Structure and physical properties of the new layered ternary chalcogenides tantalum nickel sulfide (Ta2NiS5) and tantalum nickel selenide (Ta2NiSe5)

Redetermination of the structures of CuTaS3 and Nb2Se9

“…The description Cu2+Ta4+3S 2-with strong Cu-Ta interactions would also explain the observed insulating behavior. This is similar to the model proposed for the Ta2NiQ 5 (Q = S, Se) system (Sunshine & Ibers, 1985). Fig.…”

“…This distance is slightly longer than the 2-74 (1)A originally reported (Crevecoeur & Romers, 1964) and closer to the value of 2.804 (7)/k found in the related compound 1987). A similar M-Ta interaction is seen in TazNiQ5 (Q = S, Se) (Sunshine & Ibers, 1985).…”

“…These compounds possess structural features that may aid in the understanding of other ternary chalcogenides. In particular, the structure of Nb2Se 9 contains a fivemembered polyselenide ion and CuTaS 3 possesses a short Cu-Ta distance [2.781 (2)/~] similar to the Ni--Ta distance of 2.747 (7)/k in Ta2NiS 5 (Sunshine & Ibers, 1985). Because of the significance of these two compounds the redetermination of their structures was undertaken.…”

Excitonic Bose-Einstein condensation inTa2NiSe5above room temperature

“…1(b), Ta 2 NiSe 5 has a quasi-one-dimensional (quasi-1D) structure where Ni and Ta atoms are arranged in one dimensional chains. [16,17] Resistivity of Ta 2 NiSe 5 is insulating below 500 K and exhibits an anomaly around 325 K which is assigned as second-order or weak first-order structural phase transition without CDW. The resistivity exhibits insulating behaviors both above and below the transition temperature at 325 K.…”

Coherence Factors and Quantum Interferences in Excitonic Condensation of Ta2NiSe5

“…However, thanks to the recent development of angle-resolved photoemission spectroscopy experiments that enable us to observe band dispersions directly, the realizations of excitonic condensation in some materials have been suggested [2,3,4,5,6]. Especially, Ta 2 NiSe 5 , which is known as a direct-gap semiconductor [7,8], is regarded as one of the most promising candidates of excitonic condensation [2,3,9].A measurement of transformation of the band dispersion is, however, merely an indirect evidence for excitonic condensation. In this respect, there have been some proposals that the ultrasonic attenuation rate shows a coherence peak in a simple electron-gas model [10,11] although the connection to an actual material lacks.…”