Neutron powder diffraction analysis of (Tm0.50Ca0.50)MnO3 and (Lu0.50Ca0.50)MnO3

The crystal and magnetic structures of (Tm0.50Ca0.50)MnO3 and (Lu0.50Ca0.50)MnO3 have been investigated between 5 K and 300 K by means of high resolution neutron powder diffraction followed by Rietveld refinement and dc magnetic measurements. During cooling orbital ordering at the Mn sub-lattice takes place at T-oo similar to 280 K in both compounds, inducing an orthorhombic to monoclinic phase transition. As the temperature is further decreased an antiferromagnetic CE-type structure occurs in both compounds at T-N similar to 105 K. The comparison with other (Ln(0.50)Ca(0.50))MnO3 compounds reveals that at room temperature the average Jahn-Teller distortion increases sharply with the decrease of the ionic radius for lanthanides heavier than Sm. The ordered magnetic moment progressively decreases as the lanthanide ionic radius decreases on account of the decreased values of the Mn-O-Mn bond angles.