Tutorial 7: Cell Parameter Search

 

Objectives

 

•	To find out if the unit cell dimensions of a particular crystal match those of any entries in the CSD. The usual reason for this type of search is to find out whether a crystal of known cell dimensions has already had its structure determined.

 

Steps Required

 

•	Perform a reduced cell parameter search.

 

The Example

 

•	Find structures whose cell dimensions match those of a primitive monoclinic cell with a = 6.638, b = 7.550, c = 23.180, beta = 95.98.

 

Menu Commands Required

 

1.	Start ConQuest and hit the Unit Cell button.

 

2.	Turn on the option Yes, do a reduced cell search.

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The unit cell of a crystal can be chosen in many different ways (strictly speaking, an infinite number of ways, though not many of the choices will be convenient). In order to assess whether two structures might have the same unit cell dimensions, it is therefore necessary to convert each set of dimensions to a standard form. This is called cell reduction.

 

3.	Enter the lattice type and cell dimensions of your crystal.

•	It is essential to specify correctly the lattice type of your crystal. In this example, set the Lattice Type to Primitive.

•	Type in the values of a, b, c, alpha, beta and gamma. In this example, they are:

 

•	All the cell parameter boxes must be filled in before you can do the search, even though, in this case, the alpha and gamma angles must be 90 by symmetry.

•	Accept the default tolerance. This defines how closely the reduced cell dimensions of a database structure must match the reduced cell dimensions of your crystal in order to be considered a match. The default value has been carefully chosen so is usually best left unchanged.

 

4.	Run the search.

•	Hit Search in the Unit Cell window, then Start Search in the next window.

•	Click on the Crystal tab in View Results to display the cell parameters of hit entries.

•	Two sets of unit cell dimensions are shown for each hit: the cell parameters chosen by the author and the reduced cell parameters to which they correspond. These may or may not be identical, depending on whether the author’s choice of cell happened to correspond to the standard reduced cell.

•	Similarly, the reduced cell dimensions of a hit may or may not look like the parameters you typed in, depending on whether your choice of cell dimensions happened to be standard. In this example, it was.

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In comparing the reduced cell dimensions of a database structure with the reduced cell parameters of the input structure, only the cell lengths are compared. This is because the cell angles can be mathematically unstable (i.e. a very small change in input cell dimensions may produce large changes in reduced cell angles; the same problem does not occur for the cell lengths).

•	In this example, there are several database entries which match the input cell dimensions, once cell reduction has been performed. Any one of these might therefore be the same structure as that whose cell dimensions were typed in.

 

This ends the tutorial.