By combined magnetic susceptibility and specific heat measurements, we find two successive antiferromagnetic (AFM) transitions at ~9 K and ~5 K in Fe₂(HPO₃)₃ ⋅ 4H₂O. Through density functional theory (DFT) analysis we have calculated the AFM states. Two AFM configurations, namely AFM I and AFM II, are almost degenerate in energy and their competition leads to the two successive AFM phase transitions at low temperature, reflecting the frustrated magnetism in Fe₂(HPO₃)₃ ⋅ 4H₂O.

Abstract

We report the magnetic and optical properties of Fe₂(HPO₃)₃ ⋅ 4H₂O. By combined magnetic susceptibility and specific heat measurements, we find two antiferromagnetic (AFM) transitions (T _N1=9 K, T _N2=5 K) which originate from the AFM exchange interactions mediated by the (HPO₃)²⁻ anions. Compared with the non-hydrated compound Fe₂(HPO₃)₃, the magnetic interaction strength and ordering temperature of Fe₂(HPO₃)₃ ⋅ 4H₂O are both reduced by a factor of ~2. Through density functional theory (DFT) analysis we find almost degenerate AFM states in Fe₂(HPO₃)₃ ⋅ 4H₂O which are close in energy. The competition of these AFM states might lead to the two successive AFM phase transitions at low temperature. The low ordering temperature as well as competing AFM states both imply frustrated magnetism in Fe₂(HPO₃)₃ ⋅ 4H₂O, probably originating from the complex competing AFM interactions. The optical properties of Fe₂(HPO₃)₃ ⋅ 4H₂O are also investigated by photoluminescence and infrared spectroscopies.