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MIXED-CRYSTAL LATTICE DYNAMICS OF HAFNIUM(X)TITANIUM(1-X)SELENIUM(2)
Abstract
The first-order Raman spectra of the mixed-crystal layered compound of Hf(,x)Ti(,1-x)Se(,2) have been studied over the whole range of concentrations 0 (LESSTHEQ) x (LESSTHEQ) 1. The main concerns of the results reported here are: (1) The lattice dynamical properties of the Hf(,x)Ti(,1-x)Se(,2) mixed-crystal system; and (2) the effect of Hf doping on the displacive structural phase transition in TiSe(,2). Both pure compounds, TiSe(,2) and HfSe(,2), have the Raman-active phonons, one of A(,1g) symmetry and one of E(,g) symmetry. Both of these phonons show a "two-mode" behavior in the mixed-crystals. A modified random-element isodisplacement model involving only short-range force was developed to predict the observed concentration dependences of the frequencies of these phonons. The calculated frequencies are in good quantitative agreement with the results of the Raman measurements. Below 200(DEGREES)K, the layered compound TiSe(,2) undergoes a phase transition to a structurally-distorted state, and electron-diffraction studies show a superlattice with a = 2a(,0) and c = 2c(,0). Because of the enlarged crystalline unit cell, additional superlattice peaks are observed in the Raman spectra of pure TiSe(,2). These peaks persist in the mixed-crystal up to 10% Hf concentration for a sample temperature of 100(DEGREES)K. Such behavior is expected since resistivity measurements reveal that the anomaly due to superlattice formation is suppressed as the Hf concentration is increased. The results presented here suggest that the suppression of the superlattice phase is related to the transformation from semimetal (TiSe(,2)) to semiconductor (HfSe(,2)) which occurs in the vicinity of x (TURNEQ) 0.3. Resonant enhancement of some of the Raman-active modes in pure and lightly-doped HfSe(,2) was also observed. It was found that the size of the enhancement decreased as the level of Ti-doping increased. This indicates that Ti doping reduces the size of the energy gaps responsible for the resonant enhancement.
Subject Area
Condensation
Recommended Citation
KATKANANT, VANVILAI, "MIXED-CRYSTAL LATTICE DYNAMICS OF HAFNIUM(X)TITANIUM(1-X)SELENIUM(2)" (1983). ETD collection for University of Nebraska-Lincoln. AAI8318662.
https://digitalcommons.unl.edu/dissertations/AAI8318662