  In this work we present a new chemical route to synthesize Tm2Mn2O7
pyrochlore, a compound which is thermodynamically unstable at ambient pressure.
Differently from the reported in the past high pressure synthesis of the same
compound applying oxides as starting materials, we have obtained a pure phase
Tm2Mn2O7 by a converting TmMnO3 at 1100 C and 1300 bar oxygen pressure. The
studies of Tm2Mn2O7 performed by a high resolution neutron powder diffraction
have shown, that a pure pyrochlore cubic phase Tm2Mn2O7 (space group Fd-3m)
have been obtained. On cooling below 25K there is a transition to a
ferromagnetically (FM) ordered phase observed with an additional
antiferromagnetic (AFM) canting suggesting a lowering of the initial cubic
crystal symmetry. The magnetic transition is accompanied by small but well
visible magnetostrsiction effect. Using symmetry analysis we have found a
solution for the AFM structure in the maximal Shubnikov subgroup I41/am'd'.
