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Water ice is a molecular solid whose behavior under compression reveals the interplay of covalent bonding in molecules and forces acting between them. This interplay determines high-pressure phase transitions, the elastic and plastic behavior of H 2 O ice, which are the properties needed for modeling the convection and internal structure of the giant planets and moons of the solar system as well as H 2 O-rich exoplanets. We investigated experimentally and theoretically elastic properties and phase transitions of cubic H 2 O ice at room temperature and high pressures between 10 and 82 GPa. The time-domain Brillouin scattering (TDBS) technique was used to measure longitudinal sound velocities (V L) in polycrystalline ice samples compressed in a diamond anvil cell. The high spatial resolution of the TDBS technique revealed variations of V L caused by elastic anisotropy, allowing us to reliably determine the fastest and the slowest sound velocity in a single crystal of cubic H 2 O ice and thus to evaluate existing equations of state. Pressure dependencies of the single-crystal elastic moduli C ij (P) of cubic H 2 O ice to 82 GPa have been obtained which indicate its hardness and brittleness. These results were compared with ab initio calculations. It is suggested that the transition from molecular ice VII to ionic ice X occurs at much higher pressures than proposed earlier, probably above 80 GPa.

The composition-dependent crystal structure, elastic modulus, phase stability, and magnetic property of Ni2-xCoxMn1.60Sn0.40 (0

First-principles alloy theory, formulated within the exact muffin-tin orbitals method in combination with the coherent-potential approximation, is used to study the mechanical properties of ferromagnetic body-centered cubic (bcc) Fe$_{1-x}$M$_x$ alloys (M=Mn or Ni, $0\le x \le 0.1$). We consider several physical parameters accessible from \emph{ab initio} calculations and their combinations in various phenomenological models to compare the effect of Mn and Ni on the properties of Fe. Alloying is found to slightly alter the lattice parameters and produce noticeable influence on elastic moduli. Both Mn and Ni decrease the surface energy and the unstable stacking fault energy associated with the $\{110\}$ surface facet and the $\{110\}\langle111\rangle$ slip system, respectively. Nickel is found to produce larger effect on the planar fault energies than Mn. The semi-empirical ductility criteria by Rice and Pugh consistently predict that Ni enhances the ductility of Fe but give contradictory results in the case of Mn doping. The origin of the discrepancy between the two criteria is discussed and an alternative measure of the ductile-brittle behavior based on the theoretical cleavage strength and single-crystal shear modulus $G\{110\}\langle111\rangle$ is proposed., Comment: 14 pages, 11 figures

By using first principles in combination with atomistic spin dynamics calculational methods, we determine the temperature-dependent free energies of the L2(1)-and L1(0)-Ni-2(Mn1-xFex) Ga (0

The incorporation of cubic beta/beta(0) phase improves significantly the ductility and workability of TiAl-based alloys. However, the ductility deteriorates with the precipitation of the ordered hexagonal omega(0) phase in the beta/beta(0) phase. To avoid the formation of the omega(0) phase in alloy design, fundamentals about the beta/beta(0) to omega(0) phase transition and its alloying element dependence have to be known. In the present work, we investigated the phase transition and phase stability in Ti4Al3Nb and Ti4Al3Mo alloys using a first-principles method based on density functional theory. Our calculations indicate that for the beta(0) to omega(0) transition, the "collapse-diffusion" pathway is easier than the "diffusion-collapse" one; however, both pathways are quite difficult due to the high-energy barriers. The beta(0) to omega '' transition occurs through the "diffusion-collapse" pathway without an energy barrier (except for the activation energy for the short-range diffusion needed for atomic rearrangement). Comparing the total energies of different phases, we find that the omega(0) phase of Ti4Al3Nb is more stable than beta(0) and vice versa for Ti4Al3Mo whereas the relative stability of omega '' to beta(0) for Ti4Al3Nb is stronger than that for Ti4Al3Mo, indicating that the addition of Mo inhibits the transition from the beta(0)- to the omega-related phases in Ti4Al3Nb. Our calculations demonstrate that omega '' is more stable than omega(0) for both Ti4Al3Nb and Ti4Al3Mo, in contrast to the previously proposed picture based on some experimental observations, the possible origins of which are discussed in detail.

The magnetic ordering and its effect on the physical stability of X2Mn1+xSn1-x (0

Structure searches for new high-pressure phases of Y metal have been performed by using evolutionary algorithms in conjunction with a first-principles, pseudopotential plane-wave method based on density functional theory. The $oF16\mathrm{\text{\ensuremath{-}}}Fddd$ and $hP3\mathrm{\text{\ensuremath{-}}}P{3}_{1}21$ phases are predicted to be energetically favorable at pressures over 97 GPa. These two phases are shown to be dynamically stable by computing their phonon dispersions. We thus propose that $oF16\mathrm{\text{\ensuremath{-}}}Fddd$ and $hP3\mathrm{\text{\ensuremath{-}}}P{3}_{1}21$ are the most probable crystal structures Y may take in the 97--206 GPa range. The superconducting critical temperatures (${T}_{c}$) of the new phases are estimated using the Allen-Dynes formula. The ${T}_{c}$ is predicted to decrease with increasing pressure over about 100 GPa, in sharp contrast to its observed monotonic increase under lower pressure. The electronic origins of the stabilities of the proposed high-pressure phases have also been investigated.

