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A zirconium and a zirconium/boron containing single-source precursor were synthesized via chemical modification of a commercially available polysilazane (Durazane 1800) with tetrakis (dimethylamido) zirconium (IV) (TDMAZ) as well as with both TDMAZ and borane dimethyl sulfide complex, respectively. The polymer-to-ceramic transformation of the precursors into SiZrCN and SiZrBCN ceramics as well as the thermal evolution of their phase composition and microstructure was studied. The pyrolysis of the precursors led to the formation of amorphous SiZrCN and SiZrBCN ceramics. Interestingly, the as prepared SiZrBCN ceramic was single-phasic and fully featureless; whereas SiZrCN exhibited the presence of nano-sized ZrO₂ particles; however, only very localized in close proximity to internal surfaces. Heat treatment at higher temperatures induced crystallization processes in both prepared ceramics. Thus, at temperatures beyond 1500°C, cubic ZrCₓNy, β-Si₃N₄ as well as β-SiC were generated. It was shown that the incorporation of B into SiZrCN suppressed the crystallization of ZrCₓNy and, in addition, impeded the reaction of SiNₓ with C, resulting in an improved thermal stability of SiZrBCN compared to SiZrCN ceramic. Moreover boron was shown to be mainly located in the sp²-hybridized “free” carbon present in SiZrBCN, forming a turbostratic BCN phase which has been unequivocally detected by means of high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS).