Ground-state properties and high-pressure phase
of beryllium chalcogenides BeSe, BeTe, and BeS
A. Mun
˜
oz, P. Rodrı
´
guez-Herna
´
ndez, and A. Mujica
Departamento de Fı ´sica Fundamental y Experimental, Facultad de Fı ´sica. Universidad de La Laguna,
E-38204 La Laguna, Tenerife, Spain
Received 22 April 1996
We present an ab initio pseudopotential study within the local density approximation of the ground-state and
high-pressure phases of BeSe, BeTe, and BeS. We analyze the zinc-blende, NaCl, CsCl, NiAs, and -Sn
structures. By calculating the total energy, atomic forces, and stress tensors we determine the structural
parameters lattice constants, bulk moduli, etc. of these compounds and the transition pressure from the
zinc-blende ( B 3 ) to the NiAs ( B 8) phase. The structural parameters and transition pressure for BeSe and BeTe
compare quite well with recent experimental results. For BeS our results are predictions.
S0163-18299604038-6
I. INTRODUCTION
Little is known about beryllium chalcogenides BeS, BeSe,
and BeTe. Recent experimental results
1
report the existence
of a first-order transition between zinc-blende and NiAs
phases in BeSe and BeTe at pressures of 61.36 GPa and
39.36 GPa, respectively. Among the IIA-VI compounds ex-
perimentally investigated so far only for BeSe and BeTe has
a first-order phase transition from the zinc-blende phase to
the NiAs phase been reported; for the rest a phase transition
between the NaCl and the CsCl structure has been observed
2
except for BeS where no experimental data are available.
The beryllium compounds BeSe, BeTe, and BeS crystallize
in the cubic zinc-blende structure. The rest of the chalco-
genides of group IIA adopt the cubic NaCl structure except
for BeO and MgTe, which have the wurtzite hexagonal
structure. BeSe, BeTe, and BeS are particularly similar to the
boron compounds BN, BP and BAs, having at ambient con-
ditions the same crystal structure, wide band gap, and high
bulk moduli. The zinc-blende compounds BN, BP, and BAs
have an unusual behavior when compared to the other III-V
compound families due to the small core size and the ab-
sence of p electrons. Thus the study of the Be compound
could help in the understanding of the behavior of the B
compounds.
In this work we concentrate our efforts in the theoretical
study from ab initio pseudopotential theory of the structural
phase transition of beryllium chalcogenides BeSe, BeTe,
and BeS under pressure. We analyze the ground-state zinc-
blende structure ZB and the sixfold coordinate cubic NaCl,
the hexagonal NiAs, the cubic CsCl, and the tetragonal
-Sn structures. The calculations are performed in the frame-
work of the density functional theory with ab initio norm-
conserving pseudopotential.
3
It is well known that this
method is capable of giving accurate results for ground-state
properties of a wide variety of semiconductors and metal
materials. The ab initio pseudopotential method for total en-
ergy calculations has been shown to be capable of predicting
structural properties for group-IV elements
4,5
and III-V
compounds.
6–8
Although the agreement between experiment
and theory for group-IV elements is quite impressive, no-
table discrepancies appear for some III-V compounds. Some
difficulties arise from the fact that there is little experimental
knowledge about the structure of the high-pressure phases
and the incomplete theoretical study of these phases.
The paper is organized as follows: In the next section we
briefly describe the method of calculation and in Sec. III we
present the study of the structural properties of the ground-
state and high-pressure phases of BeSe, BeTe, and BeS. Fi-
nally our conclusions are given in Sec. IV.
II. METHOD
We have used a first principles pseudopotential method
within the local density approximation LDA formalism
3
to
calculate the total energies of zinc-blende, NaCl, NiAs,
CsCl, and -Sn phases for the beryllium compounds BeSe,
BeTe, and BeS. The Ceperley-Alder form of the local den-
sity approximation for the exchange correlation
9
was used.
Norm-conserving nonlocal pseudopotentials were con-
structed with the Kerker scheme.
10
We need to calculate small energy differences between
phases. A basis set containing all plane waves up to the
cutoff energy of 30 Ry was used for BeSe and BeTe which is
sufficient to describe the energy difference between different
phases with an accuracy of 1 meV/molecule; for BeS we
need to increase the cutoff up to 90 Ry due to the hard
potential of the sulfur atom. The Brillouin zone integrations
were replaced by discrete k space summations. We use the
standard k-points technique of Monkhorst and Pack.
11
In or-
der to perform accurate Brillouin zone integrations for the
semiconducting zinc-blende phase we use 28 k points and
168, 120, 110, and 100 k points for the Nias, CsCl, NaCl,
and -Sn phases, respectively.
The -Sn and NiAs structures have a single degree of
freedom, which we take to be the c / a ratio. For these struc-
tures we minimize the energy with respect to the internal
degrees of freedom at each volume considered. For the hex-
agonal NiAs structure we determine the two lattice constants
a and c as follows. First we choose a unit cell volume V and
PHYSICAL REVIEW B 1 NOVEMBER 1996-I VOLUME 54, NUMBER 17
54 0163-1829/96/5417/118614/$10.00 11 861 © 1996 The American Physical Society