Structure of RNi3Al9 (R = Y, Gd, Dy, Er) with either ordered or partly disordered arrangement of Al-atom triangles and rare-earth-metal atoms

ErNi3A19, Mr = 586.22,trigonal, new type, hR78, R32 -f2edc4 (No. 155), a= 7.2716 (5), c= 27.346 (3) ~, V = 1252.2(2)/k 3, Z = 6, D.,. = 4.664 Mg m -3, A(Mo Ka) = 0.71073/k, # = 17.654 mm-~, F(000) --1614, T = 293 K, wR = 0.021 for 785 contributing unique reflections.The structure is built up of three kinds of monoatomic layer perpendicular to e: Al-atom layers with triangular mesh, Ni-atom layers with triangular mesh and layers containing two rare-earth-metal atoms for one Al-atom triangle.GdNi3A19 [a = 7.3006 (9), c = 27.478(5)/~, wR = 0.020 for 791 reflections] is isotypic.YNi3AI9 [a = 7.2894 (7), c = 27.430(5) A, wR =0.035 for 635 reflections] and DyNi3AI9 [a = 7.2723 (9), c = 27.344(6)/~, wR = 0.027 for 682 reflections] crystallize in related structures with a partly disordered arrangement of Al-atom triangles and rare-earth-metal atoms.Similar monoatomic layers to those in ErNi3AI9 build up the orthorhombic structure of Y2Co3AI9 (Y2Co3Ga9 type).The stoichiometry 1:3:9 is related to that of RzT3A19 by replacing every second mixed layer of composition R2AI3 by an Al-atom layer (A13): 6RT3A19 = 6R2T3AI9 -3RzAI3 + 3A13.