Session R20: Computational Materials Design and Discovery -- Structural Disorder

Ionic Transport in Materials with Substitutional Disorder

Substitutional disorder can have a profound impact on the ionic transport properties of crystalline solids, such as the solid electrolytes of solid oxide fuel cells or cation-disordered cathode materials for lithium ion batteries (LIBs). However, the direct experimental investigation of disorder on the atomic scale is challenging, and (conventional) first-principles computational techniques cannot be directly applied to disordered materials.

Here I will show how relatively simple computational models can provide useful insight into the interplay of substitutional disorder and ionic conduction in cation-disordered LIB cathode materials. Over the last years, computational modeling has contributed to the understanding of this new class of materials and has guided the discovery of several new high-energy density cathode materials.
  

Tunable band-gap engineering of ZnSnN2 via cation disorder from first-principles calculations

Heterovalent ternary alloys offer a rich space to search for the next-generation optoelectronic materials. Unlike their III-V counterpart, cation disorder can greatly alter the electronic properties of II-IV-V₂ compounds by breaking the octet-rule locally. Thus, intentional introduction of cation disorder can provide a new route to tune the electronic band gap of heterovalent ternary compound besides conventional alloying. To demonstrate this possibility, we performed first-principles calculations based on hybrid density functional theory to study the cation disorder effect in ZnSnN₂, which is a promising solar material in the II-IV-V₂ family. We found a direct relationship between the band gap of ZnSnN₂ and the long-range order parameter S, which characterizes the Zn/Sn disorder. Such band gap variations arise from the presence of octet-rule breaking motifs in the lattice (Zn3Sn, ZnSn3, Zn4, Sn4). Our findings reveal the correlations between cation disorder and the band gap of ZnSnN₂, and suggest the possibility of alloy-free band gap tuning in heterovalent ternary compounds.   

Pristine electronic properties in multinary semiconductor alloys at magic compositions

The materials electronic structure is usually very sensitive to topological distribution of atoms in the materials. Generally, the electronic integrity is easily destroyed by defects and disorder causing charge localization. The design of multinary materials has attracted a great interest for its wide tunability in properties. This functional versality is rooted in many-fold degrees of freedom in composition and atomic distribution. At some “magic” compositions, the disordered material system can exhibit perfect short range ordered (SRO) ionic distributions that conserve the local octet rule despite the absence of long range order. In this contribution, we study the dual sublattice mixed semiconductor alloy (ZnSnN₂)_1-x:(ZnO)_2x in which perfect SRO are observed when x=0.0 and 0.25. Disordered structures were generated from Monte Carlo simulations and the energies were calculated from first principles. At the magic composition, these SRO conserving systems have a much reduced mixing enthalpy and increased band gap. More importantly, these SRO structures do not have charge localization effects near band edges, as seen in the inverse participation ratio, thereby maintaining a pristine electronic structure.   

Lattice Constant and Band Gap Tuning in BInGaN Alloys for Next-Generation LEDs

InGaN light-emitting diodes (LEDs) have enabled significant energy and cost savings, and further savings can be realized by operating at the same efficiency at higher power. However, the efficiency of currently available InGaN LEDs is lowered by the loss of carriers to Auger recombination when operating at high power. The Auger loss can be reduced by increasing the active-region volume, but the lattice mismatch between thick InGaN active layers and underlying GaN layers cause performance-degrading dislocations. Previous work has shown that this problem can be addressed by co-alloying InGaN with BN. Doing so can produce alloys that maintain an approximate lattice match to GaN while allowing for a band gap that is adjustable throughout the visible range. In this work, we expand on previous hybrid density functional theory calculations to explore the thermodynamic, structural, and electronic properties of a larger area of the B_xIn_yGa_1-x-yN composition space and examine the wurtzite, zinc blende, and planar hexagonal phases. A more thorough understanding of these properties will help to direct efforts to fabricate thick active regions for more cost-effective BInGaN LEDs.   

High precision detection of the change in intermediate range order of amorphous thin films due to annealing

Future gravitational wave detectors, such as Advanced LIGO +, will be limited in sensitivity by thermal noise associated with mechanical loss in the detectors’ amorphous mirror coatings. In order to reduce mechanical loss, we aim to understand the atomic level structures responsible for it and take a directed design approach to suggest improved coatings. Through studies on thin films of amorphous zirconia-doped-tantala (ZrO₂-Ta₂O₅), a potential coating for LIGO mirrors, we present high precision detection of the change in the intermediate range order (IRO) as a function of post-deposition annealing, using grazing incidence x-ray scattering measurements carried out at the Stanford Synchrotron Radiation Lightsource. For the first time, our atomic modeling based on x-ray scattering data is able to capture the changes in IRO. We show that the primary structural units (PSUs), which are disordered oxygen octahedra centered around the metal atoms, remain largely unchanged after annealing. We explain the observed structural changes in terms of the IRO, specifically, the change in metal-oxygen-metal bond angles at the bridging oxygen sites, and the way neighboring PSUs share bridging oxygens. Finally, we discuss correlations between the observed changes in the IRO and mechanical loss.   

