Magnetic materials are essential for applications in electronics, energy conversion and many other industrial and technological sectors. Among the permanent magnets, rare-earth magnets exhibit the strongest properties and dominate the market shares in related applications [1]. However, due to the increasing cost and unstable supply, the research on reducing the rare-earth elements in magnets attracts significant increasing interest. Rare-earth-lean samarium transition metal (TM) compounds (SmTM12 or 1: 12 for short) have made breakthroughs in 2020 and the (Sm, Zr)(Fe, Co, Ti) 12 monocrystalline particles synthesized by a Ca reduction of elemental oxides at high temperature demonstrated an order of magnitude improvement in coercivity at room temperature [2][3]. In this work, we made structural analysis of (Sm, Zr)(Fe, Co, Ti) 12 particles to elucidate the impact of synthesis conditions, microstructure and phase transformation on the evolution of magnetic properties. All (Sm, Zr)(Fe, Co, Ti) 12 particles were synthesized by reduction-diffusion process at high temperature and the resulting particles were then washed by various agents, collected and further studied. Samples were annealed for mechanistic study on the phase transformation and evolution. Five batches of particles were investigated, including 1) particles 990w64 prepared at 990 C and washed with acetic acid, 2) particles 1220w4A prepared at 1220 C and washed with acetic acid, 3) particles 1220wNA prepared at 1220 C and washed in distilled water with no acid, 4) particles U368B prepared with Ca/O mole ratio of 1.65, and 5) particles U412C prepared with Ca/O mole ratio of 1.30. SEM was performed with a Carl Zeiss Auriga® 60 CrossBeam® focused ion beam (FIB) and scanning electron microscope (SEM). SEM samples were prepared by dispersing powder samples uniformly on a double-sided carbon tape on a stub. TEM and EDS studies were made using a JEM-2010F (operating at 200 kV) equipped with an EDAX spectrometer run by TEAMTM software. TEM samples on lacey carbon grids were prepared with a particle suspension in distilled water after thorough sonication of the suspension. Additionally, cross-sectional TEM foils were prepared for particles embedded in resin with Auriga® 60 FIB/SEM. Samples 990w64, 1220w4A and 1220wNA were first observed in SEM. The shape of the particles differs according to the synthesis temperature. The particles prepared at 990 C appear to be short-rod shaped with chain of several grains while the particles synthesized at 1220 C are individual single crystals (Figure 1a and 1b). The particles washed with and without acid were further observed in TEM, and less oxide was detected on the surface of washed particles (Figure 1c and 1d). TEM images of U368B and U412C are shown in Figure 2a and b, respectively. No significant crystal defects, such as twining, were detected in either of the two samples, which contradicts with other studies on similar materials that identified twining as a key feature [4][5]. Also, both samples contain polycrystalline particles (U368B with 78% single crystal, U412C with 79% single crystal, and the rest being polycrystalline particles or particles containing Fe cores). U412C includes relatively equiaxed grains with sharper edges, while U368B contains more irregularly shaped particles. A core-shell structure with Sm-free α-Fe phase in the middle was observed in both samples (Figure 2c-2e). Such structure was found in other research on reduction-diffusion reactions [6]. Figures 2c-2e suggest that the 1: 12 structure starts to form from the surface of the α-Fe particle. TEM characterization of the 1: 12 phase …