Above-Room-Temperature LiNbO₃-Type Polar Magnet Stabilized by Chemical and Physical Pressure

Abstract

LiNbO 3 (LN)-type polar magnets are technologically important but require stringent and costly high-pressure synthesis with very limited sample yields. We develop a chemical strategy to reduce the physical synthesis pressure. LN-type polar magnets require 7 GPa to stabilize in the high-pressure Mn 2 FeNbO 6 (MFNO) phase. Here, MFNO was successfully stabilized in the isostructural LN matrix at intermediate physical pressure (below 5 GPa) at gram levels for each run by dilution with LN according to (Li 1−x Mn x )(Fe x/2 Nb 1−x/2 )O 3 (x = 0.18, 0.33, 0.46, 0.57). LN-diluted MFNO demonstrates ferromagnetism above room-temperature (magnetic ordering temperature T C ≈ 516−554 K) and has large estimated spontaneous polarization (P S ≈ 18−63 μC/cm 2 ). Irreversible c-axis near-zero thermal expansion stemming from magnetostriction was observed around the magnetic transition temperature region upon heating at ambient pressure, which irreversibly elongates the distance between the face-sharing (Li/Mn) and (Fe/Nb) octahedral centroids along the c-axis and thus weakens the magnetic interactions. The magnetic ordering temperature drops in the annealed samples. The findings in (Li 1−x Mn x )(Fe x/2 Nb 1−x/2 )O 3 show that polar magnets can be made by chemical pressure together with soft physical pressure and shed light on large-scale and lower cost stabilization of high-pressure phases.

Chemical pressure in functional materials

“…14, applying chemical pressure not only changes the lattice size itself, but can also change the spin, orbital, and charge orderings of the relevant atoms. These changes alter material structures at various levels, including lattice symmetry 53,97 , local structure 8,9,82 , electronic structure 98,99 , and phonon structure 38,46 , and hence influences material properties by virtue of structure-property relationship (Fig. 14).…”

Methodological Approach to the High-Pressure Synthesis of Nonmagnetic Li2B4+B′6+O6 Oxides

“…As enumerated in Figure -(4), several approaches are possible. Chemical methods, such as external (compressively interfacial strain/stress) − ,− and internal (doping with smaller size ions, being equivalent to the formation of a solid solution of the HP phase within adopted (locally) isostructural matrix) − chemical pressure approaches, , and spatially confined nanoscale way (surface energy effect), have been experimentally applied to achieve large-scale metastable HP polymorphs at AP. Here, we take the interception of R 3-LTTO at AP as an example to calculate the proper substrate/lattice orientation and critical size of nanoparticles to guide future experimental work.…”

Metastable γ-Li2TiTeO6: Negative Chemical Pressure Interception and Polymorph Tuning of SHG

“…17−20 (a) Solid-solution strategy is effective to trap metastable phases in the (locally) isostructural matrix and tune the physical properties by modulating the ionic ordering degree, where the lattice mismatch between the host and guest phases evokes volumetric compression/expansion to capture the metastable phases. 8,21,22 (b) Topotactic ion exchange is a chemical approach that replaces cations, anions, or ionic groups with new ones under relatively mild conditions, without breaking the original structure frameworks. This approach is T h i s c o n t e n t i s 24 the only two reported topotactic products from three-dimensional (3D) close-packed precursors.…”

“…There are four commonly used chemical approaches (Figure ) to stabilize metastable polymorphs, including solid-solution (bulk trapping, Figure a), − topotactic reaction (ionic exchange, Figure b), − interfacial strain (epitaxial growth, Figure c), − and nanoparticles (surface energy effect, Figure d). − (a) Solid-solution strategy is effective to trap metastable phases in the (locally) isostructural matrix and tune the physical properties by modulating the ionic ordering degree, where the lattice mismatch between the host and guest phases evokes volumetric compression/expansion to capture the metastable phases. ,, (b) Topotactic ion exchange is a chemical approach that replaces cations, anions, or ionic groups with new ones under relatively mild conditions, without breaking the original structure frameworks. This approach is usually applied for pseudo-two-dimensional (2D) or one-dimensional (1D) materials because of their low energy barriers of ionic migration, except for Ni 0.5 TaO 3 and Li 0.1 Fe 0.45 NbWO 6 , the only two reported topotactic products from three-dimensional (3D) close-packed precursors.…”