A mechanically switchable metal-insulator transition in Mg_2NiH_4 discovers a strain sensitive, nanoscale modulated resistivity connected to a stacking fault

摘要

The band gap in semiconducting Mg_2NiH_4 was found to be dependent on subtle structural differences. This was discovered when investigating if thin film samples of Mg_2NiH_4 could be used in a switchable mirror or window device by utilizing a high to low temperature transition at about 510 K. In powder samples; this transition between an FCC high temperature phase, with dynamically disordered NiH4-complexes, and a monoclinic distorted low temperature phase, with ordered Mg_2NiH_4-complexes, has been demonstrated in a mechanical reversible conductor-insulator transition (Blomqvist and Noreus (2002) [7]). Black monoclinic Mg_2NiH_4 powders were found to have a band gap of 1.1 eV. Pressed tablets of black monoclinic Mg_2NiH_4 powders are conductive, probably from doping by impurities or non-stoichiometry. Thin film Mg_2NiH_4 samples were produced by reacting hydrogen with magnetron sputtered Mg_2Ni films on quartz glass or CaF_2 substrates. The Mg_2NiH_4 films on the other hand were orange, transparent with a band gap of 2.2 eV and a cubic unit cell parameter almost identical to the disorder HT phase but with lower symmetry. If black Mg_2NiH_4 powder is heated above the phase transition at 510 K and subsequently cooled down, the conductivity is lost and the powder turns brown. After this heat treatment TEM pictures revealed a multiple stacking fault having a local pseudo-cubic arrangement separating regions of monoclinic symmetry. The loss of conductivity and colour change is attributed to a higher band gap in the strained areas. The structure on each side of the stacking fault is related by a mirror plane as a consequence of the possibility for the NiH_4-complexes to order with different orientations. This leads to a mismatch in the long range ordering and strain is probably creating the stacking faults. Strain is important for forming the cubic modification. A severely strained film was revealed with optical microscopy in reflected light, indicating that strain prevents it from relaxing back into the monoclinic structure. This was supported by multiple twinned red translucent Mg_2NiH_4 crystals grown with cubic symmetry at elevated temperatures in a LiH flux. When cooled to ambient conditions, the "crystals" had the same cubic symmetry as the films, probably held together by their neighbours. When they were ground to a fine powder to prepare TEM samples, they relaxed and reverted back to the conventional monoclinic unit cell. This interesting nanoscale modulated resistance could possibly be developed into novel memory devices if properly controllable.