Composition and microstructure of holmium monosulfide compacts processed by reaction sintering

With its specific physical property of antiferromagnetism, holmium monosulfide (HoS) is considered as an attractive material for potential application in electronic and magnetic recording devices. The HoSx synthesis through the direct interaction of the Ho metal with S does not always attain a homogeneous state because of the large difference between the vapor pressures of Ho and S. We report herein the HoSx synthesis (0.68 ≤ x ≤ 1.2) by the direct reaction of Ho2S3 and Ho using a pulse electric current sintering apparatus. The sintering compacts of the HoS single phase are obtained when the preparation ratios of the Ho metal range from 16.16 (HoS1.19) to 38.15 (HoS0.99) mass% at a sintering temperature of 1973 K. The effect of the Ho addition on the lattice constant, melting point, and electrical resistivity is studied within the homogeneity range of the HoS phase. Consequently, we found that the lattice constant and the melting temperature gradually increase when the composition is close to the stoichiometric composition and reach the values of 5.458 Å and 2608 K of HoS1.01, respectively. All the homogeneous HoSx compacts, except HoS0.99, show metallic behaviors down to 50 K, whereas a semiconducting-like upturn can be observed below 20 K. On the other hand, HoS0.99 shows a metallic behavior to 4 K, which indicates that the lattice imperfection considerably affects the electric properties of HoSx. The advantages of the sintering process for the HoSx compact preparation with higher density and a peculiar interface are demonstrated. •Synthesis of homogeneous non-stoichiometric HoS compacts by reaction sintering.•Optimized preparation and sintering conditions to improve quality of HoSx compacts.•Investigated solid–liquid–vapor reactions forming the HoSx oxygen-free compacts.•Modified process could be applied for synthesizing other rare-earth monosulfides.