This paper discusses the relation between the surface morphologies and negative crystal structures as well as the assembling or unlinking of growth units in PMNT crystals. The growth units in relaxor ferroelectric single crystals PMNT and PZNT, which belong to lead-based complex perovskite structures, are considered as coordination octahedra [BO~6] and the growth process as the assembling of these units. These crystals are formed via a normal growth mechanism when their units grow on the {111} faces. However, the layer-by-layer growth mechanism is in operation when the units grow on {001} faces. This decides the anisotropy of growth rates and the crystal morphologies. The negative crystal structures of the PMNT single crystals with regular facets grown by the Bridgman technique could occur when the crystals grew from the melt surrounded by frozen crystals. The special surface morphologies, including positive and negative ones, could form during the period of crystal growth. Furthermore, a regular negative configuration on the surface of crystal plates as if being etched, could stem from the decomposition of PbO after annealing at high temperature. All of these phenomena could be rationalized by the mechanism of the assembling and unlinking of the [BO~6] octahedral growth units.

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