We report on the crystallographic structure of the layered compound Pb3Mn7O15. Previous analysis based on laboratory X-ray data at room temperature gave contradictory results in terms of the description of the unit cell. Motivated by recent magnetic
bulk measurements of this system, we re-investigated the chemical structure with high-resolution synchrotron powder diffraction at temperatures between 15 K and 295 K. Our results show that the crystal structure of stoichiometric Pb3Mn7O15 has a pronounced 2-dimensional character and can be described in the orthorhombic space group Pnma.
Crystal and magnetic structures of a series of novel quantum spin trimer system Ca3Cu3xNix(PO4)4 (x=0,1,2) were studied by neutron powder diffraction at the temperatures 1.5-290 K. The composition with one Ni per trimer (x=1) has a monoclinic structu
re (space group P 21 /a, no. 14) with the unit cell parameters a = 17.71 A, b = 4.89 A, c = 8.85 A and = 123.84 deg at T=290 K. The (x=2) composition crystallizes in the C 2/c space group (no. 15) with the doubled unit cell along c-axis. Each trimer is formed by two crystallographic positions: one in the middle and the second one at the ends of the trimer. We have found that the middle position is occupied by the Cu2+, whereas the end positions are equally populated with the Cu2+ and Ni2+ for (x=1) while in the (x=2) the trimers were found to be of only one type Ni-Cu-Ni. Below TN = 20 K the (x=2) compound shows an antiferromagnetic ordering with propagation vector star {[1/2,1/2, 0], [-1/2,1/2, 0]} The magnetic structure is very well described with the irreducible representation tau22 using both arms of the star {k} with the magnetic moments 1.89(1) muB and 0.62(2) muB for Ni2+ and Cu2+ ions, respectively. We note that our powder diffraction data cannot be fitted by a model containing only one arm of the propagation vector star. The Cu/Ni-spins form both parallel and antiparallel configurations in the different trimers, implying substantial effect of the inter-trimer interaction on the overall magnetic structure.
