Search

The ordered crystalline Invar alloy Fe3Pt is in a special magnetic critical state, under which the lattice dynamic stability of the system is extremely sensitive to external pressures. We studied the pressure dependence of enthalpy and magnetism of Fe3Pt in different crystalline alloys by using the first-principles projector augmented-wave method based on the density functional theory. Results show that the P4/mbm structure is the ground state structure and is more stable relative to other structures at pressures below 18.54 GPa. The total magnetic moments of L12, I4/mmm and DO22 structures decrease rapidly with pressure and oscillate near the ferromagnetic collapse critical pressure. At the pressure of 43 GPa, the ferrimagnetic property in DO22 structure becomes apparently strengthened and its volume increases rapidly. The lattice dynamics calculation for L12 structures at high pressures shows that the spontaneous magnetization of the system in ferromagnetic states induces the softening of the transverse acoustic phonon TA1 (M), and there exists a strong spontaneous volume magnetostriction at pressures below 26.95 GPa. Especially, the lattice dynamics stability is sensitive to pressure, in the pressure range between the ferromagnetic collapse critical pressure (41.9 GPa) and the magnetism completely disappearing pressure (57.25 GPa), and near the pressure of phase transition from L12 to P4/mbm structure (27.27 GPa). Moreover, the instability of magnetic structure leads to a prominent elastic modulus oscillation, and the spin polarizability of electrons near the Fermi level is very sensitive to pressures in that the pressure range. The pressure induces the stability of the phonon spectra of the system at pressures above 57.25 GPa.

Pressure effect on the magnetic, mechanical and lattice dynamical properties of the ferromagnetic L12-Fe3Pd crystalline alloy was investigated by first-principles calculations. The calculated properties under ambient pressure are in good agreement with available theoretical results. A two-step pressure-induced collapse of magnetic moment was observed, accompanied by the collapse of the volume. The first one is at about 34.2 GPa, and the other is at about 67 GPa. While the magnetic moment disappears around 84 GPa. The lattice dynamics calculations under high pressure revealed that the dynamically stable and unstable cases appear alternately. The system is dynamically unstable at low pressures due to the spontaneous magnetization of the system, and then the system is dynamically stable in the pressure range of 33.5 to 66.9 GPa, and this pressure range is between the first collapse pressure and second collapse pressure. The system becomes dynamically unstable again at pressure ranging from 67 to 83.7 GPa, and afterwards becomes stable. Elastic constants and their dependence on pressure and related mechanical properties were investigated. Stability criteria show that ferromagnetic L12-Fe3Pd crystalline alloy is mechanically stable over the entire studied pressure range. The bulk modulus exhibits pronounced softening with pressure, and present minimum values at magnetic collapse pressures. The mechanism of elastic softening was analyzed with the magnetoelastic force. From Pugh’s ratio and Poisson’s ratio, the system exhibits brittleness at the vicinity of magnetic collapse pressures. The linear thermal expansion coefficient was calculated based on the quasi-harmonic approximation. The results reveal that the alloy does not exhibit thermal Invar behavior under zero pressure and high pressure. Finally, a high-anisotropy tetragonal structure with space group symmetry P4/mbm was obtained by soft-mode phase transition theory, and this structure is dynamically and mechanically stable at zero pressure. Theoretical prediction suggested that the P4/mbm structure is energetically favorable than the previously reported structures at the pressure below 32 GPa and also showed that above 32 GPa the hexagonal D019 structure becomes the energetically most stable structure.

A transverse Ising model in the framework of the mean field approximation is developed to analyze the polarization offsets phenomena in temperature-graded ferroelectric materials. A function of two-spin exchange interaction strength has been introduced to describe the ferroelectric distortion due to the distribution of temperature gradients in materials. Comparisons of the computational results with the experimental data reveal some fundamental factors in the formation of polarization offsets. It is shown that ferroelectric distortion has influenced much on polarization offsets in temperature-graded ferroelectric materials. When quantum fluctuation effect as well as ferroelectric distortion is considered, we have successfully reproduced the experimental observations qualitatively, especially for the indistinguishable polarization offsets from the background at small temperature gradients, which were not successfully reproduced in prior theoretical studies.

B4N is expected to be a superhard material with outstanding mechanical properties like B4C. The longstanding uncertainty in its stable structural information impeded the understanding of its physical and chemical properties. Here, we systematically investigated the thermodynamically stable phases of B4N and their corresponding functional properties by using the particle swarm optimization method combined with first-principles calculations. Two new icosahedra-based B4N phases that different from the rhombohedral boron carbide type structure were predicted to be more thermodynamically stable in the pressure range of 0–300 GPa. Their dynamical stability has been identified by the theoretical phonon dispersion curves. By contrast, the long-assumed rhombohedral boron carbide type structure of B4N was currently determined to have much higher formation enthalpy and large imaginary phonon branches at zero pressure. As a result, it is obviously less stable than the newly predicted two phases. The calculated electrical and mechanical properties of these two new phases show that they possess both metallic and superhard characters, which will stimulate further high-pressure studies on synthesis and characterization.

