LONDON A group of researchers claim they have settled a long-standing debate over the source of the unusual electronic properties of silver niobate that potentially has great importance for wireless communications.
The study led by scientists at the National Institute of Standards and Technology (NIST) in the U.S. with support from the Rutherford Appleton Laboratory and Sheffield University in England, and researchers from Argonne, Los Alamos and ISIS in the U.S is said to pave the way to improved electronic components for smaller, higher performance wireless devices.
The researchers suggest the work also serves as an example of understanding how subtle nanoscale features of a material can give rise to major changes in its physical properties.
Silver niobate is a ceramic dielectric used to make capacitors, filters and other basic components of wireless communications equipment and other high-frequency electronic devices.
In the gigahertz range of the radio spectrum , silver niobate-based ceramics are the only materials known that combine a high, temperature-stable dielectric constant with sufficiently low dielectric losses.
However, as the researchers point out, silver niobate's novel dielectric properties are temperature dependent the dielectric constant peaks in a broad range near room temperature in these ceramics, which makes them suitable for practical applications.
Earlier studies were unable to identify the structural basis of the unusual dielectric response because no accompanying changes in the overall crystal structure could be observed.
"The crystal symmetry does not seem to change at those temperatures," said NIST materials scientist Igor Levin, "but that's because people were using standard techniques that tell you the average structure. The important changes happen at the nanoscale and are lost in averages."
Only in recent years, says Levin, have specialized instruments and analytic techniques been available to probe nanoscale structural changes in crystals. Even so, he says, "these subtle deviations from the average are so small that any single measurement gives only partial information on the structure. You need to combine several complementary techniques that look at different angles of the problem."
Working at the Advanced Photon Source at Argonne National Laboratory, the Lujan Neutron Center at Los Alamos National Laboratory and the ISIS Pulsed Neutron and Muon Source at Rutherford Appleton Laboratory, England, the team combined results from several high-resolution probes using X-rays, neutrons and electrons tools that are sensitive to both the local and average crystal structure to understand silver niobate’s dielectric properties.
The results revealed an intricate interplay between the oxygen atoms, arranged in an octahedral pattern that defines the compound’s crystal structure, and the niobium atoms at the centers of the octahedra.
At high temperatures, the niobium atoms are slightly displaced, but their average position remains in the center—so the shift is not seen in averaging measurements. As the compound cools, the oxygen atoms cooperate by moving a little, causing the octahedral structure to rotate slightly.
This movement generates strain which 'locks' the niobium atoms into off-centered positions but not completely. The resulting partial disorder of the niobium atoms gives rise to the dielectric properties.
The researchers say their results may have potential for engineering similar properties in other compounds.