Guided design of copper oxysulfide superconductors

We describe a framework for designing novel materials, combining modern first-principles electronic structure tools, materials databases, and evolutionary algorithms capable of exploring large configurational spaces. Guided by the chemical principles introduced by Antipov, \emph{et. al.}, for the design and synthesis of the Hg-based high-temperature superconductors, we apply our framework to design a new layered copper oxysulfide, Hg(CaS)$_2$CuO$_2$. We evaluate the prospects of superconductivity in this oxysulfide using theories based on charge-transfer energies, orbital distillation and uniaxial strain.Comment: 5 pages, 5 figure

Tetrahedral rotations in alkaline-earth metal orthovanadates

The alkaline-earth orthovanadate Sr$_3$V$_2$O$_8$ with the palmierite structure is reported to host a dielectric anomaly as well as a structural phase transition above the room temperature. With V$^{5+}$ ions and tetrahedral oxygen coordination, the crystal structure of this compound is not studied in detail from first principles yet. In this work, we perform a detailed analysis of the crystal structure and instabilities of M$_3$V$_2$O$_8$ ($\text{M} = \text{Ca, Sr, Ba}$) orthovanadates with the palmierite structure using first principles density functional theory. We find that as the M$^{2+}$ cation size decreases, a significant structural distortion that changes the symmetry from $R\bar{3}m$ to $C2/c$ emerges. This change is accompanied with a rotation of the oxygen tetrahedra. Our calculations also indicate that the polar instability in these compounds are suppressed by these tetrahedral rotations

Effect of (001) and (111) Epitaxial Strain on \emph{Pnma} Perovskite Oxides

With recent advances in strain engineering and its widespread applications, it is becoming increasingly important to understand the effect of biaxial strain on the most common structural phase of perovskites -- the orthorhombic $Pnma$ structure. In this work, by using a combination of group theory and first principles density functional theory, we study the effect of biaxial strain on (001) and (111)-oriented CaTiO$_3$, SrSnO$_3$ and SrZrO$_3$ films. We observe manifestly different behaviors depending on the strain orientation, with common trends emerging between different materials. In addition to many different structural phases observed in individual compounds before, we identify a transition between two different phases with the same space group name ($P2_1/c$) but different symmetries in (111)-strained materials. We also find that allowing the relaxation of the out-of-plane monoclinic angles, often ignored in first principles studies, lead to significant stabilization of certain phases and is essential to correctly determine the structural ground state