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B2CN was one of the synthesized light element compounds, which was expected to be superhard material with a metallic character due to its electron deficienct nature. However, in this work, we discovered two novel semiconducting superhard B2CN phases using particle swarm intelligence technique and first-principles calculations, which were reported to have three-dimensional and four coordinated covalent diamond-like structures. These two new phases were calculated to be dynamically stable at zero and high pressures, and can be deduced from the previously reported Pmma phase by pressure-induced structural phase transitions. More importantly, unlike the previously proposed metallic B2CN structures, these two new phases combine superhard (the calculated Vickers hardness reached ∼55 GPa) and semiconducting character. The semiconducting behavior of the newly predicted B2CN phases breaks the traditional view of the metallic character of the electron deficient diamond-like B-C-N ternary compounds. By a detail analyzation of the electron localization functions of these two new phases, three-center bonds were reported between some B, C and B atoms, which were suggested to be the primary mechanism that helps the compound overcome its electron-deficient nature and finally exhibit a semiconducting behavior.

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.

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.

Boron carbide (B4C) is one of the hardest materials known to date. The extreme hardness of B4C arises from architecturally efficient B12 or B11C icosahedrons and strong inter-icosahedral B–C bonding. As an excellent material for use in ballistic armor, the mechanic limit of B4C and possible phase transitions under extreme stress conditions are of great interest. Here we systematically explored the post-icosahedral solid structures of B4C under high pressure, using an unbiased structure search method. A new structure composed of extended framework of B and zigzag chains of C is predicted to be stable above 96 GPa. The new structure was predicted to have a high Vickers hardness of 55 GPa and simultaneously to retain a metallic ground state. The exceptional mechanical properties found in this structure are attributed to strong sp 3 covalent network formed under extreme pressure conditions. The predicted structure represents a new type of superhard boron carbides that form under high pressure without the presence of boron icosahedrons, which encourages experimental exploration in this direction.