Monoclinic ferroelectric NaNbO3 at room temperature: Crystal structure solved by using super high resolution neutron powder diffraction

Abstract

The crystal structure of NaNbO3 at ambient temperature was solved using time-off-flight neutron data obtained from Super High Resolution Neutron Powder Diffractometer in J-PARC. The solved structure was a novel monoclinic type with the space group Pm and lattice parameters of a=5.50453(1), b=15.52975(3), c=5.56779(1)Å, and β=90.0645(2)°. Bond valence calculation indicates that the solved structure is chemically valid. This solved structure corresponds with the observed ferroelectric behaviors (Ps=11–25μC/cm2, S=0.03–0.04%). The calculated Ps (~21μC/cm2) using the refined atomic parameters for one-quarter unit cell conform well to the observed value. Cooperative NbO6 octahedral tilting accompanied by the phase transition from the parent structure Pcmb to the Pm mainly contributes to the spontaneous polarization.