  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 structure (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.
