Impact Objectives• Establish basic theory for mixed anion compounds• Create new functionalities, discovering and developing the roots ofchemical and physical functions that will meet the needs of futuresociety• Create new materials and develop material design concepts andmethodologies that will lead to practical applications for the future

Advancing mixed anion
compounds
An ambitious collaborative effort between 40 research groups across Japan is focused on understanding
the complicated properties of mixed anion compounds and applying their data to new and refinedtechnological applications
What is yourbackground andhow has that ledto your role in thisproject?
I have a chemistry background, but in theearly stages of my career, I was interested
in the physical properties of transition
metal oxides such as high temperature
superconductivity and quantum phenomenain frustrated magnets. After that, I came tobelieve that what I really wanted to do was
to explore new materials and I switched
my research focus from condensed matterphysics to solid state chemistry. Using avariety of techniques from conventional
solid-state reactions to topochemical
low-temperature reactions as well as
high-pressure reactions, my group inKyoto prepares various types of inorganic
materials, some of which are mixed anion
compounds. Luckily for me, several of themixed anion compounds exhibit exotic
chemical and physical properties such as
low-temperature hydride (H-) migration intitanium oxyhydrides and selective heavy
metal capturing in titanium phosphidetelluride. With these discoveries and thoseof other researchers, I became convincedthat mixed anion compounds can be
emerging materials with potential nextgeneration applications.

Can you give us a little background on thisproject and explain why we need more
research in this area?
We have known about mixed anion
compounds for many years and they areeven found as minerals. Little attention
has been paid to them in the last century.
However, several landmark discoveries
relating to them have been reported since
2000. These discoveries prompted us toconsider that mixed anion compounds
should be given more attention in a wide
variety of research areas, ranging fromchemistry to physics. Another reason isthat the guiding principles for the studyof mixed anions in terms of synthesis,analysis and function are still undiscovered
and most of our knowledge of synthesis,
analysis and function has been developed
for oxide materials. In many cases, however,these cannot be applied to mixed anioncompounds.
What are the key objectives of this project?
In this new area of research, we will
create new materials and establish thebasic theory of mixed anion compounds,
developing material design concepts andmethodologies that will lead to practicalapplications in the future, 10 to 30 years
from now. In addition, we will create newfunctionality, discovering and developing
the roots of chemical and physical functions

that will meet the needs of society in thenext 10 to 30 years. At present, we know
only a little about mixed anion systems.
There is much more to explore. Three years
from now, in year five of the project, I hopethat we will have established material designconcepts and methodologies as well as newfunctions.
Have you had any particularly interestingor surprising findings in your investigations
so far?
The introduction of H- ions in barium titanate(BaTiO3), the most well-known functionaloxide, induces catalytic activity in terms ofammonia synthesis. Titanium has been
regarded as a ‘dead’ element for ammonia
synthesis since titanium nitride (Ti-N)
bonding is too strong. It means that the Hanion in oxides (oxyhydride) has the ability
to break the triple bond in the N2 molecule,the most difficult part of the process in NH3
synthesis. In other words, the introductionof the H- in oxides can break the so-called‘scaling rule’. Another important finding isthe discovery of H- size flexibility. Our x-ray
diffraction measurement under high pressure
revealed that the H- anion is at least twice
as flexible as the oxide anion (O2-). Thus,
the H- anion can be compared to a ‘balloon’
while the other anions are the ‘footballs’. We
found that strontium vanadium oxide-hydride(SrVO2H) exhibits novel insulator-to-metaltransition under high pressure.
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New and sustainable
technologies
Sustainable technologies capable of changing the future of electronic devices, batteries and displays can
now be created in the laboratory, thanks to the vast variety of mixed anion compounds available

