Metastable phase diagram of Bi probed by single-energy x-ray absorption detection and angular
dispersive x-ray diffraction
E. Principi, M. Minicucci, and A. Di Cicco
CNISM and Dipartimento di Fisica, Università degli Studi di Camerino, via Madonna delle Carceri 9, 62032 Camerino (MC), Italy
A. Trapananti, S. De Panfilis, and R. Poloni
European Synchrotron Radiation Facility, Boîte Postale 220, F-38043 Grenoble, France
Received 2 May 2006; published 2 August 2006
In this paper we report the results of a detailed experimental study of samples composed of micrometric Bi
droplets providing an insight into the metastable phase diagram of Bi. To this purpose we have used the
single-energy x-ray absorption detection technique in combination with angular dispersive x-ray diffraction
available at the BM29 beamline of the European Synchrotron Radiation Facility. This unconventional approach
has given proof of being a different and reliable tool for detecting subtle structural modifications in condensed
matter. The investigation has revealed a large variety of metastable Bi polymorphs in a broad range of
pressures and temperatures 25–500 °C, 0–6 GPa and the occurrence of a Bi crystalline structure isomorphic
to the -tin structure. We have shown that the range of undercooling of liquid Bi strongly depends upon
pressure and the underlying solid stable and metastable phases. As a final result a Bi-phase diagram including
metastable phases is proposed, which takes into account all structural information obtained from this
experiment.
DOI: 10.1103/PhysRevB.74.064101 PACS numbers: 61.10.Ht, 61.10.Nz, 64.60.My, 64.30.+t
I. INTRODUCTION
Bismuth Bi is a metal of the V group well known for
exhibiting a large number of polymorphic transitions within
a limited region of pressures and temperatures.
1–10
Bi crys-
tallizes at moderate pressures and temperatures in open struc-
tures, which can be regarded locally as distorted simple-
cubic lattices.
11–14
This atomic arrangement, typical of
elements of the V group, is mainly due to the external elec-
tronic configuration s
2
p
3
, which promotes covalent bond-
ing by the p electrons spread along the three Cartesian
axes.
11,14
The spatial localization of electrons induced by co-
valent bonding makes Bi a “poor metal” and it favors an
open local environment. The presence of a locally open
structure is favored also in the ambient pressure liquid phase,
which has been found to have a low coordination number
N 9.
11,15
This value differs from the typical coordination
number of pure liquid metals 10 N 12, suggesting pos-
sible structural similarities with covalent liquids like Si
group IV
16–18
and Se group VI.
13,19
The partial covalent
bonding and the relativistic effects caused by its nuclear
mass
20
strongly affect the Bi response to pressure and tem-
perature determining a more complex behavior respect to the
isovalent elements and other pure metals. The most striking
characteristic is evidently the large negative slope of the
melting curve in the low-pressure region P 1.7 GPa.
The unusual structural characteristics observed in Bi, are
common within a class substance such as water, Si, Ge, Ga,
Sb, GaSb, InSb, InAs,¼ termed “ice-type,” which is at the
present time an object of great interest within the scientific
community.
21
These materials show a relatively low density
at low pressures, because of their open-packed local struc-
tures, but they easily experience phase transitions toward
denser spatial arrangements under the action of pressure.
Moreover, the polymorphism observed in the stable solid
phases is suspected to affect also the liquid phase. Also, liq-
uid Bi has been found to undergo resistivity anomalies at
high pressures and temperatures.
22
Such unusual behavior of
Bi and ice-type substances is understood in terms of a “two-
fluid model” in which two different local atomic arrange-
ments coexist in the liquid.
23–25
These are a low-density liq-
uid LDL which locally resembles the low-pressure solid
phase structure, and a high-density liquid HDL which can
be associated with the structure of the high-pressure solid
polymorphs. The relative concentration of these two fluids
varies upon pressurization and this change could be associ-
ated with a liquid-liquid phase transition as observed in liq-
uid P.
26
This holds also for metastable liquids. In fact, liquids
can be strongly undercooled in the region of stability of a
low-density solid phase, and a LDL-HDL transition may oc-
cur even in this metastable regime.
In this scenario the investigation of the metastable region
of Bi becomes extremely attractive. Solid Bi exhibits a wide
set of local atomic arrangements in a limited range of pres-
sures and temperatures and it is a natural candidate for mani-
festing polymorphism also in the metastable phases. The aim
of this study was to obtain information on the atomic struc-
ture of metastable Bi in a broad range of temperatures and
pressures 25–500 °C, 0–6 GPa investigating for structural
anomalies and possible unknown atomic arrangements both
in the liquid and in the solid phase. For this purpose we have
exploited an advanced experimental setup installed at the
BM29 beamline of the European Synchrotron Radiation Fa-
cility ESRF combining x-ray absorption spectroscopy
XAS and angular dispersive x-ray diffraction ADXD.
These techniques are often used separately, limiting the sen-
sitivity to the short-range order XAS or to the long-range
order ADXD. The present study confirms the importance of
their combination to provide a complete and reliable deter-
mination of the atomic arrangement upon structural modifi-
cations induced by extreme pressure and temperature condi-
tions.
PHYSICAL REVIEW B 74, 064101 2006
1098-0121/2006/746/0641017 ©2006 The American Physical Society 064101-1