Mixed valence states in lead perovskites

May 9, 2017, Tokyo Institute of Technology
Crystal structure of Pb2+Pb4+3Co2+2Co3+2O12 where Pb and Co have charge orderings despite the simple PbCoO3 chemical composition and the valence distribution changes for PbMO3 (M: 3d transition metal). Credit: Journal of the American Chemical Society

Scientists at Tokyo Institute of Technology, the Kanagawa Academy of Science and Technology have reported an unusual charge distribution of Pb2+Pb4+3Co2+2Co3+2O12 for a perovskite PbCoO3 synthesized at 12 GPa, with charge orderings in the A and B sites of an ABO3 perovskite. This strategy can possibly lead to the production of next-generation materials with fascinating properties such as superconductivity, colossal magnetoresistance, and high thermopower.

Transition metals (TMs) exhibit charge degree of freedom, resulting in interesting properties, such as charge ordering related to metal-insulator transitions, high-temperature superconductivity, , and high thermopower. Metal ions with half-integer valence tend to split into two spatially ordered integer valence ions. To realize a half-integer valence state and charge ordering in the B site of a perovskite ABO3, two or more elements with different valences need to be mixed in the A .

A group of researchers, Prof. Masaki Azuma from Tokyo Institute of Technology, and Dr.Yuki Sakai at the Kanagawa Academy of Science and Technology and colleagues, have reported an unusual half-integer average charge system Pb3.5+M2.5+O3 with charge ordering in the A and B sites of a perovskite PbCoO3. Furthermore, the charge orderings in these sites were stabilized by tuning the energy levels of the Pb 6s and TM 3d orbitals. Bond valence sum calculations revealed a valence distribution of Pb2+Pb4+3Co2+2Co3+2O12, with Pb and Co exhibiting charge ordering despite the chemical composition of PbCoO3. As expected, the average oxidation state was Pb3.5+Co2.5+O3, with half-integer valences in both A and B sites of the perovskite structure stabilized by the balanced Pb 6s and Co 3d levels. The valence distribution of PbMO3 was controlled by tuning the depth of the d level of M. The complex valence distribution is expected to change on perturbations, e.g., pressure and chemical modification. For instance, when the Co charge ordering is melted, Pb2+0.25Pb4+0.75Co2.5+O3 is formed, Pb2+0.5Pb4+0.5Co3+O3 is first formed by the intermetallic charge transfer between Pb and Co and then possibly Pb2+Co4+O3 under pressure.

In the future, the application of the strategy of realizing mixed valence states in the A and B sites of compounds via the tuning of the energy difference between Pb 6s and transition metal 3d orbitals will be reported for other systems with valence-skipping elements, e.g., Au, Tl, and Sb.

Explore further: Revealing the ligand-hole localization behaviors in oxides with unusual high-valence Fe

More information: Yuki Sakai et al, A-Site and B-Site Charge Orderings in anLevel Controlled Perovskite Oxide PbCoO, Journal of the American Chemical Society (2017). DOI: 10.1021/jacs.7b01851

Related Stories

Solar cells with nanostripes

May 2, 2017

Solar cells based on perovskites reach high efficiencies: They convert more than 20 percent of the incident light directly into usable power. On their search for underlying physical mechanisms, researchers of the Karlsruhe ...

An unlikely route to ferroelectricity

May 18, 2012

(Phys.org) -- Ferroelectricity, which was first observed in the 1940s, is an interesting phenomenon involving the spontaneous (non-induced) formation of charge polarization (separation of charge) in certain materials. This ...

Recommended for you

Can China keep it's climate promises?

March 26, 2019

China can easily meet its Paris climate pledge to peak its greenhouse gas emissions by 2030, but sourcing 20 percent of its energy needs from renewables and nuclear power by that date may be considerably harder, researchers ...

What happened before the Big Bang?

March 26, 2019

A team of scientists has proposed a powerful new test for inflation, the theory that the universe dramatically expanded in size in a fleeting fraction of a second right after the Big Bang. Their goal is to give insight into ...

Cellular microRNA detection with miRacles

March 26, 2019

MicroRNAs (miRNAs) are short noncoding regulatory RNAs that can repress gene expression post-transcriptionally and are therefore increasingly used as biomarkers of disease. Detecting miRNAs can be arduous and expensive as ...

In the Tree of Life, youth has its advantages

March 26, 2019

It's a question that has captivated naturalists for centuries: Why have some groups of organisms enjoyed incredibly diversity—like fish, birds, insects—while others have contained only a few species—like humans.


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.