Raman spectra of titanium dioxide

Medium Effects on Charge Transfer in Metal Complexes.

This paper reports a methodology for analyzing the solvent effect from empirical measurements of solvent acidity (SA), basicity (SB), dipolarity (SdP), and polarizability (SP). The proposed methodology departs from the traditional single-parameter procedures for estimating nonspecific solvent effects by splitting them into a polarizability term and a dipolarity term. In this work, we examined the SA, SB, SP, and SdP values for 160 solvents, the gas phase (the absence of solvent) being the origin of these scales. As shown in this paper, this information allows one not only to accurately describe the solvent effect experienced by any solute—whether polar or nonpolar and exhibiting some or no specific interaction with the solvent—but also to understand the nature of the well-known solvent parameters ET(30), π*, S′, and SPP, which are frequently used to describe the overall nonspecific contribution of solvents in terms of a single parameter. The high potential of the proposed empirical methodology is illustra...

Toward a Generalized Treatment of the Solvent Effect Based on Four Empirical Scales: Dipolarity (SdP, a New Scale), Polarizability (SP), Acidity (SA), and Basicity (SB) of the Medium

The displacement of electronic absorption and luminescence spectra (solvatochromic shifts) are related to the solute—medium interactions. These interactions can be non-specific (dielectric interactions) when they depend only on multiple and polarizability properties of the solute and solvent molecules; but specific associations such as hydrogen bonding can also be important. A number of examples of solvatochromic shifts are shown and discussed according to the various solute—medium interactions. The properties of solvent mixtures and those of rigid media are considered, as well as the “thermochromic shifts” which result from the change in the temperature of the medium. The use of solvatochromic shifts for the determination of the dipole moment and of the polarizability of electronically excited molecules has been important for an understanding of electron distribution changes in such states; examples of such determinations are given, together with references to the original literature. In the final section some limitations of the theories of solvent shifts and possible improvements are discussed.

Invited review solvatochromic shifts: The influence of the medium on the energy of electronic states

The Carbonate Minerals

Raman vibrational spectra at pressures as high as 40 kbar are reported for the scheelite and previously unknown high‐pressure phases of CaMoO4 and CaWO4. The pressure derivatives of the vibrational frequencies, dν / dP, are all between 0.0 and 1.0 cm−1·kbar−1 which is normal for ionic crystals; however, the observed pressure dependences of the frequencies of the internal modes are qualitatively different from those predicted according to Scott's interpretation of the splittings of the scheelite spectra. The appearance of the high‐pressure phases is indicated by splittings of Eg modes at 145 and 797 cm−1 in CaMoO4 and 117 and 797 cm−1 in CaWO4. The spectra of the high‐pressure phases and the dependences of the apparent transition pressures upon orientation of the crystallographic axes relative to the small stress anisotropy in the high‐pressure cell suggest that the high‐pressure phases have structures similar to that of NiWO4.

Vibrational Raman Spectra of CaMoO4 and CaWO4 at High Pressures

This research was supported by the U. S. Atomic Energy Commission. Present address of Jean F. Durana: Department of Chemistry, Yale University, New Haven, Conn. $^{1}$J. F. Scott, J. Chem. Phys. 49, 98 (1968).""

VIBRATIONAL RAMAN SPECTRA OF $CaMoO_{4}$ AND $CaWO_{4}$ at High PRESSURES

Raman spectra of the rutile and high‐pressure orthorhombic (TiO2 II) phases of titanium dioxide have been studied at pressures as high as 40 kbar. The frequency of the B1g mode of rutile has an unusual negative pressure dependence, dυ / dp = − 0.3 ± 0.1 cm−1·kbar−1. Onset of the rutile:TiO2 II transition is observed at pressures above 26 kbar at room temperature, although the apparent transition pressure and extent of transformation depend upon shear stresses applied to the sample. Shears applied normal to the optic (tetragonal) axis of rutile are most effective for inducing the transition.

Raman Spectrum and Polymorphism of Titanium Dioxide at High Pressures

Raman spectra of single crystals of α quartz at pressures as high as 40 kbar are reported The frequencies of the Raman lines located at 128, 207, 265, 464, 697, 795, and 807 cm−1 at atmospheric pressure increase with increasing pressure by about 06, 18, 05, 09, 08, 08, and 08 cm−1·kbar−1, respectively, with a probable accuracy of about 20% due to an uncertainty about the pressure scale These frequencies of the α‐quartz spectrum also are significantly sensitive to temperature The frequencies of all other Raman lines of α quartz, except three at 145, 450, and 509 cm−1 that could not be studied, are independent of pressure

Raman Spectrum of α Quartz at High Pressures

Fluorescence spectra of ultrapure crystals of naphthalene, anthracene, and phenanthrene have been studied at pressures as great as 50 kbar. These spectra exhibit complex irreversible effects superimposed upon the expected reversible shifts in the energies of the normal fluorescence spectra. The irreversible effects include a loss of the intensity of the normal fluorescence with increasing pressure and the almost simultaneous appearance of a broad, featureless emission at energies about 3000–6000 cm−1 lower than that of the normal fluorescence. This emission is assigned as the fluorescence of excimers formed upon optical excitation of the crystals under high pressure. The crystals continue to exhibit this excimer fluorescence after the pressure is reduced to atmospheric pressure, but the irreversible effects can be removed by thermal annealing of the crystal at atmospheric pressure. The irreversibility is attributed to trapping of pairs of molecules in an excimer‐like orientation as a crystal defect after they lose the excitation energy of the excimer. These defects then act as traps for excitation energy in the crystal. Other possible interpretations of the irreversible effects are discussed.

Malcolm Nicol

Papers

Vibrational Raman Spectra of CaMoO4 and CaWO4 at High Pressures

VIBRATIONAL RAMAN SPECTRA OF $CaMoO_{4}$ AND $CaWO_{4}$ at High PRESSURES

Raman Spectrum and Polymorphism of Titanium Dioxide at High Pressures

Raman Spectrum of α Quartz at High Pressures

Excimer Emission of Naphthalene, Anthracene, and Phenanthrene Crystals Produced by Very High Pressures