
Astatine: Halogen or Metal? - nkurz
http://quantumchymist.blogspot.com/2014/02/astatine-halogen-or-metal-part-1.html
======
ridgeguy
The post says astatine's radioactivity is so high a small sample would
vaporize itself, so it can't exist in a condensed phase that would be
convenient to study. It can be made by exposing bismuth-209 to an alpha
particle beam.

As the post says, this is a cooling problem. I wonder if it would work to
deposit a thin film of Bi-209 on a thin diamond plate, the other side cooled
by liquid nitrogen. Bombard the bismuth with alpha to make astatine. The
diamond's very high thermal conductivity (peaks at over 40,000W/m-°K at ~
100°K [1]) could keep the mixed bismuth/astatine film in solid form.

Since you'd want to do the alpha irradiation in a vacuum, the setup would
allow for a wide range of surface characterization techniques.

[1]
[https://en.wikipedia.org/wiki/Material_properties_of_diamond...](https://en.wikipedia.org/wiki/Material_properties_of_diamond#Thermal_conductivity)

------
bborud
If it were not for Randal Munroe, Astatine would probably not have been so
fascinating to so many of us.

    
    
      “There’s no material safety data sheet for astatine. If
      there were, it would just be the word “NO” scrawled over
      and over in charred blood.” 
        ― Randall Munroe, What If?: Serious Scientific Answers to Absurd Hypothetical Questions

~~~
hpcjoe
One of my favorites is Derek Lowe's write-ups on things he won't work with.
Things like FOOF[1], Peroxide Peroxides[2], Azidoazide Azides[3].

The first reference is quite entertaining ...

"“Being a high energy oxidizer, dioxygen difluoride reacted vigorously with
organic compounds, even at temperatures close to its melting point. It reacted
instantaneously with solid ethyl alcohol, producing a blue flame and an
explosion. When a drop of liquid 02F2 was added to liquid methane, cooled at
90°K., a white flame was produced instantaneously, which turned green upon
further burning. When 0.2 (mL) of liquid 02F2 was added to 0.5 (mL) of liquid
CH4 at 90°K., a violent explosion occurred.” And he’s just getting warmed up,
if that’s the right phrase to use for something that detonates things at -180C
(that’s -300 Fahrenheit, if you only have a kitchen thermometer). The great
majority of Streng’s reactions have surely never been run again. The paper
goes on to react FOOF with everything else you wouldn’t react it with: ammonia
(“vigorous”, this at 100K), water ice (explosion, natch), chlorine (“violent
explosion”, so he added it more slowly the second time), red phosphorus (not
good), bromine fluoride, chlorine trifluoride (say what?), perchloryl fluoride
(!), tetrafluorohydrazine (how on Earth. . .), and on, and on. If the paper
weren’t laid out in complete grammatical sentences and published in JACS,
you’d swear it was the work of a violent lunatic. I ran out of vulgar
expletives after the second page. "

[1]
[http://blogs.sciencemag.org/pipeline/archives/2010/02/23/thi...](http://blogs.sciencemag.org/pipeline/archives/2010/02/23/things_i_wont_work_with_dioxygen_difluoride)

[2]
[http://blogs.sciencemag.org/pipeline/archives/2014/10/10/thi...](http://blogs.sciencemag.org/pipeline/archives/2014/10/10/things_i_wont_work_with_peroxide_peroxides)

[3]
[http://blogs.sciencemag.org/pipeline/archives/2013/01/09/thi...](http://blogs.sciencemag.org/pipeline/archives/2013/01/09/things_i_wont_work_with_azidoazide_azides_more_or_less)

------
hpcjoe
Relativistic DFT ... Brings back memories. I did non-relativistic DFT to model
III-V materials in the early 90s. Basically building GaAs, InAs, etc. on
supercomputers using a set of codes we got from another research group, that
we modified to do the calcs we needed.

What I remember from this time was that our calculations were ridiculously
intense because we could never get a machine with enough ram to store, or
computational power to compute on the fly, all the matrix elements
<phi_1|Operator|phi_2> we needed, so we wound up setting up huge tables on
disk, and doing interpolation.

My 64 atom supercells would take, initially a week of runtime to get 100
timesteps. After a few years, my SGI R8000 could do that calc about 90 seconds
per time step. 10 years after I finished up, my home computer could do 10s
wall clock per time step. I've not tried it recently, could be fun.

Dirac eqn is a harder thing to deal with in general. We used Schrodinger eqn
parameterized for soft pseudopotentials. Also had a restriction on how many
k-points we could handle, and ionization was sadly impossible in the code, as
the repeated volume of supercells made any unbalanced charge effectively
infinite. I came up with some ways to handle this, but we lacked computer
power for that. Likely it would work today.

Honestly, I miss that work. It was fun.

~~~
plus
> ionization was sadly impossible in the code, as the repeated volume of
> supercells made any unbalanced charge effectively infinite. I came up with
> some ways to handle this, but we lacked computer power for that.

Calculations of charged periodic systems are possible through the use of Ewald
summation techniques (or particle mesh Ewald/related techniques). It basically
introduces a homogeneous neutralizing background charge, which is unphysical
and makes it so that a the total energy of a charged particle in a box
converges to the "true" result as a function of 1/volume. This is applicable
to both classical and quantum simulations.

There is no way to do a "true" charged periodic system, but honestly that's
never what you want (because true infinite systems with net charge don't
exist!). You can do better than homogeneous neutralizing background charge by
introducing a neutralizing electrolyte through a continuum solvent model.
Here's the implementation I'm most familiar with
([https://arxiv.org/abs/1601.03346](https://arxiv.org/abs/1601.03346)), but I
know the fully open-source JDFTx also implements this (which has a bunch of
other fancy approaches to incorporating solvation into periodic DFT
calculations).

------
tritium
_tl;dr_ In practical terms, no manner of direct evidence via physical tests or
experimentation are believed to be possible. Theory suggests that valence
electron shells point ambiguously toward metal, but metal nonetheless, and not
halogen. Even computational experiments are still prohibitively expensive,
given that there’s no expectation to be able to apply this science in any
useful context.

~~~
zipwitch
So, this is a question best resolved by invoking Newton's Lightsaber, at least
for now?

