| The appearance of a charge-density wave (CDW) and associated phase transitions has been identified in TiNi(Fe), a binary alloy. Bulk thermal and transport measurements have revealed that the "pre-martensitic instability" is composed of two distinct phase transitions. In TiNi(3.2% Fe), these occur at 232 K and 224 K. The Fe impurities are added to produce a large temperature separation between these transitions and the martensitic transformation. Elastic neutron diffraction and x-ray diffraction measurements have shown that the 232 K transition is associated with the appearance of incommensurate 1/3(1-d)(110) and 1/3(102d)(111) superlattices with d(DBLTURN).005 at the onset. The displacements of the 1/3(110) and 1/3(111) superlattices are transversely and longitudinally polarized, respectively. This transition is second order and is associated with a discontinuous change in the slope of the magnetic susceptibility. At 224 K, the lattice undergoes a sudden rhombohedral distortion which makes the lattice and superstructure commensurate. This transition is coincident with the prominent specific heat and resistivity anomalies. On further cooling, the rhombohedral distortion increases, but the superlattice remains commensurate. The total distortion is small; at 200 K, alpha is 89.5 degrees. The crystal domains in the rhombohedral phase according to a random choice of the dilated body diagonal.;Preliminary inelastic neutron diffraction measurements have shown 1/3(110) TA and 1/3(111) LA phonon anomalies. The soft 1/3 (110) phonon is particularly sharp and deep. The fermi surface of TiNi has saddle points spanned by a 1/3(110) wavevector. The 1/3(111) and 1/3(110) CDW's are believed to be coupled. |
