Antiferromagnetic ordering in the novel Ho3Ge5 and HoGe1.85 compounds studied by X-ray and neutron diffraction

The crystal structures and magnetic ordering of the novel binary compounds Ho3Ge5 (Y3Ge5 structure type, space group Fdd2) and HoGe1.85 (DyGe1.85 structure type, space group Cmcm) have been studied by X-ray and neutron powder diffraction. Both compounds have two rare-earth sites which order simultaneously below T-N=6 K for Ho3Ge5 and 12 K for HoGe1.85. Ho3Ge5 orders with the wave vector q (1/2 1/2 1/2) or with an eightfold cell enlargement (2aX2bX2c) with a canted antiferromagnetic arrangement comprising ten sublattices. The two He-sites split into six independent sets (orbits), two for Ho-1:8(a) (1, 2) and four for Ho-2:16(b) (1'-4'). The two Ho-1 orbits have a uniaxial moment arrangement along the z-axis, the four Ho, orbits have their moments in general directions and two distinct moment values. The ordered moment values at 1.4 K for the three (1, 2), (1', 2') and (3', 4') He-orbits decrease in the sequence mu(1) = 10.3(2) mu(B)/Ho>mu(3') = 7.3(2) mu(B)/Ho> mu(1') =6.1(2) mu(B)/Ho. The opposite relation holds for the tilt angle of the moments away from the z-axis: phi(1z) = 0<phi(3z)'=47(6)degrees<phi(1z)' =56(3)degrees. The total canted antiferromagnetic structure is described by the triclinic magnetic space group F(s)1 and results from the strong geometrical frustration of the Ho moments due to triangular atomic arrangement with negative interactions. HoGe1.85 orders antiferromagnetically below T-N=12 K with the wave vector q=(1/2 1/2 0). The two 4(a) He-sites order simultaneously with a uniaxial moment arrangement along the z-axis described by the monoclinic magnetic space group C11(2(1)/m)i. At 1.4 K the ordered magnetic moment value is 7.87(6) mu(B)/Ho. A comparison of the magnetic ordering of several studied compounds of the RGe2-delta series is made. (C) 1998 Elsevier Science S.A. All rights reserved.