Structure of the microdomain boundaries in RFe 6 Ge 6 (R = Tb, Ho or Er) crystals from diffuse X-ray scattering

Abstract

Abstract The richness of diffraction features in RFe6Ge6 crystals (R = Tb, Ho or Er) (sharp main reflections, broad satellites and diffuse streaks) originates from a complex twinned microstructure. The two structural subsystems, namely hexagonal FeGe network (host) and chains of the R and Ge atoms (guest), give rise to the main reflections of the average structure (YCo6Ge6 type, in an orthohexagonal setting: Ammm, (c, 3½ a, a)h). Ordering in the guest subsystem leads to a superstructure (Cmcm, (2c,2 × 3½ a, a)h) of the YCo6Ge6 type and gives rise to the satellites. This superstructure is arranged into submicroscopically twinned domains with three 120°-rotated orientations. The microdomains are separated by microdomain boundaries scattering into diffuse streaks. The microdomain boundaries form a new superstructure (Cmmm, (2c, 3½ a, 4a)h) of the YCo6Ge6 type which is solved in a simple and original way. The microdomain boundaries have three 120°-rotated orientations, extend preferentially parallel to the {(011). (011), (010)} planes of the average structure and have limited coherency normal to these planes. The maximum and minimum correlation lengths estimated from the different diffraction features are as follows: host subsystem, 106.6(8) and 82.4(6) Å; guest subsystem. 76.9(2) and 58.4(1) Å for the microdomains, and 56(2) and 18.8(1) Å for the microdomain boundaries. Coherency of the distant microdomains restricts the distribution and size of the microdomains and microdomain boundaries. A microstructure model is proposed and confirmed by computer simulations.