Abstract
The first-principles theory calculation was used to investigate the structural, electronic, mechanical, and thermodynamic properties of intermetallics ZrBe2, ZrBe5, ZrBe13, and Zr2Be17 in Zr-Be binary alloy system, based on the density functional theory with generalized gradient approximation (GGA) approach. Results show that the optimized lattice parameters at 0 K are in agreement with the available experimental results, indicating the calculation reliability. The calculated formation enthalpy and cohesive energy indicate that all the intermetallics are formed spontaneously at 0 K, among which ZrBe5 has the strongest alloying ability and ZrBe2 has the best structural stability. Subsequently, the electronic density of states (DOS) was also used to investigate the intermetallic stability. The stress-strain method was adopted to calculate the independent elastic constants of the intermetallic. Based on that, the mechanical parameters of polycrystal, such as bulk modulus B, shear modulus G, Young's modulus E, Poisson's ratio ν, and anisotropy value A can be deduced by Voigt-Reuss-Hill approximation. In addition, according to Pugh's criterion, Poisson's ratio, and Cauchy pressure, the ductile behavior of intermetallic was analyzed. As for the thermodynamic properties, all the phonon dispersion curves illustrate the dynamic stability of the intermetallic, and the lattice vibration energy, bulk modulus, thermal expansion coefficient, and specific heat varying with temperature change were calculated by the quasi-harmonic approximation (QHA).
Science Press
In the past few decades, the intermetallics have attracted considerable interest because of their excellent electronic, magnetic, mechanical, and thermal properties, such as good ductility, thermal stability, high tensile strength, and high corrosion resistanc
The beryllium (Be) has a very low density, low thermal neutron absorption cross-sectional area, high elastic modulus, and high specific strength. Therefore, the Zr-Be binary alloy is a promising materia
In this research, the first-principles calculations were performed according to the framework of electronic density functional theory (DFT) through Simulation Package (VASP) with Vienne A

Fig.1 Schematic diagrams of crystalline structures of ZrBe2 (a), ZrBe5 (b), ZrBe13 (c), and Zr2Be17 (d) intermetallic compounds
The crystalline structure parameters and optimized lattice parameters of these intermetallic compounds are listed in
In order to investigate the thermodynamic stability of the Zr-Be intermetallic compounds, the formation enthalpy and cohesive energy were calculated. In general, the alloying ability of the intermetallic compound can be determined by its formation enthalpy. The formation energy of Zr-Be intermetallic compounds can be evaluated through the composition-averaged energies of the pure elements in their equilibrium crystal structures, as follows:
(1) |
where ∆H is the formation enthalpy of ZrxBey intermetallics; EZrxBey is the total energy of ZrxBey intermetallics; and are the total energies of the most stable ground state structures of Zr and Be, respectively. Therefore, the cohesive energy of ZrxBey intermetallics can be obtained, as follows:
(2) |
where and represent the energy of the isolated Zr and Be atoms, respectively.
The calculated formation enthalpies and cohesive energies of different intermetallic compounds are shown in Fig.2. All the formation enthalpies are negative, suggesting that the structure of these intermetallic compounds is stable. Furthermore, it can be concluded that ZrBe5 phase has the strongest alloying ability, and the alloying ability of intermetallic compounds is arranged by descending order as follows: ZrBe5>Zr2Be17>ZrBe13>ZrBe2. The calculated cohesive energies of intermetallic compounds are arranged by descending order as follows: ZrBe2>ZrBe5>ZrBe13>Zr2Be17. Thus, ZrBe2 has the largest cohesive energy, indicating that ZrBe2 phase is the most stable phase.

The total density of states (TDOS) and partial density of states (PDOS) of different intermetallic compounds were calculated and shown in Fig.3. All ZrxBey phases exhibit metallic features due to the finite density of states (DOS) values at the Fermi level EF, and the electronic states near the Fermi level are dominated by Zr 4d states. As for ZrBe2, there is a strong hybridization between Zr 4d and Be 2s states around -4 eV, which is beneficial to structural stability and mechanical property improvement for the Zr-Be intermetallic compounds.

