Lattice dynamics of CoO from first principles

Abstract

CoO: strongly correlated system, charge transfer insulator Paramagnetic @ RT : NaCl structure a = 4.26 Å AFII ordering below T N = 293 K Spins aligned along <001> Trigonal lattice distortion along <111> below T N Energy gap: 2.5-2.8 eV Total magnetic moment on Co ion: ~ 3.8 µ B Calculated quantities agree with experiment On-site Coulomb interactions between 3d electrons are crucial for proper description of the band structure and lattice dynamics. Hubbard U determines phonon DOS and phonon dispersion curves of CoO Small U eff (U eff = U-J) Underestimation of LO and TO modes due to low repulsion in 3d shell Unphysical mode softening of phonon branches (especially LO modes) Γ-ray-Ref. 4 X-ray-Ref. 5 Mössbauer-Ref. 6 Calculated and experimental specific heat for CoO. Experimental data are taken from Ref. 3.

Magnetic excitations of spin and orbital moments in cobalt oxideSpecial issue on Neutron Scattering in Canada.

“…The coherent crosssection for TA phonons with q=<0.5 0.5 0.5> also has the same value at these three wavevectors despite their different |Q|. Hence we believe that this lowest peak arises largely from coherent phonon scattering, as well as incoherent phonon scattering since it coincides with the possible peak in the density of phonon states 15 . To separate the phonon from the j=1/2 to j=3/2 spinorbit transition (orbiton) and spin excitations, inelastic scans were also performed at 450 K well above the Néel transition temperature.…”

Cobalt oxide surface chemistry: The interaction of CoO(100), Co3O4(110) and Co3O4(111) with oxygen and water

“…The intense satellite structure of the Co 2p monoxide spectrum depends upon hybridization between highspin Co 2+ 3d 7 and lattice anion O 2p states [21][22][23]:…”

“…The more complex structure in the CoO photoemission is a direct result of the nature of the valence band produced by octahedrally coordinated, high-spin Co 2+ oxides. The highspin character allows for strong electron correlation resulting in a range of closely lying states with vary d-d coupling, and the overlap of cobalt valence orbitals with surrounding O 2p allows for charge-transfer [21][22][23] from O 2p to the Co 3d leading to the possibility of different final states in the photoemission mechanism. These effects are not present in either low spin, octahedrally coordinated Co 3+ or tetrahedrally coordinated Co 2+ .…”

Phonons calculated from first-principles

“…Crystal with point defects can also be treated easily, like NiAl with Fe replacing Ni and Al [34]. Recent progress in handling the strong electron coupling by applying the LDA+U, or GGA+U approach allowed to calculated satisfactory phonons in such crystals as CoO [35], F e 3 O 4 , having a Vervey phase transition [36], iron containing mineral F e 2 SiO 4 [37], and 5f electron systems P uCoGa 5 [27,38]. The DFT calculations are not limited to bulk systems.…”