Ni-2(Mn1-xFex)Ga ferromagnetic shape memory alloy shows unusual composition-dependent martensitic transformation temperature (T-M), namely, T-M decreases with increasing e/a ratio. In hope of understanding this unusual behavior, we investigated the composition dependence of the heat of formation (H-f) and shear elastic modulus (C') of the cubic austenite as well as the stability of the five-layer modulated (5M) tetragonal martensite relative to the austenite, using first-principles exact muffin-tin orbital method in combination with coherent potential approximation. Our calculations demonstrated that H-f of the austenite increases with the Fe content x. C' increases slightly with x up to 0.05 but decreases thereafter. The composition dependence of both H-f and C' cannot fully account for the trend of T-M against x although such correlations have been proposed in literature for other Ni-Mn-Ga based alloys. The structure of 5M martensite phase of Ni-2(Mn1-xFex)Ga with 0 < x < 0.2 is determined by optimizing both the shear (changing c/a) and wavelike shuffle of atoms in (110) planes along [1 (1) over bar0] direction adopting the experimentally determined modulation function. The energy difference Delta E-AM between the austenite and 5M phases decreases with increasing x up to 0.05, following the lower Delta E-AM corresponding to lower T-M rule. However, with x larger than 0.05, Delta E-AM increases, against the experimental T-M similar to x behavior. We propose that, if taking the temperature effect and the spin-orbital coupling into account, the Delta E-AM similar to x curve might be altered and may explain the unusual composition dependence of Ni-2(Mn1-xFex)Ga.

The temperature dependence of the elastic properties of Ni(2+x)Mn(1-x)Ga and Ni(2)Mn(Ga(1-x)Al(x)) (x = 0.0, 0.1, and 0.2) random alloys are investigated by using the first-principles exact muffin-tin orbitals method. At 0 K, the calculated equilibrium parameters in both cubic L2(1) and nonmodulated tetragonal beta'''-Ni(2)MnGa are in good agreement with the available experimental data and other theoretical results. Separating the thermal effects into single electron excitation, volume expansion, phonon smearing, and magnetic terms, we find that phonon smearing gives the dominating positive tetragonal elastic constant (C') versus temperature (T) slope for the cubic phase. For Ni(2+x)Mn(1-x)Ga, the competition between the negative alloying effect (partial derivative C'/partial derivative x 0) leads to nearly constant C'[x, T(M)(x)] at the martensitic transition temperature T(M)(x). For Ni(2)Mn(Ga(1-x)Al(x)), where both partial derivative C'/partial derivative x and partial derivative C'/partial derivative T are positive, however, due to the significantly decrease of T(M)(x), the critical C'[x, T(M)(x)] slightly decreases with Al doping. Furthermore, it is demonstrated that both the composition and the temperature dependence of C' are indispensable to get a reasonable theoretical T(M)(x).

The structural phase transitions of yttrium under high pressure up to 300 GPa have been studied by using the first-principles, pseudopotential plane-wave method based on density functional theory. The experimental rare-earth structure sequence with increasing pressure is successfully reproduced. The yet unknown distorted face-centered cubic (dfcc) phase is determined to be of the R (3) over barm space group. In addition, the P6(2)22 phase is predicted to be energetically more stable beyond the pressure range for dfcc. Accurate phonon-dispersion calculations demonstrate that the P6(2)22 phase is dynamically stable beyond pressure of about 206 GPa. The corresponding projected band structures, density of states, and charge density differences are given in order to clarify the electronic origins of the stability of the P6(2)22 phase under pressure.

The site preference and elastic properties of Fe-, Co-, and Cu-doped Ni(2)MnGa alloys are investigated by using the first-principles exact muffin-tin orbital method in combination with coherent-potential approximation. It is shown that Fe atom prefers to occupy the Mn and Ni sublattices even in Ga-deficient alloys; Co has strong tendency to occupy the Ni sublattice in all types of alloys; Cu atoms always occupy the sublattice of the host elements in deficiency. For most of the alloys with stable site occupations, both the electron density n and the shear modulus C' can be considered as predictors of the composition dependence of the martensitic transition temperature T(M) of the alloys. The physics underlying the composition-dependent C' are discussed based on the calculated density of states.

The interfacial energies (gamma) between the Cr-rich alpha'-FexCr1-x and Fe-rich alpha-Fe1-yCry phases (0 < x, y < 0.35) are calculated to be between similar to 0.02 and similar to 0.33 J m(- ...

