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Through single-crystal x-ray diffraction methods, the crystal structure of TiNi has been determined in the temperature range −70° to 900°C. Contrary to what has been assumed from previous work based on the powder pattern methods, the TiNi crystal structure is not a simple CsCl type. Rather, it has an a0=9Å superlattice and an a0=3Å sublattice with 54 atoms per unit cell complex structure. The 9Å superlattice undergoes, at about 166°C, a ``martensitic'' pseudo order-disorder transition which is not accompanied by a crystallographic transformation. Through the understanding of this unique transition the apparent contradicting observations made on TiNi by various past investigators can now be reconciled and the unusual physical properties associated with the alloy are explained qualitatively.

Through an investigation of the transport and related thermodynamic properties of TiNi at and around its ``martensitic'' transition temperature, the existence of a critical range extending over a 60°C interval is established. Within this critical range, the phase transition is second order. Irreversibility of various properties within the critical range is interpreted in terms of irreversible shear movement of atoms. On the basis of transport data, the band structure of TiNi is inferred to be a single or ``nearly'' single positive band within the temperature range investigated. The postulate that some of the valence electrons undergo a ``covalent''→``conduction'' electronic transformation in the course of the second-order transition is consistent in large measure with experimental data.

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Based on combined x-ray and neutron-diffraction data, an inference for the coexistence of the B2(CsCl) and P3̄m1 structures in the TiNi-II phase is made. Other experimental evidence indicates the proportion of the B2 to P3̄m1 structures to depend critically on the thermal and mechanical history of the alloy. Through a mechanism of an inhomogeneous-but-cooperative atomic shear in the 〈111〉B2 direction and a factor sequence, −(0−1−1̄)n− in the (110)B2 plane, the B2 and P3̄m1 structures transform into P1̄ abd P6/m structures, respectively, during the TiNi-II to TiNi-III martensitic transformation. Subsequent submicrotwinning at atomic level of the P1̄ structure leads to the P1 structure. Therefore, the TiNi-III (``martensite'') phase may consist of the P1̄, P1, and P6/m structures.