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Colloquium
Speaker
D. D. Sarma (Indian Institute of Science, Bangalore)
Date & Time
13 February 2017, 15:00 to 16:00
Venue
Emmy Noether Seminar Room, ICTS Campus, Bangalore
Resources
Abstract

It is well known that lattice parameters of most alloyed solids can be interpolated almost linearly with the composition between those of the two end-members. This, confirmed within a reasonable degree of accuracy by a huge body of structural investigations over many decades using the x-ray diffraction technique, is the celebrated Vegard's Law. In fact, many properties, such as the bandgap of an alloyed semiconductor, are also known to show a similar linear behaviour between the end-members with composition. These observations form the basis of the vast literature aimed at tuning of properties by making solid solutions. There is a closely associated concept of chemical pressure achieved by the substitution of typically a cation in a solid with another ion of dissimilar size defining a dilute alloy. This technique not only allows to exert a positive pressure by doping a smaller cation akin to the physical pressure, but also makes it feasible to explore of the negative pressure regime inaccessible otherwise with the help of a larger sized dopant. While these concepts are universally accepted and much used in designing new compounds with tailor-made properties, a closer, microscopic inspection reveals many surprises not fully appreciated in the past. Based on some of our recent studies, I shall illustrate the limits of these concepts and provide a rationale for their apparent success in spite of several conceptual difficulties.