Article pubs.acs.org/IC
Pressure-Induced Stable Beryllium Peroxide Shoutao Zhang,† Fei Li,† Haiyang Xu,*,† and Guochun Yang*,†,‡ †
Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, China ‡ State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, China S Supporting Information *
ABSTRACT: Beryllium oxides, at ambient pressure, have been extensively studied due to their unique chemical bonds and applications. However, the long-desirable target beryllium peroxide (BeO2) has not been reported, thus far. Currently, the application of pressure has become a powerful tool in finding unusual stoichiometric compounds with exotic properties. Here, swarm structural searches in combination with firstprinciples calculations disclosed that the reaction of BeO and oxygen, at pressures above 89.6 GPa, yields BeO2. Interestingly, this reaction pressure is lower than the phase transition pressure (106 GPa) of pure BeO. BeO2 crystallizes in FeS2type structure, whose remarkable feature is that it contains peroxide group (O22−) with an O−O distance of 1.40 Å at 100 GPa. Notably, O22− is maintained in the pressure range of 89.6− 300 GPa. The chemical bonding analysis shows that the uniformly distributed ionic Be−O and covalent O−O bonding network plays a key role in determining its structural stability. BeO2 is a direct band gap nonmetal, and its band gap becomes larger with increase of pressure, which is in sharp contrast with BaO2. Moreover, phase diagram of Be−O binary compounds with various BexOy (x = 1−3, y = 1−6) compositions at pressures of up to 300 GPa was reliably built. Our results are also important for enriching the understanding of beryllium oxides. Various oxygen-rich composition compounds (e.g., BeO,17 BeO2,18 Be2O2,19 Be2O4,20 BeO4,21 and BeO621) have been reported. Notably, most of these compounds stabilize in molecular forms except for BeO. Inspired by heavier alkaline earth peroxides,8,22,23 great efforts have been made to synthesize beryllium peroxide (BeO2) at ambient conditions; however, no evidence suggested its existence in crystal form, thus far.24 The application of pressure has become a powerful tool in obtaining unusual stoichiometric compounds with exotic chemical and physical properties.25−33 This is properly because that pressure can effectively overcome reaction energy barriers34 and reorder atomic orbital energy levels.35 Recently, first-principles structural prediction methods can reliably discover novel compounds with unusual chemical compositions, especially at high pressure.36−39 For example, the predicted Na3Cl and NaCl3 compounds at high pressure have been confirmed experimentally.25 The predicted H3S, with Im3̅m symmetry,40 successfully explains high-temperature superconductivity (203 K) of compressed hydrogen sulfide.41 To our best knowledge, only phase transition and electronic property of BeO compound under high pressure has been reported.42−51 High-pressure phase diagram and relative stability of Be−O binary compounds have not been explored,
1. INTRODUCTION Peroxide group (O22−) contains an oxygen−oxygen single bond (O−O), in which each atom has an oxidization state of −1.1 O22− induces exotic properties in its compounds, which are of great significance in diverse fields (e.g., chemistry, energy storage, material science, industry, and medicine). For instance, hydrogen peroxide (H2O2) has widely been used as a disinfectant, bleaching agent, and oxidizer.2,3 H2O2 can also be used as a precursor to prepare other peroxide compounds. Lithium peroxide (Li2O2) is the main discharge product in Li− air battery. Li−air battery, with extremely high specific energy, becomes one of the most promising candidates for future electric vehicles.4,5 Magnesium peroxide (MgO2) is widely used in agricultural and environmental industries, because it stably releases oxygen.6 Very recently, ionic crystals consisting of O22− can be used in solid oxide fuel cells.7,8 As a consequence, a constant endeavor has been made to find novel compounds containing O22−. Owing to the small atomic radius and high ionization energy of beryllium element, the structure and chemical bond of beryllium compounds are of great interest.9,10 Moreover, covalent interactions play the important role in stabilizing beryllium compounds, unlike the other alkaline earth metals (Mg−Ba).11 Among the various beryllium compounds, beryllium oxides have drawn significant attention, because of their unique bonds and broad applications (e.g., coating, nanodevice, catalyst, and moderator in nuclear reactor).12−16 © 2017 American Chemical Society
