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Physical review / B 101(16), 165406 (1-12) (2020). doi:10.1103/PhysRevB.101.165406, We determined the lattice dynamics of metastable, surface-stabilized $α$-phase FeSi$_2$ nanoislands epitaxially grown on the Si(111) surface with average heights and widths ranging from 1.5 to 20 nm and 18 to 72 nm, respectively. The crystallographic orientation, surface morphology, and local crystal structure of the nanoislands were investigated by reflection high-energy electron diffraction, atomic force microscopy, and x-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS), obtained by nuclear inelastic scattering, exhibits a pronounced damping and broadening of the spectral features with decreasing average island height. First-principles calculations of the polarization-projected Si- and Fe-partial phonon dispersions and PDOS enable the disentanglement of the contribution of the $xy$- and $z$-polarized phonons to the experimental PDOS. Modeling of the experimental data with the theoretical results unveils an enhanced damping of the $z$-polarized phonons for islands with average sizes below 10 nm. This phenomenon is attributed to the fact that the low-energy $z$-polarized phonons couple to the low-energy surface/interface vibrational modes. The thermodynamic and elastic properties obtained from the experimental data show a pronounced size-dependent behavior., Published by Inst., Woodbury, NY

Journal of applied physics 121(4), 044901 (2017). doi:10.1063/1.4974106, We report on the characterization of high quality sapphire single crystals suitable for high-resolution X-ray optics at high energy. Investigations using rocking curve imaging reveal the crystals to be of uniformly good quality at the level of ∼10$^{−4}$ in lattice parameter variations, δd/d. However, investigations using backscattering rocking curve imaging with a lattice spacing resolution of δd/d∼5×10$^{−8}$ show very diverse quality maps for all crystals. Our results highlight nearly ideal areas with an edge length of 0.2–0.5 mm in most crystals, but a comparison of the back reflection peak positions shows that even neighboring ideal areas exhibit a relative difference in the lattice parameters on the order of δd/d=10–20×10$^{−8}$; this is several times larger than the rocking curve width. Stress-strain analysis suggests that an extremely stringent limit on the strain at a level of ∼100 kPa in the growth process is required in order to produce crystals with large areas of the quality required for X-ray optics at high energy., Published by American Inst. of Physics, Melville, NY

We performed nuclear forward and inelastic scattering of synchrotron radiation by elemental Os utilizing the nuclear excited state of Os187 which is otherwise inaccessible using any practical radioactive decay process. The lifetime of the excited state, 3.06(8)ns, and the energy of the transition, 9.778(3)keV, are refined. The nuclear quadrupole moment of the excited state, Q3/2=1.46(10)b, is determined. The density of phonon states for elemental Os, which is herein experimentally determined, suggests that the Os lattice is a model for the lattice dynamics of hcp-Fe. The combination of the low energy of the nuclear transition and the large nuclear mass leads to a high recoil free fraction, fLM=0.95(1), at room temperature, a large value that strongly supports the viability of nuclear resonance scattering as a reliable method to study electronic, magnetic, and elastic properties of Os compounds, including Os organometallics.

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using x-ray and neutron diffraction, 151Eu-Mössbauer spectroscopy, and 151Eu nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by abinitio theoretical studies. The perovskite symmetry, Pm3¯m, is unstable at the M- and R-points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: I4/mcm, Imma, R3¯c with relative relaxation energy around −25meV. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at 11.5meV can be modeled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO3, however, although previous studies reported the structural phase transition to the tetragonal I4/mcm phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities.

The lattice dynamics in Bi2Te3 and Sb2Te3 were investigated both microscopically and macroscopically using Sb-121 and Te-125 nuclear inelastic scattering, x-ray diffraction, and heat capacity measurements. In combination with earlier inelastic neutron scattering data, the element-specific density of phonon states was obtained for both compounds and phonon polarization analysis was carried out for Bi2Te3. A prominent peak in the Te specific density of phonon states at 13 meV, that involves mainly in-plane vibrations, is mostly unaffected upon substitution of Sb with Bi revealing vibrations with essentially Te character. A significant softening is observed for the density of vibrational states of Bi with respect to Sb, consistently with the mass homology relation in the long-wavelength limit. In order to explain the energy mismatch in the optical phonon region, a similar to 20% force constant softening of the Sb-Te bond with respect to the Bi-Te bond is required. The reduced average speed of sound at 20 K in Bi2Te3, 1.75(1) km/s, compared to Sb2Te3, 1.85(4) km/s, is not only related to the larger mass density but also to a larger Debye level. The observed low lattice thermal conductivity at 295 K, 2.4 Wm(-1)K(-1) for Sb2Te3 and 1.6 Wm(-1)K(-1) for Bi2Te3, cannot be explained by anharmonicity alone given the rather modest Gruneisen parameters, 1.7(1) for Sb2Te3 and 1.5(1) for Bi2Te3, without accounting for the reduced speed of sound and more importantly the low acoustic cutoff energy.

The high pressure structural phase diagram of EuTiO3 is revisited by means of single crystal x-ray diffraction at pressures below 30GPa. We report a transition at about 3GPa with a surprisingly small lattice distortion. At 23GPa we have detected a yet unknown transition into a new orthorhombic phase (space group Ibam). Combining our observations on structures obtained on single crystals and polycrystalline samples with complementary methods, such as electrical resistivity and nuclear forward scattering of synchrotron radiation measured on polycrystalline samples, we address issues relevant to chemistry and disorder.

Physical review / B covering condensed matter and materials physics 93(20), 201110(2016). doi:10.1103/PhysRevB.93.201110, Magnetic properties of NiO have been studied in the multimegabar pressure range by nuclear forward scattering of synchrotron radiation using the 67.4 keV Mössbauer transition of $^{61}$Ni. The observed magnetic hyperfine splitting confirms the antiferromagnetic state of NiO up to 280 GPa, the highest pressure where magnetism has been observed so far, in any material. Remarkably, the hyperfine field increases from 8.47 T at ambient pressure to ∼ 24 T at the highest pressure, ruling out the possibility of a magnetic collapse. A joint x-ray diffraction and extended x-ray-absorption fine structure investigation reveals that NiO remains in a distorted sodium chloride structure in the entire studied pressure range. Ab initio calculations support the experimental observations, and further indicate a complete absence of Mott transition in NiO up to at least 280 GPa., Published by Inst., Woodbury, NY