Below the NBel temperature the ferrous oxalate dihydrate is characterized by a large electric quadrupole interaction comparable with the dipole magnetic one /1 to 4/. This implies an ambiguity in the determination of the asymmetry parameter p and the angles (8 , (b )describing the orientation of the internal magnetic field (HefJ with respect to the principal axes of the electric field gradient (EFG) /5, 6/. Mossbauer spectra of oc -FeC2O4-2D20 have been measured at 300 and 4.2 K in /4/ to study the deuterium isotope effect. The analysis of the spectrum a t 4.2 K gives the ambiguity region: 76 O< 8 < 90°, Oo< (9 < 18O, and 0.46 < ' 1 c 0.72./7/ has shown, below TN = (16 11' deviation of the magnetic moment direction from the crystal b-axis. Each Fe ion in w-FeC2O4. 2D20 (space group C%/c) is located a t the centre of a distorted octahedron with vertices occupied by four O2 ions of the oxalate groups and arranged in a rectangle with the longer edges parallel to the b-axis. The two other 0 ions, belonging to the water molecules, are situated along the a-axis. To a good approximation, the orthorhombic symmetry of the octahedron may be assumed with the z, y, x axes parallel to the a , b, and c' = c sin B macroscopic axes, respectively. A comparison of the angular ambiguity region with the neutron diffraktion result implies that the Vxx axis of the EFG tensor is directed along the b-axis. Based on the above data it is difficult to decide the orientation of the remaining EFG axes. Therefore the following two possibilities were taken into account in calculations using a computer program /a/: (ii) Vzz parallel to the c' -axis (perpendicular to the a-b plane). Then 8 = = (84 2 1)' and (9 = (11 ? 1)'. The best fit has been obtained for ' 2 = 0.58 2 0.2.On the other hand, the recent neutron diffraction study of a -FeC204-2D20 1) K, the collinear magnetic structure with

S e c o n d -O r d e r S p i n -H a m i l t o n P a r a m e t e r s of M n 2 + in Z i n c O x a l a t e D i h y d r a t e BY I. gLEDZINSKA ( a ) , K . MACETKA ( b ) , and L. TYKARSKI ( a )