Ferroaxial transitions in glaserite-type NaBa(PO) ( = Mg, Mn, Co, and Ni)

Kajita, Yoichi*; Nagai, Takayuki*; Yamagishi, Shigetada*; Kimura, Kenta*; Hagihara, Masato   ; Kimura, Tsuyoshi*

Ferroaxial transitions of NaBa(PO) ( = Mg, Mn, Co, and Ni) with the glaserite structure, featuring rotational distortions of PO tetrahedra, are investigated. Neutron powder diffraction (NPD) measurements on polycrystalline samples were carried out in the temperature range from room temperature to 800 K. Structure analyses on the NPD data revealed that all of the compounds studied here have a ferroaxial structure (space group: )at room temperature and undergo a ferroaxial transition into a non-ferroaxial phase (space group: 1) upon heating. Structural parameters change continuously at the transition temperature, which suggests that the ferroaxial transition of NaBa(PO) systems is of the second order. Furthermore, to examine the structural stability, ab initio phonon calculations were carried out for NaBaMg(PO). The calculated result shows that the rotational phonon mode instability inherent in the non-ferroaxial 1 phase leads to the phases as a ground state, which well explains the experimental result. This study revealed that NaBa (PO) systems with the glaserite structure are a class of ferroaxial compounds showing a second-order pure ferroaxial transition.