The incommensurately modulated image, image, solid solution: The ‘missing link’ between the NiP and MnP structure types

The image-d incommensurately modulated structures of four members of the image solid solution field have been successfully refined from X-ray powder diffraction data (image; 1.52/4.25; 1.27/3.44 and 2.00/4.03 for image, 0.5, 0.6 and 0.7, respectively). The 4-d superspace group symmetry is image (image; image, 5.0188(2), 4.9796(2) and image; image, 6.0576(2), 6.0183(2) and image; image, 3.4812(2), 3.4593(1) and image; image, 0.7467(1), 0.7241(1) and 0.7046(1) for image, 0.5, 0.6 and 0.7, respectively). The underlying average structure is of NiAs type while the (in general) incommensurate primary modulation wave-vector, image, varies continuously and smoothly with composition. The two largest amplitude displacive atomic modulation functions (AMFs), for all samples, were the Ni displacement along b AMF and the Ge/P displacement along a AMF. The refined amplitude of the former was found to systematically increase with P content from image for image to image for image while the magnitude of the latter was found to increase with P content from image for image to image for image. These displacive shifts significantly modulate the local crystal chemistry i.e. the local interatomic distances and co-ordination polyhedra. This continuously variable, incommensurately modulated, intermediate structure type is shown to provide a natural link or bridge between the two extreme end-member structures i.e. NiGe (of MnP structure type) and NiP by simply choosing the commensurate options with image and image respectively.