Bct Structure Research Articles

Constitutive modeling of the slip-twinning transition in martensitic transformations

Martensite is the result of a diffusionless displacive phase transition. In Fe-based alloys it transforms the FCC to the BCC, BCT, or HCP structures and occurs at high strain rates. It has two typical morphologies, known as lath and plate. A quantitative constitutive description of the slip-twinning transition in the martensitic transformation is presented. It is based on the temperature and strain-rate sensitivities of slip, which are much higher than those for twinning. Thus, twinning becomes a favored deformation mechanism at low temperatures and high strain rates. The Hall-Petch coefficient, for the inclusion of grain size effects, is two times larger for twinning than slip. Constitutive relationships for slip and twinning are presented and applied to the martensitic transformation in steels; the lath-to-plate morphology change observed with increasing carbon content is successfully predicted as a function of grain size by calculations incorporating the two modes of deformation. A simple calculation of the strain rates during martensitic transformation is also provided. This methodology is applied to the Fe–C system and can be extended to the Fe–Ni–C system and to thermoelastic martensites, where twinning is favored over slip to enhance reversibility.

Formation of a Body-Centered Tetragonal Structure With High Magnetic Anisotropy and Coercivity in Fe–Co–V Films With High C Contents

A high amount of C (0–20 at.%) was added to Fe–Co–V films via sputtering to transform their body-centered cubic (bcc) structure to a face-centered cubic (fcc) lattice through a body-centered tetragonal (bct) structure. The Fe–Co films without V retained a bcc or bct crystal structure with an axial ratio ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/a</i> ) of 1.03 even at high C contents close to 20 at.%. Meanwhile, in the Fe–Co films containing 10 at.% of V, a bct structure with 1.06 ≤ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c/a</i> ≤ 1.30 was formed after adding 5–12 at.% of C, which was subsequently transformed to an fcc structure with increasing C content. The saturation magnetization of the Fe–Co–V–C films with the bct structure was approximately 1000–1500 kA/m, and their uniaxial magnetic anisotropy constant ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> ) was approximately 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> J/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> (the films with high Fe contents exhibited high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> values). The bct Fe–Co–V–C films with high <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">u</sub> were microfabricated into nanodot array patterns via electron beam lithography. The coercivity of the pattern increased with decreasing dot size, and that of the pattern with a dot diameter of 85 nm was nearly an order of magnitude higher than that of the continuous film.

Improved mechanical performance of quasi-cubic lattice metamaterials with asymmetric joints

In this paper, we propose a simple method for the modification of the unit cells in the lattice metamaterials that provides an improvement of their impact strength. The idea is based on the introduction of small mutual offsets of the interconnected struts inside the unit cells. In such way, the joints between the struts become asymmetric and the overall geometry of the unit cells can be defined as the quasi-cubic with the axis of chirality. Considering four types of cubic lattices with BCC, BCT, FCC and octahedron structures, we modified their geometry and investigated the influence of the offsets and the unit cell size on the overall performance in static and dynamic tests. From the experiments we found that the small offsets (less than the strut diameter) can allow to increase the impact strength of 3d-printed polymeric specimens in 1.5–3 times remaining almost the same density and static mechanical properties. Based on the numerical simulations, we show that the explanation of the observed phenomena can be related to the increase of plastic deformations and damage accumulation in the unit-cells with asymmetric joints leading to the transition from the quasi-brittle to the ductile type of fracture in tested specimens.

