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 highpressure 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 degrees 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.