Question

\(\mathrm{HgSe}\) is a semiconductor which can crystallize in both the zinc blende and wurtzite structures. At high temperatures the hexagonal close- packed wurtzite structure is preferred, but at lower temperatures the cubic close-packed form dominates. (i) Describe what is meant by the underlined terms, and hence, differentiate between the two types of close-packed structures. (ii) Draw a projection of the cubic close-packed structure of \(\mathrm{HgSe}\). (iii) \(\mathrm{CeO}_{2}\) crystallizes with the fluorite structure at all temperatures where no hexagonal close-packed polymorph exists. By consideration of the type of hole being filled, explain why this could be predicted.

Short answer

Zinc blende and wurtzite structures are ways of arranging atoms in crystals, both involve each atom being surrounded by four neighbors. Cubic Close Packed (CCP) and Hexagonal Close Packed (HCP) structures are methods of efficiently packing equal-sized spheres - CCP follows ABCABC while HCP follows ABAB. A Cubic Close Packed HgSe structure is made up of a face-centered cubic cell with an atom at each corner and face of the cube. CeO2 crystallizes in a Fluorite structure where cerium ions are in a cubic close pack arrangement and oxygen ions fill all the tetrahedral holes, ruling out the possibility of a HCP structure.

Step-by-step solution

Understand the Terminologies

Zinc blende and wurtzite structures are the ways in which atoms in a crystal are arranged. - 'Zinc blende' is a cubic crystal structure where each atom is surrounded by four neighbors in a tetrahedral arrangement. - 'Wurtzite structure' is a hexagonal crystal structure with each atom also surrounded by four neighbors in a tetrahedral arrangement. Hexagonal close-packed (HCP) and cubic close-packed (CCP) structures refer to the packing of equal-sized spheres in the most efficient arrangement. - In CCP structures, the packing arrangement follows ABCABC pattern, while in HCP, the packing follows ABAB pattern.

Draw Cubic Close-Packed Structure

Although it's challenging to visualize without images, a basic verbal description of a Cubic Close Packed (CCP) structure would be as follows: - A face-centered cubic cell is selected, with an atom at each corner and an atom centered on each face of the cube. - In the CCP structure, each atom is in direct contact with 12 other atoms: 4 above, 4 below and 4 around in its own layer. This forms a three-dimensional network.

Explain CeO2 Crystallization

Fluorite structure is a type of cubic crystal structure. In the Fluorite structure of CeO2, cerium ions are in a cubic close pack arrangement and oxygen ions occupy all the tetrahedral holes. There is no room in this structure for oxygen ions to occupy octahedral holes as would be required for it to form Hexagonal close-packed (HCP) structure, thus it can be predicted that no HCP polymorph of CeO2 exists.