Black arsenic. Image: TUM |
Phosphorus
and arsenic are on top of each other in one group of the periodic
table, so they have many similar properties. In addition to tubular
forms, phosphorus is found in white, red, black, and purple structural
forms. At room temperature, black phosphorus is the stable form; the
others are metastable. According to textbooks, arsenic occurs in gray,
yellow, and black forms.
However,
the existence of black arsenic, which should be analogous to black
phosphorus, has never been indisputably proven. In the journal
Angewandte Chemie, researchers from Technische Universitaet Muenchen,
University of Regensburg and Lausitz University of Applied Sciences have
now demonstrated that black arsenic is metastable in its pure form, and
that it has thus far only been obtained in a form stabilized by atoms
of other elements.
In
their studies, a team led by Tom Nilges at the Technical University of
Munich, Richard Weihrich at the University of Regensburg, and Peer
Schmidt at the Lausitz University of Applied Sciences combined quantum
chemical computations with experimental investigations of phase
formation.
The
calculations make it possible to estimate the energetic stabilities of
various structural forms of pure substances or combinations of solids,
which are called solid solutions. Which phases are formed depends not
only on this thermodynamic energy content, but also on the speed
(kinetics) with which the individual phases form and interconvert.
Metastable phases have a higher energy at defined pressures and
temperatures than the stable phase. However, because a relatively high
energy barrier must initially be overcome in their conversion, they only
slowly convert to the stable phase, if at all.
The
researchers used gas-phase reactions to study phase formation. In these
reactions, the solids are heated and the resulting pressure, which
builds through sublimation of particles from the solid, is measured.
Particles from a metastable phase enter the gas phase much more easily,
so the pressure is higher than for a stable phase. When a metastable
phase converts to a stable phase, the drop in pressure can be observed.
It is even possible to observe pathways involving multiple different
metastable intermediates.
The
researchers were thus able to identify all metastable and stable phases
of solid solutions of arsenic and phosphorus in all possible ratios.
They were thus able to demonstrate that black arsenic is metastable in
its pure form.
The
results of such experiments do not only provide fundamental academic
knowledge, they are also helpful in the development of targeted
synthetic pathways for desirable metastable phases. This is of interest
for the production of innovative materials, since metastable phases
often demonstrate interesting properties. One current example of a
metastable phase is an extremely hard diamond that can theoretically
spontaneously convert to graphite—at room temperature—but actually never
does.
Synthesis and Identification of Metastable Compounds: Black Arsenic—Science or Fiction?
