Satoshi Iikubo

Giant negative thermal expansion is achieved in antiperovskite manganese nitrides when the sharp volume change associated with magnetic ordering is broadened by substitution. In this Letter, we address the unique role of the ''magic" element, Ge, for such broadening in Mn3Cu1-xGexN. We present evidence for a local lattice distortion well described by the low-temperature tetragonal (T4) structure of Mn3GeN for a range of x, where the overall structure remains cubic. This structural instability sh

Magnetic structures in an antiperovskite system, ${\mathrm{Mn}}_{3}{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}\mathrm{N}$, with a large magnetovolume effect above $x=0.15$ have been studied by neutron powder diffraction measurement. The present neutron study revealed that not only a cubic crystal structure but also a ${\ensuremath{\Gamma}}^{5g}$ antiferromagnetic spin structure are key ingredients of the large magnetovolume effect in this itinerant electron system. The large magnetovolume

The phase stability of the long periodic structures in Mg has been investigated at finite temperature by means of first-principles calculations. Free-energy calculation, including the lattice vibration effect, clearly reveals that 14H and 18R type long periodic structures become more stable than 2H-Mg. Furthermore, the stacking fault energies from a structure of ABA (hcp) to ABC (fcc) were calculated for the isotropic lattice variation. We found that the stacking fault energy decreased by lattic

A deep understanding of the effect of the A-site cation cross-exchange on the hot-carrier relaxation dynamics in perovskite quantum dots (PQDs) has profound implications on the further development of disruptive photovoltaic technologies. In this study, the hot carrier cooling kinetics of pure FAPbI₃ (FA⁺ , CH(NH₂ )₂ ⁺ ), MAPbI₃ (MA⁺ , CH₃ NH₃ ⁺ ₊ ), CsPbI₃ (Cs<sup>+

To aid the development of Pb-free perovskite solar cells, the stability of the iodide perovskite structure ABI3 has been investigated by first-principles calculations, Bader charge analysis, and the cluster expansion method. At the A sites, methylammonium (MA, CH3NH3+), formamidinium (FA, CH(NH2)2+), and Cs+ were modeled, while at the B sites, one or two elements from Pb, Sn, Ge, In, Ga, Bi, and Sr were examined. For the partially substituted system A(B,B′)I3, we found that the stability strongl

Calculated specific heats of several carbides and nitrides with a B1 structure have been compared with those of experimental data up to 3000 K. The specific heats at constant pressure are calculated with the quasiharmonic approximation, using phonon dispersions calculated from direct method, and pseudopotential plane-wave method. The calculated results of HfC, TaC, TiC, ZrC, and ZrN are in excellent agreement with the experimental data up to 3000 K. For NbC, TiN, and VC, the calculated results a

The lattice dynamics of CsSnX3 (X = Cl, Br, and I) and CsPbI3, which are low-thermal-conductivity materials, are investigated using first-principles phonon calculations. Because of the strong lattice anharmonicity and the accompanying instability of high-temperature cubic phases, the self-consistent phonon theory, which can incorporate the effect of lattice anharmonicity at a mean-field level, is applied in this study. The calculated lattice thermal conductivity reproduced a low thermal conducti

In this study, thermodynamic properties of the Mg–RE–Zn (RE = Y, La) ternary hcp phase at finite temperature have been investigated by means of first-principles calculations combined with the cluster variation method (CVM). Free energy calculation, including the configurational entropy, shows that the Mg–Y–Zn ternary hcp phase has a tendency to phase separation. Conversely, the Mg–La–Zn ternary system does not exhibit such behavior even around room temperature. Furthermore, the calculated spinod

Magnetic and transport properties of the pyrochlore molybdate R 2- x R x ′ Mo 2 O 7 (R, R ′ = rare earth, Y), which exhibits the metal-insulator (M-I) transition with varying x , have been studied. The transition from an antiferromagnetic spin-glass state to a ferromagnetic one occurs at x far from the M-I phase boundary, indicating that the intimate correlation of the magnetic state with the conducting nature is not so obvious. In Nd 2 Mo 2 O 7 , the ordering of Mo spins takes place at about 95

Motivated by the discovery of superconductivity in F-doped LaFeAsO, we investigated the magnetic fluctuations in a related compound Fe(Se1-xTex)0.92 (x = 0.75, 1) using neutron scattering techniques. Non-superconducting FeTe0.92 shows antiferromagnetic (AF) ordering with the ordering vector |Q| = 0.97 A-1 (Q = (0.5,0,0.5)) at TN ~ 70 K and a structural transition between tetragonal and monoclinic phases. In the AF monoclinic phase, the low-energy spectral weight is suppressed, indicating a possi