Fermi-Level Instability as a Way to Tailor the Properties of La₃Te₄

Abstract

Traditionally, the formation of off-stoichiometric compounds is believed to be the growth effect rather than the intrinsic tendency of the system. However, here, using the example of La 3 Te 4 , we demonstrate that in n-type gapped metals having a large internal gap between principal band edges and the Fermi level inside of the principal conduction band, Fermi-level instability can develop, resulting in a decrease in the formation energy for acceptor defects. Specifically, La vacancies in La 3 Te 4 form spontaneously to produce the acceptor states and remove a fraction of free carriers from the principal conduction band via electron−hole recombination. Such a unique self-doping mechanism allows stabilization of a range of off-stoichiometric La 3−x Te 4 compounds, which have different electronic properties. Moreover, we thus show how controlling synthesis conditions can be used as a knob to reach the target functionality, including the controllable metal-to-insulator transition.

Spontaneous off-stoichiometry as the knob to control the dielectric properties of gapped metals

“…1a) can develop off-stoichiometry due to the decay of conduction electrons to the acceptor level (known as Fermi level instability). 9,14,24 Such behavior can lead to the formation of a range of non-stoichiometric compounds due to the intrinsic non-thermal tendency of the system, even at low temperatures. For instance, previous experimental works 25,26 and our recent first-principles calculations 24 showed that La 3 Te 4 (I% 43d, No.…”

“…This approach does not introduce any additional charge to the system upon the removal of a La atom. Interestingly, due to the presence of conduction band electrons, the defect formation energy for these vacancies is negative, 24 and the formation of La vacancies increases the thermodynamic stability of the compound even at low temperature. We then conducted DFT calculations on each unique system resulting from at least 40 different randomly selected atomic configurations for each composition.…”

“…In contrast, for all La 3− x Te 4 compounds, ε 1 ( ω ) = 0 at ∼4.3 eV is clearly not affected by the presence of free carriers, implying that it is the intrinsic tendency of the system originated from band-to-band transitions (only one taken into account at these frequencies for all compounds). We note as well that generally the absolute values of crossing frequency should be taken into account with caution as the calculations presented here are done within the DFT framework resulting in the underestimation of the internal gap between principal band edges 24 or even sensitivity to scattering rate in the Drude model. Despite this, the demonstrated knob reflects the universal approach that can be adopted for a wide class of gapped metals to tailor their properties.…”

“…9,14,24 Such behavior can lead to the formation of a range of non-stoichiometric compounds due to the intrinsic non-thermal tendency of the system, even at low temperatures. For instance, previous experimental works 25,26 and our recent first-principles calculations 24 showed that La 3 Te 4 (I% 43d, No. 220) is an example of an n-type gapped metal (Fig.…”