Packing fraction for multi-atom basis

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Discussion Overview

The discussion revolves around calculating the maximum packing fraction for a unit cell containing two different types of atoms. Participants explore methods for determining the packing fraction in this context, including considerations of atomic radii and nearest neighbor distances.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant, James, asks how to calculate the maximum packing fraction for a basis with two different types of atoms, noting the challenge of determining atomic radii.
  • Another participant suggests starting with a unit cell and refers to an external example to clarify the concept.
  • James elaborates on his understanding of the unit cell and expresses uncertainty about finding the radii of the atoms that would yield the maximum packing fraction.
  • A different participant states that there is no unique method to determine the radii, proposing that one could set the radii equal to half the nearest neighbor distance or analytically maximize the packing fraction with respect to the radii under a specific constraint.
  • James acknowledges the suggestion but expresses uncertainty about how to proceed with the proposed methods.

Areas of Agreement / Disagreement

Participants do not reach a consensus on a specific method for calculating the maximum packing fraction, indicating that multiple approaches exist and uncertainty remains regarding the best method to apply.

Contextual Notes

The discussion highlights the dependence on definitions of atomic radii and the nearest neighbor distance, as well as the lack of resolution on the mathematical steps required to maximize the packing fraction.

james477
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Hi,

I understand that the maximum packing fraction for a particular atomic structure can be calculated assuming the nearset neighbours are touching but my question is how can the maximum packing fraction be calculated for a basis containing two different types of atoms?

Thanks, James
 
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Well, begin with a unit cell.
Does http://en.wikipedia.org/wiki/Atomic_packing_factor" example clarify matters?
 
Last edited by a moderator:
alxm said:
Well, begin with a unit cell.
Does http://en.wikipedia.org/wiki/Atomic_packing_factor" example clarify matters?

So say I have a unit cell containing 1 atom of type X and 1 of type Y - this means that i can find the total volume of atoms per unit cell and i know the volume of the cell. But how would i go about finding the radii of these atoms which results in the maximum packing fraction? This is where i get lost..

Thanks, James
 
Last edited by a moderator:
There is no unique way to do it. You could simply choose R_x = R_y = R_NN/2 where R_NN is the nearest neighbor distance. Or you could work out the packing fraction analytically, and then maximize it with respect to R_x and R_y subject to the constraint that R_x + R_y = R_NN.
 
kanato said:
There is no unique way to do it. You could simply choose R_x = R_y = R_NN/2 where R_NN is the nearest neighbor distance. Or you could work out the packing fraction analytically, and then maximize it with respect to R_x and R_y subject to the constraint that R_x + R_y = R_NN.

Thanks, That makes sense - not quite sure how i would go about it but i'll give it some more thought.
 

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