Electric Transport Properties of NaAlB(14 )with Covalent Frameworks br

A synthetic protocol was developed for obtaining a single phase of polycrystalline NaAlB14with strongly connected intergrain boundaries. NaAlB14has a unique crystal structure with a tunnel-likecovalent framework of B that traps monovalent Na and trivalent Al ions. Owing to the atmosphericinstability and volatility of Na, the synthesis of polycrystalline NaAlB14and its physical properties have notbeen reported yet. This study employed a two-step process to achieve single-phase polycrystalline NaAlB14.As afirst step, a mixture of Al and B with excess Al was sintered in the Na vapor atmosphere followed byHCl 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 andsuppressed intergranular resistance. As a result, the HP-annealed sample achieved superior transport properties (0.1 k Omega cm at 300K) to the vacuum-annealed sample (260 k Omega cm). Furthermore, from the viewpoint of its crystal structure and DFT calculations, themost probable site for the defect was suggested to be the Na site.