Simultaneous Control of Ionic and Electronic Conductivity in Materials: Thallium Bromide Case Study

Achieving simultaneous control of ionic and electronic conductivity in materials is one of the great challenges in solid state ionics. Since these properties are intertwined, optimizing one often results in degrading the other. In this Letter, we propose a method to limit ionic current without impacting the electronic properties of a general class of materials, based on codoping with oppositely charged ions. We describe a set of analyses, based on parameter-free quantum mechanical simulations, to assess the efficacy of the approach and determine optimal dopants. For illustration, we discuss the case of thallium bromide, a wide band gap ionic crystal whose promise as a room-temperature radiation detector has been hampered by ionic migration. We find that acceptors and donors bind strongly with the charged vacancies that mediate ionic transport, forming neutral complexes that render them immobile. Analysis of carrier recombination and scattering by the complexes allows the identification of specific dopants that do not degrade electronic transport in the crystal.

Figure 1

Formation energies of Pb (a) and Se (b) defects in TlBr. The lowest energy configuration was tested in association with the oppositely charged vacancy. The slopes of the lines give the charge state of the defects. The vertical dashed lines indicate the calculated positions of the band edges in pristine TlBr, and the vertical solid lines bound the region of the Fermi level in which the formation energies of the vacancies are positive (see Fig. S1 in the Supplemental Material [27]). Stoichiometric conditions were considered.

Figure 2

Supercells of the TlBr crystal with defect complexes consisting of a donor dopant, ${\mathrm{D}}_{\mathrm{Tl}}$ (D=Pb), an acceptor dopant ${\mathrm{A}}_{\mathrm{Br}}$ (A=S, Se, or Te), and vacancies. Vacancies $V_{\mathrm{Br}}$ and $V_{\mathrm{Tl}}$ are indicated by translucent spheres. In configuration $a$, the vacancies are nearest neighbors to both dopants, whereas in configuration $b$ they are nearest neighbors only to the oppositely charged dopant.

Figure 3

Relative carrier scattering strengths of different intrinsic and extrinsic defects and defect complexes in TlBr as given by Eq. (6). The values are normalized to that for the Schottky pair.