Received: February 9, 2017 Published: April 11, 2017 5233
DOI: 10.1021/acs.inorgchem.7b00365 Inorg. Chem. 2017, 56, 5233−5238
Article
Inorganic Chemistry
Be−O compositions at different pressures are constructed (Figure 1a) by evaluating the average atom formation enthalpy
thus far. In view of empirical considerations on chemical pressure,52,53 oxygen-rich composition compounds are expected to become stable in crystal forms under high pressure. In this work, we conducted an extensive structure search on Be−O compounds with various BexOy (x = 1−3, y = 1−6) compositions up to 300 GPa by using the swarm-intelligence structural prediction method.36,37 For BeO compound, both the known structures and phase transition pressure were wellreproduced by our calculations. As expected, the BeO2 compound with pyrite-type structure (space group Pa3̅, four formula units per cell) becomes stable above 89.6 GPa. Notably, the character of the O22− group is maintained in the pressure range of 89.6−300 GPa. Its band gap becomes larger and larger with increasing pressure, which is completely different from the observation in BaO2. This work also represents a step forward toward fully understanding the structure and property of peroxide group.
2. COMPUTATIONAL DETAILS To find stable Be−O binary compounds, structure search up to four formula units (f.u.), in the pressure range of 0−300 GPa, was performed by using the swarm-intelligence based Crystal structure AnaLYsis by Particle Swarm Optimization (CALYPSO) structure prediction method.36,37 The main merit of this method is to find stable structures just depending on the given chemical composition. Its success has been proven in finding the stable structures of the various systems, from element solid to binary and ternary compounds.29,31,33,54−57 Detailed structural predictions can be found in the Supporting Information. The Vienna Ab initio Simulation Package (VASP) code58 within the framework of density-functional theory (DFT) was adopted to perform structural relaxations and electronic properties calculations. The Perdew−Burke−Ernzerhof59 functional in the generalized gradient approximation60 was selected. The electron−ion interactions were represented by means of the all-electron projector augmentedwave method61 with 1s22s2 and 2s22p4 treated as the valence electrons of Be and O atoms, respectively. A plane-wave basis set cutoff of 800 eV and Monkhorst−Pack scheme62 with a dense k-point grid of spacing 2π × 0.025 Å−1 in Brillouin zone were found to give converged energy less than 1 meV/atom. To test the validity of the selected PAW pseudopotentials under high pressure, the full-potential all-electron calculations of the equation of states for BeO compound were performed with the fullpotential linearized augmented plane-wave method as implemented in the WIEN2k code (Figure S1).63 The phonon calculations were performed by using a supercell approach with the finite displacement method64 as done in the Phonopy code.65 Electron localization function (ELF) was used to gauge the degree of electron localization.66 Bader’s Quantum Theory of Atoms in Molecules (QTAIM) analysis was adopted for the charge-transfer analysis.67 The elastic constants were calculated by means of strain−stress method,68 and the elastic moduli including bulk modulus and shear modulus were estimated by the Voigt−Reuss−Hill approximation.69
Figure 1. Stability of Be−O binary compounds. (a) Calculated formation enthalpies per atom of Be−O compounds with different chemical compositions with respect to elemental Be and O solids. The thermodynamically stable phases at each pressure are represented by solid symbols, which are linked by the convex hull (solid lines). Data points corresponding to the metastable phases are directly linked by dotted lines. Elemental Be solid with P63/mmc was used to calculate formation energies throughout the explored pressures.71 α- and ζphases of O with C2/m symmetry were adopted.72,73 (b) Schematic representation of the phase diagram of Be−O system in the pressure range from 0 to 300 GPa.
