The phase transformation in Ta3Ge: I

The transformation from the high temperature form of Ta3Ge (h-Ta3Ge) with the Fe3P-type structure to its low temperature form (l-Ta3Ge) with the Ti3P-type structure was studied by means of X-ray diffraction techniques and optical metallography. The transformation temperature was estimated to be about 15500C. A shear mechanism is proposed on the basis of the crystal structure relationship between the two modifications. The transformation can be classified as first order but it is not heterogeneous. The observed diffraction lines can be indexed on the basis of a tetragonal unit cell, the dimensions of which change continuously from those of h-Ta3Ge to those of l-Ta3Ge during the transformation. This is interpreted in terms of a transitional structure with regions of the h-Ta3Ge type alternating with regions of the l-Ta3Ge type within a common coherent lattice. The observed cell dimensions may depend on the relative amounts of h-Ta3Ge and l-Ta3Ge. The spread in the cell parameters observed in different samples of l-Ta3Ge are due to variations in the amount of h-Ta3Ge remaining after annealing. The number and the size of the h-Ta3Ge regions that remain are attributed to the variable ease with which the strains induced by the accommodation of the volume and shape changes accompanying the transformation can be released. (Auth.)