Studies of the growth, structure and some electrical properties of heavy rare earth dihydride films

Abstract

Conditions have been established for the epitaxial growth, by vacuum evaporation at 10-6 torr, of single crystal films of the dihydrides of gadolinium,terbium, dysprosium, holmium and erbium, on the cube, dodecahedral and octahedral faces of rock salt. The film structures have been investigated by electron microscopy and diffraction. Lattice parameter measurements on single crystal and polycrystalline specimens indicate that thin films (less then 200A) are face-centred-cubic dihydrides whereas thick films (>900A) are dominated by hexagonal close-packed metallic structure. Films of intermediate thickness exhibit a mixed (f.c.c. dihydride and h.c.p. metal) phase. The effects,on the structure of Gd and Tb dihydride films, of such parameters as substrate nature and temperature, vacuum environment, annealing and hydrogen treatment have been studied. Some nucleation and decoration studies have been conducted for gadolinium and terbium dihydrides. The electrical resistivity and temperature coefficient of resistance of polycrystalline films of the dihydrides of gadolinium, terbium, dysprosium, holmium and erbium, grown on glass slides, have been measured in vacuo. Resistivity measurements on the dihydride films, made in the temperature range 77K to 573K indicate generally metallic conduction characteristics. Studies of the changes of resistivity on annealing the dihydride films in the presence of a stream of hydrogen gas support the conclusion that the films become converted to semiconducting trihydrides having negative temperature coefficient of resistance.