Determination of the magnetic structure at 10 K of Mn3Ta2O8, and refinement of the crystal structure at 295 K, using neutron powder diffraction data

The Mn2+-containing oxide Mn3Ta2O8 crystallises in the space group I4(1)/a with a=11.2728(2) and c=9.8030(3) Angstrom, and with a structure related to the cubic fluorite type. The compound is antiferromagnetic with a Neel tem perature of 23 K. The magnetic structure at 10 K was determined from consta nt-wavelength (lambda=1.47 Angstrom) neutron powder diffraction (NPD) data and refined by the Rietveld method to R-F=2.1% and S=1.3 for 80 reflections with magnetic contributions at 2 theta less than or equal to 60 degrees. T he magnetic moments, of magnitude 4.01 (2) mu(B), lie in the ab plane and a re antiparallel with the (001) planes. The spin ordering is similar to that in the cubic fluorite structure of UO2 but with 40% of the magnetic atoms replaced with: non-magnetic Ta atoms. The non-magnetic crystal structure at 295 K was refined to R-F=0.6% using time-of-flight (TOF) NPD data, anisotr opic thermal parameters and 1943 reflections with d greater than or equal t o 0.55 Angstrom.