Fine-grained 4 at% Nd3+:YAG transparent ceramics were prepared by reactive spark plasma sintering using a two-step heating profile. The effect of key sintering parameters (heating rate, external pressure, sintering and post-annealing temperatures) on densification peculiarities, optical and mechanical properties was investigated. Special attention was paid to the behavior and nature of carbon contaminations, dispersed in the yttrium aluminum garnet matrix. It was established that the optimal heating rate to the isothermal exposure temperature was 15 °C min−1. In this case, supporting the uniformity of temperature field distribution between the punches ensures the synthesis of the single-phase product and positive densification dynamics throughout the sintering trajectory for the disk geometry of samples. It was shown that an increase in applied pressure from 30 to 70 MPa changed the color of ceramics: the samples darkened and became dark brown. This may be due to an increase in the concentration of oxygen vacancy defects (color centers). The formation of silicon oxycarbides SiOxCy (where x + y = 4; x,y ≥ 1) in ceramics at sintering temperatures T > 1400 °C was revealed by X-ray photoelectron spectroscopy. It was found that these impurity phases demonstrated a thermal stability and resistance to decomposition during post-annealing. The study showed that 4 at% Nd3+:YAG ceramics obtained by reactive spark plasma sintering at 1350°С//10 min//30 MPa//15 °C min−1 and post-annealed at 900 °C for 1 h possessed a mean grain size of 0.74 μm, a microhardness of 13.2 GPa, a residual porosity of 0.0192 vol% and in-line optical transmittance 73.7% at λ = 1064 nm, which is 0.87 from the theoretical value for a single crystal.

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