Elucidating how asymmetry confers chemical properties
You've heard the expression form follows function? In materials science, function follows form

Date:
July 1, 2020
Source:
Carnegie Institution for Science
Summary:
New research categorizes the causes of structural asymmetry, some
surprising, which underpin useful properties of crystals, including
ferroelectricity, photoluminescence, and photovoltaic ability.



FULL STORY ========================================================================== You've heard the expression form follows function? In materials science, function follows form.


==========================================================================
New research by Carnegie's Olivier Gagne' and collaborator Frank
Hawthorne of the University of Manitoba categorizes the causes of
structural asymmetry, some surprising, which underpin useful properties of crystals, including ferroelectricity, photoluminescence, and photovoltaic ability. Their findings are published this week as a lead article in
the International Union of Crystallography Journal.

"Understanding how different bond arrangements convey various useful
attributes is central to the materials sciences" explained Gagne'. "For
this project, we were particularly interested in what variations in
bond lengths mean for a material's most-exciting characteristics, and in
how to create a framework for their optimization." This was the fifth
and final installment in a series of papers by Gagne' and Hawthorne
examining variability in bond lengths of crystalline structures. This
time around they focused on compounds made up of oxygen and elements
from the category called transition metals.

Picture the periodic table. The transition metals make up its central
block - - forming a bridge linking the taller towers of elements on the
left and right sides.

Like all metals, they can conduct an electrical current. They also have
a tremendous range of chemical and physical properties, including the
emission of visible light, malleability, and magnetism. Many, like gold, platinum, and silver are prized for their value. Others, including iron, nickel, copper, and titanium are crucial for tools and technologies.

The transition metals' ability to form a variety of useful compounds is
owed in large part to the particular three-dimensional configuration
of their electrons. As such, the bonds they form in compounds can be
widely asymmetrical. But Gagne' and Hawthorne wanted to understand
whether other causes for bond-length variation were in play.

"It's a century old problem" Gagne' explained. "The likes of Linus
Pauling and Victor Goldschmidt made this topic one of their prime
research interests; however, the data simply weren't there at the time."
Gagne' and Hawthorne analyzed data on the bond lengths of 63 different transition metal ions bonded to oxygen in 147 configurations from 3,814
crystal structures and developed two new indices for contextualizing asymmetrical bonding.

"These indices allow us to pinpoint the different reasons underlying asymmetrical bonding arrangements, which will hopefully allow us to
harness the properties that they convey when predicting and synthesizing
new materials," Hawthorne explained.

To their surprise, they found that the internal structure of crystals
often spontaneously distorts as a sole function of the connectivity of
its bond network, an effect which they show occurs more frequently than distortion caused by electronic effects or any other factors.

"We suspected some bond-length variation originated from crystal-structure controls, but we didn't expect it to be the primary factor underlying
bond- length variation in inorganic solids," Gagne' explained. "It's
a mechanism that is entirely separate and unaccounted for by current
notions of solid-state chemistry; it that has been overlooked since the
early days of crystallography."

========================================================================== Story Source: Materials provided by
Carnegie_Institution_for_Science. Note: Content may be edited for style
and length.


========================================================================== Journal Reference:
1. Olivier Charles Gagne', Frank Christopher Hawthorne. Bond-length
distributions for ions bonded to oxygen: results for the transition
metals and quantification of the factors underlying bond-length
variation in inorganic solids. IUCrJ, 2020; 7 (4): 581 DOI:
10.1107/ S2052252520005928 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200701125457.htm

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