ΔHf of each composition. At a given pressure, the compounds located on the convex hull are thermodynamically stable, whereas the ones above the convex hull are metastable. With the only input of chemical compositions in our structure searching calculations, the experimental structure of BeO compound (space group P63mc, 2 f.u. per cell) is readily reproduced (Figure 3a),70 validating that our structure prediction method is suitable to the Be−O system. Moreover, the calculated lattice parameters of P63mc-structured BeO are a = b = 2.717 Å and c = 4.404 Å, in good agreement with the experimental values of a = b = 2.698 Å and c = 4.377 Å,70 which further verify the suitability of our adopted pseudopotentials and functional. At elevated pressures, our structure searching calculations not only find the reported high-pressure phase of BeO (space group Fm3m ̅ , 4 f.u. per cell) but also reproduce the phase transition pressure, as shown in Figure 1b.45,47 It is exciting to note that oxygen-rich composition of BeO2 appears to be stable at 100 GPa. Then, it still sites on the convex hull at the higher pressures of 150, 200, and 300 GPa (Figures 1a and S2). Further calculations indicate that the stable pressure region of BeO2 is from 89.6 to 300 GPa (Figure 1b). That is, BeO2 can be synthesized through the chemical reaction of BeO and oxygen at pressures above 89.6 GPa (Figure S3). Interestingly, this reaction pressure of 89.6 GPa is much lower than the phase-transition pressure (106 GPa) from P63mc to Fm3̅m of the pure BeO. It is apparent that BeO2 is much denser than BeO plus O2 in the studied pressure range (Figure 2). To further confirm this, we calculated the change of enthalpy (ΔH), of internal energy (ΔU), and of ΔPV of Pa3̅-structured BeO2 with respect to Fm3̅m-structured BeO plus O2 in the
3. RESULTS AND DISCUSSION 3.1. Thermodynamic Stability. To determine the phase stability of BexOy compounds (x = 1−3, y = 1−6), we performed structural searches on each of the considered compositions at 0 K and the selected pressures of 0, 50, 100, 150, 200, and 300 GPa. Then, the predicted stable structure of each composition is used to calculate formation enthalpy (ΔHf) relative to elemental Be and O solids according to the following formula: ΔHf = [H(BexOy) − xH(Be) − yH(O)]/(x + y). Here, H(BexOy) is the enthalpy of the considered compound, H(Be) is the enthalpy of elemental Be, and H(O) is the enthalpy of elemental O. Convex hull data for the considered 5234
DOI: 10.1021/acs.inorgchem.7b00365 Inorg. Chem. 2017, 56, 5233−5238
Article
Inorganic Chemistry
of 1.33 Å in superoxide group, shorter than 1.49 Å in peroxide group at ambient pressure.75 This O−O distance in BeO2 originates from the synergistic effect between intramolecular charge transfer and pressure. On the one hand, the transferred charges from Be to O2 occupy the anitbonding π* orbitals of quasi-molecular O2, which lengthens the O−O distance (Table S2). On the other hand, pressure shortens the O−O distance, as observed in alkali metal peroxides.76 Moreover, the O−O distance of 1.40 Å at 100 GPa in Pa3̅ BeO2 is comparable to 1.45 Å in MgO2 (magnesium peroxide) at 96 GPa.23 In view of these factors, quasi-molecular O2 in BeO2 acts as peroxide ion (O22−), which is also supported by the result of the electron structure property calculations, as will be discussed below. Therefore, the predicted BeO2 is beryllium peroxide that people have long been desiring. 3.3. Lattice Dynamic and Mechanical Stability. To examine the dynamical stability of the predicted BeO 2 compound, we calculated its phonon dispersions, as shown in Figure 4a. The absence of any imaginary frequencies, in the first
Figure 2. Variation of volume as a function of pressure. Pink line represents the pressure dependence of volume per formula unit for Pa3̅-structured BeO2, green line is the sum of the unit cell volumes of BeO and oxygen. The kink of EOS at ∼100 GPa is mainly because the BeO undergoes a structural phase transition from P63mc to Fm3̅m at 106 GPa, along with the great decrease of the volume.
