Systematics of the allotrope formation in elemental gallium films

Elemental gallium forms unusual allotropes, whose structures all differ distinctly from the close-packing of spheres typical for nearly all elemental metals. At standard pressure, four different crystalline allotropes are known where the alpha-Ga allotrope is the thermodynamically stable one. Since the other allotropes can only be obtained under special conditions such as spatial confinement or undercooling, the structural relations among the gallium allotropes are not well-explored. We present here a systematic investigation of the dependence of the crystal structure formation in elemental gallium films on the deposition temperature and the subsequent annealing procedures using in situ x-ray powder diffraction and Raman measurements, complemented by ab initio calculations. The films were prepared with the femtosecond pulsed-laser-deposition technique on a substrate kept at constant temperature in the range of-190 degrees C to 25 degrees C, followed by cooling + heating cycles in the same temperature range. Besides preparing alpha-,beta-, gamma- and amorphous Ga films as a single phase, a new gallium allotrope, beta'-Ga, was synthesized, which is a distorted derivative of the beta-Ga crystal structure. Furthermore, no direct transition between the alpha-Ga phase on one side and the beta-, beta'-, and gamma-Ga phases on the other side was observed in the solid state. Surprisingly, at room temperature amorphous gallium and below-60 degrees C the alpha-Ga allotrope, respectively, is formed in the deposited films.