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New Insights into the Low-Temperature Properties of the Ternary Halide Na2CrCl4: Magnetic Ordering and Entropy
Nick T H Ter Veer1, Ian M Berkel1, Indu Dhiman1
1Radiation Science & Technology Department, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, Delft 2629 JB, The Netherlands.
Abstract:
The structural, thermodynamic, and magnetic properties of Na2CrCl4 have been investigated to provide fundamental insights into this ternary halide relevant to chloride-based molten salt reactor systems. Room-temperature powder X-ray and neutron diffraction confirm a monoclinic (P21/c) structure and phase purity. Neutron diffraction measurements at 4.6 K reveal additional magnetic reflections indexed with , indicating the onset of long-range antiferromagnetic order. Low-temperature heat capacity measurements in the range 2-300 K show a pronounced λ-type anomaly at T N = 8.5 ± 0.5 K, with an associated magnetic entropy S mag = 11.9 ± 0.4 J K-1 mol -1 consistent with antiferromagnetic ordering of high-spin Cr2+ (S = 2), a second-order phase transition. The standard molar entropy at 298.15 K, S m °(298.15 K) = 256.8 ± 7.7 J K-1 mol -1, is slightly lower than previous CALPHAD assessments of the NaCl-CrCl2 system. Magnetic susceptibility measurements also confirm antiferromagnetic behavior, with a Curie-Weiss fit giving μeff = 5.57 ± 0.05 μB and θCW = -15.0 ± 1.0 K. Compared to the related ferromagnetic chlorides K2CrCl4, Rb2CrCl4, and Cs2CrCl4, Na2CrCl4 exhibits a distinctly lower ordering temperature and antiferromagnetic structure, likely due to variations in lattice geometry and exchange interactions. These results provide the first experimental thermodynamic parameters for Na2CrCl4, contributing to refining phase diagrams and corrosion models in chloride salt systems.
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