Structural transition of Li3N under high pressure: A first-principles study

We theoretically study the electronic properties and pressure induced solid–solid phase transition of Li₃N by first-principles calculations. The calculations indicate a pressure-induced structural phase transition above 1.5GPa from the ambient $P6/MMM$ hexagonal phase (α-Li₃N) to a layered hexagonal phase (β-Li₃N, P63/MMC) which is accompanied by a 21.6% volume collapse. Above 38.8 GPa, β-Li₃N transforms into γ-Li₃N ($Fm\stackrel{?}{3}m$), and the recently reported new ${\alpha }^{\prime }$-phase ($P\text{-}3M1$) is not stable under high pressure. The analysis of electronic density of states suggests that various Li₃N polymorphs are insulators, and the band gap is broadened with further compression.     

Pressure dependence of negative thermal expansion in Zr2(WO4)(PO4)2

Negative thermal expansion materials can experience significant stresses when they are used in composites. Under ambient conditions Zr₂(WO₄)(PO₄)₂ displays anisotropic negative thermal expansion (NTE) (${\alpha }_v=?14.0\left(10\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_a=?7.9\left(5\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_b=2.5\left(5\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_c=?8.7\left(2\right)\times 10^{?6}{K}^{?1}$ at 0 GPa). The effect of hydrostatic pressure on its thermal expansion characteristics was investigated by neutron diffraction between 300 and 60 K at pressures up to 0.3 GPa. No phase transitions were observed in the pressure and temperature range examined. The material was found to have a bulk modulus, B₀, of 61.3(8) GPa at ambient temperature, and unlike some other NTE materials, pressure had no detectable effect on thermal expansion (${\alpha }_v=?14.2\left(8\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_a=?7.9\left(3\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_b=2.9\left(5\right)\times 10^{?6}{K}^{?1}$, ${\alpha }_c=?9.2\left(2\right)\times 10^{?6}{K}^{?1}$ at 0.3 GPa).     

p(x) and Tc(x) characteristics in the Y 1−b(Ca)bBa2Cu3O6+x cuprate family

A model has been proposed to calculate the $p\left(x\right)$ and $T_c\left(x\right)$ dependences in the Y _1?b(Ca)_bBa₂Cu₃O_6+x high-$T_c$ cuprate family and applied to $b=0$, $b=0.1$, and $b=0.2$ cases, for which experimental data exist in the literature. The results obtained imply that the Ca efficiency to provide holes is independent of the basal plane oxygen concentration, which is consistent with a view that electrons from CuO₂ layers would go primarily to Ca since it is twice closer than oxygen (in addition, the chain oxygen is screened by a layer made up of Ba and O(4) ions). It is shown that, in fully oxygenized compounds ($x=1$) the average efficiency, $\chi$, of a chain oxygen to attract an electron from the two nearby layers is reduced by the Ca insertion, though not because the charge transfer mechanism is in itself weakened by Ca, but because a part of electrons that are otherwise available in CuO${}_2$ layers has already been removed by the substitution of Y ³⁺ with Ca²⁺. It has been found that the $b$-dependence of the average oxygen doping efficiency can be fairly accurately described by the following relation: $\chi \left(b\right)=0.39\times (1?0.78b)$. The calculated $p\left(x\right)$ and $T_c\left(x\right)$ dependences are in very good agreement with the available experimental data.     

First-principles study on the phase transitions,crystal stabilities and thermodynamic properties of TiN under high pressure

The structural and thermodynamic properties of titanium nitride (TiN) have been investigated by merging first-principles calculations and particle-swarm algorithm. The three phases are identified for TiN, including the B1, the $P{6}_3/mmc$, and the B2 phases. A new phase of anti-TiP structure with the space group $P{6}_3/mmc$ has been predicted. The calculated phase transition from the B1 to the $P{6}_3/mmc$ occurs at 270 GPa. The vibrational, elastic, and thermodynamic properties for the three phases have been calculated and discussed.     

