Direct observation of crystallographic and chemical changes during dehydrogenation of oxygen contaminated La2C3H1.5

The amorphous product, La2C3H1.5, formed by reaction of crystalline La2C3 with hydrogen at 170 °C has been investigated by electron microscopy. Electron beam induced hydrogen loss leads to full crystallinity via paracrystalline intermediates with distorted face centered cubic (fcc) substructure of the La atoms. Progressive transformation results in a separation into distinct phases and to significant strain, which finally produces a fragmentation of the particles. While La2C3 is recovered in the core of the particles, traces of oxygen contamination are captured in perfectly crystalline La2O3 (A- and C-type) on the surface, which is embedded in a matrix of LaC2. The preferred orientations of all crystalline phases depend on the average structure of the paracrystalline intermediate.