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We present the results of an extensive STM investigation of the parasitic crystallites appearing during the growth of ZrB2(0 0 0 1) buffer layers on Si(1 1 1) aimed at understanding their nature and preventing their growth. Owing to the identification of several of their facets, deemed to induce their nucleation with undesired orientations, we determined the epitaxial relationships with the substrate and pointed out the large variety of possible misorientations. As the short direction of the crystallites systematically corresponds to the largest misfit, the anisotropic epitaxial strain is deduced to be the origin of the anisotropic growth for most of them. We have experimentally demonstrated that the formation of the crystallites can be kinetically prevented giving rise to ZrB2 buffer layers with a dramatically improved quality. This work is therefore expected to open a way to the technologically relevant growth of defect-free GaN-based LED on Si(1 1 1).