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

Abstract

Ferroaxial order, which is characterized by a vortex of electric dipole moments, is attracting more attention in terms of a new category of ferroic order. Here we propose that the disodium hafnium borate, Na2Hf(BO3)2, shows ferroaxial order that is stabilized by the second-order Jahn–Teller (SOJT) effect. Furthermore, the dipotassium hafnium borate, K2Hf(BO3)2, the synthesis of which has never been reported previously, is predicted to have a nonferroaxial ground state from the perspective of the SOJT effect. This prediction is demonstrated through our structural refinements on polycrystalline samples and our first-principles calculations with representation theory. This study offers a chemical perspective for developing new ferroaxial materials and controlling their ferroaxial order.

Breaking the Inherent Interarrangement of [B3O6] Clusters for Nonlinear Optics with Orbital Hybridization Enhancement

“…Borate composed of π- and/or non-π-conjugated boron oxyanions offers wide tunability of its optical and electrical properties. − Modifying the properties of borates begins with the initial design of their anionic framework and cationic component. However, these component-level designs are only part of the story, as the way these anions and cations are arranged in the lattice determines the key properties of borates, with this heavily influenced by microelectronic arrangement in the lattice. − For short-wave ultraviolet and deep ultraviolet borates, the main optical-active component is their anionic frameworks, wherein the π-conjugated [BO 3 ] units with large hyperpolarizability, polarizability anisotropy, and HOMO–LUMO gap stimulated by the delocalized π electrons on all adjacent arranged p orbitals are the star primitives to design optical crystals. − Of course, when [BO 3 ] units further form the polymerized B–O groups by sharing the O atoms, such as dimeric [B 2 O 5 ], ring-type [B 3 O 6 ], and one-dimensional (1D) 1 [BO 2 ] ∞ infinite chains, an increase in optical anisotropy compared with the original isolated [BO 3 ] units can be expected, − affecting the birefringence and phase-matching wavelength of corresponding crystals. Hence, the design space for borate chemistry, when considering the atomic-scale construction and rearrangement of these anionic frameworks, in which the different π - conjugated building blocks will be the key, is, in a word, great.…”