Japan is a country rich in beauty but poorin natural resources. Unlike countries suchas Venezuela, with an estimated £10.82
trillion worth of natural gas, petroleum andiron, Japan has virtually no natural energy
resources and consequently is the world’s
largest importer of coal and liquefied naturalgas. This has a huge impact on Japan’s
worldwide industrial competitiveness.
As a result, Japanese researchers areworking to develop new, high-performancematerials that can be applied to futuretechnologies. Professor Hiroshi Kageyama,a head researcher at the Graduate Schoolof Engineering at Kyoto University, is theproject coordinator for a collaboration of40 leading Japanese research groups thatseeks to investigate and develop mixedanion compounds for new functionalities.
Currently, inorganic compounds suchas oxides and nitrides, typically binary
compounds containing oxygen or nitrides,
are base materials that support a wide
variety of industry appliances, including
electronic devices, batteries, displays andmore. Although highly functional, singleoxide and single-nitride compounds arelimited in their structural and chemicalstates because the building blocks (orcoordination geometries around a metal) in
these compounds are limited.
The group partners are working to expand
Japanese industrial development by
employing mixed anion compounds orcompounds with multiple anions (negativelycharged ions) that expand beyond
single-oxide or single-nitride compounds.
Currently, scientists do not fully understandthe guiding principles for the study of mixed
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anions and much of the scientific knowledge
on their synthesis, analysis and function
comes from single-oxide materials. In a fiveyear-long study, the group aims to create,
observe and explore the functionalities ofmixed anion compounds in order to expand
this knowledge base. With a stronger
foundation for the make-up and propertiesof mixed anion compounds, the team hopes
to develop new and advanced materials thatcan be used in practical applications in thefuture.
BEYOND SINGLE ANION COMPOUNDS
Until the year 2000, many researchers were
not paying much attention to mixed anion
compounds. This is due to the fact that theEarth’s atmosphere is far more conducive
to simpler single-anion compounds such as
metal oxides. For example, when elemental
iron is heated in the air, iron oxide – or rust– is quickly created. Complex oxides can also
be obtained by heating a mixture of binaryoxides to high temperatures in a processknown as a high-temperature solid-state
reaction. The simplicity of this method has
allowed a wide range of physicists to workwith metal oxides and create a variety of
new oxide structures and compositions.
‘On the other hand,’ Kageyama explains,‘mixed anion compounds cannot generally
be prepared in such a facile method asair.’ Inevitably, this makes controlling andcreating mixed anion compounds far moredifficult. However, the benefits that comefrom working with mixed anion compounds
far outweigh the struggle of creating them.
Unlike oxides and nitrides that only containone type of ‘building block’, that is a singularanion, mixed anion compounds containa diverse range of anions with different
key features, such as valency, ionic radius,
electronegativity and polarizability. Like astructure with different toy construction
bricks, this variability allows for a wider
range of compounds with distinct structuresand dimensions.
In order to properly work with mixed anion
compounds, Kageyama and his team havedivided their study into three subgroups:
group 1 for the creation of mixed anion
compounds; group 2 for the observation ofmixed anions and group 3 for the explorationof innovative functionalities. Group 1, led
by Dr Hiraku Ogino from the National
Institute of Advanced Industrial Science andTechnology, is focused on establishing andsynthesising mixed anion compounds using
techniques such as high-pressure synthesis,
topochemical reaction and solvothermalmethods. Additionally, this group isalso working to define the rules of anion
arrangement through both experimentaland theoretical analysis. Group 2, led
by Professor Katsuro Hayashi of Kyushu
University, is focused primarily on assessing
light elements such as hydrogen as a means
to deepen their understanding of mixed
anion compounds. By combining x-rays
and neutron diffraction methods, the teamcan evaluate the chemical state of hydrogen
within the mixed anion compounds and, indoing so, paint a picture of the geometry
within these complex compounds. Group 3,
led by Professor Kazuhiko Maeda from theTokyo Institute of Technology, is the final
step in applying mixed anion compounds inthe real world. The team’s primary objective
is to generate high-performance energy

