Growth habit of polar crystals

Using coordination polyhedron rule, growth habit of polar crystals such as ZnO, ZnS and SiO2 is investigated. It shows that the growth rates in the positive and negative polar axis directions are different. The theoretical growth habit of ZnO crystal is hexagonal prism and the growth rates of its various faces are: V{0001} >V{0111} > V{0110} > V{0111} > V{0001}. The growth habit of ZnScrystal is tetrahedron and its growth rates of different crystal faces are: V(111}>V{001}>V(001}=V{100} = V{010}>V{111}. The growth rate relationship between positive and negative polar axis directionsof SiO2 crystal is V{1120} > V{1120}. These results are in agreement with the growth habits observedunder hydrothermal conditions. The different growth rates between positive and negative polar axis directions cannot be explained by PBC theory.