High-Pressure Synthesis, Crystal Structure, and Phase Stability Relations of a LiNbO₃‑Type Polar Titanate ZnTiO₃ and Its Reinforced Polarity by the Second-Order Jahn–Teller Effect

A polar LiNbO₃-type (LN-type) titanate ZnTiO₃ has been successfully synthesized using ilmenite-type (IL-type) ZnTiO₃ under high pressure and high temperature. The first principles calculation indicates that LN-type ZnTiO₃ is a metastable phase obtained by the transformation in the decompression process from the perovskite-type phase, which is stable at high pressure and high temperature. The Rietveld structural refinement using synchrotron powder X-ray diffraction data reveals that LN-type ZnTiO₃ crystallizes into a hexagonal structure with a polar space group <i>R</i>3<i>c</i> and exhibits greater intradistortion of the TiO₆ octahedron in LN-type ZnTiO₃ than that of the SnO₆ octahedron in LN-type ZnSnO₃. The estimated spontaneous polarization (75 μC/cm², 88 μC/cm²) using the nominal charge and the Born effective charge (BEC) derived from density functional perturbation theory, respectively, are greater than those of ZnSnO₃ (59 μC/cm², 65 μC/cm²), which is strongly attributed to the great displacement of Ti from the centrosymmetric position along the <i>c</i>-axis and the fact that the BEC of Ti (+6.1) is greater than that of Sn (+4.1). Furthermore, the spontaneous polarization of LN-type ZnTiO₃ is greater than that of LiNbO₃ (62 μC/cm², 76 μC/cm²), indicating that LN-type ZnTiO₃, like LiNbO₃, is a candidate ferroelectric material with high performance. The second harmonic generation (SHG) response of LN-type ZnTiO₃ is 24 times greater than that of LN-type ZnSnO₃. The findings indicate that the intraoctahedral distortion, spontaneous polarization, and the accompanying SHG response are caused by the stabilization of the polar LiNbO₃-type structure and reinforced by the second-order Jahn–Teller effect attributable to the orbital interaction between oxygen ions and d⁰ ions such as Ti⁴⁺

High-Pressure Synthesis of <i>A</i>‑Site Ordered Double Perovskite CaMnTi₂O₆ and Ferroelectricity Driven by Coupling of <i>A</i>‑Site Ordering and the Second-Order Jahn–Teller Effect

We successfully synthesized a novel ferroelectric <i>A</i>-site-ordered double perovskite CaMnTi₂O₆ under high-pressure and investigated its structure, ferroelectric, magnetic and dielectric properties, and high-temperature phase transition behavior. Optical second harmonic generation signal, by frequency doubling 1064 nm radiation to 532 nm, was observed and its efficiency is about 9 times as much as that of SiO₂ (α-quartz). This compound possesses a tetragonal polar structure with space group <i>P</i>4₂<i>mc</i>. <i>P</i>-<i>E</i> hysteresis measurement demonstrated that CaMnTi₂O₆ is also ferroelectric. A spontaneous polarization calculated by use of point charge model and the observed remnant polarization are 24 and 3.5 μC/cm², respectively. CaMnTi₂O₆ undergoes a ferroelectric–paraelectric order–disorder-type phase transition at 630 K. The structural analysis implies that both the ordering of shift of Mn²⁺ from the square-planar and the off-center displacement of Ti⁴⁺ in TiO₆ octahedra are responsible for ferroelectricity. CaMnTi₂O₆ belongs to a new class of ferroelectrics in which <i>A</i>-site ordering and second-order Jahn–Teller distortion are cooperatively coupled. The finding gave us a new concept for the design of ferroelectric materials

New Mechanism for Ferroelectricity in the Perovskite Ca_2–<i>x</i>Mn_<i>x</i>Ti₂O₆ Synthesized by Spark Plasma Sintering

Perovskite oxides hosting ferroelectricity are particularly important materials for modern technologies. The ferroelectric transition in the well-known oxides BaTiO₃ and PbTiO₃ is realized by softening of a vibration mode in the cubic perovskite structure. For most perovskite oxides, octahedral-site tilting systems are developed to accommodate the bonding mismatch due to a geometric tolerance factor <i>t</i> = (A–O)/[√2­(B–O)] < 1. In the absence of cations having lone-pair electrons, e.g., Bi³⁺ and Pb²⁺, all simple and complex A-site and B-site ordered perovskite oxides with a <i>t</i> < 1 show a variety of tilting systems, and none of them become ferroelectric. The ferroelectric CaMnTi₂O₆ oxide is, up to now, the only one that breaks this rule. It exhibits a columnar A-site ordering with a pronounced octahedral-site tilting and yet becomes ferroelectric at <i>T</i>_c ≈ 650 K. Most importantly, the ferroelectricity at <i>T</i> < <i>T</i>_c is caused by an order–disorder transition instead of a displacive transition; this character may be useful to overcome the critical thickness problem experienced in all proper ferroelectrics. Application of this new ferroelectric material can greatly simplify the structure of microelectronic devices. However, CaMnTi₂O₆ is a high-pressure phase obtained at 7 GPa and 1200 °C, which limits its application. Here we report a new method to synthesize a gram-level sample of ferroelectric Ca_2–<i>x</i>Mn_<i>x</i>Ti₂O₆, having the same crystal structure as CaMnTi₂O₆ and a similarly high Curie temperature. The new finding paves the way for the mass production of this important ferroelectric oxide. We have used neutron powder diffraction to identify the origin of the peculiar ferroelectric transition in this double perovskite and to reveal the interplay between magnetic ordering and the ferroelectric displacement at low temperatures

A‑Site and B‑Site Charge Orderings in an <i>s–d</i> Level Controlled Perovskite Oxide PbCoO₃

Perovskite PbCoO₃ synthesized at 12 GPa was found to have an unusual charge distribution of Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ with charge orderings in both the A and B sites of perovskite ABO₃. Comprehensive studies using density functional theory (DFT) calculation, electron diffraction (ED), synchrotron X-ray diffraction (SXRD), neutron powder diffraction (NPD), hard X-ray photoemission spectroscopy (HAXPES), soft X-ray absorption spectroscopy (XAS), and measurements of specific heat as well as magnetic and electrical properties provide evidence of lead ion and cobalt ion charge ordering leading to Pb²⁺Pb⁴⁺₃Co²⁺₂Co³⁺₂O₁₂ quadruple perovskite structure. It is shown that the average valence distribution of Pb^3.5+Co^2.5+O₃ between Pb³⁺Cr³⁺O₃ and Pb⁴⁺Ni²⁺O₃ can be stabilized by tuning the energy levels of Pb 6<i>s</i> and transition metal 3<i>d</i> orbitals