First principles study of structural phase stability of wide-gap semiconductors MgTe, MgS and MgSe

Cataloged from PDF version of article.The structural stability of different crystallographic phases of three magnesium chalcogenides, MgTe, MgS and MgSe, are investigated from first principles calculations based on density functional theory (DFT) using projector augmented waves (PAW) potentials within both generalized gradient approximation (GGA) and local density approximation (LDA). Five different phases of these compounds, rock-salt (B1), cesium chloride (B2), zinc-blende (B3), wurtzite (B4) and NiAs (B8(1)), are considered in detail LDA. calculations of the ground state structure of all compounds considered are in accordance with previous studies, while calculations with GGA functionals resulted interesting perspectives. Ground state stable phases of MgS and MgSe are found as rock-salt structure, while the NiAs is the ground state of MgTe within the LDA scheme. But GGA calculations show that the ground states of MgTe and MgSe are wurtzite, while the ground state of MgS is same as in LDA calculations. There is also a structural transition observed for MgTe around similar to 1.2 GPa pressure from wurtzite to NiAs by GGA calculations. The calculated quantities show that the rock-salt-NiAs and zinc-blende-wurtzite pairs have similar energetics. The calculated structural quantities for all compounds under study in all phases agree well with the available experimental and theoretical values. (C) 2009 Elsevier B.V. All rights reserved

Liquid-liquid phase transition in Stillinger-Weber silicon

It was recently demonstrated that the Stillinger-Weber silicon undergoes a liquid-liquid first-order phase transition deep into the supercooled region (Sastry and Angell, Nature Materials 2, 739 (2003)). Here we study the effects of perturbations on this phase transition. We show that the order of the liquid-liquid transition changes with negative pressure. We also find that the liquid-liquid transition disappears when the three-body term of the potential is strengthened by as little as 5 %. This implies that the details of the potential could affect strongly the nature and even the existence of the liquid-liquid phase.Comment: 13 page

Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation

Using density functional molecular dynamics simulations, we analyze the broken chemical order in a GeS$_2$ glass and its impact on the dynamical properties of the glass through the in-depth study of the vibrational eigenvectors. We find homopolar bonds and the frequencies of the corresponding modes are in agreement with experimental data. Localized S-S modes and 3-fold coordinated sulfur atoms are found to be at the origin of specific Raman peaks whose origin was not previously clear. Through the ring size statistics we find, during the glass formation, a conversion of 3-membered rings into larger units but also into 2-membered rings whose vibrational signature is in agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.

Transformation Pathways of Silica under High Pressure

Concurrent molecular dynamics simulations and ab initio calculations show that densification of silica under pressure follows a ubiquitous two-stage mechanism. First, anions form a close-packed sub-lattice, governed by the strong repulsion between them. Next, cations redistribute onto the interstices. In cristobalite silica, the first stage is manifest by the formation of a metastable phase, which was observed experimentally a decade ago, but never indexed due to ambiguous diffraction patterns. Our simulations conclusively reveal its structure and its role in the densification of silica.Comment: 14 pages, 4 figure

Ab initio prediction of pressure-induced structural phase transition of superconducting FeSe

External pressure driven phase transitions of FeSe are predicted using \textit{ab initio} calculations. The calculations reveal that $\alpha$-FeSe takes transitions to NiAs-type, MnP-type, and CsCl-type FeSe. Transitions from NiAs-type to MnP-type and CsCl-type FeSe is also predicted. MnP-type FeSe is also found to be able to transform to CsCl-type FeSe, which is easier from $\alpha$-FeSe than the transition to MnP-type FeSe, but comparable to the transition from NiAs-type FeSe. The calculated electronic structures show that all phases of FeSe are metallic, but the ionic interaction between Fe-Se bonds becomes stronger and the covalent interaction becomes weaker when the structural phase transition occurs from $\alpha$-FeSe to the other phases of FeSe. The experimentally observed decrease in $T_{c}$ of superconducting $\alpha$-FeSe at high pressure may be due to a structural/magnetic instability, which exists at high pressure. The results suggest us to increase the $T_{c}$ of $\alpha$-FeSe if such phase transitions are frustrated by suitable methods.Comment: Accepted for publications in J. Phys.: Condens. Matter (2012

Formation of a two-dimensional layered structure in silica under shear stresses: An ab initio study

The behavior of ?-cristobalite under pure shear stresses is studied using an ab initio technique. The application of shear stress on the a-b planes yields a two-dimensional layered monoclinic phase with C2 space group via an orthorhombic phase having C2221 symmetry. In the layered structure, each silicon atom is fourfold coordinated. On the other hand, shearing of a-c and b-c planes causes first a transformation into monoclinic phases within P 21 symmetry and then structural failure. We also study the behavior of ? -cristobalite under the simultaneous application of shear stress and hydrostatic pressure. We consider six different loading conditions and find that the shear stresses have no influence on the densification mechanism until a first-order phase transformation occurs. On the other hand, the shear stresses play a significant role during the first-order phase transformation and result in the Cmcm, anataselike, and stishovite structures in our simulations. The phase transformation from ? -cristobalite to the Cmcm structure is due to the shear deformation on the a-b planes and proceeds via an orthorhombic intermediate phase having C2221 symmetry. © 2010 The American Physical Society

Pressure-induced phase transformation in CdO: An ab initio constant-pressure study

WOS: 000263598900014A constant-pressure ab initio technique is used to study the behavior of CdO at high pressures. A phase transformation from the NaCl-type structure to the CsCl-type structure via a rhombohedral intermediate state is successfully observed through the constant-pressure simulation. This phase change is also analyzed from total energy calculations. Our transition parameters and bulk properties are comparable with available experimental and theoretical data. Copyright (C) EPLA, 2008Scientific and Technical Research Council of Turkey (TUBITAK)Turkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [BIDEB-2221]The visit of MD to Ahi Evran Universitesi was facilitated by the Scientific and Technical Research Council of Turkey (TUBITAK) BIDEB-2221. The calculations were run on Sacagawea, a 128 processor Beowulf cluster, in the University of Texas at El Paso

Expanded phase of ZrO2: An ab initio constant-pressure study

WOS: 000273854100016The stability of ZrO2 in the tensile regime is studied using an ab initio constant-pressure technique. A first-order phase transformation from the baddeleyite structure to an anatase-like structure (I4(1)/amd) is predicted through the constant-pressure simulation. A transformation mechanism at the atomistic level is discussed. Furthermore this phase transformation is studied from the energy-volume calculations. Copyright (C) EPLA, 200

An ab initio constant-pressure study of pressure-induced phase transition of MgSe

WOS: 000263598800016We study the pressure-induced phase transition of MgSe using a constant-pressure ab initio technique and. find that the rocksalt-structured MgSe transforms into a FeSi-type structure. Furthermore, we. find that this phase transformation is based on a cubic intermediate state with space group of F (4) over bar 3m. The rocksalt-to-FeSi-type phase transition is also studied using total-energy calculations. Our transition pressure and bulk properties are in good agreement with the available experimental and theoretical results. Copyright (C) EPLA, 200