Temperature-dependent properties of thermoelectric clathrates

Intermetallic clathrate compounds, among them the type-I clathrate Ba₈Al_xSi_46-x, are promising candidates for high-efficiency thermoelectric applications. They exhibit a strong dependence of the electronic properties on the atomic arrangement of the substitutional Al atoms in the crystal framework [1]. At the charge-balanced composition (x=16), the ground-state configuration is semiconducting, however, configurations higher in energy are metallic. Understanding the temperature-dependent properties is essential, as semiconducting behavior is a prerequisite for thermoelectric applications. By employing the cluster expansion technique combined with Monte-Carlo samplings and the Wang-Landau method [2] we find a semiconductor-to-metal transition at around 600 K which is driven by a partial order-disorder transition. Signatures of this phase transition are observed in the temperature-dependent band structure, specific heat, and partial occupations.
[1] M. Troppenz, S. Rigamonti, and C. Draxl, Chem. Mat. 29, 2414 (2017).
[2] S. Rigamonti, M. Troppenz, M. Kuban, A. Huebner, C. Sutton, L. Ghiringhelli, M. Scheffler, and C. Draxl. CELL: a python package for cluster expansions with a focus on complex alloys, in preparation.   

Beyond the constant relaxation time

Over the years, first principles calculations have become a complementary tool for the experimental research aiming to discover high-performance thermoelectrics. This has greatly improved the understanding of the transport properties and has advanced optimization strategies based on “band engineering” [1,2]. In most cases, the constant relaxation time approximation has been used without much analysis. We investigated the limits of the approximations for the relaxation time in the electronic transport coefficients properties of well know TE materials by considering different models that include energy and temperature dependence. Transport coefficients have been computed using the recently developed PAOFLOW package (http://aflowlib.org/src/paoflow/).   

Potential Candidate for Thin-Film Photovoltaics: Alloyed Semiconductor Cu2BaGe1-xSnxSe4

Cu₂BaGeSe₄ (CBGSe) was previously identified as a potential thin-film photovoltaic (PV) candidate based on computationally scanning the I₂-II-IV-VI₄ (I = Cu, Ag; II = Ba, Sr; IV = Ge, Sn; VI = S, Se) materials family and successful experimental synthesis. This material avoids difficulties of competing PV materials like toxicity (Cd) or scarcity (In, Te) of constituent elements in Cu(In,Ga)(S,Se)₂ and CdTe. Moreover, the similar ionic sizes and coordination preferences compared to the recently identified PV material Cu₂BaSn(S,Se)₄ may provide a potential avenue to overcome the unavoidable antisite disordering that limits the competing material Cu₂ZnSn(S,Se)₄. In this work, hybrid density functional theory (HSE06) including spin-orbit coupling is used to explore the electronic properties of three cation alloying approaches (Ag for Cu, Sr for Ba, and Sn for Ge ) for unalloyed CBGSe, which has a relatively large band gap (1.91 eV). The largest band gap decrease can be achieved by Sn/Ge alloying. The minimum band gap occurs at 1.57 eV for x≈0.70 in alloyed Cu₂BaGe_1-xSn_xSe₄. The minimum occurs just prior to a structural transition from the P3₁ to the Ama2 space group just above x=0.7; the P3₁ phase is associated with a significantly lower band gap.   

First-principles study of photochromic material YHxOy

Photochromic materials refer to the materials that change color under light illumination, particularly visible light. Organic materials dominate the photochromic materials due to their highly tunable properties. However, inorganic photochromic materials are attractive for their stability and possibly high resistance to fatigue. YHxOy is a new photochromic material found a couple of years ago. However, its chemical composition, atomic structure, electronic properties have not been completely studied so far. The mechanism of the photo-induced color change is still elusive too. In this work, using first-principles calculations based on the density functional theory, we aim to provide a comprehensive understanding on this new material. Possible strategies on how to tune the properties, such as its band gap, through oxygen concentration will be discussed.