H. D. Bist

Librational Modes of Crystal Water in Hydrated Solids

Abstract The first paper on librational modes (LMs) of water in hydrated solids appeared in 1956 [l], although some earlier papers mentioned them in the context of other studies [2–4]. No detailed review is available, but articles in some books on water [5, 6], hydrogen bonding [7], and infrared (IR) spectra of inorganic and coordination compounds [8, 9] give partial coverage to the subject. The present paper reviews the up-to-date information and understanding on the topic. No attempt has been made to create a comprehensive bibliography or to discuss and evaluate individual papers. The emphasis is on the development of methodology and systematics. In several cases, work on a particular substance is discussed because it brings out the salient features which are representative of many other salts. The review, which covers work published to June 1979, is limited mainly to IR [including attenuated total reflection (ATR)] and Raman studies, with some coverage of neutron inelastic scattering (NIS) and nuclear ...

Diamond polytypes in the chemical vapor deposited diamond films

Micro‐Raman spectroscopy and x‐ray diffraction measurements have been used to identify the presence of hexagonal diamond polytypes in chemical vapor deposited diamond films. The presence of additional reflections, in addition to the normal cubic diamond reflections in the diffraction spectra and bands at 1306 and 1193 cm−1 in the Raman spectra, are attributed to the presence of diamond 6H polytype. The presence of the 1324 cm−1 band in addition to the main diamond peak in the expanded spectra is attributed to regions with hexagonal symmetry, thus supporting the presence of hexagonal diamond polytypes. The compressive strain in diamond layers on the Si substrate is believed to create regions of hexagonal symmetry and vice versa.

Photoluminescence study of deep levels in Cr-doped ZnSe

Single crystals of intrinsic ZnSe were grown by the seeded physical vapor transport method and the diffusion doping was utilized to incorporate Cr in these crystals. The radiative recombinations in these samples with Cr concentration in the range 1.0–10.2×1019 cm−3 were studied by the steady state photoluminescence technique. It was found that the Cr deep centers inhibit the band-to-band emission in Cr-doped ZnSe. Except in undoped single crystals, no emission corresponding to the band-to-band transition was observed from any of the doped samples. Instead, the higher wavelength emissions associated with Cr deep levels were obtained. This capture of photoexcited carriers by deep centers was verified using different excitation wavelengths. The role of chromium impurities in nonradiative recombination processes was also evidenced from the sharp decreases in the deep level emission intensity with increasing Cr concentration.

Effect of substrate temperature and heat treatment on the microstructure of diamondlike carbon films

Nonhydrogenated diamondlike carbon films prepared at a substrate temperature (ST) of 100, 300, and 500 °C by the laser ablation of graphite on a single‐crystal silicon substrate have been characterized by scanning tunneling microscopy for the surface structure and Raman spectroscopy for the microstructure. Distorted pentagonal and hexagonal rings are observed on the surface of the film grown at 100 °C while only hexagonal rings are observable for the one grown at 500 °C. The rise in ST is found to increase the surface roughness. To assign the various coexisting carbonaceous species formed at different growth temperatures and to check their thermal stability, heat treatment was performed at up to 1300 °C in vacuum and 600 °C in air. The changes occurring on heat treatment in vacuum in these films around 600 °C have been correlated with the release of defects from the threefold network. Likewise, 950 °C temperature has been associated with the conversion of disordered tetrahedral bonding to a distorted trig...

Spectra and structure of small ring molecules. XLVII. Rotational and vibrational spectra and conformational stability of cyclobutylcarboxylic acid fluoride

The microwave spectrum of cyclobutylcarboxylic acid fluoride, c‐C4H7CFO, has been recorded from 18.0 to 40.0 GHz. The a‐type R‐branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial‐gauche conformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm−1. From the Stark effect the dipole moment components were determined to be: ‖μa‖ =2.97±0.02, ‖μb‖ =0.84±0.01, ‖μc‖ =0.35±0.01,...

