Search

The Raman scattering of $\ensuremath{\mu}\mathrm{c}\ensuremath{-}{\mathrm{WO}}_{3}$ was investigated for pressures ranging from atmospheric to 4 GPa. The material, originally a mixture of two crystalline structures with groups ${P2}_{1}/n$ (monoclinic). and $P1\ifmmode\bar\else\textasciimacron\fi{}$ (triclinic), transforms gradually to a new variety belonging to the group ${P2}_{1}/c$ (monoclinic). This gradual process starts at a very low pressure and ends up at a pressure ${P}_{t}=1.4 \mathrm{GPa}.$ This is the only structural transformation observed to occur for pressures up to $P=4.0 \mathrm{GPa}.$ Pressure coefficients are given for ${\mathrm{WO}}_{3}$ in the phases studied. Analysis of the data clearly demonstrates the existence of a unique phase $(\ensuremath{\epsilon})$ for $\ensuremath{\mu}\mathrm{c}\ensuremath{-}{\mathrm{WO}}_{3}$ at high pressures. As the $\ensuremath{\epsilon}$-phase Raman features were found to subsist on releasing the pressure, we believe the $\ensuremath{\mu}\mathrm{c}\ensuremath{-}{\mathrm{WO}}_{3}$ retain the corresponding structure. This structure relaxes partially to the triclinic form one day after releasing the pressure to $P=0.$ No indication of size effects was found in this experiment.

This work reports Raman detection of the tetragonal $\ensuremath{\rightarrow}$ monoclinic phase transition in ${\mathrm{PbZr}}_{0.52}{\mathrm{Ti}}_{0.48}{\mathrm{O}}_{3}$ ceramics near the morphotropic phase boundary at low temperatures. The transition is characterized by changes in the frequency of lattice modes with temperature. The results presented here confirm the previous one recently reported by Noheda et al. [Appl. Phys. Lett. $74,$ 2059 (1999)] using high-resolution synchrotron x-ray powder diffraction technique and dielectric measurements. The stability of the new phase is discussed within the framework of phenomenological Landau-Devonshire theory.

Raman spectra of ${(\mathrm{AlN})}_{8\ensuremath{-}\ensuremath{\delta}}/({\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}{)}_{\ensuremath{\delta}}/{(\mathrm{GaN})}_{8\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}/({\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}{)}_{\ensuremath{\delta}}$ superlattices with interface thickness varying between $\ensuremath{\delta}=0$ and $\ensuremath{\delta}=3$ are calculated. The influence of the nonabrupt interface related broadening is described in the complete range of scattering, with special attention to the modes giving stronger contribution to the Raman intensity. It is shown that the dispersion of folded acoustic phonons does not change appreciably with the interface smoothing. For $\ensuremath{\delta}=0$ the Raman spectra display new peaks due to the enhancement of some confined optic modes.

In this work we present the uniaxial pressure-temperature phase diagram of ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ (KDP), for temperatures near the ferroelectric phase transition at 122 K. Using Raman measurements we show that under uniaxial pressure the KDP undergoes two metastable transitions: (i) from tetragonal ${D}_{2d}^{12}$ phase to orthorhombic ${C}_{2\ensuremath{\nu}}^{j\ensuremath{\ne}19}$ phase at 126 K and ${\ensuremath{\sigma}}_{6}=1.12\mathrm{kbar},$ and (ii) from orthorhombic ${C}_{2\ensuremath{\nu}}^{19}$ phase to the same orthorhombic ${C}_{2\ensuremath{\nu}}^{j\ensuremath{\ne}19}$ phase as before, at 110 K and ${\ensuremath{\sigma}}_{6}=0.3\mathrm{kbar}.$ We discuss the reversibility paths of these metastable phases, and provide an explanation to the appearance of the ${C}_{2\ensuremath{\nu}}^{j\ensuremath{\ne}19}$ phase, based on the Gibbs free energy density of the system.

${\text{Dy}}_{2}{\text{Mo}}_{4}{\text{O}}_{15}$ crystal was investigated by high-pressure Raman spectroscopy experiments. The assignment of vibrational modes at ambient pressure was made based on lattice-dynamics calculations and the results helped us also to discuss high-pressure transformations observed in this material. A pressure-induced structural phase transition was observed at about 2.0 GPa. This transition is associated with significant rearrangement of the molybdenum-oxygen sublattice and an increase in the molybdenum-oxygen coordination most likely changing from tetrahedral to pentagonal. The bond compressibility, related to the Mo-O bonds, were estimated for both $P{2}_{1}/c$ and high-pressure phase. Upon further increase in pressure ${\text{Dy}}_{2}{\text{Mo}}_{4}{\text{O}}_{15}$ exhibits irreversible amorphization above 4.7 GPa. The irreversible character of this transformation points out to either chemical decomposition or crystallographic transformation as a cause of this amorphization process. Our analysis suggests that the amorphization of ${\text{Dy}}_{2}{\text{Mo}}_{4}{\text{O}}_{15}$ may be most likely attributed to a chemical decomposition.

