Chemical Aspect of Displacive-Type Ferroaxial Phase Transition from Perspective of Second-Order Jahn–Teller Effect: NASICON Systems as an Example

Abstract

Ferroaxial order, characterized by a rotational arrangement of electric dipoles, attracts increasing attention in terms of a new family of ferroic orders. However, there has been no chemical guideline for exploring crystalline materials showing ferroaxial order, namely ferroaxial materials. Here, we present a chemical guideline grounded in staggered polyhedral connectivity, which we propose as a structural prerequisite for ferroaxial order, and the second-order Jahn–Teller (SOJT) theory extended from molecular orbitals to electronic band structures. Na-superionic conductors (NASICON) including NaM 2(PO4)3 (M = early-transition or post-transition metal) are identified as potential ferroaxial materials because of their staggered structures composed of MO6 octahedra and PO4 tetrahedra. However, ferroaxial phase transitions hardly occur in some of the NASICON systems, which offers a platform to uncover a hidden factor playing an important role in driving this system into ferroaxial states. Our first-principles calculations demonstrate that a ferroaxial phase transition in NASICON systems occurs only when SOJT interaction is symmetrically allowed, that is, energy-lowering chemical bonds are formed as a consequence of the distortion. Our proposals would be not limited to NASICON systems but applicable to a variety of compounds and provide new insight into the exploration of displacive-type ferroaxial materials.

Chemical Switching of Ferroaxial and Nonferroaxial Structures Based on Second-Order Jahn–Teller Activity in (Na,K)2Hf(BO3)2

“…The SOJT activity is evaluated using the symmetry selection rule Φ 0 ⊗Φ p ⊗Φ n , where Φ 0 , Φ p , and Φ n are the irreps of the ground states, the concerned distortion mode, and the excited states, respectively. , When the direct product result contains the totally symmetric representation, the concerned interaction of their electronic states is allowed to be SOJT-active. Furthermore, the interaction between the upper-lying ground states and low-lying excited states (e.g., VBM and CBM for an insulator) strongly contributes to the SOJT activity because it is inversely proportional to the energy difference E 0 – E n ( E 0 and E n are eigen energies of the ground state and the excited state, respectively). , Using the irreps of the VBM, CBM, and ferroaxial mode, the direct product is calculated as Γ 2 + ⊗Γ 2 + ⊗Γ 1 + = Γ 1 + , including the totally symmetric representation for space group R m . This means that the electronic state of Na 2 Hf­(BO 3 ) 2 is SOJT-active under the Γ 2 + ferroaxial distortion at least from the viewpoint of orbital symmetry.…”

“…The SOJT theory based on molecular orbitals has successfully explained the mechanism of structural distortions with wavevector q = 0 such as molecular deformation and polar displacement of perovskite structures. − Meanwhile, it has also been applied to the origin of structural distortions with a non-zero q such as a rotational displacement leading to a superlattice structure, − being recognized as the general source driving structural distortions with various q . Indeed, it has been demonstrated that the chemical origin of the displacive-type ferroaxial phase transition in Na superionic conductor (NASICON) systems is described by the SOJT theory extended to electronic band structure . These results suggest to us that switching between ferroaxial and nonferroaxial structures can be tuned by SOJT activity.…”

“…Rotational distortions in crystalline materials with staggered structures induce ferroaxial order, which is ascribed to ferri-like arranged electric toroidal moments. Furthermore, it has also been suggested that the chemical origin of displacive-type ferroaxial phase transition is the second-order Jahn–Teller (SOJT) effect . The SOJT theory based on molecular orbitals has successfully explained the mechanism of structural distortions with wavevector q = 0 such as molecular deformation and polar displacement of perovskite structures. − Meanwhile, it has also been applied to the origin of structural distortions with a non-zero q such as a rotational displacement leading to a superlattice structure, − being recognized as the general source driving structural distortions with various q .…”

“…More recently, it has been proposed that “staggered structure”, with the nonequivalent coordination polyhedra connected alternately, is essential to the emergence of ferroaxial order . Rotational distortions in crystalline materials with staggered structures induce ferroaxial order, which is ascribed to ferri-like arranged electric toroidal moments.…”

A zero-strain Na-deficient NASICON-type Na2.8Mn0.4V1.0Ti0.6(PO4)3 cathode for wide-temperature rechargeable Na-ion batteries

“…38,39 Nevertheless, the structural distortion caused by the Jahn–Teller effect hinders the diffusion of Na + ions leading to poor rate capability. 40–43 Besides, the Mn 3+ ions generated in the reaction process could transform into Mn 4+ and Mn 2+ ions under disproportionation, and the dissolved Mn 2+ in the electrolyte destroys the crystal structure with the degradation of the cathode materials, and could be deposited on the anode and catalyze the continuous growth of a solid electrolyte interphase (SEI) film, giving rise to the deterioration of full cell performance. 44–46 To make better use of the multi-electron reaction superiority of Mn ions, researchers have developed a series of Na-rich cathodes to activate the Na + ions attached to the crystal lattice to achieve high working voltage and specific capacity, such as the well-designed Na 3+ x V 2− x Mn x (PO 4 ) 3 , 18,47 Na 3+ x + y V 2− x − y Fe x Mn y (PO 4 ) 3 , 32,48 Na 3 MnTi(PO 4 ) 3 , 49,50 etc.…”