2 research outputs found
Synthesis and Structure of Three New Oxychalcogenides: A<sub>2</sub>O<sub>2</sub>Bi<sub>2</sub>Se<sub>3</sub> (A = Sr, Ba) and Sr<sub>2</sub>O<sub>2</sub>Sb<sub>2</sub>Se<sub>3</sub>
Three new compounds have been added
to the alkali chalcogenide
and oxychalcogenide families: Sr<sub>2</sub>O<sub>2</sub>Bi<sub>2</sub>Se<sub>3</sub>, Ba<sub>2</sub>O<sub>2</sub>Bi<sub>2</sub>Se<sub>3</sub>, and Sr<sub>2</sub>O<sub>2</sub>Sb<sub>2</sub>Se<sub>3</sub> were
synthesized by direct combination of SrO or BaO with Bi<sub>2</sub>Se<sub>3</sub> or Sb<sub>2</sub>Se<sub>3</sub>. The structure, determined
from laboratory X-ray powder diffraction data, consists of double
chains of edge-sharing BiSe<sub>4</sub>O square pyramids. Temperature-dependent
resistance data reveal all three compounds to be insulators, while
heat capacity data of Ba<sub>2</sub>O<sub>2</sub>Bi<sub>2</sub>Se<sub>3</sub> and Sr<sub>2</sub>O<sub>2</sub>Bi<sub>2</sub>Se<sub>3</sub>, in conjunction with the literature reports, reveal low energy phonon
modes due to bismuth lone pair effects. We propose a specific materials
design principle connecting electron count to structural dimensionality
by comparison to related chalcogenides, including the BiS<sub>2</sub> superconductors
Vacancy-Driven Disorder and Elevated Dielectric Response in the Pyrochlore Pb<sub>1.5</sub>Nb<sub>2</sub>O<sub>6.5</sub>
Lone pair-driven distortions are a hallmark of many technologically
important lead (Pb)-based materials. The role of Pb2+ in
polar perovskites is well understood and easily manipulated for applications
in piezo- and ferroelectricity, but the control of ordered lone pair
behavior in Pb-based pyrochlores is less clear. Crystallographically
and geometrically more complex than the perovskite structure, the
pyrochlore structure is prone to geometric frustration of local dipoles
due to a triangular arrangement of cations on a diamond lattice. The
role of vacancies on the O′ site of the pyrochlore network
has been implicated as an important driver for the expression and
correlation of stereochemically active lone pairs in pyrochlores such
as Pb2Ru2O6.5 and Pb2Sn2O6. In this work we report on the structural, dielectric,
and heat capacity behavior of the cation- and anion-deficient pyrochlore
Pb1.5Nb2O6.5 upon cooling. We find
that local distortions are present at all temperatures that can be
described by cristobalite-type cation ordering, and this ordering
persists to longer length scales upon cooling. From a crystallographic
perspective, the material remains disordered and does not undergo
an observable phase transition. In combination with density function
calculations, we propose that the stereochemical activity of the Pb2+ lone pairs is driven by proximity to O′ vacancies,
and the crystallographic site disorder of the O′ vacancies
prohibits long range correlation of lone pair-driven distortions.
This in turn prevents a low-temperature phase transition and results
in an elevated dielectric permittivity across a broad temperature
range