AuBe not P2(1)3?

Uncle Al UncleAl0 at hate.spam.net
Wed Dec 20 17:32:29 EST 2000



Cristal wrote:
> 
> In the CRYSTMET database (1999 edition), there is nothing
> more on AuBe than the structure described in the Cullity paper
> of which you gave the reference :
> 
> B Cullity Trans. Am. Inst. Min. Metall. Pet. Eng. 171 396 (1947)
> 
> But if you have a sample, the cell could be checked with more
> modern diffractometers...


For wholly perverse reasons, thought experiment
http://www.mazepath.com/uncleal/eotvos.htm  (revision to be posted by
next week) I desire

   1) chiral space group
   2) smallest unit cell possible
   3) heaviest non-radioactive atoms possible (Bi, Pb, Hg, Au, Pt, W,
Ta)
   4) no traceable atomic helices with opposite chirality to the main
twist.  
   5) single crystals of both chiralities with smallest dimension >2
cm.

AuBe is extremely friendly - 95.6 wt-% gold, mp of 780 C, hence
Bridgeman-Stockbarger or Czochralski growth of large single crystals -
unless AuBe is really achiral tetragonal.  Most intermetallics and
silicides with appropriate structures are nasty, with extraordinary mp
and ceramic hardness.  PtAl retains facets like a gemstone
(Platigems).

I haven't found anything as good as AuBe.  A chiral space group with
fewer independent symmetry elements would be nicer (PX, PX(n), IX,
IX(n), C2).  Enantiomorphic pairs P3(1)/P3(3), P4(1)/P4(3),
P6(1)/P6(5), P6(2)/P6(4), would be nicer still.  Heavy atoms and high
material densities don't seem to do that sort of thing within my
needs.

No molecular solid can begin to compete (intermolecular empty space)
with an ionic or metallic solid, nor can any organic (light atoms,
empty space).  I already have lithium iodate (P6(3)) and bismuth
triborate (C2) as low density chiral representatives.  AuBe with a
unit cell volume of 101.72 A^3 and 13.45 g/cm^3 is extraordinary.
 
> PS - Let me apologize for the cold reception you obtained at the
> sci.technique.xtallography.

A glassblower can be "persuaded" with a sixpack of good beer or a
fifth of good Scotch.  With crystallographers, you get in line.  Then
again, I bet some 5% of the crystallographic structure literature is
open to, ah, further interpretation.  One wouldn't think finding gold
on a Patterson map would be a hardship.

Modern equipment (CCD data collection) and software will resolve the
issue if and when.  You folks can see fractions of an electron now.  I
expect to submit my chiral Eotvos experiment proposal as a Letter to
Nature in January.  Good data is better than data.  "8^>)

-- 
Uncle Al
http://www.mazepath.com/uncleal/
http://www.ultra.net.au/~wisby/uncleal/
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