Chromium germanides: formation, structure and properties

Abstract

The reaction of Cr and Ge films leads to the successive formation of Cr5Ge3, CrGe (FeSi structure), and Cr11Ge19 (Mn11Si19 structure). The first one of the phases could not be identified unambiguously because of difficulties relating to the proper crystalline structure of this compound. The formation of both Cr5Ge3 and CrGe (respectively at 400-500°C, and 600°C) leave the surface of the sample absolutely smooth strongly implying that the rate-limiting mechanism is diffusion. On the contrary, the formation of Cr11Ge19 occurs in island-like fashion, islands that spread laterally to consume the remaining CrGe, in a process that is quite characteristic of nucleation-controlled reactions. The growth of Cr5Ge3 proceeds as t1/n with n greater than 2. It is possible for this to occur if the rate-controlling mechanism is grain-boundary diffusion, with grains growing simultaneously with the thickness of the compound. The upper limit for the resistivities of Cr5Ge3, Cr11Ge8, CrGe an Cr11Ge19 were found to be 200, 160, 150 and 430 μω · cm, respectively. The last one of these phases is ferromagnetic with a Curie temperature of 91 K. © 1991.