Since my project launched, many researchers, including organic chemists
and company workers in Japan, have come to understand what a ‘mixed
anion compound’ ismaterials that can both produce and saveenergy. With these new developments, group
3 hopes to establish usable technologies
that are far more diverse than single anion
compounds and can sustain our futuresociety.
DISCOVERING MIXED ANIONPROPERTIES
A recent paper by Kuriki et al., publishedin the Journal of the American ChemicalSociety (volume 140, page 6648-6655,2018) showcases one successful example
of extensive collaboration between 40
research groups. This work, involving thefive PIs of this project (Associate ProfessorKazuhiko Maeda, Dr Kengo Oka, AssistantProfessor Kenta Hongo, Dr Ryo Maezono
and Kageyama), describes the discoveryof an oxyfluoride semiconductor known
as pyrochlore (Pb2Ti2O5.4F1.2), that has anunprecedented small band gap of around
2.4 eV and the ability to act as a stablesemiconductor photocatalyst, opening up
new opportunities within materials researchfor heterogeneous photocatalysis with visible
light. In February 2018, Kageyama, Hayashi
and Maeda co-authored a review in Nature
Communications (https://www.nature.com/articles/s41467-018-02838-4) outlining theopportunities that have been enabled byrecent advances in mixed anion research.
He found that the recent increase in interest
in solid-based mixed anion compounds canvery likely lead to significant contributions
to the fields of energy conversion, electronic
devices and catalysis.
These findings have come with their own
set of challenges. ‘One challenge,’ describes
Kageyama, ‘is how to tune anion order/disorder and local coordination. In somecases, multiple anions are completelyordered. In other cases, they are complexlydisorganised.’ To address this, groups 1, 2and 3 are working to develop new methodsthat can tackle this problem and deal with‘correlated disorder’ within mixed anion
compounds. Additionally, the synthesis ofmixed anion compounds is a complicatedaffair. Thus, the team is focusing on using
multiple tools and multistep processes
that can provide a more diverse range ofplatforms on which to manipulate thesecompounds.
MAKING HEADWAY
The intriguing qualities of mixed anion
compounds can be leveraged for a wide
variety of applications. However, there is stillmuch to discover about the principles andapplications of these chemical structures.
‘Most of the researchers are either notfamiliar with mixed anion materials or they
are not aware of mixed anion compounds,’
Kageyama states. ‘Since my project
launched, many researchers, includingorganic chemists and company workers
in Japan, have come to understand what a‘mixed anion compound’ is.’This is key in future technologicaldevelopments that are both efficient andsustainable. Specifically, in the fields
of chemistry and materials science,
understanding mixed anion compounds
can mean establishing new design concepts
and methodologies. This important study
harnesses the expertise and collaborative
efforts of over 100 researchers both in Japanand abroad and may prove to be the big
step forward in uncovering the complicated,
yet compelling properties of mixed anion
compounds and applying them for futureuse. ●Project Insights
FUNDING
The project was support by funding fromthe Japan Society for the Promotionof Science (JSPS) Grant-in-Aid for
Scientific Research on Innovative Areas
programme, grant numbers JP16H6438,
JP16H6439, JP16H6440 and JP16H6441.
CONTACT
Professor Hiroshi Kageyama
Project Coordinator
T: +81 75383 2506
E: kage@scl.kyoto-u.ac.jp
W: https://www.mixed-anion.jp (theproject website), http://www.ehcc.kyoto-u.
ac.jp/eh10 (the Kageyama lab in Kyoto)
BIOS
Professor Hiroshi Kageyama graduatedfrom Kyoto University in Japan with a BSin chemistry in 1993 and followed this up
with an MS in chemistry in 1995 and PhDin 1998, both also from Kyoto University.
He has been awarded a number ofprestigious awards and prizes, including
the CSJ Award for Creative Work in 2017
and the Japan Society for the Promotionof Science (JSPS) prize in 2014. Kageyama
currently serves as Professor at theGraduate School of Engineering, Kyoto
University, as well as Adjunct Professor at
the Institute for Integrated Cell-Material
Sciences at Kyoto University.

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