Neutron scattering study of the charge-density wave transitions in 2H-TaSe/sub 2/ and 2H-NbSe/sub 2/

Journal Article · Fri Jul 15 04:00:00 EDT 1977 · Phys. Rev., B; (United States)

DOI:https://doi.org/10.1103/PhysRevB.16.801· OSTI ID:7308899

Moncton, D E; Axe, J D; DiSalvo, F J

We have used the triple-axis neutron-scattering technique to study 2H-TaSe/sub 2/ and 2H-NbSe/sub 2/, which undergo charge-density wave transitions at T/sub 0/ = 122.3 and = 33.5 K, respectively. The transitions in both compounds appear to be second-order and involve atomic displacements of ..sigma../sub 1/ symmetry. At inception, the superlattices in both compounds have nearly identical incommensurate wave vectors with magnitude q/sub delta/ = 1/3(1-delta) a*, with delta approx. 0.02. The NbSe/sub 2/ superlattice remains incommensurate to 5 K, but TaSe/sub 2/ undergoes a first-order lock-in transition where delta ..-->.. 0 at 90 K. The temperature dependence of the superlattice wave vector q in the incommensurate phase and the lock-in transition are discussed using a free energy involving third-order ''umklapp'' terms and a secondary order parameter. The secondary lattice distortion which is predicted in this model is observed experimentally. Most phonon branches having energies less than 10 meV with propagation vectors in the (zeta00) and (00zeta) directions have been measured at 300 K. Strong anomalies are found in the ..sigma../sub 1/(zeta00) phonon branches in both materials near the wave vector q/sub c/ = (1/3, 0, 0) characteristic of the low-temperature superlattices. Substantial softening of this phonon is observed as the transition is approached. In addition, the spectral profile exhibits a central peak which is not measurably inelastic.

• Research Organization: Physics Department, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
• OSTI ID: 7308899
• Journal Information: Phys. Rev., B; (United States), Journal Name: Phys. Rev., B; (United States) Vol. 16:2; ISSN PLRBA
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602* -- Other Materials-- Structure & Phase Studies
BASIC INTERACTIONS
CHALCOGENIDES
COHERENT SCATTERING
CRYSTAL-PHASE TRANSFORMATIONS
DIFFRACTION
ELECTROMAGNETIC INTERACTIONS
ENERGY LEVELS
FERMI LEVEL
INTERACTIONS
NEUTRON DIFFRACTION
NIOBIUM COMPOUNDS
NIOBIUM SELENIDES
PHASE TRANSFORMATIONS
PHONONS
QUASI PARTICLES
SCATTERING
SELENIDES
SELENIUM COMPOUNDS
TANTALUM COMPOUNDS
TANTALUM SELENIDES
TEMPERATURE DEPENDENCE
TRANSITION ELEMENT COMPOUNDS
UMKLAPP PROCESSES