First-principles study on the effects of atomic configuration on the magnetic anisotropy energy of (Fe,Co)16(N,C)2 alloys

Herein, the first-principles technique combined with a coherent potential approximation is utilized to investigate the magnetic anisotropy energy (MAE) of (Fe1-xCox)16(N,C)2 alloys. The alloys contain an ordered α″-phase and partially ordered α′-phase, where α-Fe is distorted to form a bct structure with c/a = 1.1 owing to the random intrusion of (N, C) atoms. For the α″-phase, the MAE is sensitive to Co substitution sites and reaches approximately 3 MJ/m3 at an x of 0.25 when Co atoms occupy the 4d sites. For the α′-phase, the lattice distortion c/a = 1.1 alone cannot provide a positive MAE when x = 0 (FeN0.125 and FeC0.125). Evidently, both Co substitution and further increasing the c/a over 1.1 must be performed to realize a positive MAE in the α'-phase. Finally, the importance of local distortion of the (Fe, Co) lattice in further increasing the c/a beyond 1.1 is discussed as it increases the effective c/a even when the global c/a is fixed at 1.1.

Bct Structure and Uniaxial Magnetic Anisotropy of Fe-Co Films with V-N Added

bct Structure and Uniaxial Magnetic Anisotropy of Fe-Co Films with V-N Added

Microstructural Evolution of SK85 Pearlitic Steel Deformed by Heavy Cold Rolling.

The microstructural evolution of SK85 pearlitic steel cold-rolled up to a 90% rolling reduction was characterized by scanning electron microscopy with electron backscattered diffraction (EBSD) and X-ray diffraction (XRD). SK85 steel exhibits excellent cold rolling performance. The interlamellar spacing of pearlite is refined obviously and a tensile strength of 2318 MPa can be reached for SK85 steel after 90% rolling reduction, an increase of 83% from 1264 MPa before rolling. The EBSD observation indicates that the {001} <110> texture becomes pronounced at a 90% rolling reduction in cold-rolled Sk85 steel. A propagation and multiplication of dislocations occur during rolling as the kernel average misorientation (KAM) angles significantly increase from 0.72° to 2.11°. The XRD analysis reveals that bcc ferrite is transformed into a bct structure at a 90% rolling reduction. The strengthening mechanism was discussed.

Microstructure characterization of Au7Cu5Al4 shape memory alloys

The crystal structures and microstructures of parent and martensite phases of Au7Cu5Al4 alloy were investigated using X-ray diffraction and transmission electron microscopy. Premartensitic tweed striations with 6 M modulated structures were observed in the austenitic phase. Non-modulated martensite variants and partially modulated (8 M) martensite variants were found in the alloy that was slowly cooled (10 K/min) to –10 ℃. By contrast, in addition to non-modulated martensite plates, perfect periodic modulated (8 M) martensites were observed when the alloy was quenched into liquid nitrogen. The crystal structures of martensites with or without modulation can be basically indexed with a body-centered tetragonal (bct) structure. Whereas the tetragonality ratio (c/a) for the martensite was found to be affected by the modulation structure. These results resolve the controversy regarding the structure of martensite.

Evolution of Cu-rich phase in Al-modified ferrite stainless steel during short-term ageing

In this article, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy were used to characterize the precipitation behaviour of the Cu-rich phase in Al-free and 1Al Cu-bearing ferrite stainless steels; and further, the effects of Al addition on the precipitation and coarsening of the Cu-rich phase and its mechanism were analysed. The results show that Al increases the nucleation rate of the Cu-rich phase, and refines Cu-rich nanoprecipitates after ageing for 1 min. The Cu-rich phase with four crystal structures-BCC, 9R, twin FCC, and non-twin FCC were found in the Al-free sample during ageing. However, only Cu-rich phase with BCC and BCT structures were observed after Al addition. There are high-density edge dislocations in Al-containing Cu-rich nanoprecipitates. These dislocations can be used as a diffusion channel for solute atoms, increasing the coarsening rate of the Cu-rich phase. However, with the extension of the ageing time, dislocations in the Cu-rich phase will be annihilated owing to the thermal activation and the decrease of the Al concentration in the Cu-rich phase, which will lead to a decrease in the dislocation density in Al-containing Cu-rich phase. Finally, the promotion effect of Al on the coarsening of the Cu-rich phase is weakened.

