The high bond Polarity is responsible for favoring of the wurtzite structure instead of zinc blende structure in case of ZnO. Can anyone please describe this phenomenon in more detail..
Thank you
Hi Jagruti,
This paper gives the conditions under which GaN will form either zinc-blende or wurtzite. Hope it is useful.
http://scitation.aip.org/content/aip/journal/apl/67/25/10.1063/1.115374
The tetrahedrally oordinated bonding geometry plus the bond polarity determines the ZnO crystal structure which is hexagonal wurtzite type. Each Zn ion has four O neighbor ions and vice versa. This type of bonding is known in diamond type structure and is covalent. However if the partners (Zn and O here) have very different electronegativity then there can be a considerable amount of polarity. O is higly electronegative as high as 3.5 on the Pauling scale whereas Zn has quite a low value of electronegativity 0.91. So Zn and O can be considered as Zn^2+ and O^-2 ions and therefore the bonding is not covalent but somewhat ionic. The bond polarity menifests itself by the effective charg Z* which is about Z* = 1.15. This high bond polarity favors the wurtzite structure instead of zinc-blende structure for ZnO. The zinc-blende structure on the other hand is favored for tetrahedrally oriented bonds with lower polarity.
The wurtzite structure is related to the zinc-blende structure as hexagonal close-packing is to cubic close -packing.If we take the lattice complex of zinc-blende i.e. a Zn atom and O atom and use the hexagonal close-packing operator (ABAB rather than ABCABC) then we get wurtzite structure. The strong bond polarity favors ABAB stacking (wurtzite) wheras weak polarity or no polarity favors ABCABC (zinc-blende) stacking. This is just chemst's simplified version of explanation. The proper way of understanding would be to calculate by DFT with GGA approximation the energies of different competing structures which are rock-salt, zinc-blende or wurtzite in the present case. You can do these caculations I suppose and become famous. The softwares are free.
Another important difference from the zinc-blende and wurtzite structure is that whereas zinc-blende structure is not uniaxial, the wurtzite structure is. The unique axis in wurtzite structure has consequences in the physical properties.