The composition-dependent lattice parameters and elastic constants of In1-xTlx(0

The composition dependent lattice parameter, phase stability, elastic moduli, and magnetic transition temperature of the Ni2+xMn1-xGa shape-memory alloys are studied by using the first-principles e ...

The site occupancy and elastic modulus of off-stoichiometric Ni2MnGa alloys are investigated by the use of the first-principles exact muffin-tin orbital method in combination with coherent-potentia ...

The elastic moduli c' and c(44) of beta-Ti with respect to external pressure P (up to about 138 GPa) are calculated with a first-principles plane-wave pseudopotential method. The accuracy of the calculations regarding the plane-wave cut-off energy, k-point mesh, and transferability of the pseudopotentials is carefully tested. It is found that the critical pressure beyond which beta-Ti satisfies the elastic stability conditions is about 60 GPa. The Mulliken population analysis shows that both s and p electrons transfer to the d orbitals with increasing pressure, however, the number of s electrons starts to increase when the pressure exceeds about 70 GPa. The number of d electrons at the critical pressure is about 2.96, in perfect agreement with the critical number of d electrons for a stable bcc Ti-V alloy, which demonstrates the correlation between the stability of bcc metals and their d orbital occupation. The bonding charge density calculations show charge accumulation on the d-t(2g) orbitals under high pressure, which may improve the elastic stability of beta-Ti.

The effect of Zr on the martensitic transformation (MT) behavior and mechanical properties of (Ti(0.5-x)Zr(x))Ni(0.5) alloys is investigated by calculating the elastic constants and elastic moduli in the B2 phase as a function of x for 0

A first-principles plane-wave pseudopotential method based on density functional theory is used to investigate the concentrations of point defects and their interaction in TiNi. The calculations show that, in the thermal equilibrium state, TiNi develops antisite-type point defect configurations. In accordance with the sequence of the stability of the B2, B19, and B19(') phases, the concentrations of the point defects in stoichiometric TiNi at the identical temperature decrease from B2 to B19 to B19('). At high temperature, there are considerable amounts of vacancies in Ti-rich compounds. The calculated interaction energies show that the nearest-neighboring Ni antisite and Ti antisite are attractive to each other, whereas the second-nearest-neighboring Ni antisites repel each other and Ti antisites mutually attract, which indicates that a Ti-rich domain may appear in stoichiometric TiNi, and also explains the high solubility of excess Ni and low solubility of excess Ti in TiNi. The electronic structure mechanisms behind the interactions between the point defects are discussed based on charge density calculations. It was shown that the attractive interaction between Ti antisites is due to the accumulation of electron density between the Ti antisites, whereas the repulsion between Ni antisites is due to the depletion of electron density between them.

By using a first-principles plane-wave pseudopotential method, the energetics and electronic structure of Sigma5(210) grain boundary (GB) and the (210) surface of undoped as well as B- and/or H-doped Ni3Al are investigated. The geometric structures of the GBs and surfaces are fully relaxed by minimizing the total energy and interatomic force. The results show that B induces a large lattice expansion but H does not. Both B and H "prefer" to occupy the Ni-rich hole at the GB or surface but not the Ni-deficient one. The segregation energies of B and H as well as the interaction energy between them at the GB and surface are calculated. The calculation indicates that B segregates more strongly to the GB than to the surface, which results in an increase in the Griffith work of the GB and, therefore, in agreement with the experiments, improves the ductility of Ni3Al. Contrary to the case of B, H segregates more strongly to the surface than to the GB, which results in a decrease in Griffith work and confirms H as an embrittler for Ni3Al. The calculation of the interaction energy between B and H demonstrates that B and H repel each other. Consequently, B may block the site of occupation of H at the GB, and restrain the H-induced embrittlement. To understand the mechanism of the obtained energetic features, the electronic densities of states (DOSs) are calculated. A comparison of the total DOSs between the B-doped GB and undoped as well as H-doped ones shows that B increases the hybridization of the GB, which contributes to the enhanced binding of the B-doped GB over the undoped and H-doped ones. When the site of B changes from bulk to GB to surface, the hybridization between B and Ni decreases accordingly. It is proposed that the segregation behavior of B at the GB and surface is dominated by the competition between B(p)-Ni(d) bond energy and the strain energy induced by B. The preference of B for the Ni-rich interstice in Ni3Al is explained by the repulsive interaction between B and Al atoms resulting from the hybridization between their electrons when they are close to each other. The repulsion between B and H can also be explained by the same electronic structure mechanism as that for the B-Al interaction. The segregation of B at surface shifts the DOS of its nearest neighbor Ni to lower energy. This may increase the chemisorption potential energy of H2O on Ni3Al surface and, therefore, decrease the reactivity of the surface, inhibiting the environmental embrittlement of Ni3Al.