If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

ChemWiki: The Dynamic Chemistry E-textbook > Under Construction > Schaller > Part I: Structure in Organic, Biological & Inorganic Chemistry > IC. Ionic Compounds > IC5. Solid State Structures

Copyright (c) 2006-2014 MindTouch Inc.

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa

IC5. Solid State Structures

Table of Contents

The structure of ionic solids is determined by how the cations and anions can pack together. Generally, one of the ions adopts a standard packing structure, like the metal atoms in a metallic solid. The counterions then fit into the holes or interstitial spaces among these ions. It is pretty common for the anions to form a close-packed structure, and for cations to find room in the resulting holes, but sometimes it is the other way around.

Problem IC5.1
Why might anions more commonly pack into a close-packed structure, rather than the cations?

The holes between the atoms have particular coordination numbers and geometries. There are many possible holes of different shapes where a counterion can find room. However, some of these geometries are very common.

For example, in the center of a simple cube, there is room for an additional atom.  This atom is described as sitting in a cubic hole.


Figure IC5.1. An ion in a cubic hole.

The above drawing emphasizes the relationship between the central atom and the atoms that form the corners of the cube. The central atom is in a cubic coordination geometry. Alternatively, we could describe the coordination number of the central atom. Instead of describing the shape formed by the surrounding atoms, we simply count the number of near neighbours. In this case, the coordination number of the central atom is 8. We can think of the central atom as sharing ionic bonding with its eight near neighbours.

An atom in a cubic hole might be viewed more easily if we draw lines between the various atoms at the corners of the cube, however.  In that way,we can see more clearly the cubic shape of the cage in which the central atom is sitting.


Figure IC5.2. An alternative view of an ion in a cubic hole.

Another common interstitial space in ionic solids is an octahedral hole. An octahedral hole forms in between two close-packed layers.  Because the atoms in the layers are packed more tightly than in a simple cube, this octahedral hole is generally a little smaller than a cubic hole. 


Figure IC5.3. An octahedral hole.


Figure IC5.4. An atom in an octahedral hole.

A third, common type of interstitial space is a tetrahedral hole. Tetrahedral holes, like octahedral holes, are found between two close-packed layers.


Figure IC5.5. A tetrahedral hole.


Figure IC5.6. An atom in a tetrahedral hole.

Problem IC5.2.
In the following structures, the anions are represented in red and the cations are represented in blue. For each structure,
  1. identify the type of unit cell that is visible
  2. identify the type of hole occupied by the counter ion
  3. identify the fraction of those holes that are occupied
  4. identify the number of anions and cations in the unit cell
  5. state the empirical formula (the lowest possible ratio of atoms in the material)

i.  The ions are formed from cesium and chlorine.



ii.  The ions are formed from sodium and chlorine.



iii.  The cations are formed from calcium and the anions are formed from fluorine.



iv.  The cations are formed from zinc and the anions are formed from sulfur.



v.  The cations are formed from zinc and the anions are formed from sulfur.




Additional Information on solid structures:

Liverpool Solid State

      Visualization of solid state structures: unit cells, etc.

Davidson College Crystals

 Visualization of solid state structures: unit cells, etc.

Oxford University Solid Structures

Visualization of solid state structures: unit cells, etc.

You must to post a comment.
Last modified
19:59, 26 May 2014


This page has no custom tags.


Lower Divisional

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use