Quaternary Rare-Earth Oxyselenides RE 4 Ga 2 Se 7 O 2 ( RE = Pr, Nd) with Trigonal Bipyramidal GaSe 5 Units: Evaluation of Optical, Thermoelectric, and Electrocatalytic Properties

Mishra, Vidyanshu; Abdullahi, Ibrahim Munkaila; Mumbaraddi, Dundappa; Jomaa, Mohammed; Guérin, Louis; Nath, Manashi; Mar, Arthur

doi:10.1021/acs.chemmater.3c02842

Structural distortions such as cation off-centering are frustrated in the pyrochlore structure due to the triangular arrangement of cations on the pyrochlore lattice. This geometric constraint inhibits a transition from a paraelectric to ferroelectric phase in majority of pyrochlore oxide materials. Few pyrochlore materials can overcome this frustration and exhibit polar crystal structures, and unraveling the origin of such leads to the understanding of polarity in complex materials. Herein we hypothesize that frustration on the pyrochlore lattice can be relieved through A -site doping with rare earth cations that do not possess stereochemically active lone pairs. To assess if frustration is relieved, we have analyzed cation off-centering in various Bi 2−x RE xTi 2 O 7 ( RE = Y 3+ , Ho 3+ ) pyrochlores through neutron and X-ray total scattering. Motivated by known distortions from the pyrochlore literature, we present our findings that most samples show local distortions similar to the β-cristobalite structure. We additionally comment on the complexity of factors that play a role in the structural behavior, including cation size, bond valence, electronic structure, and magnetoelectronic interactions. We posit that the addition of magnetic cations on the pyrochlore lattice may play a role in an extension of the real-space correlation length of electric dipoles in the Bi-Ho series, and offer considerations for driving long-range polarity on the pyrochlore lattice.

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