While reading about fusion reactors, I noticed that an alternative molten salt can be used, this is a 1:1:1 mixture of LiF, NaF and BeF2. I expect that this will be rather similar to Li2BeF4 but I could not resist having a look in the crystallography database. I found that a report exists of Na3Li.2[BeF4] (J. Vicat, Duc Tran Qui, S. Aleonard and P. Richard, Acta Crystallographica B, 1974, 30(11), 2678-2682).
This is a more complex solid that the Li2BeF4, the lithium is five coordinate (distorted trigonal based pyramidal).
Two of the sodiums has a distorted square based pyramidal geometry. (Na1 and Na2). While Na3 is six coordinate the geometry is so distorted away from octahedral that I can not tell what it is. Here is a picture of Na1′s coordination environment.
One of the berylliums (Be1) is has a nice and regular looking tetrahedral environment. I think that the important thing is that with the Be:F ratio of 1:4 we have the nice tetrahedrons.
The other (Be2) has a distorted tetrahedral environment (one bond is longer than the others, this is the bond to the flourine bonding to the sodium and the lithium).
Also a report exists of a Na2LiBeF7 by G. Brunton, Materials Research Bulletin, 1972, 7, 641-646. I will get onto that one in the next post on Be chemistry.