This paper discusses the relation between the splitting and assembling of the same type growth units and the fluctuation in composition and structure in relaxor ferroelectric single crystals PMNT and PZNT, which belong to lead-based complex perovskite structures. Their growth units are considered as coordination octahedra [BO~6]. Depending on their stability, different [BO~6] growth units in the melt may have different probability to exist. [NbO~6]^7^- and [TiO~6]^8^- are more stable or more favorable octahedral units than [MgO~6]^1^0^- and [ZnO~6]^1^0^-. Various [BO~6] units exhibit remarkable difference in stability, ion size and ion electrovalence. Therefore, the growth interfaces with electric charges would make a choice among those [BO~6] growth units, resulting in the short range of fluctuation in composition and structure at the stage of crystal growth and preparing for the subsequent formation of ordered domains and other chemical defect clusters. Adding the dopant PbTiO~3 to solid solution could adjust the composition and structure of micro-regions and suppress the formation of pyrochlore, which could contribute to the resemblance between the octahedral units [TiO~6]^8^- and [NbO~6]^7^- and the stability effect of [TiO~6]^8^- in crystal structures. This become an important basis for using the modified Bridgman technique to grow pure perovskite PMNT single crystals directly from melt. The evident distinction in growth difficulty between PMNT and PZNT crystals is attributed to the differences between the growth units [MgO~6]^1^0^- and [ZnO~6]^1^0^-. The stability of growth units or structure ones should be taken into consideration when choosing the growth methods.

Key words: PEROVSKITE TYPE STRUCTURE, FERROELECTRIC CRYSTALS, CRYSTAL GROWTH, RELAXATION, MAGNESIUM OXIDE, NIOBIUM OXIDE, LEAD OXIDE