Ab initio aided design of novel quaternary, quinary and senary high-entropy borocarbides

High-entropy materials have attracted considerable interest due to their unique, improved properties and large configurational entropy. Out of these, high-entropy ceramics (HECs) are of particular interest since the independent solubility of cations and anions results in increased configurational entropy. However, most HEC research considers only a single element occupying the anion sublattice, which limits the maximum attainable configurational entropy. Here, we expand our previous work on high-entropy borocarbides where both boron and carbon occupy the anion sublattice. By applying an ab initio based screening procedure, we identify six elements Li, Ti, V, Zr, Nb and Hf suitable for forming high-entropy borocarbides. With these elements, we propose six novel HEC compositions, and by computing their entropy forming ability, we identify that three are likely to form single-phase during synthesis. Material properties and lattice distortions for all proposed compositions are studied using density functional theory calculations with special quasirandom structures. The directional lattice distortions, a concept we introduce in this work, show that lattice distortions have an elemental and directional preference for certain HEC compositions. We also show that the novel inclusion of Li improves the mechanical properties of the proposed HECs, the details of which are studied using the electron localization function.

SIMULATION OF STRUCTURE OF LINBO CRYSTALS GROWN WITH USING OF BO FLUX

Показано, что элемент B в следовых количествах (≈10 масс.%) может встраиваться в составе группы [BO ] в грани кислородных тетраэдров кристаллической структуры LiNbO. При этом бор заметно искажает анионный каркас кристалла, изменяя длины < O - O > связей, улучшает упорядочение структурных единиц катионной подрешетки, изменяет поляризуемость кислородно-октаэдрических кластеров MeO (Me - Li, Nb), определяющую сегнетоэлектрические и нелинейнооптические свойства кристалла. It is shown that the B element is able to incorporate into the facets of oxygen tetrahedra of LiNbO, crystal structure [BO ] in a trace amounts (≈10 wt. %). In this case, boron noticeably distorts the anion sublattice of the crystal, changing the lengths of the < O - O > bonds, increasing the ordering of structural units of the cation sublattice. At the same time, boron changes the polarizability of the oxygen-octahedral MeO clusters (Me - Li, Nb) which determines the ferroelectric and nonlinear optical properties of the crystal.

Anion Charge and Lattice Volume Maps for Searching Lithium Superionic Conductors

<p><a>The effects of anion charge and lattice volume (lithium-anion bond length) on lithium ion migration have been investigated by utilizing the density functional theory calculations combined with the anion sublattice models, e.g. <i>fcc</i>, <i>hcp</i> and <i>bcc</i>. It is found that the anion charge and lattice volume have great impacts on the activation energy barrier (E_a) of lithium ion migration, which is validated by some reported sulfides. For the tetrahedrally occupied lithium, the less negative anion charge is, the lower the lithium ion migration barrier is likely to be. While for the octahedrally occupied lithium, the more negative anion charge is, the lower the lithium ion migration barrier is. There are opposite effects of anion charge on E_a and optimum lattice volumes for minimum E_a of lithium ion migration along the <i>Tet-Oct-Tet</i> and <i>Oct-Tet-Oct </i>pathways in the <i>hcp</i>-type sublattices. Based on the full understandings of anion sublattice model, general design strategies for developing lithium superionic conductors were proposed. Adjusting the electronegativity difference between the anion element and non-mobile cation element by selecting the most suitable non-mobile cation element without changing the crystal structure sublattice can achieve low E_a for lithium ion migration. For the desired lithium superionic conductors with tetrahedrally occupied lithium ions, the fine non-mobile cation element should give preferences to those elements located at the right top of the periodic table of elements with large electronegativities. For the lithium superionic conductors with octahedrally occupied lithium ions, the fine non-mobile cation element should give preferences to the elements located at the left bottom of the periodic table with small electronegativities.</a><br></p>

