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 po sitive and negative polar axis directions are different. The theoretical gr owth habit of ZnO crystal is hexagonal prism and the growth rates of its va rious faces are:V-{0001} > V-{01 (1) over bar(1) over bar} > V-{01 (1) over bar0} > V-{000 (1) over bar}. The growth habit of ZnS crystal is tetrahedr on and its growth rates of different crystal faces are: V-{111) > V-{001} > V-{001} = V-{100} = V-{010} > V-{(1) over bar(1) over bar(1) over bar}. Th e growth rate relationship between positive and negative polar axis directi ons of SiO2 crystal is V-[(1) over bar(1) over bar 20] > V-[11 (2) over bar 0]. These results are in agreement with the growth habits observed under hy drothermal conditions. The different growth rates between positive and nega tive polar axis directions cannot be explained by PBC theory.