Neutron Scattering Study of Charge-Ordering in <i>R</i><sub>1/3</sub>Sr<sub>2/3</sub>FeO<sub>3</sub>(<i>R</i>=La, Pr, Nd, Sm, and Y)
The series of compounds R1/3Sr2/3FeO3 (R = La, Pr, Nd, and Sm) undergo a Verwey transition along with an antiferromagnetic ordering of Fe spins as well as a slight structural distortion below the Verwey transition temperature, TV. The charge disproportionation occurs via 3Fe3.67+=>2Fe3++Fe5+. The driving force of the charge ordering was determined from the magnetic and phonon spectra measured using inelastic neutron scattering. The data indicate a dominant role of the magnetic energy behind the charge ordering of Fe ions in La1/3Sr2/3FeO3and Pr1/3Sr2/3FeO3. Although Nd1/3Sr2/3FeO3 has a larger antiferromagnetic exchange interaction, the charge ordering state can still be driven by the magnetic energy alone. The Fe-Fe Coulomb interaction in this system is discussed by changing R-site ions from La3+ to Nd3+. The effect of the electron-lattice interaction also plays a role to this ordering state.