pressure range of 106−110 GPa, as shown in Figure S4. It apparently shows that the lower enthalpy value of BeO2 mainly comes from the contribution of PV term. Moreover, the covalent bond in O22− and ionic bond between Be and O atoms also reduce the total energy, as will be discussed later. 3.2. Crystal Structure. BeO2 crystallizes in FeS2-type (pyrite) structure74 (space group Pa3,̅ 4 f.u. per cell; see Figure 3c). This structure contains one equivalent Be atom sitting at Figure 4. Lattice dynamic stability of BeO2 compound. Phonon dispersion curves (a) and PHDOS (b) projected on Be and O atoms for Pa3̅ structure of BeO2 at 100 GPa, where the pink, blue, and green lines indicate the PHDOS of unit cell, Be, and O atoms, respectively.
Brillouin zone, confirms its dynamical stability. On the basis of the analysis of phonon density of states (PHDOS; Figure 4b), the low-frequency bands below 30.7 THz come mainly from the strongly coupled vibrations between Be and O atoms, whereas high-frequency band at ∼34.0 THz mainly originates from the O−O stretching mode, which is very similar to those observed in Li2O2.75,77 This observation again verifies the existence of peroxide group. Subsequently, we tested its mechanical stability. For a cubic structure, the elastic constant Cij matrix should satisfy the following criteria: C11 > 0, C44 > 0, C11 > |C12|, and (C11 + 2C12) > 0.78 The calculated elastic constants Cij of BeO2 at 100 GPa, obtained by the strain−stress method, are C11 = 936.6, C12 = 211.7, and C44 = 374.0 GPa. Obviously, its elastic constants satisfy the mechanical stability criteria. The calculated bulk modulus (B) and shear modulus (G) of BeO2 at 100 GPa are 453 and 369 GPa, respectively. In view of its high B and G values and the reported BeB279,80 as a hard material, we calculated its mechanical properties at ambient pressure (Table S3). The calculated hardness value is 25.7 GPa, indicating that Pa3-̅ structured BeO2 is a potential hard material. 3.4. Electronic Properties and Chemical Bonding. To provide insights into the electronic properties of the predicted BeO2, the electronic band structure and the corresponding projected density of states (PDOS) were calculated at 100 GPa. Pa3̅-structured BeO2 is a direct band gap nonmetal with its maximum of valence band and minimum of conduction band at Γ point, as shown in Figure 5a. It is noted that DFT/PBE
Figure 3. Stable structures of the predicted Be−O compounds. (a) Low-pressure phase of WZ-type BeO with P63mc symmetry. (b) Highpressure phase of RS-type BeO with Fm3̅m symmetry. (c) Polyhedron view of BeO2 phase with Pa3̅ symmetry along the a-axis direction. (d) View of peroxide group (O22−) in Pa3̅-structured BeO2.
4a (0.5000, 0.0000, 0.5000) position, and one equivalent O atom occupying 8c (0.3955, 0.3955, 0.3955) site. Figure 3d shows clearly that all of the Be atoms occupy face-centered sites of the lattice. For each Be atom, there are six nearest neighbors of O atoms, forming a Be−O octahedron. These octahedrons are interconnected by sharing vertex oxygen. The most striking feature of this structure is that the nearest-neighbor O atoms exist in quasi-molecular O2 form with an O−O distance of 1.40 Å at 100 GPa, which is slightly longer than the O−O distance 5235
DOI: 10.1021/acs.inorgchem.7b00365 Inorg. Chem. 2017, 56, 5233−5238
Article
Inorganic Chemistry
electron localization function (ELF)66 was calculated, as implemented in the VASP code.58 In general, the large ELF values (>0.5) correspond to the lone electron pairs, core electrons, or covalent bonds, whereas the ionic bonds are represented by smaller ELF values (