Precipitation behaviors and property variations of Cu-3.0 wt% Ti fabricated by a novel short-processing non-vacuum heating-cooling combined mold continuous casting

A Cu-3.0 wt% Ti alloy is fabricated by a novel non-vacuum short-process based on heating-cooling combined mold (HCCM) continuous casting in this study. Similar properties including tensile strength, hardness and electrical conductivity are obtained by this newly developed process, compared to its counterpart alloy prepared by conventional vacuum smelting. The tensile strength, total elongation and conductivity of the HCCM prepared Cu-3 wt% Ti alloy were 895 MPa, 13% and 13.00% IACS, respectively, after 95% deformation and aging treatment at 450 °C for 1 h. The cast materials fabricated by HCCM method can be directly cold rolled and aged after solution treatment without hot deformation such as hot forging. Through the HCCM continuous casting method, not only the Cu-Ti alloy can be fabricated under non-vacuum conditions, but also the fabrication process can apparently shortened by shortening the solid solution treatment time. Similar precipitation process is also found in the Cu-3.0 wt% Ti alloy prepared by this method. The precipitation of the ordered, metastable and coherent β'-Cu4Ti phase with body-centered tetragonal (bct) structure are formed in the early stage of aging treatment, which is transformed into stable β-Cu3Ti with an orthogonal structure in the over-aging stage. The precipitation hardening of β'-Cu4Ti phase, i.e., Orowan strengthening effect, is believed to be the leading strengthening mechanism in this alloy.

TEM-based crystal structure analysis of body-centered tetragonal structure in non-epitaxial FeCo film with added V and N

FeCo with a tetragonally distorted, body-centered tetragonal (bct) structure has been grown epitaxially for investigating the basic magnetic properties; however, it is necessary to form the bct-FeCo without epitaxial growth for allowing their use in permanent magnet-based applications. In this study, we investigated the effect of adding V and N to non-epitaxial FeCo films by directly depositing FeCoVN films on an amorphous SiO2 substrate. Crystal structure analysis performed through transmission electron microscopy revealed a bct lattice with an axial ratio of 1.07 at 10 at.% V and 1.5 at.% N. The tetragonal distortion stability was evaluated using first-principles calculations. The results indicated that bct-FeCo was formed via non-epitaxial growth, and the addition of V and N stabilized the bct structure.

Effect of V addition on the body-centered cubic to body-centered tetragonal to face-centered cubic structural transformation of N-containing Fe and FeCo films

In this study, the effect of adding V to N-containing Fe and FeCo on the stability of the body-centered tetragonal (bct) structure was investigated. The crystal structures of FeN, FeVN, FeCoN, and FeCoVN films with a thickness of 20 nm were evaluated, and the body-centered cubic to bct to face-centered cubic structural transformation was observed based on the N content. Results indicate that V addition stabilizes the bct structure and the Co content effectively promotes the formation of the bct structure. The uniaxial magnetic anisotropy of approximately 1.0 MJ/m3 was obtained in the FeCoVN films with the bct structure.

Analysis of quasistatic squeeze behavior of magnetorheological fluid from the microstructure variations

Magnetorheological fluid is a novel functional material, of which quasistatic squeeze behavior needs to be quantitatively controlled in industrial applications. Since the quasistatic squeeze behavior has a close relation with microstructure variations, thus it is modeled from a microscopic approach. By analyzing compression of single chains, aggregation from single chains to BCT structure and compression of BCT structure, the initial stress [Formula: see text], yield stress [Formula: see text], yield strain [Formula: see text], and stress in post-yield stage [Formula: see text] are respectively modeled. It is found that they have an exponential dependence on magnetic field strength H and particle volume concentration [Formula: see text], including [Formula: see text], [Formula: see text], and [Formula: see text], etc. By comparing predicted results with measured results, the micro-macro stress model on quasistatic squeeze behavior is well validated. This model can be used to design, manufacture, and control industrial magnetorheological devices.

