arXiv subjects

Joseph T. Schick

Publications and source records attributed to Joseph T. Schick.

4 recordsLinked to original sources

arXiv2017

Descriptors for thermal expansion in solids

Thermal expansion in materials can be accurately modeled with careful anharmonic phonon calculations within density functional theory. However, because of interest in controlling thermal expansion and the time consumed evaluating thermal expansion properties of candidate materials, either theoretically or experimentally, an approach to rapidly identifying materials with desirable thermal expansion properties would be of great utility. When the ionic bonding is important in a material, we show that the fraction of crystal volume occupied by ions, (based upon ionic radii), the mean bond coordination, and the deviation of bond coordination are descriptors that correlate with the room-temperature coefficient of thermal expansion for these materials found in widely accessible databases. Correlation is greatly improved by combining these descriptors in a multi-dimensional fit. This fit reinforces the physical interpretation that open space combined with low mean coordination and a variety of local bond coordinations leads to materials with lower coefficients of thermal expansion, materials with single-valued local coordination and less open space have the highest coefficients of thermal expansion.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced charge ordering transition in doped CaFeO3 through steric templating

We report density functional theory (DFT) investigation of $B$-site doped CaFeO$_3$, a prototypical charge-ordered perovskite. At 290 K, CaFeO$_3$ undergoes a metal-insulator transition and a charge disproportionation reaction 2Fe$^{4+}$$\rightarrow$Fe$^{5+}$+Fe$^{3+}$. We observe that when Zr dopants occupy a (001) layer, the band gap of the resulting solid solution increases to 0.93 eV due to a 2D Jahn-Teller type distortion, where FeO$_6$ cages on the $xy$ plane elongate along $x$ and $y$ alternatively between neighboring Fe sites. Furthermore, we show that the rock-salt ordering of the Fe$^{5+}$ and Fe$^{3+}$ cations can be enhanced when the $B$-site dopants are arranged in a (111) plane due to a collective steric effect that facilitates the size discrepancy between the Fe$^{5+}$O$_6$ and Fe$^{3+}$O$_6$ octahedra and therefore gives rise to a larger band gap. The enhanced charge disproportionation in these solid solutions is verified by rigorously calculating the oxidation states of the Fe cations with different octahedral cage sizes. We therefore predict that the corresponding transition temperature will increase due to the enhanced charge ordering and larger band gap. The compositional, structural and electrical relationships exploited in this paper can be extended to a variety of perovskites and non-perovskite oxides providing guidance in structurally manipulating electrical properties of functional materials.

cond-mat.mtrl-sci↗

arXiv2016

Classical model of negative thermal expansion in solids with expanding bonds

We study negative thermal expansion (NTE) in model lattices with multiple atoms per cell and first- and second-nearest neighbor interactions using the (anharmonic) Morse potential. By exploring the phase space of neighbor distances and thermal expansion rates of the bonds, we determine the conditions under which NTE emerges. By permitting all bond lengths to expand at different rates, we find that NTE is possible without appealing to fully rigid units. Nearly constant, large-amplitude, isotropic NTE is observed up to the melting temperature in a classical molecular dynamics model of a $\mathrm{ReO}_3$-like structure when the rigidity of octahedral units is almost completely eliminated. Only weak NTE, changing over to positive expansion is observed when the corner-linked octahedra are rigid, with flexible second-neighbor bonds between neighboring octahedra permitting easy rotation. We observe similar changes to thermal expansion behavior for the diamond lattice: NTE when second-neighbor interactions are weak to positive thermal expansion when second-neighbor interactions are strong. From these observations, we suggest that the only essential local conditions for NTE are atoms with low coordination numbers along with very low energies for changing bond angles relative to bond-stretching energies.

cond-mat.mtrl-sci↗

arXiv2014

Coupling between octahedral rotations and local polar displacements in WO3/ReO3 superlattices

We model short-period superlattices of WO$_3$ and ReO$_3$ with first-principles calculations. In fully-relaxed superlattices, we observe that octahedral tilts about an axis in the planes of the superlattices do not propagate from one material, despite the presence of the corner-shared oxygen atoms. However, we find that octahedral rotation is enhanced within WO$_3$ layers in cases in which strain couples with native antiferroelectric displacements of tungsten within their octahedral cages. Resulting structures remain antiferroelectric with low net global polarization. Thermodynamic analysis reveals that superlattices with sufficiently thick ReO$_3$ layers, the absolute number being three or more layers and the Re fraction $\geq 50\%$, tend to be more stable than the separated material phases and also show enhanced octahedral rotations in the WO$_3$ layers.

cond-mat.mtrl-sci↗