THE ANTIFERROMAGNETIC STRUCTURE OF TRICLINIC COPPER(II) PHOSPHATE

The antiferromagnetic structure of triclinic Cu3(PO 4)2 (TN=22.2(5)K) has been determined at 2 K by unpolarised neutron diffraction from powdered and single-crystal samples prepared by hydrothermal synthesis. The triacute reduced chemical unit cell has a=4.855(1), b=5.288(1), c=6.184(1) AA, alpha =72.34(1), beta =86.99(1) and gamma =68.54(1) degrees at ambient temperature and a=4.848(1), b=5.280(1), c=6.183(1) AA, alpha =72.30(1), beta =86.90(1) and gamma =68.59(1) degrees at 6 K: it contains one formula unit and its space group is P1. The magnetic propagation vector is (001/2) referred to this cell and, within the accuracy of the determination, the magnetic structure is collinear with equal moments of 0.68(1) mu B on each of the two crystallographically inequivalent Cu2+ ions. The Cu1 ion is at an inversion centre which is preserved in the magnetic space group P2c1. The Cu1 moment points in a direction theta =72(2) degrees , phi =135(2) degrees with the Cu2 moments antiferromagnetically coupled to the Cu1 moment; here theta and phi are the polar angles referred to the orthogonal crystal axes with (001) as the polar axis (z), and x along a*. The phi -angles are measured from the x, z plane. The magnetic form factor is found to be expanded by 14(3)% relative to that calculated for free Cu2+ ion wave functions. The magnetic exchange paths through the structure are discussed.