Crystal and magnetic structures of the spin-trimer compounds Ca3 Cu3xNix (PO4)4 (x=0,1,2)

TL;DR

This study investigates the crystal and magnetic structures of Ca3Cu3xNix(PO4)4 compounds with varying Ni content, revealing their space groups, magnetic ordering below 20 K, and complex spin configurations influenced by inter-trimer interactions.

Contribution

The paper provides detailed neutron diffraction analysis of the crystal and magnetic structures of Ca3Cu3xNix(PO4)4 with different compositions, highlighting the magnetic ordering and spin arrangements.

Findings

Ca3Cu3xNix(PO4)4 compounds have distinct space groups depending on Ni content.

The x=2 compound exhibits antiferromagnetic order below 20 K.

Magnetic moments differ for Ni2+ and Cu2+ ions, with complex spin configurations.

Abstract

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.