Normal Mode Determination of Perovskite Crystal Structures with Octahedral Rotations: Theory and Applications

Nuclear site analysis methods are used to enumerate the normal modes of $ABX_{3}$ perovskite polymorphs with octahedral rotations. We provide the modes of the fourteen subgroups of the cubic aristotype describing the Glazer octahedral tilt patterns, which are obtained from rotations of the $BX_{6}$ octahedra with different sense and amplitude about high symmetry axes. We tabulate all normal modes of each tilt system and specify the contribution of each atomic species to the mode displacement pattern, elucidating the physical meaning of the symmetry unique modes. We have systematically generated 705 schematic atomic displacement patterns for the normal modes of all 15 (14 rotated + 1 unrotated) Glazer tilt systems. We show through some illustrative examples how to use these tables to identify the octahedral rotations, symmetric breathing, and first-order Jahn-Teller anti-symmetric breathing distortions of the $BX_{6}$ octahedra, and the associated Raman selection rules. We anticipate that these tables and schematics will be useful in understanding the lattice dynamics of bulk perovskites and would serve as reference point in elucidating the atomic origin of a wide range of physical properties in synthetic perovskite thin films and superlattices.Comment: 17 pages, 3 figures, 17 tables. Supporting information accessed through link specified within manuscrip

Vibrational Behavior of the M\u3csub\u3en+1\u3c/sub\u3e\u3cem\u3eAX\u3csub\u3en\u3c/sub\u3e\u3c/em\u3e Phases from First-Order Raman Scattering (\u3cem\u3eM\u3c/em\u3e=Ti,V,Cr, \u3cem\u3eA\u3c/em\u3e=Si, \u3cem\u3eX\u3c/em\u3e=C,N)

We report on the Raman spectra of Ti3SiC2 (312), M2AlC(211) (M=Ti, V, Cr, and Nb) and Ti4AlN3 (413), as representative compounds from the family of Mn+1AXn phases. Intense and narrow first-order Raman peaks are observed, and we present an analysis of the spectra based on symmetry considerations and from results of first-principles calculations of phonon frequencies. The agreement between experimental and calculated mode energies is excellent. The identification of the modes enables application of Raman scattering as a diagnostic tool for the detailed study of the structural and physical properties of this family of compounds and their engineered solid solutions

Infrared reflection spectroscopy and effective medium modeling of as-anodized and oxidized porous silicon carbide

Abstract. We present a study of the infrared reflectance of porous silicon carbide (PSC) formed by the electrochemical dissolution of silicon carbide substrates of both 6H and 4H polytypes. The reflectance from n-PSC, both as-anodized and passivated, is reported for the first time. The passivation of PSC has been accomplished using a short thermal oxidation. Fourier transform infrared (FTIR) reflectance spectroscopy is employed ex situ after different stages of the thermal oxidation process. The characteristics of the reststrahlen band normally observed in bulk SiC are altered by anodization; further changes in the reflectance spectra occur following oxidation for different periods of time. An effective medium theory model that includes air, SiC and SiO 2 as component materials is shown to characterize the observed changes in the reflectance spectra after different stages of PSC oxidation

Enhanced Raman scattering from individual semiconductor nanocones and nanowires

Physical Review Letters, 96(15): pp. 4.We report strong enhancement (similar to 10(3)) of the spontaneous Raman scattering from individual silicon nanowires and nanocones as compared with bulk Si. The observed enhancement is diameter (d), excitation wavelength (lambda(laser)), and incident polarization state dependent, and is explained in terms of a resonant behavior involving incident electromagnetic radiation and the structural dielectric cross section. The variation of the Raman enhancement with d, lambda(laser), and polarization is shown to be in good agreement with model calculations of scattering from an infinite dielectric cylinder

Crystallization engineering as a route to epitaxial strain control

The controlled synthesis of epitaxial thin films offers opportunities for tuning their functional properties via enabling or suppressing strain relaxation. Examining differences in the epitaxial crystallization of amorphous oxide films, we report on an alternate, low-temperature route for strain engineering. Thin films of amorphous Bi–Fe–O were grown on (001)SrTiO3 and (001)LaAlO3substrates via atomic layer deposition. In situ X-ray diffraction and X-ray photoelectron spectroscopy studies of the crystallization of the amorphous films into the epitaxial (001)BiFeO3 phase reveal distinct evolution profiles of crystallinity with temperature. While growth on (001)SrTiO3 results in a coherently strained film, the same films obtained on (001)LaAlO3 showed an unstrained, dislocation-rich interface, with an even lower temperature onset of the perovskite phase crystallization than in the case of (001)SrTiO3. Our results demonstrate how the strain control in an epitaxial film can be accomplished via its crystallization from the amorphous state

Visible Thermal Emission from Sub-Band-Gap Laser Excited Cerium Dioxide Particles

Cerium dioxide particles excited in air with sub-band-gap radiation emit very broad radiation in the visible spectrum above a threshold intensity that decreases with increasing ambient temperature. Concomitant with this emission is the near disappearance of the Stokes and anti-Stokes Raman scattering peaks. Both phenomena are reversible in air up to just above threshold, and are seen for nanoparticles and several-micron-diameter particles with particle diameter comparable to or smaller than the laser focus. Temperature estimates using the Stokes/anti-Stokes scattering intensity ratio suggest there is laser heating due to small intragap absorption and possible nonlinear processes, given the very slow thermal conduction. The broad emission in this loose powder may well be due to thermal emission, on the basis of spectral fitting of the high-energy part of the spectrum to a blackbody radiator at ϳ1200-1400°C, although luminescence from a new phase is a possibility. The sudden decrease in Raman scattering and increase in emission in air are consistent with a transition to a new, possibly luminescent, phase, as is the continued disappearance of the Raman peaks in forming gas when the laser power is reduced below the upstroke threshold. Oxygen point defects and their complexes may play an important role in many of these processes