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1793mation about the direction of the segment. Some models of imaginary small crystals comprising tens of chain segments that are set to make the three-dimensional lattice of orthorhombic PE were considered in the calculation. The (100) contact plane of PE whose chain axis orients in the [OlO] direction of (CH,),, is the most probable. In the case of a normal paraffin (c36H74), the most stable orientation is obtained when a set of chain segments with the (110) contact plane orient in the (110) directions of C3&4 and the second most stable is the [OlO] orientation with a (110) contact plane. These stable contact planes are also predicted by lattice mismatching. The contact planes thus predicted are in good agreement with those observed in surface decoration of monoclinic cycloparaffins ((cH2)36 and (cH2)6,,) and orthorhombic normal paraffins (C36H74 and C,4H,,).69s~9 Acknowledgment.ABSTRACT The structures of poly(p-hydroxybenzoic acid) (PHBA) at the two high-temperature phase transitions (ca. 340 and 430 'C) have been investigated by X-ray powder diffraction methods (X-ray tube focusing and synchrotron parallel beam), chain conformation/packing models, and dielectric measurements. The structure of PHBA between ca. 340 and 430 OC, derived from the X-ray patterns and available electron diffraction results, is satisfactorily accounted for by the following model: orthorhombic cell, a = 9.24 A, b = 5.28 A, and c = 12.50 A, with two successive phenylene planes along the c-axis rotated 60" from each other in the unit cell; the phenyl groups in the a-b plane are oriented in an ordered herringbone-type packing in each phenyl layer with their planes 60" and 120° with respect to the a-axis; and each chain segment (of two monomer repeats) in the unit cell is positioned randomly in the a-b projection among the two different sites involving a twofold axis of reorientation through an angle T. The dielectric constants at high frequencies (100 kHz) indicate rapid chain mobility accompanying this high-temperature phase. This structure of PHBA therefore follows quite well the model of the smectic-E phase of small rodlike molecules. The second transition around 430 "C involves no change in the packing order along the chain axis or chain conformations but only the loss of long-range phenyl orientation order in the a-b plane, very much like the smectic-E to smectic-B transition of small rodlike molecules. The coherence length along the chain axis of PHBA crystals at room temperature is estimated to be ca. 2700 A, according to the Sherrer formula, and this packing order along the chain axis is maintained throughout the two high-temperature transitions. Relevance of the smectic-type high-temperature structures of PHBA to the molecular order in aromatic copolyesters comprising majority HBA monomers is then discussed.

The oxidation of tin layers deposited onto alumina substrates is investigated with the aim to identify the different steps of the process and obtain information on the sample homogeneity, phase segregation, and degree of oxidation. It is shown that at least three phases coexist at 450 °C, Sn, SnO, and SnO2, and remarkable inhomogeneities, already visible at an optical inspection, are found in the thin film. A micro-Raman mapping of the layer shows that these inhomogeneities are related to the presence of different Sn oxidation states, as evidenced by the inhomogeneous distribution of SnO and SnOx Raman bands. The thin film becomes homogeneous after annealing treatments above 550 °C, where only the SnO2 cassiterite phase is detected.

Abstraer. Paramagnetic singly ionized oxygen vacancies V(; and chemisorbed Sn4+-O 2 species were detected by electron paramagnetic resonance measurements on SnO 2 and transition metal (Pt, Ru)-doped SnO 2 thin film that had been reduced with CO at different temperatures and then brought into contact with oxygen. The amounts of the two paramagnetic species were evaluated and are discussed asa function of the film annealing temperature in air, the reduction temperature under CO, and the type and concentration of the doping transition element. A1so the structural properties of the film were identified through glancing incidence X-ray diffraction analysis. Measurements of the electrical sensitivity S (S = RJRco, where Ra~ , and Rco are the resistance under air and under CO(800 ppm)/air respectively) show that the trend of the sensitivity values vs. the reduction temperature with CO could be predicted by the parallel trend of the number of Sn4+-O~ centers.