Takahiro Yamada

Single crystals are important model materials for studying layered metal dichalcogenides (TMDs); however, strategies for phase-selective growth remain limited for the MoX2 (X = S, Se, Te) family. In this study, a rapid (∼48 h) one-step chloride-flux method was developed for the phase-selective growth of MoX2 single crystals and chalcogen-dependent structure–growth relationships governing crystal morphology were established. Single-crystal X-ray diffraction, Raman spectroscopy, and elemental analyses confirmed the crystal structure and composition of the obtained MoX2 single crystals. NaCl–CsCl flux produced 2H-MoS2 single crystals with lateral dimensions of 10–50 μm, whereas NaCl flux yielded 2H-MoSe2 single crystals with lateral dimensions of 5–10 μm. By contrast, MoTe2 crystallized preferentially in the monoclinic 1T′ phase, reflecting its chalcogen-dependent phase stability. Density functional theory calculations revealed pronounced facet-energy anisotropy in MoS2 and MoSe2, which promoted lateral platelet growth, whereas the reduced anisotropy of MoTe2 accounted for its distinct morphology and polymorphism. Furthermore, the flux-grown MoS2 crystals were exfoliated into thin nanosheets. The present chloride-flux growth provides an effective method for the phase-selective synthesis of MoX2 single crystals, and the results of this study offer valuable insights into the crystal growth behavior of layered TMDs.

Electronic nematic and smectic liquid crystals are spontaneous symmetry-breaking phases that are seen to precede or coexist with enigmatic unconventional superconducting states in multiple classes of materials. In this Letter we describe scanning tunneling microscopy observations of a short ranged charge stripe (smectic) order in NaAlSi, whose superconductivity is speculated to have an unconventional origin. As well as this we resolve a clear spatial modulation of the superconducting gap amplitude, which arises due to the intertwined superconducting and smectic orders. Numerical calculations help to understand the possible driving mechanism as a suppression of kinetic energy on the Fermi surface formed in part by two large, flat-topped hole pockets of p-orbital character.

Cr 2 N and CrN were synthesized using chromium oxides and boron nitride (BN) as starting materials. Cr 2 N powder with particle sizes of 0.1-5 μm was obtained together with Na 3 BO 3 (melting point (m. p.): 675°C) by heating a mixture of Cr 2 O 3 , BN, and Na in an Ar atmosphere at 500-1100°C. CrN powder with particle size of 1 μm and NaBO 2 (m. p.: 968°C) were obtained as the main phases by heating NaCrO 2 and BN in an Ar atmosphere at 900-1000°C, while Cr 2 N crystal grains with sizes of 10-100 μm and NaBO 2 were obtained at 1100°C. The formation reactions of the chromium nitrides are metathesis reactions in which the O in chromium oxides and N in BN are exchanged, resulting in the formation of sodium borates (Na 3 BO 3 and NaBO 2). The melts of these sodium borates were found to accelerate the formation reactions of chromium nitrides, and the NaBO 2 melt, in particular, contributes significantly to the grain growth of Cr 2 N.

Semiconducting BaTiO ₃ ‐based ceramics for positive temperature coefficient (PTC) thermistors have conventionally been sintered at ∼1400°C in a reducing atmosphere. In the present study, sintered BaTiO ₃ ‐based semiconductor ceramics were obtained by exposing green compacts of BaTiO ₃ powder to Na vapor in an Ar atmosphere. The Na vapor functioned as both a reducing agent and a sintering accelerator. Densification and grain growth of BaTiO ₃ proceeded during heating at 800°C or higher. PTC thermistor properties with 1.9 orders of magnitude were observed at temperatures near the Curie temperature of BaTiO ₃ (120°C). Contamination of sodium in the BaTiO ₃ ‐based ceramics was rather small (∼ 0.2 wt%) and tiny areas containing Na were found in some of the fracture surfaces of ceramics.