Anion Charge and Lattice Volume Maps for Searching Lithium Superionic Conductors

<p><a>The effects of anion charge and lattice volume (lithium-anion bond length) on lithium ion migration have been investigated by utilizing the density functional theory calculations combined with the anion sublattice models, e.g. <i>fcc</i>, <i>hcp</i> and <i>bcc</i>. It is found that the anion charge and lattice volume have great impacts on the activation energy barrier (E_a) of lithium ion migration, which is validated by some reported sulfides. For the tetrahedrally occupied lithium, the less negative anion charge is, the lower the lithium ion migration barrier is likely to be. While for the octahedrally occupied lithium, the more negative anion charge is, the lower the lithium ion migration barrier is. There are opposite effects of anion charge on E_a and optimum lattice volumes for minimum E_a of lithium ion migration along the <i>Tet-Oct-Tet</i> and <i>Oct-Tet-Oct </i>pathways in the <i>hcp</i>-type sublattices. Based on the full understandings of anion sublattice model, general design strategies for developing lithium superionic conductors were proposed. Adjusting the electronegativity difference between the anion element and non-mobile cation element by selecting the most suitable non-mobile cation element without changing the crystal structure sublattice can achieve low E_a for lithium ion migration. For the desired lithium superionic conductors with tetrahedrally occupied lithium ions, the fine non-mobile cation element should give preferences to those elements located at the right top of the periodic table of elements with large electronegativities. For the lithium superionic conductors with octahedrally occupied lithium ions, the fine non-mobile cation element should give preferences to the elements located at the left bottom of the periodic table with small electronegativities.</a><br></p>

A Lattice Dynamical Approach for Finding the Lithium Superionic Conductor Li3ErI6

<p>Driven by the increasing attention that the superionic conductors Li₃MX₆ (M = Y, Er, In, La; X = Cl, Br, I) have gained recently for the use of solid-state batteries, and the idea that a softer, more polarizable anion sublattice is beneficial for ionic transport, here we report Li₃ErI₆, the first experimentally-obtained iodine-based compound within this material system of ionic conductors. Using a combination of synchrotron and neutron diffraction, we elucidate the structure, the lithium positions and possible diffusion pathways of Li₃ErI₆. Temperature-dependent impedance spectroscopy shows low activation energies of 0.37 and 0.38 eV alongside promising ionic conductivities of 0.65 mS·cm^-1 and 0.39 mS·cm^-1directly after ball milling and the subsequently annealed Li₃ErI₆, respectively. Speed of sound measurements are used to determine the Debye frequency of the lattice as a descriptor of the lattice dynamics and overall lattice softness, and Li₃ErI₆ is compared to the known material Li₃ErCl₆. The softer, more polarizable framework from the iodide anion leads to improved ionic transport, showing that the idea of softer lattices holds up in this class of materials. This work provides Li₃ErI₆ as an interesting novel framework for optimization in the class of halide-based ionic conductors.</p>

A Lattice Dynamical Approach for Finding the Lithium Superionic Conductor Li3ErI6

<p>Driven by the increasing attention that the superionic conductors Li₃MX₆ (M = Y, Er, In, La; X = Cl, Br, I) have gained recently for the use of solid-state batteries, and the idea that a softer, more polarizable anion sublattice is beneficial for ionic transport, here we report Li₃ErI₆, the first experimentally-obtained iodine-based compound within this material system of ionic conductors. Using a combination of synchrotron and neutron diffraction, we elucidate the structure, the lithium positions and possible diffusion pathways of Li₃ErI₆. Temperature-dependent impedance spectroscopy shows low activation energies of 0.37 and 0.38 eV alongside promising ionic conductivities of 0.65 mS·cm^-1 and 0.39 mS·cm^-1directly after ball milling and the subsequently annealed Li₃ErI₆, respectively. Speed of sound measurements are used to determine the Debye frequency of the lattice as a descriptor of the lattice dynamics and overall lattice softness, and Li₃ErI₆ is compared to the known material Li₃ErCl₆. The softer, more polarizable framework from the iodide anion leads to improved ionic transport, showing that the idea of softer lattices holds up in this class of materials. This work provides Li₃ErI₆ as an interesting novel framework for optimization in the class of halide-based ionic conductors.</p>

Al5+αSi5+δN12, a new Nitride compound

The family of III-Nitride semiconductors has been under intensive research for almost 30 years and has revolutionized lighting applications at the dawn of the 21st century. However, besides the developments and applications achieved, nitride alloys continue to fuel the quest for novel materials and applications. We report on the synthesis of a new nitride-based compound by using annealing of AlN heteroepitaxial layers under a Si-atmosphere at temperatures between 1350 °C and 1550 °C. The structure and stoichiometry of this compound are investigated by high resolution transmission electron microscopy (TEM) techniques and energy dispersive X-Ray (EDX) spectroscopy. Results are supported by density functional theory (DFT) calculations. The identified structure is a derivative of the parent wurtzite AlN crystal where the anion sublattice is fully occupied by N atoms and the cation sublattice is the stacking of 2 different planes along <0001>: The first one exhibits a ×3 periodicity along <11–20> with 1/3 of the sites being vacant. The rest of the sites in the cation sublattice are occupied by an equal number of Si and Al atoms. Assuming a semiconducting alloy, a range of stoichiometries is proposed, Al5+αSi5+δN12 with α being between −2/3 and 1/4 and δ between 0 and 3/4.