Polarized Raman scattering studies and lattice-dynamics calculations were performed at ambient pressure and temperature. These results allowed us to propose the normal-mode symmetries and assignments. High-pressure Raman scattering studies were further performed revealing the onset of two reversible phase transformations near 1.2 and 4.5 GPa with different structural characteristics. The pressure dependence of Raman bands provides strong evidence that the first phase transition involves significant shifts of the potassium atoms associated with small shifts of the niobium atoms whereas the second phase transition is associated mainly with significant shifts of the Nb atoms toward the center of the ${\text{NbO}}_{6}$ octahedra. In the structure stable above 4.5 GPa the Nb atoms are dynamically disordered among symmetry equivalent positions.

Lattice dynamics calculations and high-pressure Raman scattering study of ${\text{Bi}}_{2}{\text{W}}_{2}{\text{O}}_{9}$, which is an $m=2$ member of the cation-deficient bismuth layered Aurivillius family of compounds, are presented. These studies showed the onset of two reversible second-order phase transitions near 2.8 and 4.8 GPa. The pressure dependence of Raman bands provides strong evidence that the first phase transition involves the loss of the ${\text{WO}}_{6}$ tilt mode around pseudotetragonal axis. The second transition is likely related to some shift of the W atoms and/or tilting of the ${\text{WO}}_{6}$ octahedra plus shift of the Bi atoms or the whole ${\text{Bi}}_{2}{\text{O}}_{2}$ layers. In contrast to the $m=1$ cation-deficient bismuth layered compounds, ${\text{Bi}}_{2}{\text{WO}}_{6}$ and ${\text{Bi}}_{2}{\text{MoO}}_{6}$, no soft-mode behavior was observed for ${\text{Bi}}_{2}{\text{W}}_{2}{\text{O}}_{9}$ and the transition at 4.8 GPa leads to symmetry lowering.

The effect of 170 keV Si and 100 keV C ion bombardment on the structure and properties of highly pure, double-wall carbon nanotubes has been investigated using resonance Raman spectroscopy. The implantations were performed at room temperature with ion doses ranging between $1\ifmmode\times\else\texttimes\fi{}{10}^{13}\text{ }\text{ions}/{\text{cm}}^{2}$ and $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ions}/{\text{cm}}^{2}$. As expected, the Si irradiation created more disorder than the C irradiation for the same ion fluence. For both species, as the ion-implantation fluence increased, the $D$-band intensity increased, while the $G$-band intensity decreased, indicating increased lattice disorder, in analogous form to other forms of graphite and other nanotube types. The frequency of the $G$ band decreased with increasing dose, reflecting a softening of the phonon mode due to lattice defects. With increasing ion fluence, the radial breathing modes (RBMs) of the outer tubes (either semiconducting or metallic) disappeared before the respective RBM bands from the inner tubes, suggesting that the outer nanotubes are more affected than the inner nanotubes by the ion irradiation. After Si ion bombardment to a dose of $1\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ions}/{\text{cm}}^{2}$, the Raman spectrum resembled that of highly disordered graphite, indicating that the lattice structures of the inner and outer nanotubes were almost completely destroyed. However, laser annealing partially restored the crystalline structure of the nanotubes, as evidenced by the re-emergence of the $G$ and RBM bands and the significant attenuation of the $D$ band in the Raman spectrum.

Magnetic properties of graphenic carbon nanostructures, which are relevant for future spintronic applications, depend crucially on doping and on the presence of defects. In this paper we study the magnetism of the recently detected substitutional Ni (Nisub) impurities. Nisub defects are nonmagnetic in flat graphene and develop a nonzero-spin moment only in metallic nanotubes. This surprising behavior stems from the peculiar curvature dependence of the electronic structure of Nisub. A similar magnetic-nonmagnetic transition of Nisub can be expected by applying anisotropic strain to a flat graphene layer., E.J.G.S., A.A., and D.S.P. acknowledge support from the Basque Government and UPV/EHU Grant No. IT-366-07, CSIC, the Spanish MEC Grant No. FIS2007-66711-C02-02, and the Basque Government and Diputación Foral de Guipuzcoa through the ETORTEK program. A.G.S.F. acknowledges the support from Brazilian agencies FUNCAP, CAPES/FAPERGS, CNPq Grant No. 306335/2007-7, Rede Nacional de Pesquisa em Nanotubos de Carbono, Rede de Nanobioestruturas, and Instituto do Milenio de Nanotecnologia CNPq/MCT-Brazil.

