3 ms·

Astatine: Halogen or Metal?

76 points by nkurz 9y ago

tritium 9y ago
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 9y ago
  So, this is a question best resolved by invoking Newton's Lightsaber, at least for now?
bborud 9y ago
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
- deleted 9y ago
  [deleted]
- digi_owl 9y ago
  And just now it dawned on me that my rss reader had been failing to process the xkcd feed for some time. I guess i am way past the saturation point of the stick figures humor.
- hpcjoe 9y ago
  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/things_i_wont_work_with_dioxygen_difluoride http://blogs.sciencemag.org/pipeline/archives/2010/02/23/thi... [2] http://blogs.sciencemag.org/pipeline/archives/2014/10/10/things_i_wont_work_with_peroxide_peroxides http://blogs.sciencemag.org/pipeline/archives/2014/10/10/thi... [3] http://blogs.sciencemag.org/pipeline/archives/2013/01/09/things_i_wont_work_with_azidoazide_azides_more_or_less http://blogs.sciencemag.org/pipeline/archives/2013/01/09/thi...
ridgeguy 9y ago
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#Thermal_conductivity https://en.wikipedia.org/wiki/Material_properties_of_diamond...
hpcjoe 9y ago
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 9y ago
  > 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).

76 points by nkurz 9y ago

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 9y ago
So, this is a question best resolved by invoking Newton's Lightsaber, at least for now?

bborud 9y ago

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

deleted 9y ago
[deleted]
digi_owl 9y ago
And just now it dawned on me that my rss reader had been failing to process the xkcd feed for some time. I guess i am way past the saturation point of the stick figures humor.
hpcjoe 9y ago
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/things_i_wont_work_with_dioxygen_difluoride http://blogs.sciencemag.org/pipeline/archives/2010/02/23/thi... [2] http://blogs.sciencemag.org/pipeline/archives/2014/10/10/things_i_wont_work_with_peroxide_peroxides http://blogs.sciencemag.org/pipeline/archives/2014/10/10/thi... [3] http://blogs.sciencemag.org/pipeline/archives/2013/01/09/things_i_wont_work_with_azidoazide_azides_more_or_less http://blogs.sciencemag.org/pipeline/archives/2013/01/09/thi...

ridgeguy 9y ago

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#Thermal_conductivity https://en.wikipedia.org/wiki/Material_properties_of_diamond...

hpcjoe 9y ago

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 9y ago
> 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).