Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb_1.5Nb₂O_6.5

Area of Science:

• Materials Science
• Solid State Chemistry
• Crystallography

Background:

• Lead (Pb)-based materials exhibit lone pair-driven distortions crucial for applications.
• Controlling lone pair behavior in complex pyrochlore structures, unlike simpler perovskites, remains challenging.
• Vacancies on the O' site are suspected to influence lone pair activity in pyrochlores.

Purpose of the Study:

• To investigate the structural, dielectric, and heat capacity behavior of Pb1.5Nb2O6.5 upon cooling.
• To understand the role of lone pair distortions and O' vacancies in pyrochlore properties.
• To elucidate the factors preventing low-temperature phase transitions in cation- and anion-deficient pyrochlores.

Main Methods:

• Experimental analysis of structural, dielectric, and heat capacity properties.
• Crystallographic and geometric analysis of the pyrochlore structure.
• Density functional theory calculations to model electronic structure and bonding.

Main Results:

• Local distortions, described by cristobalite-type cation ordering, are present at all temperatures and persist upon cooling.
• The material remains crystallographically disordered, showing no observable phase transition.
• Pb2+ lone pair activity is linked to proximity to O' vacancies, with disorder preventing long-range correlation.

Conclusions:

• O' vacancy disorder in Pb1.5Nb2O6.5 prohibits long-range correlation of lone pair distortions.
• This disorder prevents a low-temperature phase transition, leading to broad-range dielectric permittivity.
• Understanding lone pair-vacancy interactions is key to designing functional Pb-based pyrochlores.