Influence of anisotropy, tilt and pairing of Weyl nodes: the Weyl semimetals TaAs, TaP, NbAs and NbP

By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character.Graphical abstract

TEM observation of nitrogen-tunable bcc–bct–fcc transformation of iron-cobalt with added vanadium

We used transmission electron microscopy (TEM) to observe tunable bcc–bct–fcc transformation of FeCo films caused by the addition of auxiliary elements. The crystal structure of the FeCo with added V was bcc, and the transformation of the bcc structure through the bct to fcc was found to depend on the N content (x). TEM observation revealed that bct structure with c/a = 1.12 was obtained for x = 1.7 atomic%, while fcc structure with c/a = 2 was obtained for x = 9.6 atomic%. The c/a-values depended on x, so the bcc–bct–fcc transformation of the FeCoV was tunable by N content. This research was made possible by special preventative measures taken during preparation to avoid compressive stress, which could otherwise induce the bct FeCo lattice of axial ratio c/a &amp;gt; 1.00 to transform to bcc structure. A FeCo coating of thickness 20 nm was therefore deposited directly on a MgO substrate without any other metallic layers.

Theoretical investigation of fundamental inherent physical and optoelectronic properties of ZnSnSb2 chalcopyrite semiconductor

Here in, we have investigated fundamental inherent physical properties like as structural, electronic, optical, elastic, thermal etc of the ZnSnSb2 by using the accurate full potential linearized augmented plane wave (FP-LAPW) method. These materials have higher energy gaps and lower melting points as compared to their binary analogues, because of which they are considered to be important in crystal growth studies and device applications. For structural properties, the minimization has been done in two steps, first parameter u is minimized by the calculation of the internal forces acting on the atoms within the unit cell until the forces become negligible, for this MINI task is used, which is included in the WIEN2K code. Second, the total energy of crystal is calculated for a grid of volume of the unit cell (V) and c/a values. Five values of c/a are used for each volume and a polynomial is fitted to the calculated energies to calculate the best c/a ratio. We have presented the electronic and optical properties with the recently developed density functional of Tran and Blaha. Furthermore, optical features such as dielectric functions, refractive indices, extinction coefficient, optical reflectivity, absorption coefficients, optical conductivities, were calculated for photon energies up to 40 eV. We have used WC and TB-mBJ exchange correlation potential for these properties and yield a direct band gap of 0.46 eV for this material and the obtained electronic band gap matches well with the experimental data. The TB-mBJ potential gives results in good agreement with experimental values that are similar to those produced by more sophisticated methods, but at much lower computational costs. The main peaks of real part of the electronic dielectric function ε1(ω) which is mainly generated by electronic transition from the top of the valence band to the bottom of conduction band, occurs at 1.59 eV and ε1(ω) spectra further decreases up to 4.99 eV. The imaginary part of the electronic dielectric constant ε2(ω) is the fundamental factor of the optical properties of a material. The proposed study shows that the critical point of the ε2(ω) occurs at 0.42 eV, which is closely related to the obtained band gap value 0.46 eV. The maximum reflectivity occurs in region 3.74-11.33 eV. This material has non-vanishing conductivity in the visible light region (1.65 eV-3.1 eV), the main peak occurs at 3.80 eV, which fall in the UV region. The elastic constants at equilibrium in BCT structure have also determined. The elastic stiffness tensor of chalcopyrite compounds has six independent components, because of the symmetry properties of the space group, namely C11, C12, C13, C33, C44 and C66 in Young notation. The thermal properties such as thermal expansion, heat capacity, Debye temperature, entropy, Grüneisen parameter and bulk modulus were calculated employing the quasi-harmonic Debye model at different temperatures and pressures and the silent results were interpreted. To determine the thermodynamic properties through the quasi-harmonic Debye model, a temperature range 0 K 500 K has been taken. The pressure effects are studied in the 0–7 GPa range. Similar trends have been observed in the considered temperature range, but above 600 K trends get disturbed which may be due to melting of material. Based on the semi-empirical relation, we have determined the hardness of the materials, which attributed to different covalent bonding strengths. Most of the investigated parameters are reported for the first time.

