Less than 50% sublattice polarization in an insulating S=3/2 kagome antiferromagnet at T approximate to 0

We have found weak long-range antiferromagnetic order in the quasi-two-dimensional insulating oxide KCr3(OD)(6)(SO4)(2) which contains Cr3+ S=3/2 ions on a kagome lattice. In a sample with approximate to 76% occupancy of the chromium sites the ordered moment is 1.1(3)mu(B) per chromium ion which is only one-third of the Neel value g mu(B)S=3 mu(B). The magnetic unit cell equals the chemical unit cell, a situation which is favored by interplane interactions. Gapless quantum spin fluctuations (Delta/k(B)<0.25 K) with a bandwidth of 60 K much greater than T-N=1.6 K are the dominant contribution to the spin correlation function S(Q,omega) in the ordered phase.