Theoretical research reveals that experimental crystal structure of an important energy material is impossible

Dec 14, 2012
Left – the newly established (I41/acd) structure of the δ-phase. Right – previously proposed structure.

(Phys.org)—An international team led by Xiang-Feng Zhou and Artem R. Oganov, PhD, theoretical crystallographers in the Department of Geosciences and Department of Physics and Astronomy at Stony Brook University, have established the structure of one of the most important high-energy-density materials, magnesium borohydride or Mg(BH4)2. Their findings, "First-Principles Determination of the Structure of Magnesium Borohydride," have been published in the December 13 edition of Physical Review Letters.

"Experimental determination is often viewed as a routine task with a guaranteed correct result, but we successfully challenged the 'experimental' structure of δ-Mg(BH4)2 ," said Zhou. "This material contains nearly 15 wt. % hydrogen, which makes this an important energy material," added Oganov.

Structures of several modifications of Mg(BH4)2 were known from high-quality powder diffraction data, a rather standard method for determining crystal structures of materials. Researchers used Prof. Oganov's breakthrough evolutionary method for crystal structure prediction, aiming to find the most stable structures of Mg(BH4)2 at different conditions.

To Zhou's surprise, among the theoretically predicted structures he did not find the structure earlier proposed by experimentalists for the δ-phase. He then investigated the experimental structural model and found it to be very unfavorable compared to the theoretically predicted models. Even worse, the "experimental" structure was found to be unable to sustain its own lattice vibrations - predicted to fall apart as a result of atomic thermal motion. This indicates that the "experimental" structure is absolutely impossible – even as a .

Comparing the of the theoretically predicted structure with experiments, Zhou realized that there is a perfect match. Subsequently, he found yet another structure that matches experimental data. Thus, there are at least three completely different crystal structures that match experimental diffraction data, but one of them – the one claimed by experimentalists – has been ruled out. The other two structures were shown to explain another mystery - the existence of two almost indistinguishable phases called δ and δ' (previous experiments were unable to propose any solution for the latter). Zhou and colleagues determined the structures of these phases to have symmetries I41/acd and P-4.

"It is indeed surprising that experimental work, based on high quality data, failed to correctly solve these simple and highly symmetric crystal structures, containing only six non-hydrogen atoms," said Zhou. "We were also surprised to see completely different structures having identical diffraction patterns. In such situations, which may be more common than we expect, theoretical structure searching will play a major role."

"Crystal structure is the basis for understanding the behavior of materials," said Oganov. "The possibility to predict crystal structures is a major breakthrough of our time and will prove crucial for the future discovery of new materials."

Explore further: Synthesis of a new lean rare earth permanent magnetic compound superior to Nd2Fe14B

More information: prl.aps.org/abstract/PRL/v109/i24/e245503

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geokstr
1.8 / 5 (5) Dec 14, 2012
I refer you to the first two of Arthur C. Clarke's Rules of Prediction:

1. When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.

2. The only way of discovering the limits of the possible is to venture a little way past them into the impossible.

The third Rule is also interesting:

3. Any sufficiently advanced technology is indistinguishable from magic.
tatiana_covington_7
1 / 5 (1) Dec 14, 2012
So it's impossible... so what! Just do it anyway.
Lurker2358
1 / 5 (1) Dec 14, 2012
Thus, there are at least three completely different crystal structures that match experimental diffraction data, but one of them – the one claimed by experimentalists – has been ruled out.


Either this article is very poorly written, or the theorist is an idiot. I haven't figured out which.

Now given the choice between believing the observations and believing the opposing "predictions" of the theory, I will believe the observations.

Since when does a THEORY disprove an observation? That's outrageous.

Now observations can disprove other observations, by showing that an error was made in measurement or apparatus, etc, but a theory cannot disprove an observation.

This is an absurdity...
Torbjorn_Larsson_OM
not rated yet Dec 15, 2012
@ Lurker: It is poorly written. AFAIU the theorists found alternative structures matching the data, while the one the experimenters found wouldn't work, and matching some other data as well. Win-win.

@ geokstr: Clarke's 3d rule is purely philosophical though. Magic, outside artistry of make belief, is essentially proposals of breaking the 1st law of thermodynamics. And we can test for that.