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Citation for fin al p u blish e d ve rsion:
Dzade, N elson Y. a n d d e Lee uw, Nora H. 20 1 7. Periodic DFT + U investig a tion
of th e b ulk a n d su rface p r op e r ties of m a r c asite (FeS2). Physical C h e mistry
Chemical Physics 1 9 (40) , p p. 27478-27488. 1 0.1039/C7CP04413E file
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1
Periodic DFT+U investigation of the bulk and surface properties of
marcasite (FeS
2
)
Nelson Y. Dzade*
1
and Nora H. de Leeuw
1, 2*
1
Department of Earth Sciences, Utrecht University, Princetonplein 9, 3584 CC, Utrecht, The
Netherlands
2
School of Chemistry, Cardiff University, Main Building, Park Place, CF10 3AT, Cardiff, United
Kingdom
E-mail:
N.Y.Dzade@uu.nl (NYD); deLeeuwN@cardiff.ac.uk (NHdL)
ABSTRACT
Marcasite FeS
2
and its surfaces properties have been investigated by Hubbard-corrected Density
Functional Theory (DFT+U) calculations. The calculated structural parameters, interatomic bond
distances, elastic constants and electronic properties of the bulk mineral were determined and
compared with earlier theoretical reports and experimental data where available. We have also
investigated the relative stability, interlayer spacing relaxations, work function, and electronic
structures of the {010}, {101}, {110} and {130} surfaces under dehydrated and hydrated
conditions. Using the calculated surface energies, we have derived the equilibrium crystal shape
of marcasite from a Wulff construction. The {101} and {010} surfaces dominate the marcasite
crystallite surface area under both dehydrated and hydrated conditions, in agreement with their
relative stabilities compared to the other surfaces. The simulated scanning tunneling microscopy
(STM) images of the {101} and {010} facets are also presented, for comparison with future
experiments.
KEY WORDS: Marcasite (FeS
2
); Elastic constants; Crystal morphology; Surface structures, STM
images; Density Functional Theory (DFT)
2
GRAPHICAL ABSTRACT
1. INTRODUCTION
Iron sulfide minerals are abundant in nature and exist in a variety of phases with stoichiometries
that range from the sulfur-deficient mackinawite FeS
1−x
through iron-deficient pyrrhotite Fe
1−x
S
to greigite (Fe
3
S
4
) and pyrite (FeS
2
).
1
Iron disulfide (FeS
2
) occurs naturally as two polymorphs;
pyrite (p−FeS
2
) crystallizes in the cubic space group, Pa3 while marcasite (m−FeS
2
) belongs to
orthorhombic Pnnm.
2
Iron pyrite has received much attention as a promising photovoltaic material
because of its suitable band gap (E
g
=0.95 eV), high abundance, nontoxicity, and strong light
absorption (~10
5
cm
−1
for hν > 4 1.4 eV).
3−12
Marcasite, the lesser known polymorph, is often considered to be an undesired contaminant phase
for photovoltaic applications,
13,14
because of its reported small band gap of 0.34 eV.
15
Wadia and
co-workers have speculated that the presence of trace amounts of marcasite in pyrite would
significantly lower the band gap and therefore deteriorate the material’s photovoltaic
performance.
13, 14
However, recently published studies have thrown doubt on the earlier reported
band gap of 0.34 eV for marcasite.
16−21
Theoretical investigations have predicted that marcasite
should have a band gap that is quite similar to that of pyrite (around 0.8–1.1 eV),
16−20
whereas
3
recent diffuse reflectance spectroscopy (DRS) measurements of natural marcasite samples have
estimated the optical absorption gap to be approximately 0.83 ± 0.02 eV,
21
which is similar to the
band gap of pyrite (0.95 eV).
11
These recent findings suggest that marcasite, which co-exists with
pyrite and was originally regarded as a detrimental impurity, may actually be a highly useful semi-
conductor and photocatalyst in its own right.
The development of an efficient phot-catalyst, however, requires an atomic-level understanding of
the structure and composition, as well as information about the relative stabilities of its major
surfaces as they dictate its morphology and reactivity towards adsorbing species.
22−24
Detailed
information regarding the structure, electronic and mechanical stability of the bulk material is also
required. In earlier studies, the phase stability and thermoelectric properties of the naturally
occurring marcasite phase of FeS
2
under ambient conditions has been investigated using first-
principles calculations.
18, 25
Total energy calculations show that marcasite FeS
2
was stable at
ambient conditions, and that it undergoes a first-order phase transition to pyrite FeS
2
at around
3.7−5.4 GPa at 0 K.
18, 25
Reich and Becker have also employed first-principles and Monte Carlo
calculations to investigate the thermodynamic mixing properties of arsenic into bulk pyrite and
marcasite.
26
From their calculated enthalpies, configurational entropies and Gibbs free energies of
mixing, it was shown that the two-phase mixtures of FeS
2
(pyrite or marcasite) and FeAsS
(arsenopyrite) are energetically more favorable than the solid solution Fe(S,As)
2
(arsenian pyrite
or marcasite) for a wide range of geologically relevant temperatures.
26
There also exist significant
information in the literature on the oxidation and chemistry of different stoichiometric and
defective pyrite surfaces using ab initio theoretical calculations
27−31
and experimental
32−34
investigations. Hydration and early oxidation of the surfaces of mackinawite,
35, 36
greigite,
37, 38
and
violarite (FeNi
2
S
4
)
39
have also been investigated using DFT calculations. However, to date, no
4
systematic theoretical study has been conducted to investigate the structures and stabilities of the
major surfaces of marcasite, which makes this investigation timely.
In this study, we have employed Density Functional Theory calculations, with Hubbard corrections
for the electron correlation in the localized d-Fe orbitals (DFT+U), to first investigate the
structures, electronic and mechanical properties of bulk marcasite. Secondly, the composition and
structure, as well as the relative stabilities of the major surfaces of marcasite have been
characterized systematically under dehydrated and hydrated conditions. The electronic properties
of each surface, including the work function (Φ) have also been determined and are discussed.
Using the calculated surface energies, we have derived the equilibrium morphology of marcasite
crystals using Wulff construction.
40
Finally, we have used the HIVE program
41
to simulate the
topographical Scanning tunneling microscopy (STM) images of the {101} and {010} surfaces,
which are the dominant growth facets expressed in the marcasite crystal morphology.
2. COMPUTATIONAL DETAILS
The optimized structures were determined using plane-wave density functional theory (PW-DFT)
calculations within the Vienna Ab-initio Simulation Package (VASP code).
42−45
The interactions
between the valence electrons and the ionic core were described with the projected augmented
wave (PAW) method
43, 46
and the electronic exchange-correlation potential was calculated using
the Perdew−Burke−Enzerhof (PBE) generalized gradient approximation (GGA) functional,
47,48
with Hubbard U correction
(PBE+U).
49−51
The +U correction term provides an accurate treatment
of the electron correlation in the localized d-Fe orbitals, which is crucial for a proper description
of the structural and electronic properties of these materials. We have used an effective U of 2 eV,
which has been shown to give an accurate description of the structural parameters and the
