Kenji Kawashima

The solid solution Rh_1-xPt_xSb, bridging the structurally distinct MnP-type RhSb and NiAs-type PtSb, was systematically investigated. A temperature-induced structural phase transition between the MnP- and NiAs-type phases occurs at x ≈ 0.1 near room temperature, accompanied by pronounced resistivity hysteresis. Superconductivity emerges for x ≥ 0.2, and the critical temperature (T_c) reaches a maximum value of 4.25 K at x = 0.4─the highest T_c among transition-metal monoantimonides. The electron-phonon coupling constant (λ_ep ≈ 0.6) and normalized specific-heat jump (ΔC_el/γT_c ≈ 1.6) classify Rh_1-xPt_xSb as a weak-coupling superconductor. The compositional dependence of both the Debye temperature (Θ_D) and the electronic density of states at the Fermi level (N(E_F)) correlates closely with T_c, suggesting that they are responsible for the T_c enhancement. A comparison with the high-entropy analogue M_1-xPt_xSb (M = equimolar Ru, Rh, Pd, and Ir) revealed distinct differences in Θ_D and N(E_F), which can account for the observed difference in T_c between the two systems.

Calcium (Ca) intercalation into graphite (C) has been considered to require elevated temperatures, and its occurrence at room temperature (RT) has long been regarded as highly unlikely. Here, we demonstrate that sodium (Na) catalysis enables the formation of superconducting CaC6 even at RT. In mixtures of Ca, Na, and graphite, the gradual development of superconducting diamagnetism and the emergence of x-ray diffraction peaks confirm the formation of CaC6 during storage at RT. The superconducting transition temperature increases with storage time, and the volume fraction of the formed CaC6 scales proportionally to the square root of time, indicating that CaC6 formation proceeds via a diffusion-controlled reaction. These findings provide new insights into both the superconducting mechanism of CaC6 and the intercalation chemistry of graphite. Finally, we propose a possible scenario explaining the Na-catalyzed Ca intercalation process.

Alkali metal-intercalated C₆₀, A ₃C₆₀ (A = K, Rb, Cs, and their combinations), holds significant potential for practical applications due to its high superconducting transition temperature (33 K), high upper critical field (900 kOe), and isotropic superconductivity. However, application-oriented research has been limited by the lack of an efficient A ₃C₆₀ synthesis process. In this study, we demonstrate a rapid and scalable synthesis of A ₃C₆₀ (A = K, Rb, and Cs_1/3Rb_2/3) via direct mixing of A and C₆₀, realizing the fabrication of high-quality sintered A ₃C₆₀ pellets within just 1 h of heating at 200 °C–300 °C. The pellets exhibited large superconducting shielding volume fractions with sharp transitions, and the relationship between the lattice constant and transition temperature was in good agreement with previous reports. This direct mixing method enables simple and rapid production of large quantities of A ₃C₆₀, which is expected to accelerate research into applications such as superconducting wires and bulk magnets.

Lanthanides (Ln) are notoriously difficult to intercalate into graphite. We investigated the possibility of using Na to catalyze the formation of Ln-intercalated graphite and successfully synthesized LnC₆ (Ln = Sm, Eu, and Yb) significantly rapidly in high yields. The synthesis process involves the formation of the reaction intermediate NaC_x, through the mixing of Na and C, which subsequently reacts with Ln upon heating to form LnC₆. Well-sintered LnC₆ pellets with low residual Na concentrations (Ln:Na ≈ 98:2) were fabricated by the two-step method. The pellets enabled the evaluation of LnC₆ by powder X-ray diffraction and electrical resistivity measurements. This study highlights the versatility of the Na-catalyzed method and lays the foundation for the rapid mass production of LnC₆, with potential applications in superconducting and rechargeable battery materials.