Persistent luminescence and synchrotron radiation study of the Ca2MgSi2O7:Eu2+,R3+ materials

The tetragonal ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+},{\mathrm{R}}^{3+}$ persistent luminescence materials were prepared by a solid state reaction. The UV excited and persistent luminescence was observed in the green region centred at 535 nm. Both luminescence phenomena are due to the same ${\mathrm{Eu}}^{2+}$ ion occupying the single ${\mathrm{Ca}}^{2+}$ site in the host lattice. The ${\mathrm{R}}^{3+}$ codoping usually reduced the persistent luminescence of ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$, which differs from the ${\mathrm{M}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Sr},\mathrm{Ba})$ and ${\mathrm{MAl}}_2{\mathrm{O}}_4$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Ca},\mathrm{Sr})$ materials. Only the ${\mathrm{Tb}}^{3+}$ ion enhanced slightly the persistent luminescence. With the aid of synchrotron radiation, the band gap energy of ${\mathrm{Ca}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ was found to be about 7 eV that is very similar to those of the ${\mathrm{M}}_2{\mathrm{MgSi}}_2{\mathrm{O}}_7$:${\mathrm{Eu}}^{2+}$ $(\mathrm{M}=\mathrm{Sr},\mathrm{Ba})$ materials. Thermoluminescence results suggested that the ${\mathrm{R}}^{3+}$ ions might act as electron traps, but only the TL peaks created by ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Sm}}^{3+}$ can be found in the temperature range accessible. Lattice defects (e.g. oxygen vacancies) are also important, since the same main thermoluminescence peak was observed at about $100{}^{\circ }\mathrm{C}$ with and without ${\mathrm{R}}^{3+}$ codoping.     

An orthorhombic chromium-center in the spinel MgAl2O4

In Cr-doped MgAl₂O₄ most of the Cr³⁺-ions occupy normal B-sites. In addition to this axial center we report here the presence of an orthorhombic Cr³⁺-center. The ESR-spectra of this new center can be described by the spin Hamiltonian:

$?=\mathit{D}\left[S_z^2?\frac{1}{3}\mathit{S}\left(S+1\right)\right]+g_x\beta H_x{S}_x+g_y\beta H_y{S}_y+g_z\beta H_z{S}_z+\mathit{E}\left(S_x^2?S_y^2\right)$

, with S=3/2. The principal axes of the center are along [111]=Z, $\left[111\right]=Z,\left[1\overline{1}0\right]=Y$ and $\left[\overline{1}\overline{1}2\right]=X$. The values of the parameters are: g_z=1·950±0·005, g_x=g_y=1·915±0·015, D=+0·903cm^?1±0·003 cm^?1 and E=+0·084 cm^?1±0·005 cm^?1. The intensity of the lines is about ten times lower than that of the axial Cr³⁺-center.     

Thermoelectric and topological properties of half-Heusler compounds ZrIrX(As,Sb, Bi)

The strain dependent electronic structures, thermoelectric and topological properties of the half-Heusler compounds ZrIrX(X=As, Sb, Bi) are investigated by the first-principle calculations. At the equilibrium lattice constants, all the three compounds are trivial insulators and good thermoelectric materials with the Seebeck coefficient S and the power factor over relaxation time $S^2\sigma /\tau$ as large as 1180 (μV/K) and 4.1 (${10}^{11}W{m}^{?1}{K}^{?2}{s}^{?1}$), respectively. The compressive strain enhances the band gap, while the tensile strain decreases the band gap. At some specific tensile strains, the compounds become Dirac-semimetals, with the s-type band ${\mathrm{\Gamma }}_6$ below p-type band ${\mathrm{\Gamma }}_8$, in the cubic phase. When we compress the a(b)-axis and elongate the c-axis of the compounds, they become the type-I Weyl semimetals. For ZrIrAs, the eight Weyl-Points (WPS) locate at (± K_x, 0, ± K_z), (0, ± K_y, ± K_z), ${\mathrm{K}}_x={\mathrm{K}}_y=0.008{\mathrm{\AA }}^{?1}$, ${\mathrm{K}}_z=0.043{\mathrm{\AA }}^{?1}$.     

