We propose to study the magnetic order and temperature dependent structural properties of the ThCr2Si2-type phases BaFeCrAs2 and BaCr2As2 at instruments E9 and E2. These compounds are related to BaFe2As2, an itinerant antiferromagnet which can be converted into a superconductor by doping or high pressure. BaCr2As2 is known to be an antiferromagnet too, but the spin structure has not been determined experimentally yet. Recent electronic structure calculations have led to contradictory predictions for the spin structure of BaCrFeAs2. A ferromagnetic as well as a fully compensated half-metallic antiferromagnetic ground state have been suggested. We intend to clarify the spin structure of BaFeCrAs2 and to relate the results to our 57Fe Mössbauer spectra. Special attention will paid to magnteostructural coupling effects which are known to be important in such transition metal pnictides.