PACS. 79.60 -Photoemission and photoelectron spectra. PACS. 71.30 -Metal-insulator transitions. Abstract. -The temperature-dependent electronic structure of 1T-TaS2 near the Fermi level, determined by means of angle-resolved photoemission with high-energy resolution of 35 meV, exhibits a distinct energy gap associated with a metal-semiconductor transition at 180 K on cooling, whereas on warming up the retransformation occurs in the electronic structure at 280 K. The complex phase transformation

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... behaviour including the resistivity can be consistently explained between 400 K and 120 K. For the metal-semiconductor transition Mott-Hubbard localization of the d conduction electrons driven by critical interlayer coupling seems most likely. The layered transition metal dichalcogenide 1 T-TaS2 has been intensively investigated in the past few years because of the formation of strong charge density waves (CDWs) [l, 21 and electronic localization at low temperatures [3]. 1T-TaS2 is known to exhibit three main CDW phases well known from electron and X-ray diffraction: the low-temperature commensurate CDW phase (CCDW) below about 180 K the high-temperature incommensurate phase (ICDW) above 350K, and in between the nearly commensurate phase (NCCDW) present at room temperature. The appearance of the NCCDW phase and the peculiar temperature behaviour of the conductivity above and below the NCCDW-CCDW phase transition account for the extraordinary physical properties of lT-TaS2 in comparison with other transition metal dichalcogenides. These result from the possibility of a domainlike CDW structure with discommensurations in the NCCDW phase [4,5], eliminating all of the Fermi surface with decreasing temperature, and/or from the localization being of collective nature (Mott-Hubbard localization) or caused by disorder (Anderson localization) [3,6-81. Recently, the situation became even more complex by the observation of an additional phase transition at about 280 K during the warming-up cycle, accompanied by a new, so-called T phase between 220 K and 280 K [9-111. Angle-resolved photoemission with high resolution in energy and angle is known to be a powerful tool for studying the electronic structure just below the Fermi level, EF [12-141. Recent photoemission measurements of 1T-TaS2 have shown that the commensurate CDW phase is semiconducting due to a rearrangement of the electronic structure in the vicinity of EF[14-161. This was concluded from a comparison of spectra taken at room