Zur Darstellung und Struktur von M1?LnTa3O9 (Ln = Pr,Nd) Röntgenographische und elektronenmikroskopische Untersuchungen

Preparation and Structure of M1? LnTa₃O₉ (Ln = Pr, Nd), X-Ray and Electronmicroscopical Investigations New ternary compounds M1? LnTa₃O₉ (Ln = Pr, Nd) could be prepared by chemical transport reaction in a temperature gradient T₂ → T₁ (T₂ = 1100°C; T₁ = 1000°C; CI₂ as transport agent). M1 NdTa₃O₉ crystallizes in the monoclinic space group P 2₁/m with a = 5.3840(9) Å, b = 7.550(1) Å, c = 8.1911(9) Å and β = 92.46(1)°. The structure was refined to give R = 6.29% and R_w = 6.20%. It is built of double and single chains of corner-sharing TaO₆ octahedra extended along the b-axis. Tunnels running along 010] are created by the framework of TaO₆ octahedra. Ln (Ln = Pr, Nd) is located in these tunnels to levels of y = 1/4 and 3/4. A structure refinement for isostructural M1? PrTa₃O₉ led to a = 5.4051(7) Å, b = 7.5680(2) Å, c = 8.1964(9) Å, β = 92.38(2)° and R = 7.72%, R_w = 7.57%. By grinding in an agate mortar M1? LnTa₃O₉ transforms into M2? LnTa₃O₉, a new modification with a higher density. High resolution transmission electron microscopy images of the M1? PrTa₃O₉ structure were made along the 010] direction. They could be interpreted by comparing them with images calculated on the basis of the multi-slice method.