A mechanically switchable metal–insulator transition in Mg 2NiH 4 discovers a strain sensitive, nanoscale modulated resistivity connected to a stacking fault

Abstract

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 NiH 4-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 Noréus (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.