I4/mmm phase of MgF2: An ab initio molecular dynamics study

WOS: 000365031800014A constant pressure ab initio technique is applied in order to study the structural response of magnesium fluoride (MgF2) under the hydrostatic pressure up to 800 GPa. The orthorhombic-to-tetragonal phase transformation of MgF2 is obtained at 720 GPa. This phase transition is also analyzed from the total energy and enthalpy calculations, and it is found that this phase change should occur around 260 GPa. As far as we know, this tetragonal phase has not been obtained in any previous studies.Ahi Evran UniversityAhi Evran University [PYO-Fen.4010.14.008]This study was supported by the Ahi Evran University under Scientific Support, project no. PYO-Fen.4010.14.008

Structural and electronic properties of BiOF with two-dimensional layered structure under high pressure: Ab initio study

WOS: 000454538900007In this work, the crystal structure of the BiOF is studied under high hydrostatic pressure using ab initio calculations. Pressure-volume relationships and structural transitions are investigated using Siesta method. A first-order phase transition from the tetragonal matlockite PbFCl-type structure with space group P4/nmm to the orthorhombic structure with space group Cmcm is successfully observed for BiOF. This phase transition which occur around 19.6 GPa is also analyzed from the total energy and enthalpy calculations. In addition, electronic properties of BiOF are researched during the pressure. By analyzing the energy band structures, it is found that the band gaps P4/nmm and Cmcm phases for the BiOF are 2.74 and 2.47 eV, respectively

High-pressure structural phase transitions, electronic propertie and intermediate states of CaSe

WOS: 000473803900013In this study, ab initio calculations have been carried out to understand the effect of extreme external pressure on the crystal structure of CaSe. The crystal structure of CaSe, a calcium chalcogen, is studied using density functional theory (DFT) with the generalized gradient approximation (GGA) up to 250 GPa under high hydrostatic pressure. Structurally CaSe crystallizes in cubic NaCI-type (BI) structure (space group: Fm (3) over barm) at ambient conditions. The results indicated that CaSe undergoes a structural phase transition from this cubic structure to another cubic CsCl-type (B2) structure (space group: Pm (3) over barm) at high pressure. This transformation is based on two intermediate states with space group R (3) over barm and C2/m. Additionally, the electronic band structures and density of states for the obtained B1 and B2 structures of CaSe have been calculated. According to these calculations, obtained band gap values are in good agreement with the values reported in the literature

An Ab-initio Study of Structural and Electronic Properties of CaTe under High Pressure

WOS: 000493607000021The crystal structure of the CaTe compound is studied up to 150 GPa under high hydrostatic pressure using the density functional theory (DFT) with the generalized gradient approximation (GGA). Pressure-volume relationships, structural transitions and electronic properties in CaTe compound are investigated using Siesta method. CaTe crystallizes in the NaCl-type (B1) structure (space group:) at ambient conditions, and transforms to CsCl-type (B2) structure (space group:) at high pressure. This transformation is based on a intermediate state with space group. Moreover, the electronic band structure of the B1 and B2 structures of CaTe have been calculated. According to this calculation, obtained band gap values are in good agreement with the values reported in the literature

Structural phase transformations and new intermediate phases of MgF2 under high-pressures applied via conjugate-gradient method

WOS: 000336606000030An ab initio constant pressure technique is performed in order to study the structural behavior of MgF2 under the hydrostatic pressure up to 700 GPa. Two high-pressure phases of MgF2 are successfully observed through constant pressure simulations. MgF2 undergoes a phase transformation from the rutile-type structure to the cubic-fluorite-type structure with space group Fm(3) over bar m at 280 GPa. Another phase transformation from the cubic-fluorite-type structure to the cotunnite-type structure with space group Pnma occurs at 500 GPa. The first transformation proceeds via three intermediate phases with space groups Pnnm, Immm and R(3) over bar m. We also investigate the stability of these phases from the total energy and enthalpy computations. According to these investigations, the phase transformations should occur from the rutile-type structure to the cubic-fluorite-type structure around 20 GPa and from the cubic-fluorite-type structure to the cotunnite-type structure around 45 GPa. (C) 2014 Elsevier B.V. All rights reserved.Ahi Evran University under Scientific Research Project (BAP)Ahi Evran University [FBA-10-14]This study was supported by the Ahi Evran University under Scientific Research Project (BAP) No.: FBA-10-14

High-pressure structural phase transitions and intermediate phases of magnesium fluoride