Enriching the material variation often contributes to the progress of materials science. We have discovered for the first time antiperovskite arsenide SrPd₃As and revealed a hidden structural and superconducting phase in Sr(Pd_1-xPt_x)₃As. The Pd-rich samples (0 ≤ x ≤ 0.2) had the same noncentrosymmetric (NCS) tetragonal structure (a space group of I4₁md) as SrPd₃P. For the samples with 0.3 ≤ x ≤ 0.7, a centrosymmetric (CS) tetragonal structure (P4/nmm) identical to that of SrPt₃P was found to appear, accompanied by superconductivity at a transition temperature (T_c) up to 3.7 K. In the samples synthesized with Pt-rich nominal compositions (0.8 ≤ x ≤ 1.0), Sr₂(Pd,Pt)_8-yAs_1+y with an intergrowth structure (CS-orthorhombic with Cmcm) was crystallized. The phase diagram obtained for Sr(Pd,Pt)₃As was analogous to that of (Ca,Sr)Pd₃P in which superconductivity (T_c ≥ 2 K) occurred in the CS phases induced by substitutions to the NCS phases. This study indicates the potential for further material variation expansion and the importance of elemental substitutions to reveal hidden phases in related antiperovskites.

In this paper we report the results of the first-principles calculations for (Ca_1-x Sr _x )Pd₃P for various crystal structures observed in experiment. The calculated electron specific heat coefficient shows a good agreement with the experimental values. We found the centrosymmetric phase has a higher density of state at the Fermi level than that of the non-centrosymmetric phase. Possible parity mixing in the non-centrosymmetric phase is estimated as small in this system.

It is generally difficult to quantify the amount of light elements in materials because of their low X-ray-scattering power, as this means that they cannot be easily estimated via X-ray analyses. Meanwhile, the recently reported layered superconductor, Sc₂₀C_8–xB_xC₂₀, requires a small amount of boron, which is a light element, for its structural stability. In this context, here, we quantitatively evaluate the optimal x value using both experimental and computational approaches. Using the high-pressure synthesis approach, which can maintain the starting composition even after sintering, we obtain the Sc₂₀(C,B)₈C₂₀ phase by the reaction of the previously reported Sc₁₅C₁₉ and B (Sc₁₅B_yC₁₉). Our experiments demonstrate that an increase in y values promotes the phase formation of the Sc₂₀(C,B)₈C₂₀ structure; however, there appears to be an upper limit to the nominal y value to form this phase. The maximum critical temperature (T_c = 7.6 K) is found to correspond with the actual x value of x ≈ 5 under the assumption that the sample with the same T_c as the reported value (7.7 K) possesses the optimal x amount. Moreover, we construct the energy convex hull diagram by calculating the formation enthalpy based on first principles. Our computational results indicate that the composition of Sc₂₀C₄B₄C₂₀ (x = 4) is the most thermodynamically stable, which is reasonably consistent with the experimentally obtained value.

In this study, we succeeded in synthesizing new antiperovskite phosphides MPd₃P (M = Ca, Sr, Ba) and discovered the appearance of a superconducting phase (0.17 ≤ x ≤ 0.55) in a solid solution (Ca_1–xSr_x)Pd₃P. Three perovskite-related crystal structures were identified in (Ca_1–xSr_x)Pd₃P, and a phase diagram was built on the basis of experimental results. The first phase transition from centrosymmetric (Pnma) to noncentrosymmetric orthorhombic (Aba2) occurred in CaPd₃P near room temperature. The phase transition temperature decreased as Ca²⁺ was replaced with a larger-sized isovalent Sr²⁺. Bulk superconductivity at a critical temperature (T_c) of approximately 3.5 K was observed in a range of x = 0.17–0.55; this was associated with the centrosymmetric orthorhombic phase. Thereafter, a noncentrosymmetric tetragonal phase (I4₁md) remained stable for 0.6 ≤ x ≤ 1.0, and superconductivity was significantly suppressed as samples with x = 0.75 and 1.0 showed T_c values as low as 0.32 K and 57 mK, respectively. For further substitution with a larger-sized isovalent Ba²⁺, namely, (Sr_1–yBa_y)Pd₃P, the tetragonal phase continued throughout the composition range. BaPd₃P no longer showed superconductivity down to 20 mK. Since the inversion symmetry of structure and superconductivity can be precisely controlled in (Ca_1–xSr_x)Pd₃P, this material may offer a unique opportunity to study the relationship between inversion symmetry and superconductivity.