Stoichiometry of the diamond/silicon interface and its influence on the silicon content of diamond films

Thin films of diamond were grown by microwave plasma chemical vapor deposition at growth pressures of 10, 20, 40, and 60 Torr keeping the substrate temperature constant at 975 °C. Increase in the growth pressure reduced the size of the plasma ball resulting in an increase in the microwave power density (MPD). The films were characterized by scanning electron microscopy, micro-Raman, and photoluminescence (PL) spectroscopy. A systematic variation was observed in surface morphology and quality of the films. The intensity of the peak at 1.68 eV in the PL spectra of the films, which is assigned to Si impurities was also observed to increase consistently with the MPD. The stoichiometry of the diamond/silicon interface was studied by x-ray photoelectron spectroscopy (XPS) and found to be a sensitive function of the MPD. XPS results showed the formation of nonstoichiometric SiC along with other carbon phases in the initial stages of the growth. A correlation was observed between the composition of the interface ...

Micro‐Raman and infrared reflectivity spectra of YBa2Cu3O7−δ deposited on LaAlO3 and stainless steel/inconel substrates

The motivation for this article has been to try to understand the relative phenomenological differences between the behavior of the films of the high‐temperature superconductors like YBa2Cu3O7−δ deposited on (a) relatively insulating substrates (like LaAlO3) through sputtering technique and (b) highly metallic substrates (like stainless steel and inconel) through laser ablation method. It is observed that the most prominent Raman bands on LaAlO3 substrate falling near 337, 440, and 503 cm−1 have half widths (defined as the full width at half the maximum intensity) of 22, 42, and 38 cm−1, respectively. The half width gets nearly doubled on the metallic substrates. These results have been attributed to the metal‐superconductor interfacial interactions. Further, infrared reflectance measurements and the Kramers–Kronig analysis of the spectra reveal a large difference in the conductivity of the film on LaAlO3 compared to the films on metallic substrates.

A Raman study of the high temperature phase transition in lithium ammonium sulphate

Abstract The temperature dependence of the Raman scattering from LiNH 4 SO 4 has been studied to investigate the phase transition at 459.5 K, which is found to be of second-order type. The crystals show increased ordering of the molecular ions at room temperature, the sulphate ion sublattice being severely distorted in the ferroelectric state. The reorientational motion of the sulphate ions appears to be mainly responsible for the observed order-disorder transition. The ferroelectric state of the crystal is characterized by stronger hydrogen bonds.

Intercalation of Bi- and Tl-oxide superconductors with conjugated ring-shaped organic molecules: X-ray, Raman scattering, microwave and magnetic studies

Abstract The characterization of Bi- and Tl-oxides, intercalated with either benzene or metal-phthalocyanines MPc (M=Cu, Ni; Pc=C 32 N 16 H 8 ), by X-ray diffraction, Raman scattering and microwave absorption spectroscopies, as well as by DC and AC magnetization techniques, is reported. All the samples exhibit unusual magnetic behaviour at 300 K, similar to those reported earlier for the benzene-intercalated Bi-Oxides, observed in magnetization and microwave studies. The spectroscopic fingerprints of the phase responsible for the anomalies are identified. The broad vibrational modes at higher frequencies above 700 cm -1 were observed in the intercalated samples and are at different frequencies from those of the pure organic species. We speculate that the magnetic anomalies are most likely not due to the host oxides, but rather to the appropriate spatial arrangement of the specific organic molecules, favoring the orbital magnetism of delocalized π-electrons over the molecules.

Fermi resonance in M(CH3COO)2.4H2O (M Mg, Co and Ni)☆

Abstract A comprehensive analysis of the vibrational spectra (i.r. and Raman) suggests Fermi resonance interactions between fundamental modes of the acetate group and two-phonon modes associated with water in the tetrahydrates of Mg, Co and Ni acetates. The interaction is confirmed through the variations in positions and intensity ratios of the concerned bands with temperature and isotopic dilutions. It is found that the Fermi interaction constant is almost the same in all these compounds and is unaffected by lowering the temperature.