Crystal and magnetic structure of orthorhombic HoMnO3

o-HoMnO₃ crystallizes in the GdFeO₃-type structure (Pbnm) with no crystal structure anomalies below 295 K and with the Mn-O-Mn angle as low as 144.1°. The Mn³⁺ and Ho³⁺ magnetic moments are ordered below T_N=38 K and T_Ho=11 K, respectively. The ordering is incommensurate with a propagation vector k=k_yb^*, where k_y is between 0.395 and 0.431 depending on temperature. Symmetry analyses have been carried out for both magnetic sites. The ordering was determined to be A_y for the Mn³⁺ moments, modulated in a longitudinal sinusoidal wave along the propagation vector. The Ho³⁺ moments order with the same propagation vector as Mn³⁺, but the site is split in two orbits of two ions, each ordered in an a_xf_y pattern with a phase factor between them. At the lowest temperatures an antiferromagnetic (commensurate) magnetic phase appears in competition with the main (incommensurate) magnetic phase. The arrangement of the moments is discussed in terms of the magnetic exchange interactions and compared with the closely related magnetic structures of LaMnO₃, TbMnO₃, and o-YMnO₃. k_y is found to increase with decreasing size of the rare-earth component.