The multi-atom per unit cell average t-matrix approximation is used to study evolution of the Ti(Ni1-xFex) and Ti(Ni1-xPdx) Fermi surfaces, as the concentration x is increased. The possible relation of the Fermi surface peculiarities with the B-R martensitic transformation in these alloys is discussed. It is theoretically shown that the appearance of the charge-density wave with wavevector Q1=2/3 pi (1, 1, 1) in the presence of a rhombic distortion can be caused by softening of the phonon mode with wavevector Q2=2/3 pi (1, 1, 0). On this basis a new microscopic mechanism for the R-phase formation in TiNi is proposed.
BYThe distinctive feature of the pseudobinary Ti( Nil-xCux) alloys is the "shape memory" effect connected with the martensitic transitions (MT) / l , 21. The initial phase of these alloys has the CsCl structure in the range 06xSO.56, and the composition x variation essentially influences on the sequence of the MT 12/.The mechanism of the MT in TiNi-based alloys is assumed to be of the electronic nature 131. There have been a few theoretical investigations of the electronic structure (ES) of the binary ordered TiNi / 3 to 61.For the analysis of the ES behaviour connected with the alloy composition variation and the long-range order ( q ) change the multi-atom per unit cell average t-matrix approximation I 7 I calculations are performed for Ti( NiO. 8Cuo. 2) and for disordered TiNi for q = 0.75.
The total density of the states (DOS) is calculated as follows:I where no(E) is the free electron DOS, tsl = Z Cas is the effective t-matrix at the sublattice S, and CaS are the average site t-matrix and the probability of finding an a-type atom at the site of the sublattice S, respectively, ' BZ gLs,L~ s~ (k) are the structure constants for a complex lattice, L {l,m} and is the Brillouin zone volume.The probabilities Cas are defined as follows: CTil = 1, CNi2 = 1-x, Ccu2 = x for Ti(NilmxCux) and CaS = ( 1 t q ) l Z for equiatomic disordered TiNi (the sign case). The calculations of the averages and the structure constants are carried out similar to 181.The calculated DOS for Ti(Ni0.8Cu0.2) (Fig. l a ) is similar to the DOS calculated in / 4 / for ordered TiNi (Fig. l a , inset). The DOS at the Fermi energy is found for the ternary alloy to be equal to 20.25 states/(Ryd cell spin) (the respective value a * 11+1r ( It -11 ) is taken for Ti at the sublattice S = 1 ( S = 2), and for N i in the opposite l ) Akademicheskii 8, SU-634055 Tomsk, USSR.
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