Neutron powder diffraction investigation of the structural and magnetic properties of (La1-yYy)FeAsO(y=0.10,0.20,and 0.30)

The structural, magnetic and resistive properties of (La1-yYy)FeAsO compounds (y = 0.10, 0.20, 0.30) have been investigated by means of X-ray and neutron powder diffraction as well as by resistivity measurements. The temperatures at which the tetragonal to orthorhombic transition and magnetic ordering take place progressively reduce by similar amounts with increasing Y substitution on account of the progressive chemical pressure increase. We propose that the structural transition could be originated by a cooperative Jahn-Teller distortion involving the alignment of the fully occupied Fe orbitals in the Fe plane along the y axis, leading to the branching of the cell parameters a and b. The magnetic structure develops after the occurrence of the structural transition, but before its completion.