α-SiAlON, as a solid solution of α-Si₃N₄, is a valuable ceramic material due to its excellent mechanical and functional properties. The structure of α-SiAlON is necessarily stabilized by so-called stabilizing cations. To date, various stabilizing cations, including Li, Mg, Ca, Sr, and most rare earth elements, have been used in α-SiAlON, but no α-SiAlON stabilized with transition metal cations has been reported. In this work, we report Mn-α-SiAlON as a new member of the α-SiAlON family and, to the best of our knowledge, the first example of α-SiAlON stabilized with transition metal cations. Single-phase Mn-α-SiAlON with a nominal chemical composition of Mn_0.75Si_9.75Al_2.25O_0.75N_15.25 was prepared by spark plasma sintering from a precursor powder mixture of α-Si₃N_4, AlN, and MnO. The formation of Mn-α-SiAlON isostructural to α-Si₃N₄ was confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The new Mn-α-SiAlON exhibited a superior Vickers hardness of 23.1 GPa, good fracture toughness of 6.7 MPa·m^1/2, and thermal conductivity of 7.60 W/(m·K). We hope that the discovery of Mn-α-SiAlON will open new possibilities for tailoring the chemical composition and optimizing the properties of α-SiAlON.

The fabrication of Si₃N₄ ceramics typically requires high temperatures (above 1700 °C) and prolonged sintering time to achieve densification, resulting in high energy consumption and increased manufacturing costs. Moreover, reports on the fabrication of dense Si₃N₄ ceramics with good mechanical properties under MPa-level pressure and low temperatures are rare. In this work, we propose a low-temperature rapid spark plasma sintering strategy involving the introduction of fine-grained β-Si₃N₄ powder with high lattice strain energy as an “additive”. Dense biphasic Si₃N₄ ceramics, predominantly α-Si₃N₄, were successfully fabricated at a mechanical pressure of 200 MPa and a temperature of 1300 °C, achieving a relative density of 97%. The application of high pressure promoted particle rearrangement and uniform liquid‒phase distribution, providing additional driving forces for sintering. The introduction of β-Si₃N₄ seeds facilitated an in-situ solution–reprecipitation process, enabling rapid densification with a minimal liquid phase and without significant grain growth, resulting in nanometer-scale grains. The Si₃N₄ sample prepared at 1350 °C exhibited a desirable combination of high hardness (18.5 $\pm$0.3 GPa) and fracture toughness (6.7 $\pm$0.2 MPa·m^1/2). The results demonstrate that by adjusting the sintering temperature and time, the phase composition and mechanical properties of the ceramics can be flexibly tailored. This work holds significant potential for industrial manufacturing and provides valuable insights into low-temperature strategies for ceramic fabrication.