We investigated the crystal structures of high-and low-temperature phases in Sr 4 [Al 6 O 12 ]SO 4 , and their thermal behavior by high-temperature X-ray powder diffraction (Cu K¡ 1 ), differential thermal analysis, and temperature-dependent Raman spectroscopy. The crystal structure at 298 K was isomorphous with that of Ca 4 [Al 6 O 12 ]SO 4 (space group Pcc2 and Z = 4). The structural model at 573 K (space group I23 and Z = 2) was characterized by the positional disordering of oxygen atoms that form [SO 4 ] tetrahedra. The maximum-entropy method-based pattern fitting method was used to confirm the validity of these structural models, in which conventional structure bias caused by assuming intensity partitioning was minimized. The starting temperature of the cubic-to-orthorhombic transformation during cooling (= 524 K) was slightly higher than that of the reverse transformation during heating (= 519 K). The negative thermal hysteresis (= ¹5 K) strongly suggested the transformation being thermoelastic. At around the transformation temperature during heating, the vibrational spectra, corresponding to the Raman-active [SO 4 ] internal stretching mode, showed the continuous and gradual change in the slope of full width at half maximum versus temperature curve. This strongly suggests that the phase transformation would be principally accompanied by the statistical disordering of oxygen-atom positions, without distinct dynamical reorientation of the [SO 4 ] tetrahedra.
We have investigated the deposition characteristics of silicon oxide (SiO x) layers in atmospheric pressure (AP) argon (Ar)-based plasma at a substrate temperature of 120 °C. A 150 MHz, very high-frequency (VHF) power is effectively used for exciting stable and uniform hexamethyldisiloxane (HMDSO)-oxygen (O2) fed plasma under AP. The microstructure of the SiO x layers is discussed in comparison with that by using helium (He)-based plasma. In the case of depositions with He/HMDSO/O2 plasma, SiO2-like films with uniform thickness, which have sufficient compactness to be used as the gate dielectrics of bottom-gate thin film transistors, can be obtained by moving substrate at a constant speed during the plasma operation. However, the decrease in the total gas flow rate (increase in the gas residence time in the plasma) causes the increase in the participation of nanoparticles to the film growth, which might result in the deterioration of film quality. Shortening the electrode length is effective for avoiding the incorporation of nanoparticles into the growing SiO x films. On the other hand, when Ar/HMDSO/O2 plasma is used, no deterioration of film compactness is observed irrespective of the gas flow rate. The results obtained in this study demonstrate the effectiveness of the VHF excitation of AP plasma on the generation of stable and uniform glow discharge without using He, which will lead to the development of a highly efficient and reduced cost formation process of good-quality SiO x films.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.