Articles you may be interested inTransient reflectivity as a probe of ultrafast carrier dynamics in semiconductors: A revised model for lowtemperature grown GaAs

Modulated elemental reactants containing iron and antimony were found to react at low temperature (T < 200 degrees C) forming either FeSb(2) or FeSb(3) depending on both the layer thicknesses and the overall composition of the initial reactant. For films containing 75% antimony and 25% iron, the metastable compound FeSb(3) was observed to nucleate and grow if the layer thickness was below approximately 35 A. Above this critical thickness for the modulation, annealing led to the formation of FeSb(2). This, combined with low-angle diffraction data, suggests that the initial interdiffusion between iron and antimony layers in an elementally modulated reactant results in the formation of an amorphous reaction intermediate if the layering thickness is less than 35 A. For modulated reactants with composition between 70 and 90 atomic % antimony and below this critical layer thickness, the metastable compound FeSb(3) formed. In more iron-rich modulated reactants FeSb(2) nucleates. The nucleation temperature and the nucleation energy of FeSb(3) were found to be a function of the composition of the amorphous intermediate, while those of FeSb(2) were found to be relatively independent of composition.

A new metastable binary compound with the skutterudite crystal structure has been synthesized from modulated elemental reactants, through an amorphous intermediate, using a novel low-temperature synthesis technique. The amorphous reaction intermediate undergoes nucleation at 87 degrees C, an extremely low temperature for solid-state reactions. When heated above 350 degrees C, the metastable phase NiSb(3) disproportionates into the thermodynamically stable phases NiSb(2) and Sb. Also, if the sum of the individual elemental layer thicknesses is greater than 30 A, a mixture of different phases forms. Simulation of the high-angle powder X-ray diffraction spectrum confirms that NiSb(3) is isostructural with CoSb(3).

The nucleation energy of a series of La(x)Fe(y)Sb(z) modulated elemental reactants was measured as a function of the Fe/Sb ratio over a large composition range while holding the La content constant. The nucleation energy of the ternary compound La(0.5)Fe(4)Sb(12) with the skutterudite crystal structure was found to depend very strongly on the Fe/Sb ratio in the modulated elemental reactant, with a higher nucleation energy as the Fe/Sb ratio is moved away from the 1:3 stoichiometric value. When the results of this study are compared with those from Fe(y)Sb(z) modulated reactants, the addition of lanthanum was found to suppress the nucleation of FeSb(2), thereby broadening the Fe/Sb composition range in which the ternary skutterudite compound La(x)Fe(4)Sb(12) nucleates. This suppression of nucleation of a binary phase on addition of a ternary component to an amorphous intermediate is in agreement with theoretical arguments. The observed suppression of nucleation also provides rational for the observed nucleation of metastable ternary and higher-order compounds from homogeneous amorphous reactants.

A series of samples ((AB)(x)(CD)(y))(z) were prepared containing both short repeat units (AB and CD) and long repeat units ((AB)(x)(CD)(y)), where the short repeat units were designed to have the composition appropriate to form square M(4)Sb(12) skutterudites (M = Fe, Co, or Ir; square = vacancy, La, or Y). X-ray diffraction and reflectivity were used to follow the evolution of the films from amorphous, layered materials to crystalline skutterudite superlattices as a function of annealing temperature and time. In all cases, the short repeat units interdiffused and crystallized the expected skutterudite, while the long repeat period persisted after annealing. The skutterudites crystallize with random crystallographic orientation with respect to the substrate. The observed splitting of the peaks in the high-angle diffraction data from the IrSb(3)/CoSb(3) sample indicates the formation of a novel superlattice structure with each grain having a random crystallographic orientation of the skutterudite lattice with respect to the superlattice direction.