WOS: 000333083400024We investigate the structural behavior of magnesium fluoride (MgF2) under the hydrostatic pressure using constant pressure ab initio technique up to 130 GPa. Through constant pressure simulations, two high-pressure phases of MgF2 are estimated. MgF2 undergoes a phase transformation from the rutile-type structure to the CaCl2-type structure with space group Pnnm at 10-20 GPa. Another phase transformation from the CaCl2-type structure to the alpha-PbCl2-type structure with space group Pnma occurs at 80-90 GPa. The later transformation is based on three intermediate phases with space groups P2(1)/m, P2(1) and P2(1)2(1)2(1). These phase transitions are also analyzed from the total energy and enthalpy calculations. These phase changes should occur around 9 and 35 GPa from enthalpy calculations, respectively. (C) 2014 Elsevier B.V. All rights reserved.Ahi Evran UniversityAhi Evran University [FBA-10-14]This work was supported by the Ahi Evran University under Scientific Research Project (BAP) No: FBA-10-14. The authors are grateful to the referees for many suggestions which led to an improved version of this paper

Structural evolution and electronic properties of CaS: An ab initio study

WOS: 000462471400003CaS crystallizes in cubic NaCl (B1) type structure with symmetry Fm (3) over barm. In this work, the structural and electronic properties of CaS were investigated by considering the Density Functional calculations within the framework of Generalized Gradient Approximation (GGA) under high pressure. The structural change was found at the B1 type structure of CaS. B1 type structure transformed into another cubic CsCl (B2) type structure with symmetry Pm (3) over barm at 36.6 GPa. An intermediate state with symmetry R (3) over barm was predicted during this transition. Besides, the effects of the pressure on the electronic properties of CaS were also studied. Both the B1 and B2 type structures exhibited semiconducting behaviors with direct band gaps at the Gamma-point and R-point, respectively. Intermediate state was searched during this phase change first time in detail. The obtained results were compared with experimental and theoretical ones in the literature

Pressure-induced phase transitions and structural properties of CoF2: An ab-initio molecular dynamics study

WOS: 000371347700005The crystal structure of CoF2 was studied theoretically using first-principles density functional theory (DFT) methods within the generalized gradient approximation (GGA) and local density approximation (LDA) under rapid hydrostatic pressure up to 144 GPa. CoF2 undergoes a structural phase transformation from the rutile-type tetragonal parent phase with space group P4(2)/mnm to the CaCl2-type orthorhombic parent phase with space group Pnnm at 64 GPa with GGA and at 96 GPa with LDA methods. Another phase transformation occurs from the CaCl2-type structure to monoclinic parent phase with space group P2(1)/c at 96 GPa with a GGA method. These phase transitions are also studied by enthalpy and total energy calculations. According to these calculations, we obtained the first phase transformation at about 6.5 GPa both GGA and LDA methods and the later phase transformation at about 45 GPa with the GGA method. (C) 2016 Elsevier Ltd. All rights reserved

Structural phase transition and electronic properties of CaO under high pressure

WOS: 000445944900002The crystal structure of the CaO compound is studied up to 300 GPa under high hydrostatic pressure using the density functional theory (DFT) with the generalized gradient approximation (GGA). Pressure-volume relationships, structural transitions and electronic properties in CaO compound are investigated using Siesta method. CaO crystallizes in the NaCl-type (B1) structure (space group: Fm (3) over barm) in ambient conditions. CaO transforms from this structure to CsCl-type (B2) structure (space group: Pm (3) over barm) at high pressure. This transformation is based on a intermediate state with space group R (3) over barm. Moreover, the electronic band structures of the B1 and B2 structures of CaO have been calculated. According to this calculation, obtained band gap values are in good agreement with the values reported in the literature

Theoretical calculations of high-pressure phases of NiF2: An ab initio constant-pressure study

WOS: 000388216600007We have studied the structural properties of the antiferromagnetic NiF2 tetragonal structure with P4(2)/mnm symmetry using density functional theory (DFT) under rapid hydrostatic pressure up to 400 GPa. For the exchange correlation energy we used the local density approximation (LDA) of Ceperley and Alder (CA). Two phase transformations are successfully observed through the simulations. The structures of XF2-type compounds crystallize in rutile-type structure. NiF2 undergoes phase transformations from the tetragonal rutile-type structure with space group P4(2)/mnm to orthorhombic CaCl2-type structure with space group Pnnm and from this orthorhombic phase to monoclinic structure with space group C2/m at 152 GPa and 360 GPa, respectively. These phase changes are also studied by total energy and enthalpy calculations. According to these calculations, we perdict these phase transformations at about 1.85 and 30 GPa