Tetragonal to monoclinic transition in the metallic antiferromagnet (formula presented)

The structural changes associated with the ferromagnetic (FM)/metallic to antiferromagnetic (AFM)/insulating transition of (Formula presented) are described in detail. Based on high-resolution neutron-diffraction data, in the (Formula presented) phase Mn1 and Mn2 octahedra have the same charge and are identically deformed. The electronically induced transition to the antiferromagnetic A-type structure at (Formula presented) is the result of a stretchinglike distortion of the octahedra that (i) contracts the Mn-O2 bond perpendicular to the FM planes by deviating (Formula presented) from (Formula presented) and (ii) injects electrons into Mn-O1 and Mn-O3 bonds. The observed microscopic changes are completely different to the structural transition in the A-type (Formula presented) and are discussed in the light of the orbital polarization previously proposed. © 1999 The American Physical Society.