N = 2.4 \, \text{K}$. The inset shows the magnetic part of the heat capacity in red and the computed magnetic entropy in blue below $25 \, \text{K}$. The magnetic part of the heat capacity extrapolated to zero is shown as a dashed red line. The magnetic entropy lost through the transition is consistent with the doublet ground state of the cerium $4f$-electron level structure.
• Figure 5: Heat capacity measured around the magnetic transition for various applied magnetic fields. (a) Heat capacity of crystal B with field aligned perpendicular to the c-axis. (b) Heat capacity of crystal C with field aligned parallel to the c-axis.