FePd crystalline alloy with chemical stoichiometry of 1:1 is important permanent magnets and ultrahigh magnetic recording material, however there is few reports on its stable crystal structures under normal and high pressures. Thus, the stable crystal structures of FePd alloy were explored here by using particle swarm optimization algorithm and the soft-mode phase transition theory. Four new phases of FePd were discovered, namely, Imma, Cmmm-I, Cmmm-П, and P4/nmm. The structure, magnetisim, elasticity and lattice dynamics of the system were systematically investigated by first-principles calculations. It was found that the ferromagnetic state of these structures is more energetically favorable than the non-magnetic state below their respective critical pressure of ferromagnetic collapse. The phonon dispersion curves have no imaginary frequency and elastic constants fulfill Born’s criteria demonstrated that these four structures are dynamically and mechanically stable at pressure range of 0 to 59.1 GPa. These four new phases were found have negative formation enthalpy and the three orthorhombic structures were identified to be meta-stable in the pressure range of 0 to 59.1 GPa since they possess very small enthalpy difference which could not be firmly distinguished by our calculations relative to the experimentally known L10 phase. After 59.1 GPa, P4/nmm structure became the energetically most stable structure accompanied by simultaneous collapse of magnetic moments and lattice volume. The underlying mechanism was analyzed and attributed to be the simultaneously collapse of magnetoelastic force of Fe-Fe.

We have systematically explored the pressure dependence of magnetism, electronic structure, elasticity and lattice dynamics of the ferromagnetic L10-FePd crystalline alloy by employing first-principles calculations. All predicted properties of the ground state are well consistent with available experimental and theoretical results. The total magnetic moment decreases slowly with pressure before the ferromagnetic collapse critical pressure of about 164 GPa and then sharply decreases near the 164 GPa, and the (C44-C66) value changed from positive to negative. The phonon dispersion relations and phonon densities of state are also investigated at zero and high pressures. Owing to the spontaneous magnetization of the system, the ferromagnetic L10-FePd crystalline alloy is dynamically stable at pressures below 171 GPa (above which the magnetism completely disappears). When the pressure was greater than 171.4 GPa, the system dynamics became unstable. The calculated elastic properties indicate that the L10-FePd alloy is mechanically stable up to 180 GPa. The ferromagnetic L10-FePd crystalline alloy has good ductility and metallicity outside the ferromagnetic collapse pressure region, but the L10-FePd crystalline alloy has an abnormal phenomenon of a sudden sharp increase in brittleness near 162 GPa. We found this anomaly from the Pugh ratio B/G and Poisson's ratio of the system and the change of Cauchy pressures (C12-C66) and (C13-C44) with pressure, and this anomaly was analyzed with compression factor and magnetoelastic force.

In the frame work of Ginzburg-Landau-Devonshire theory, the dielectric and pyroelectric properties of ferroelectric thin films coated with two semiconducting electrodes are studied. Due to the depolarization effect produced by the surface polar charges cannot be completely screened, the contribution of the semiconducting electrodes is included in the free-energy functional. The dielectric susceptibility and the pyroelectric coefficient are calculated respectively as functions of temperature, film thickness and the bias electric field. A comparison with the corresponding results performed for perfectly conducting electrodes has been made, which shows that the effects of semiconducting electrodes can induce a transition from the ferroelectric phase to paraelectric phase, change the phase transition from the second order to first order, and greatly improve the dielectric susceptibility and pyroelectric coefficient. The work of this paper may pave a way to control the phase state and dielectric and pyroelectric properties of ferroelectric thin films by choosing the appropriate electrode materials.

AlN nanowires with hexagonal structure were successfully synthesized by direct reaction of aluminum with nitrogen gas using arc discharge method. The wurtzite AlN nanowires have an average diameter of 40 nm and a length of several tens micrometer. The growth direction of most single-crystalline AlN nanowires is perpendicular to the [0 0 1] direction, while it is also found that the AlN nanowires grow along [0 0 1] direction. The vapor–solid growth mechanism can explain the formation of the AlN nanowires. Raman spectroscopy studies of the AlN nanowires reveal that the stress is rather low and the crystallinity is close to bulk AlN. The UV spectrum of the AlN nanowires shows that the absorption edge at 6.23 eV is comparable with that of the bulk AlN. The photoluminescence of the AlN nanowires suggests that the emission band at 506 nm may be ascribed to the deep level defect due to nitrogen vacancy.

Nanocrystalline chromium nitride (CrN) with the cubic rock-salt structure was synthesized by the arc discharge method in nitrogen gas (N2). The product was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It was found that the nitrogen gas pressure is a crucial factor for the synthesis of cubic CrN. At relatively low N2 pressure, cubic CrN was formed. With the increase of N2 pressure, hexagonal Cr2N and metal Cr were gradually formed. It indicated that the formation of CrN apparently favors a low nitrogen pressure environment, and the diffusion of nitrogen atoms into the Cr was lowered with the increase of N2 pressure. We explain this experimental observation in terms of the evaporation rate of anode Cr and the ionization of nitrogen.