We synthesized a Na–Pt–Si compound, NaPt ₃ Si, that exhibits catalytic activity toward the oxygen evolution reaction (OER) in an acidic electrolyte. The compound crystallizes in a monoclinic structure with space group C 2/ m and lattice constants of a = 21.4066(9) Å, b = 11.0626(4) Å, c = 10.2833(4) Å, and β = 92.866(2)°. The structure consists of a three-dimensional Pt–Si tunnel framework containing Na atoms, similar to the structures of previously reported Na–Pt–Si compounds such as Na _0.28 PtSi and Na ₃ Pt ₁₀ Si ₅ ; however, the shortest Pt–Pt distance in NaPt ₃ Si is 2.628 Å, which is shorter than the corresponding distances in Na ₃ Pt ₁₀ Si ₅ and Na _0.28 PtSi (2.800 and 2.876 Å, respectively) and even shorter than that of elemental Pt (2.775 Å). The Tafel plot for the OER suggested that NaPt ₃ Si exhibited a comparable overpotential to elemental Pt. Such high catalytic activity possibly originated from a uniquely reconstructed surface structure of NaPt ₃ Si.

This work comprised detailed assessments of the thermoelectric properties and crystal structure of the Zintl compound Na ₂ CdSn ₅ . A polycrystalline sample having a relative density of 98% was prepared by pressure sintering, and was found to exhibit p-type semiconductor characteristics with a low thermal conductivity of 0.58 W m ^–1 K ^–1 at room temperature. Single-crystal X-ray diffraction analyses at 90, 200, and 300 K showed that Na atoms within tunnel structures had large atomic displacement parameters, indicating that a rattling phenomenon occurred to a significant degree, promoting phonon scattering. A phase transition from tetragonal (space group: I 4 ¯ 2 d) to orthorhombic (space group: P2 ₁ 2 ₁ 2 ₁ ) symmetry was observed by cooling from 200 to 90 K, accompanied by Na atomic ordering. Partial replacement of Cd atoms with Ag to produce Na ₂ Cd _{1− x} Ag _x Sn ₅ ( x = 0.01–0.05) resulted in hole doping and a 30–50 fold increase in electrical conductivity, while maintaining low thermal conductivity. An optimally doped sample ( x = 0.02) had a dimensionless figure of merit, ZT , of 0.23 at 393 K, representing a fourfold enhancement relative to the undoped compound.

The first experimental realization of KBaBi, a ternary Zintl compound theoretically predicted to exhibit promising thermoelectric properties, is reported. Single crystals are successfully synthesized from constituent elements, and structural analysis reveals that KBaBi crystallizes in a hexagonal ZrBeSi‐type structure (space group P6₃/mmc, Z = 2) with lattice parameters a = 6.0884(5) Å and c = 7.4868(6) Å at 300 K. In this structure, Ba and Bi atoms form planar honeycomb lattices stacked along the c‐axis with K atom layers sandwiched between them. Notably, K atoms exhibit remarkably large thermal vibrations, with the equivalent isotropic atomic displacement parameter approximately twice than those of Ba and Bi atoms in the temperature range of 100–300 K, suggesting potential “rattling” behavior beneficial for thermoelectric performance. Transport measurements on a sintered polycrystalline sample (70% relative density) reveal electrical resistivity increasing from 3.89 to 8.56 mΩ cm⁻¹ and positive Seebeck coefficients ranging from 48.8 to 88.1 μV K⁻¹ between 296–486 K, indicating that KBaBi is a p‐type narrow‐gap semiconductor.

δ-NbN powder with particles of approximately 1 µm in size was obtained by heating a mixture of NaNbO₃ and BN powders at 1373 K for 1 h, while agglomerates composed of ε-NbN platelet grains up to 50 µm in size were obtained by heating the mixture at the same temperature for 10 h. Metathesis reaction in which sodium borate was formed with replacement of the oxygen and nitrogen from NaNbO₃ and BN occurred with the NbN formation. Wavelength-dispersive X-ray analysis showed that the molar ratios of Nb to N in the synthesized δ-NbN and ε-NbN were approximately 1:1. Magnetic susceptibility measurements showed that the δ-NbN powder was a superconductor with a transition temperature of approximately 16 K. In comparison, the ε-NbN agglomerates did not exhibit superconductivity above 2 K.