A complex oxide Ho2Mn2/3Nb4/3O7 with a pyrochlore-related structure was synthesized from Ho2O3, MnO, and Nb2O5 in Ar containing 1% H-2 atmosphere. Its structure was investigated by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The oxide, which was determined to be Ho(2.0)Hn(0.6)Nb(1.40)O(7.1) by an electron probe microanalyzer (EPMA), had a monoclinic symmetry (space group C2/c) with lattice parameters a = 1.2813(3), b = 0.7397(1), c = 1.1625(2) nm, and beta = 100.575(8)degrees. The oxide had a large number of lamellar domains bounded by planar defects on (001), and such domains were composed of twinned structures on the twin axis <110>. (C) 2000 Elsevier Science Ltd.