arXiv subjects

Matthew Jankousky

Publications and source records attributed to Matthew Jankousky.

5 recordsLinked to original sources

arXiv2025

Effective bands and band-like electron transport in amorphous solids

The localization of electrons caused by atomic disorder is a well-known phenomenon. However, what circumstances allow electrons to remain delocalized and retain band-like characteristics even when the crystal structure is completely absent, as found in certain amorphous solids, is less well understood. To probe this phenomenon, we developed a fully first-principles description of the electronic structure and charge transport in amorphous solids by combining a novel representation of the amorphous state with the state-of-the-art many-body (QSGW) electronic structure theory. Using amorphous In2O3 as an example, we demonstrate the accuracy of our approach in reproducing the band-like nature of the conduction electrons as well as their disorder-limited mobility. Our approach reveals the physical origins responsible for the electron delocalization and the survival of the band dispersions despite the absence of long-range order.

cond-mat.mtrl-sci↗

arXiv2025

Stability, growth, and doping of In$_{2}$(Si, Ge)$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors

In this paper we investigate, computationally and experimentally, the phase stability, electronic structure properties, and the propensity for n-type doping of In$_{2}$X$_{2}$O$_{7}$ (X=Si, Ge) ternary oxides. This family of materials contains promising novel wide-gap semiconductors based on their estimated high $n$-type Baliga figures of merit and acceptable thermal conductivity for power electronics applications. Here, we find that both In$_{2}$Si$_{2}$O$_{7}$ and In$_{2}$Ge$_{2}$O$_{7}$ to be n-type dopable, with Zr providing between 10$^{16}$ and above 10$^{21}$ cm$^{-3}$ net donor concentrations under O-poor conditions, depending on the chemistry, structure (ground-state thorvetite or high-pressure pyrochlore) and synthesis temperature. Initial thin-film growth and annealing leads to polycrystalline In$_{2}$Ge$_{2}$O$_{7}$ thin films in thorvetite structure with band gap over 4 eV, and confirms Zr doping predictions by achieving electron concentrations at 10$^{14}$-10$^{16}$ cm$^{-3}$ under O-rich condition. While future epitaxial growth development is still needed, this study establishes In$_{2}$X$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors for power electronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Ion exchange synthesizes layered polymorphs of MgZrN$_2$ and MgHfN$_2$, two metastable semiconductors

The synthesis of ternary nitrides is uniquely difficult, in large part because elemental N$_2$ is relatively inert. However, lithium reacts readily with other metals and N$_2$, making Li-M-N the most numerous sub-set of ternary nitrides. Here, we use Li$_2$ZrN$_2$, a ternary with a simple synthesis recipe, as a precursor for ion exchange reactions towards AZrN$_2$ (A = Mg, Fe, Cu, Zn). In situ synchrotron powder X-ray diffraction studies show that Li$^+$ and Mg$^{2+}$ undergo ion exchange topochemically, preserving the layers of octahedral [ZrN$_6$] to yield a metastable layered polymorph of MgZrN$_2$ (spacegroup $R\overline{3}m$) rather than the calculated ground state structure ($I41/amd$). UV-vis measurements show an optical absorption onset near 2.0 eV, consistent with the calculated bandgap for this polymorph. Our experimental attempts to extend this ion exchange method towards FeZrN$_2$, CuZrN$_2$, and ZnZrN$_2$ resulted in decomposition products (A + ZrN + 1/6 N$_2$), an outcome that our computational results explain via the higher metastability of these phases. We successfully extended this ion exchange method to other Li-M-N precursors by synthesizing MgHfN$_2$ from Li$_2$HfN$_2$. In addition to the discovery of metastable $R\overline{3}m$ MgZrN$_2$ and MgHfN$_2$, this work highlights the potential of the 63 unique Li-M-N phases as precursors to synthesize new ternary nitrides.

cond-mat.mtrl-sci↗

arXiv2024

Synthesis pathways to thin films of stable layered nitrides

Controlled synthesis of metastable materials away from equilibrium is of interest in materials chemistry. Thin film deposition methods with rapid condensation of vapor precursors can readily synthesize metastable phases, but often struggle to yield the thermodynamic ground state. Growing thermodynamically-stable structures using kinetically-limited synthesis methods in important for practical applications in electronics and energy conversion. Here, we reveal a synthesis pathway to thermodynamically-stable ordered layered ternary nitride materials, and discuss why disordered metastable intermediate phases tend to form. We show that starting from elemental vapor precursors leads to a 3D long-range disordered MgMoN2 thin film metastable intermediate structure, with a layered short-range order that has a low-energy transformation barrier to the layered 2D-like stable structure. This synthesis approach is extended to ScTaN2, MgWN2 and MgTa2N3, and may lead to the synthesis of other layered nitride thin films with unique semiconducting and quantum properties.

cond-mat.mtrl-sci↗

arXiv2024

All "roads" lead to rocksalt structure

Rocksalt is the most common crystal structure among binary compounds. Moreover, no long-lived, metastable polymorphs are observed in compounds with the rocksalt ground state. We investigate the absence of polymorphism and the dominance of the rocksalt structure via first-principles random structure sampling and transformation kinetics modeling of three rocksalt compounds: MgO, TaC, and PbTe. Random structure sampling reveals that for all three systems the rocksalt local minimum on the potential energy surface is far larger than any other, making it statistically the most significant structure. The kinetics modeling shows that virtually all other relevant structures, including most of the 457 known A$_1$B$_1$ prototypes, transform rapidly to rocksalt, making it the only option at ambient conditions. These results explain the absence of polymorphism in binary rocksalts, answer why rocksalt is the structure of choice for high-entropy ceramics and disordered ternary nitrides, and help understand/predict metastable states in crystalline solids more generally.

cond-mat.mtrl-sci↗