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.
The recent surge of interest in phase change materials GeTe, Ge2Sb2Te5, and related compounds motivated us to revisit the structural phase transition in GeTe in more details than was done before. Rhombohedral-to-cubic ferroelectric phase transition in GeTe has been studied by high resolution neutron powder diffraction on a spallation neutron source. We determined the temperature dependence of the structural parameters in a wide temperature range extending from 309 to 973 K. Results of our studies clearly show an anomalous volume contraction of 0.6% at the phase transition from the rhombohedral to cubic phase. In order to better understand the phase transition and the associated anomalous volume decrease in GeTe we have performed phonon calculations based on the density functional theory. Results of the present investigations are also discussed with respect to the experimental data obtained for single crystals of GeTe.
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