Electric Transport Properties of NaAlB₁₄ with Covalent Frameworks

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB₁₄ with strongly connected intergrain boundaries. NaAlB₁₄ has a unique crystal structure with a tunnel-like covalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmospheric instability and volatility of Na, the synthesis of polycrystalline NaAlB₁₄ and its physical properties have not been reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB₁₄. As a first step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed by HCl treatment to remove excess Al as a second step. For obtaining bulk samples with strong grain connection, vacuum or high-pressure (HP) annealing was employed. HP annealing promoted bandgap shrinkage due to the crystal strain and defect levels and suppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 kΩ cm at 300 K) to the vacuum-annealed sample (260 kΩ cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, the most probable site for the defect was suggested to be the Na site.