Monopoles on the Bryant–Salamon G2-manifolds

$G_2$-monopoles are solutions to gauge theoretical equations on noncompact $7$-manifolds of $G_2$ holonomy. We shall study this equation on the $3$ Bryant–Salamon manifolds. We construct examples of $G_2$-monopoles on two of these manifolds, namely the total space of the bundle of anti-self-dual two forms over the ${\mathbb{S}}^4$ and ${\mathbb{C}\mathbb{P}}^2$. These are the first nontrivial examples of $G_2$-monopoles.Associated with each monopole there is a parameter $m\in {\mathbb{R}}^{+}$, known as the mass of the monopole. We prove that under a symmetry assumption, for each given $m\in {\mathbb{R}}^{+}$ there is a unique monopole with mass $m$. We also find explicit irreducible $G_2$-instantons on ${\Lambda }_{-}^2\left({\mathbb{S}}^4\right)$ and on ${\Lambda }_{-}^2\left({\mathbb{C}\mathbb{P}}^2\right)$.The third Bryant–Salamon $G_2$-metric lives on the spinor bundle over the $3$-sphere. In this case we produce a vanishing theorem for monopoles.     

Investigation of structural and electronic properties of β- HgS: Molecular dynamics simulations

The pressure induced phase transition of β-HgS is studied using an ab initio molecular dynamics simulation. The structural phase transformation from the zinc-blende structure to the NaCl-type structure (space group $Fm\overline{3}m$) and from this structure to CsCl-type structure ($Pm\overline{3}m$) with the application of hydrostatic pressure is predicted. Additionally, the electronic properties of HgS and various physical properties such as the lattice constants, the bulk modulus and the pressure derivative of the bulk modulus are revealed. Furthermore, these phase transitions are obtained using the total energy and enthalpy calculations. According to these calculations these transformations are occurring at about 20?GPa and 28?GPa for $F\overline{4}3m$ →$Fm\overline{3}m$ and $Fm\overline{3}m$ →$Pm\overline{3}m$, respectively.     

Lattice dynamics and phase transition of NiTi alloy

We have performed detailed first-principles calculations to investigate the structural and lattice dynamical properties of NiTi alloy. The calculated static structures consist well with the experimental data and other theoretical results. With quasi harmonic approximation, the phase boundary between B19′ and BCO phases can be described as a five order polynomial $T=100?89.28P+296.75P^2?717.94P^3+734.62P^4?274.25P^5$. The change of vibrational entropy is $0.07k_B$/atom at the transition temperature 100 K under zero pressure.     

Disorder dependence electron phonon scattering rate of V82Pd18 − xFex alloys at low temperature

We have systematically investigated the disorder dependence electron phonon scattering rate in three dimensional disordered V₈₂Pd_18 ? xFe_x alloys. A minimum in temperature dependence resistivity curve has been observed at low temperature $T=T_{\mathrm{m}}$. In the temperature range $5\text{ K}\le T\le T_{\mathrm{m}}$ the resistivity correction follows ${{\rho }_{\mathrm{o}}}^{5/2}{T}^{1/2}$ law. The dephasing scattering time has been calculated from analysis of magnetoresistivity by weak localization theory. The electron dephasing time is dominated by electron–phonon scattering and follows anomalous temperature (T) and disorder $({\rho }_0)$ dependence behaviour like ${\tau }_{\mathrm{e}\text{-}\mathrm{ph}}^{?1}\propto T^2/{\rho }_0$, where ${\rho }_0$ is the impurity resistivity. The magnitude of the saturated dephasing scattering time $({\tau }_0)$ at zero temperature decreases with increasing disorder of the samples. Such anomalous behaviour of dephasing scattering rate is still unresolved.     

Spin-glass behavior in the antiperovskite manganese nitride Mn3CuN codoped with Ge and Si

Antiperovskite manganese nitrides ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_x{\mathrm{Ge}}_{0.4?x}\right)N$ ($x=0.05$, 0.1, 0.15) were prepared and their negative thermal expansion, magnetic and specific heat properties were investigated. A frozen state with a freezing temperature was found at ～207 K in ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$. This indicates that ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$ exhibits a spin glass state at low temperatures. We discussed the cause of spin glass behavior and correlated this spin glass behavior with broadening of the negative thermal expansion operation-temperature window of the manganese nitrides ${\mathrm{Mn}}_3\left({\mathrm{Cu}}_{0.6}{\mathrm{Si}}_{0.15}{\mathrm{Ge}}_{0.25}\right)N$.