X-RAY DIFFRACTION STUDIES OF LAYERED CRYSTAL STRUCTURES: GRAPHITE AND 1T - TANTALUM SULFIDE (DEBYE TEMPERATURE, CHARGE-DENSITY-WAVES)
Abstract
We have measured the x-ray Bragg reflections of the (0018) in graphite and the (0016) in 1T-TaS(,2) as functions of temperature from room temperature ((TURN)296(DEGREES)K) down to liquid helium temperature ((TURN)4.2(DEGREES)K). The mean-square amplitudes of thermal vibrations in both cases are found to be linear in temperature for T > 120(DEGREES)K and lower than the values predicted by the Debye model at low temperatures (T < 30(DEGREES)K). This result is believed to be due to the anisotropic character of a layered structure. The Debye temperature deduced for graphite is 572 (+OR-) 5(DEGREES)K. The graphite data are also discussed with the surface values found by another group. Changes in CDW's structures are found to have no effect on the thermal attenuation of the (0016) Bragg reflection in 1T-TaS(,2) except that the (0016) integrated intensity is found to be 44% weaker than the value calculated for a mosaic crystal. A possible modulation factor due to CDW's is discussed. We also measured the integrated intensities vs. temperature for the (040) and (050) of 1T-TaS(,2), which show an anomalous thermal behavior and indicate a strong CDW's effect. A transition between the quasi-commensurate and the commensurate phase of CDW's is observed within a wide interval, between 140(DEGREES)K and 190(DEGREES)K. The measurements were done on an absolute basis. Using the simple CDW's sinusoidal model, we estimate that the Ta-ion displacement along the basal planes is (TURN).160 (+OR-) 0.001(ANGSTROM) at room temperature and (TURN).172 (+OR-) 0.001(ANGSTROM) near 6(DEGREES)K.
Degree
Ph.D.
Subject Area
Condensation
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