Synthesis and Crystal Structures of Ca₄SiN₄ and New Polymorph of Ca₅Si₂N₆

Single crystals of Ca₄SiN₄ were found in the product prepared by heating Ba, Ca, Si, NaN₃, and Na at 900 °C. Ca₄SiN₄ [space group <i>P</i>2₁/<i>c</i> (No. 14), <i>Z</i> = 4, <i>a</i> = 9.1905(4) Å, <i>b</i> = 5.9775(3) Å, <i>c</i> = 11.0138(7) Å, β = 116.4054(17)°] is isotypic with Ca₄GeN₄ and K₄SiO₄. Isolated [SiN₄]^8– tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 °C, a new polymorph of Ca₅Si₂N₆ crystallized. The space group of the polymorph [<i>C</i>2/<i>m</i> (No. 12), <i>Z</i> = 4, <i>a</i> = 6.2712(5) Å, <i>b</i> = 10.0175(8) Å, <i>c</i> = 12.0287(8) Å, β = 99.303(2)°] is different from <i>C</i>2/<i>c</i> previously reported for Ca₅Si₂N₆, while both polymorphs are composed of Ca²⁺ and edge-sharing double tetrahedra [Si₂N₆]^10–

Synthesis and Crystal Structures of Ca₄SiN₄ and New Polymorph of Ca₅Si₂N₆

Single crystals of Ca₄SiN₄ were found in the product prepared by heating Ba, Ca, Si, NaN₃, and Na at 900 °C. Ca₄SiN₄ [space group <i>P</i>2₁/<i>c</i> (No. 14), <i>Z</i> = 4, <i>a</i> = 9.1905(4) Å, <i>b</i> = 5.9775(3) Å, <i>c</i> = 11.0138(7) Å, β = 116.4054(17)°] is isotypic with Ca₄GeN₄ and K₄SiO₄. Isolated [SiN₄]^8– tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 °C, a new polymorph of Ca₅Si₂N₆ crystallized. The space group of the polymorph [<i>C</i>2/<i>m</i> (No. 12), <i>Z</i> = 4, <i>a</i> = 6.2712(5) Å, <i>b</i> = 10.0175(8) Å, <i>c</i> = 12.0287(8) Å, β = 99.303(2)°] is different from <i>C</i>2/<i>c</i> previously reported for Ca₅Si₂N₆, while both polymorphs are composed of Ca²⁺ and edge-sharing double tetrahedra [Si₂N₆]^10–

Synthesis and Crystal Structures of Ca₄SiN₄ and New Polymorph of Ca₅Si₂N₆

Single crystals of Ca₄SiN₄ were found in the product prepared by heating Ba, Ca, Si, NaN₃, and Na at 900 °C. Ca₄SiN₄ [space group <i>P</i>2₁/<i>c</i> (No. 14), <i>Z</i> = 4, <i>a</i> = 9.1905(4) Å, <i>b</i> = 5.9775(3) Å, <i>c</i> = 11.0138(7) Å, β = 116.4054(17)°] is isotypic with Ca₄GeN₄ and K₄SiO₄. Isolated [SiN₄]^8– tetrahedra were identified in the structure by single-crystal X-ray diffraction. After reheating the product at 900 °C, a new polymorph of Ca₅Si₂N₆ crystallized. The space group of the polymorph [<i>C</i>2/<i>m</i> (No. 12), <i>Z</i> = 4, <i>a</i> = 6.2712(5) Å, <i>b</i> = 10.0175(8) Å, <i>c</i> = 12.0287(8) Å, β = 99.303(2)°] is different from <i>C</i>2/<i>c</i> previously reported for Ca₅Si₂N₆, while both polymorphs are composed of Ca²⁺ and edge-sharing double tetrahedra [Si₂N₆]^10–

Crystal Growth Conditions of Types I and II Na–Si Clathrates by Evaporation of Na from a Na–Si–Sn Solution

Single crystals of the type I clathrate Na₈Si₄₆ were synthesized by the evaporation of Na from a Na–Si–Sn solution (Na:Si:Sn = 6:2:1 molar ratio) at 723–873 K under an Ar atmosphere of 10⁵ Pa. In the mixture of type I and type II clathrates prepared by heating at 773 K for 72 h, type I single crystals with sizes of up to 5 mm were found. A single phase of single crystals of the Na₂₄Si₁₃₆ type-II clathrate having a {111} crystal facet of about 2 mm on one side was obtained by heating at 873 K for 9 h

High-Pressure Diffusion Control: Na Extraction from NaAlB₁₄

A novel synthesis technique, called the high-pressure diffusion control (HPDC) method, was developed in this study. The method combined the high-pressure synthesis using a cubic anvil apparatus and an anisotropic diffusion control technique; the electrical processing in high-pressure and high-temperature environments of up to 4 GPa and over 1000 °C is enabled by simultaneously adjusting the temperature, pressure, and voltage. This nonequilibrium state is effective in creating metastable materials. The developed novel technique was applied to polycrystalline NaAlB14 with a boron covalent framework. Although electronic conduction is dominant in this material and no Na-ion conduction is observed even at high temperatures, the HPDC method successfully extracted Na ions by utilizing the difference in bond strength between Na and B, creating the metastable material AlB14 while maintaining its basic crystal structure. During the decrease in the Na concentration, applying a high pressure compressed the sample according to the volume change and maintained good contact at the intergrain boundary in the polycrystalline sample, promoting Na-ion diffusion. The Na extraction functioned as electron carrier modulation and significantly reduced the electrical resistivity. The developed HPDC method is expected to be applicable to various compounds with a difference in the bond strength between constituent elements and has the potential to open up new avenues in the inorganic synthesis of polycrystalline metastable materials with dense sintered states and modulate their physical properties