Crystal and magnetic structures of the spin-trimer compounds Ca3Cu3-xNix(PO4)4 (x=0,1,2)

Crystal and magnetic structures of a series of quantum spin-trimer system Ca3Cu3-xNix(PO4)(4) (x=0,1,2) were studied by neutron powder diffraction at temperatures of 1.5-290 K. The composition with one Ni per trimer (x=1) has a monoclinic structure (space group P2(1)/a, No. 14) with the unit cell parameters a=17.71 A, b=4.89 A, c=8.85 A, and beta=123.84 degrees at T=290 K. The x=2 composition crystallizes in the C2/c space group (No. 15) with the doubled unit cell along the 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 T-N=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 tau(2) using both arms of the star {k} with the magnetic moments of 1.89(1) and 0.62(2) mu(B) 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 intertrimer interaction on the overall magnetic structure.