High-pressure Raman scattering and luminescence studies of ${\mathrm{Bi}}_{2}\mathrm{W}{\mathrm{O}}_{6}:{\mathrm{Eu}}^{3+}$, which is a member of the bismuth layered Aurivillius family of ferroelectrics, are presented. These studies showed the onset of two reversible second order phase transitions near 3.4 and $6.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. The pressure dependence of Raman bands provides strong evidence that the first phase transition involves the loss of the $\mathrm{W}{\mathrm{O}}_{6}$ tilt mode around the pseudotetragonal axis. This structural change may be the same as that observed at ambient pressure at $660\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ (from the $Pca{2}_{1}$ to $B2cb$ structure). The second phase transition is associated with the instability of a low wave number mode, which behaves as soft mode. The discovery of the soft mode in ${\mathrm{Bi}}_{2}\mathrm{W}{\mathrm{O}}_{6}$ that corresponds most likely to the ${E}_{u}$ mode responsible for ferroelectricity in Aurivillius family of compounds proves that this transition is of displacive type. Our studies also suggest that the structure above $6.2\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$ is orthorhombic and centrosymmetric and, consequently, the phase transition can be classified as a pressure-induced ferroelectric to a paraelectric phase transition.

The structural and electronic properties of $\mathrm{Cr}{\mathrm{O}}_{3}$ interacting with graphene layer are calculated using ab initio methods based on the density functional theory. The $\mathrm{Cr}{\mathrm{O}}_{3}$ acts as an electron acceptor modifying the original electronic and magnetic properties of the graphene surface through an adsorption process. The changes induced in the electronic properties are strongly dependent on the $\mathrm{Cr}{\mathrm{O}}_{3}$ adsorption site, and for some sites, it is possible to open a gap in the electronic band structure. Spin polarization effects are also predicted for some adsorption configurations for which the binding energy is lower and charge transfer is higher.

A Raman-scattering study of the optical coupled soft mode in ${R}_{b}{\mathrm{H}}_{2}{\mathrm{AsO}}_{4}$ is reported. It was found that in the temperature range 110\char21{}220 K the soft mode frequency has the behavior ${\ensuremath{\omega}}_{1}^{2}=29.5{\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}/\mathrm{K}(T\ensuremath{-}91$ K). Therefore, there is a large gap between the temperature at which \ensuremath{\omega} extrapolates to zero (91 K) and the transition temperature of the mechanically free crystal ${(T}_{c}=110$ K). We predict that 91 K is the transition temperature of the mechanically clamped crystal, not yet measured, and that the large gap between these temperatures is the result of unusually large coupling between the optical soft mode and the shear $\mathrm{xy}$ acoustic mode.

The structural and electronic properties of FeCl3 interacting with single-wall carbon nanotubes SWNTs were investigated by ab initio methods. By using first-principles spin-polarized calculations, we studied the structural and electronic behavior of FeCl3 adsorbed on both semiconducting and metallic SWNTs. It was found that the FeCl3 molecule behaves as an electron acceptor. The binding energy is very small, thus suggesting that the interaction is through a physisorption regime.

$\mathrm{K}{\mathrm{H}}_{2}\mathrm{P}{\mathrm{O}}_{4}$ (KDP) crystals doped with Mn ions were studied by an x-ray multiple diffraction technique using synchrotron radiation at room temperature. Asymmetric peaks were observed for both undoped and doped crystals, a characteristic of perfect crystals. X-ray powder diffraction at room temperature and analysis by Rietveld refinement were also performed, showing that doped KDP has the same tetragonal structure as undoped KDP, but with contraction of the lattice parameter for [100], [010], and [001] directions. The polarized Raman spectra of KDP and KDP:Mn were obtained for temperatures in the range $10--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Some of the features observed in the spectra were interpreted as consequences of phase transitions. Although group theory analysis shows that both doped and undoped crystals have the same symmetry at room temperature, at low temperature they have different symmetries: a previously reported phase transition for KDP at $60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ was not observed for KDP:Mn.

M\"ossbauer and Raman measurements for ${\mathrm{Li}}_{x}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$ and ${\mathrm{Li}}_{x}{M}_{0.03}{\mathrm{Fe}}_{0.97}\mathrm{P}{\mathrm{O}}_{4}$ $(M=\mathrm{Cr},\mathrm{Cu},\mathrm{Al},\mathrm{Ti})$ were performed. The results for the ${\mathrm{Li}}_{0.11}\mathrm{Fe}\mathrm{P}{\mathrm{O}}_{4}$ oxide require an unknown additional contribution, besides the high-spin ${\mathrm{Fe}}^{2+}$ doublet and the high-spin ${\mathrm{Fe}}^{3+}$ doublet, for the M\"ossbauer profile to be properly accounted for. The corresponding Raman scattering suggests a two-phase material and an additional disordered structure present in the mixture. The M\"ossbauer-transformed patterns indicate the unknown doublet to be due to a disordered phase also. In addition, the observation of a high amount of ${\mathrm{Fe}}^{2+}$ sites for the low-lithium-content ${\mathrm{Li}}_{x}{M}_{0.03}{\mathrm{Fe}}_{0.97}\mathrm{P}{\mathrm{O}}_{4}$ oxides is analyzed on the basis of the electronic charge configuration of the Li-lacking octahedral units to suffer small changes upon lithium extraction.

