Tribochemische und thermische Umwandlungen von LnTa3O9 (Ln = Pr,Nd) – röntgenographische und elektronenmikroskopische Untersuchungen

Tribochemical and Thermal Transitions of LnTa₃O₉ (Ln = Pr, Nd) — X-ray and Electron Microscopic Investigations Upon grinding crystals of M1? LnTa₃O₉ (Ln = Pr, Nd) 3] undergo a tribochemical phase transition. This leads to a new modifikation M2? LnTa₃O₉ with a significant higher density. We tried to find out more about the structure with high resolution electron microscopic investigations. According to electron diffraction and powder patterns the lattice parameters are (CuKα₁, λ = 1,54051 Å): M2? PrTa₃O₉: a = 6.2545(7) Å, b = 7.6736(7) Å, c = 6.5316(8) Å, β = 93.93(9)°; M2? NdTa₃O₉: a = 6.2552(5) Å, b = 7.6598(7) Å, c = 6.5103(4) Å, β = 94.096(7)°; (Z = 2). Using the intensities of powder patterns two structure models were calculated (space group P2₁/m, P2/m; R < 20%, heavy metal positions only). A through focus series of high resolution images was in better agreement with the first model (space group P2₁/m). Both models show a remarkable similarity to the structure of M? CeTa₃O₉ 4]. A thermal phase transition leads to M? PrTa₃O₉ and M? NdTa₃O₉ which are both isostructural to M? CeTa₃O₉.