Sequential insulating-metal-insulating phase transition of NbO2 by doping photoexcited carrier

A large number of free carriers are introduced in the photoexcitation process and the complex interaction between photoexcited carriers and lattice can lead to an ultrafast phase transition in the solid. Niobium dioxide is an important optical material, which exhibiting a threshold switching phenomenon that can be explained by insulating-metal phase transition. It can be used as an ultrafast optical switches for telecommunication and optoelectronic devices. Here, we investigated the effects of injecting photoexcited carriers on consecutive insulator-metal-insulator phase transition of NbO2 by applying the GGA and GGA + U energy functions on the basis of density functional theory (DFT). Compared with electron injection, the effect of hole doping on phase transition in NbO2 is significantly more effective. The length of Nb-Nb dimer (long or short) is nearly unchanged upon electron doping, while the length of Nb-Nb_l is gradual shortened from 3.366 to 3.039 Å and the Nb-Nb_s is gradual stretched from 2.733 to 3.035 Å upon hole doping below a critical level of doping 12 holes per conventional cell. The structure phase changed from a body-center tetragonal (bct) to a rutile. Upon a higher hole doping, the structure kept the rutile structure until the number of doping holes reached 28 and it began to change to bct structure. We studied the electronic properties of NbO2 at a certain carriers concentration and reported a controlled and reversible insulator-metal-insulator phase transition by carries doping. The coexistence of the strong electron correlation and the Nb-Nb dimerization due to lattice distortion obscures the nature of the phase transition in NbO2. By analyzing the evolutions of lattice parameters and electronic properties of NbO2 at different charge carrier concentration, we got a conclusion that this sequential phase transition of NbO2 is not just a Mott phase transition or Peierls phase transition, but a collaborative of Peierls-Mott phase transition. Our work confirms that the observed structural transformations are driven primarily by hole doping, and the dominant lattice distortion caused by hole doping is the off-centering of Nb atoms in NbO6 octahedral.

An answer to the carbon cluster in low-temperature aged ferritic low-carbon steel

The morphology and crystal structure of the carbon cluster in a low-temperature aged low-carbon ferritic steel were clarified by performing conventional and in-situ TEM, and atomically-resolved ADF-STEM observation. The carbon cluster grow parallel to <001> direction of the matrix, whose morphology was presumed to be either disk or plate. Lattice constant of the carbon cluster was measured by peak fitting of atomic columns observed by ADF-STEM. The result clarified that the structure of the carbon cluster was martensite-like body-centered tetragonal (bct) structure. This martensite-like bct structure suggested that the carbon cluster was supersaturated with carbon atoms occupying specific octahedral sites.

Coexistence of hcp and bct Phases during In Situ Superlattice Assembly from Faceted Colloidal Nanocrystals.

We study the in situ self-assembly of faceted PbS nanocrystals from colloidal suspensions upon controlled solvent evaporation using time-resolved small-angle X-ray scattering and X-ray cross-correlation analysis. In our bulk-sensitive experiment in transmission geometry, the superlattice crystallization is observed in real time, revealing a hexagonal closed-packed (hcp) structure followed by formation of a body-centered cubic (bcc) superlattice. The bcc superlattice undergoes continuous tetragonal distortion in the solvated state shortly after its formation, resulting in the body-centered tetragonal (bct) structure. Upon solvent evaporation, the bct superstructure becomes more pronounced with the still coexisting hcp phase. These findings corroborate the existing simulations of assembling cuboctahedral-shaped particles and illustrate that we observed the predicted equilibrium states. This work is essential for a deeper understanding of the fundamental forces that direct nanocrystal assembly including nanocrystal shape and ligand coverage.