The stress-strain approach was employed to calculate the elastic properties in this research. Based on Hook's law, a linear relationship exists between stress (σ) and strain (ε). Thus, the proportion of elastic stiffness Cij can be simplified, as follows:
(3) |
where subscript i and j are positive integer in a specific range. Based on the strain-stress method, the elastic constants can be calculated under pressure of 0 MPa and temperature of 0 K. A small finite strain was applied to the optimized structure, and then the elastic constants were obtained from the stress. The mechanical stability of cubic, hexagonal, and trigonal structures was examined firstly by the Born-Huang mechanical stability criteria, as expressed by Eq.(
(4) |
(5) |
(6) |
The elastic constants of different intermetallic compounds are shown in
The calculated elastic modulus E and Poisson's ratio ν of these intermetallic compounds are listed in
The brittle and ductile behavior of materials can be predicted by the ratio of bulk modulus B to shear modulus
The phonon frequency is one of the most important aspects for investigation of stability, phase transformation, and thermodynamic properties of the crystalline structures. In order to obtain the phonon dispersion curves with the high-symmetry directions in Brillouin zone, the Hellmann-Feynman theorem and the direct metho
As shown in Fig.4, there is no imaginary frequency, which indicates the dynamical stability of these intermetallics. The primitive cells of ZrBe2, ZrBe5, ZrBe13, and Zr2Be17 intermetallic compounds contain 3, 6, 28, and 19 atoms, and hence, these intermetallics have 9, 18, 84, and 57 independent modes of vibrations, respectively. It is also found that the TDOS is mainly determined by Zr at low frequency, whereas that is dominated by Be at high frequency, because Be atoms are lighter than Zr atoms.
The Helmholtz free energies of the intermetallics are calculated by
(7) |

where E0 is the free energy at 0 K, which can be calculated using VASP directly; Fel denotes the electronic contribution to the free energ
(8) |
where q and v are the wave vector and band index, respectively; ωq,v is the phonon frequency; T, kB, and ћ denote the temperature, Boltzmann constant, and the reduced Planck's constant, respectively.
The vibrational specific heat at constant volume CV and entropy S can be calculated as a function of temperature, as follow
(9) |
(10) |
The specific heat at constant pressure CP can be expressed as follows:
(11) |
where α, B, V, and T represent the linear thermal expansion coefficient, bulk modulus, volume, and temperature of the system, respectively. The linear thermal expansion coefficient α and the volume expansion coefficient αV can be calculated by
(12) |
The bulk modulus can be calculated by
(13) |
Based on these equations, the Helmholtz free energies of these intermetallic compounds were calculated under the volume value near the equilibrium volume. In

Fig.5 Relationship of free energy-crystal volume at different temperatures of different Zr-Be intermetallic compounds: (a) ZrBe2, (b) ZrBe5, (c) ZrBe13, and (d) Zr2Be17
The free energy at 0 K is simply the internal energy of the system, namely the zero-point energy of the system. The free energy of intermetallic compounds is decreased, whereas the internal energy is increased with increasing the temperature. As the internal energy of each intermetallic compound is beyond 200 K, the difference in their free energies is due to different vibrational entropies. The zero-point energy of ZrBe2, ZrBe5, ZrBe13, and Zr2Be17 is 3.38 kJ/mol. The ZrBe2 phase is the most stable intermetallic, so its free energy is more negative than others.
The bulk modulus as a function of temperature for all intermetallic compounds is shown in

Fig.6 Bulk moduli of different Zr-Be intermetallic compounds
The linear thermal expansion coefficient α of all intermetallic compounds is shown in Fig.7. Initially, the linear thermal expansion coefficient of all intermetallic compounds increases sharply. Once the temperature is over 300 K, the thermal expansion coefficient increases slowly, and tends to become a constant value. It can also be found that over 600 K, the thermal expansion coefficient of ZrBe2 is obviously less than that of other intermetallic compounds, which is desirable for the metallic waste form.


The specific heat at constant volume and specific heat capacity at constant pressure of the intermetallic are shown in Fig.8 and Fig.9, respectively. It can be seen that at low temperature, CV of all intermetallic compounds increases rapidly at first and once the temperature is over 400 K, CV becomes almost constant of ~25 J·mo

1) All the Zr-Be intermetallic compounds can be formed spontaneously at 0 K, among which ZrBe5 has the strongest alloying ability, and ZrBe2 has the best structural stability.
2) According to different criteria, all the Zr-Be intermetallic compounds show mechanical stability and are brittle at 0 K.
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