Magnetic ordering in a new double ordered Perovskite

An international team of researchers is paving the path to discovering and developing new materials with polar crystal structure: a critical structure for compounds in ferroelectricity, dielectricity, piezoelectricity, nonlinear optical effect and more.

Led by Changqing Jin, professor in the Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences (IOPCAS), scientists fabricated a Pb-based B-site-ordered double perovskite under high pressure and high temperature conditions.  The new material shows polar crystal structure at room temperature and structural transition from non-centrosymmetric to centrosymmetric at 400 Kelvin.

The results were published on Chemistry of Materials 34, 97(2022)

The research team focused on the crystal structure of perovskite, named for a mineral. Perovskites and derivatives present rich physical properties owing to the cross interplay and correlation of lattice, charge, spin, and orbital degrees of freedom.  

“Pb is a main group element with a charge degree of freedom stemming from the possibility of having either the Pb²⁺ or the Pb⁴⁺ electronic configuration. Their ions are called valence-skipping (or negative-U) ions because the Pb³⁺ is prohibited. Pb-based oxides with a stereo-chemically active lone pair of Pb²⁺ cation have been the subject of long-standing interest due to large remnant polarization and high-temperature structural/electronic transitions, as found in Pb(Zr,Ti)O₃,” said paper co-author Yunze Yu, professor in IOPCAS.

“Finding Pb-based perovskite with polar structure via conventional solid-phase reaction is a challenge in material design and synthesis,” Jin said. “With the advancement of high-pressure synthesis technology, many Pb-based perovskites with interesting physical, chemical and mineral properties become possible.”

To build up the desired structure of B-site-ordered perovskites, the researchers applied more than 60,000 times the atmospheric pressure to synthesize a new material composed of oxides of lead, nickel, molybdenum at about 1,273 Kelvin for 30 minutes. The new material, dubbed PNMO, shows polar structure with large distortion induced by the lone pair electron effect of Pb²⁺ with 6s² electronic configuration.

“PNMO shows an unprecedentedly low antiferromagnetic temperature and unusual two-step antiferromagnetic transitions due to multiple competing magnetic super-exchange interactions paths, which is attributed to polar structure with large distortion of NiO₆ and MoO₆,” said first author Jianfa Zhao, a postdoc in IOPCAS.

According to Zhao, this result is key because it could allow researchers to introduce more stereo-chemically active lone pair cations in even more new perovskite type materials by the same strategy, which could have other useful properties.

“This work has guiding significance for designing and searching new functional materials with stereo-chemically active lone pair cations and opens a new direction to search for polar materials,” Zhao said. “Advanced high-pressure synthesis technology is promising to broaden the scope of forefront of condensed matters via building up new structures with unusual properties.”

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