Stabilisation of tetragonal FeCo structure with high magnetic anisotropy by the addition of V and N elements

The development of magnetic materials with high saturation magnetization (Ms) and uniaxial magnetic anisotropy (Ku) is required for the realisation of high-performance permanent magnets capable of reducing the power consumption of motors and data storage devices. Although FeCo-based materials with the body-centred cubic structure (bcc) exhibit the highest Ms values among various transition metal alloys, their low Ku magnitudes makes them unsuitable for permanent magnets. Recent first-principles calculations and experimental studies revealed that the epitaxial FeCo thin films with the body-centred tetragonal (bct) structure and thicknesses of several nanometres exhibited Ku values of 106 J·m−3 due to epitaxial stress, which required further stabilisation. In this work, the FeCo lattice stabilised via VN addition were characterised by high Ku magnitudes exceeding 106 J·m−3. The obtained bct structure remained stable even for the films with thicknesses of 100 nm deposited on an amorphous substrate, suggesting its possible use in bulk systems.

First-principles calculations of high pressure phase transition of Ce-La-Th alloy

The lanthanide and actinide metals and alloys are of great interest in experimental and theoretical high-pressure research, because of the unique behavior of the f electrons under pressure and their delocalization and participation in bonding. Cerium (Ce) metal is the first lanthanide element with a 4f electron. It has a very complex phase diagram and displays intriguing physical and chemical properties. In addition, it is expected to be an excellent surrogate candidate for plutonium (Pu), one of the radioactive transuranic actinides with a 5f electron. The bulk properties and phase transformation characteristics of Ce-based alloys are similar to those of Pu and its compounds. Thus, the investigations of Ce-based alloys are necessary and can potentially advance the understanding of the behavior of Pu. In this work, the equation of state, phase transition, elastic and thermodynamic properties of Ce&lt;sub&gt;0.8&lt;/sub&gt;La&lt;sub&gt;0.1&lt;/sub&gt;Th&lt;sub&gt;0.1&lt;/sub&gt; alloy at high pressure are investigated by using first-principles calculations based on the density-functional theory. The structural properties of the Ce&lt;sub&gt;0.8&lt;/sub&gt;La&lt;sub&gt;0.1&lt;/sub&gt;Th&lt;sub&gt;0.1&lt;/sub&gt; alloy are in good agreement with the available experimental and theoretical data. The lattice constant &lt;i&gt;a&lt;/i&gt; decreases with pressure increasing, while &lt;i&gt;c&lt;/i&gt; shows an opposite variation. It is found that the lattice parameter &lt;i&gt;c&lt;/i&gt; shows abnormal jump. And the critical volume is located at 20.1 Å&lt;sup&gt;3&lt;/sup&gt;. The axial ratio jumps from a value of about &lt;inline-formula&gt;&lt;tex-math id="M90"&gt;\begin{document}$\sqrt 2 $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182128_M90.jpg"/&gt;&lt;graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="8-20182128_M90.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt; (corresponding to the fcc structure) to a higher value, which indicates that the fcc-bct transition occurs. And the corresponding transition pressure is located at ~31.6 GPa. When the pressure rises to 34.9 GPa, the bct structure displays a saturated &lt;i&gt;c/a&lt;/i&gt; axial ratio close to about 1.67. The Young's modulus &lt;i&gt;E&lt;/i&gt;, shear modulus &lt;i&gt;G&lt;/i&gt; and the Debye temperature of the fcc phase tend to be " softened” around the phase transition pressure. The vibrational free energy is obtained by using the quasi-harmonic Debye model. And then the thermodynamic properties including the thermal equation of state, heat capacity and entropy under high pressure and high temperature are also predicted successfully. The results show that the heat capacity and entropy increase rapidly with temperature increasing, and decrease with pressure increasing. The high pressure can suppress part of the anharmonicity caused by temperature.