We calculate structural and electronic properties of ${\mathrm{CaCu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ from ambient pressure up to 10.0 GPa using total-energy first-principles methods. Analysis of the simulated high-pressure structure suggests a pressure-induced structural phase transition between 3 and 4 GPa with the crystalline structure changing from $\mathrm{Im}\overline{3}$ (cubic) to $R\overline{3}$ (rhombohedral). By analyzing the electronic band structure, we have observed a pressure-induced semiconductor-metallic transition between 6 and 7 GPa.

High pressure Raman scattering and x-ray diffraction studies of the molybdate ${\mathrm{Sc}}_{2}({\mathrm{MoO}}_{4}{)}_{3}$ are presented. A sequence of changing symmetry effects is observed through two structural phase transitions ending up with an amorphous state. The observed two structural phase transformations are reversible. The crystal to amorphous transition is irreversible. Our results point out that the amorphization process in ${\mathrm{Sc}}_{2}({\mathrm{MoO}}_{4}{)}_{3}$ may be due to a kinetic hindrance of a phase transition rather than due to chemical decomposition effects as have been proposed to occur for some molybdates and tungstates.

Polarized Raman scattering and infrared reflectivity of the modified pyrochlore single crystal CsInMgF 6 have been carried out at room temperature. The spectra showed the appearance of additional Raman and infrared bands,which are not predicted from group theory for the pyrochlore cubic structure. A deeper analysis, by taking into account the occupational disorder of Mg 2 + and In 3 + in octahedral sites, indicates that the overall features can be explained by a lowering of the crystal symmetry from F3dm (O 7 h ) to R3m (D 5 3 d ). This result should have important consequences on the understanding of the unusual magnetic behavior of this system.

In this paper we present a Raman-scattering study of the phase transitions in the ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ systems around the morphotropic phase boundary over a wide temperature range. The boundary between rhombohedral and monoclinic phases was found to be a quasivertical line between $x=0.46$ and $x=0.47.$ We also studied the monoclinic-tetragonal phase boundary and our spectroscopic results agree very well with those reported by using x-ray diffraction.

The Raman spectra of zinc-blende InN/GaN superlattices were calculated assuming the existence of an interface region with thickness \ensuremath{\delta} varying from one to three monolayers. The acoustic branches are weakly affected by interfacing, but the optical branches can present frequency shifts up to $60 {\mathrm{cm}}^{\ensuremath{-}1}.$ A downward shift is observed for the higher frequency and an upward shift for the lower frequency modes. As a consequence, the Raman peaks collapse together in the middle frequency range giving rise to a most prominent structure in the spectrum, for $\ensuremath{\delta}=3.$ These effects are tracked to the localization of atomic displacements at the direct and inverse interface regions. The localization effects are strong in the InN/GaN superlattices because of the wide gap observed in the phonon density of states for both constituent materials.

The Raman spectra of ${\mathrm{PbZr}}_{1\ensuremath{-}x}{\mathrm{Ti}}_{x}{\mathrm{O}}_{3}$ ceramics with titanium concentration varying between $x=0.40$ and 0.60 were measured at 7 K. By observing the concentration-frequency dependence of the vibrational modes, we identified the boundaries among the rhombohedral, monoclinic, and tetragonal ferroelectric phases. The analysis of the spectra was made in the view of group theory analysis making possible the assignment of some modes for the monoclinic phase.

Raman spectra of ${\mathrm{LiNH}}_{4}$${\mathrm{SO}}_{4}$ in frequency range from 600 to 1200 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ were measured from atmospheric pressure up to a pressure at 82 kbar. Two distinct phases are identified in the ranges below 13 kbar (\ensuremath{\beta} phase) and above 35 kbar (\ensuremath{\delta} phase). In the range between 13 and 35 kbar the spectrum changes qualitatively in a continuous manner, both as a function of time for fixed pressure and as a function of pressure when the latter is rapidly changed. The results suggest a dynamical reorientation of the ${\mathrm{SO}}_{4}$${\mathrm{}}^{2\mathrm{\ensuremath{-}}}$ ions taking place in this pressure interval.