UC Riverside

Two-dimensional chromium ditelluride (CrTe2) is a promising ferromagnetic layered material that exhibits long-range ferromagnetic ordering in the monolayer limit. The formation energies of the different possible structural phases (1T, 1H, 2H) calculated from density functional theory (DFT) show that the 1T phase is the ground state and the energetic transition barriers between the phases, calculated by the nudged elastic band method, are large, on the order of 0.5 eV. The self-consistent Hubbard U correction parameters are calculated for all the phases of CrTe2. The calculated magnetic moment of 1T-CrTe2 with >= 2 layers lies in the plane, whereas the magnetic moment of a monolayer is out-of-plane. Band filling and tensile bi-axial strain cause the magnetic moment of a monolayer to switch from out-of-plane to in-plane, and compressive bi-axial strain in a bilayer causes the magnetic moment to switch from in-plane to out-of-plane. The magnetic anisotropy is shown to originate from the large spin orbit coupling (SOC) of the Te atoms and the anisotropy of the exchange coupling constants Jxy and Jz in an XXZ type Hamiltonian. Renormalized spin wave theory using experimental values for the magnetic anisotropy energy and Curie temperatures provides a range of values for the nearest neighbor exchange coupling.

The self-intercalated Cr1+xTe2 is a ferromagnetic layered material with complex structure and magnetic configurations. It is composed of alternating CrTe2 and intercalated Cr layers. The calculated formation energies show that Cr1+xTe2 is more stable when both the top and bottom surfaces are the CrTe2 layers. The exchange coupling constants are extracted by calculating the energies of different magnetic configurations. The direction of magnetic anisotropy depends on the inversion symmetry in Cr1+xTe2. The perpendicular magnetic anisotropy only exists in the structures with broken inversion symmetry. The exchange coupling constant J1, that is between the intercalated Cr atom and its nearest neighbor Cr atom in the CrTe2 layer, responds to applied strain in different ways in structures with different symmetries.

A systematic theoretical study of dopants in a half-delithiated lithium nickel oxide (Li0.5NiO2) cathode is performed to determine the preferred occupation sites, dopant ion migration, the improvement of structural stability, and the suppression of oxygen evolution. Dopants considered include Li, B, Na, Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Y, Zr, Nb, Mo, In, Sn, Sb, La, Ce, Ta, and W. For the non-transition metal dopants, the energy barrier governing dopant migration is correlated with the number of valence shell electrons, so that it increases from left to right across a row of the periodic table. For these dopants, the energy barrier also decreases moving down a column of the periodic table. For transition metal dopants, the energy barrier depends on the number of unpaired valence electrons of the dopant, so that the energy barrier is maximum near the middle of a transition metal row. The energy required in oxygen evolution is linearly correlated with the change in charge of the oxygen resulting from the neighboring dopant ions. In particular, oxygen release is enhanced most by cobalt doping and suppressed most by boron doping.