Pii: S0038-1098(99)00277-x

Optical reflectivity spectra of the hexagonal Ni12dS have been measured to study its electronic structures, in particular those associated with the metal–nonmetal transition in this compound. Samples with d , 0.002 and 0.02 are studied, which have transition temperatures Tt ,260 K and 150 K, respectively. Upon the transition, a pronounced dip appears in the infrared region of the reflectivity spectra. The optical conductivity spectra suggest that the nonmetallic phase is a carrier-doped semiconductor with an energy gap of ,0.2–0.3 eV. The spectra also show that the gap becomes larger with decreasing temperature, and smaller with increasing d. It is found that the overall spectrum in the nonmetallic phase can be explained in terms of a chargetransfer semiconductor, consistent with recent theoretical and photoemission studies of NiS. q 1999 Elsevier Science Ltd. All rights reserved. Keywords: D. Optical properties; D. Phase transitions The problem of the metal–nonmetal transition in the hexagonal NiS has been studied for three decades, but the transition mechanism is not completely understood yet [1]. The high temperature (HT) phase above the transition temperature, Tt , 264 K, is a paramagnetic metal. Upon cooling through Tt, the resistivity increases suddenly by a factor of ,40, associated with a slight increase in the lattice constants (0.3 % in a and 1 % in c) and the appearance of an antiferromagnetic order [2–5]. Tt is lowered sharply with increasing Ni vacancies, and the transition disappears when the vacancy content exceeds ,4% [6]. Similar behavior is observed also with an applied pressure, and the transition is not observed at pressures above ,2 GPa [7]. These behaviors are summarized in the phase diagram of Fig. 1. The nature of the low-temperature (LT) phase below Tt has been studied by many experiments. The resistivity (r) increases only slightly with cooling, with an activation energy of several meV [6]. In contrast, the optical study by Barker and Remeika [8] clearly showed the presence of an energy gap of about 0.2 eV. The Hall effect experiment by Ohtani [6] has shown that the majority carrier in the LT phase is the hole with a density of 10210 cm, which is proportional to that of Ni vacancies, with , two holes per Ni vacancy. Namely, the LT phase can be described as a p-type degenerate semiconductor, where the Ni vacancies act as acceptors. Effects of substituting Ni or S sites by other elements have also been studied in detail [9,10]. Recently, two high-resolution photoemission studies [11,12] have revealed a finite density of states (DOS) around the Fermi energy (EF) in the LT phase, but leading to contrasting interpretations: Nakamura et al. [11] have concluded that there is a small correlation-induced energy gap with an unusually sharp band edge, and that the observed finite DOS at EF is due to thermal and instrumental broadenings of the edge. On the other hand, Sarma et al. [12] have concluded that the LT phase is an “anomalous metal”. Various models have been proposed to describe the phase transition and the gap opening in NiS. At early stages, it was proposed that the transition was a Mott-type transition [13]. In this model, the Ni 3d band splits into two bands separated by a gap in the LT phase due to strong Coulomb interaction Solid State Communications 112 (1999) 91–95 0038-1098/99/$ see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-1098(99)00277-X PERGAMON www.elsevier.com/locate/ssc * Corresponding author. E-mail address: [email protected] (H. Okamura) at Ni sites. Band calculations that take into account manybody effects via various approximations have been unable to reproduce an energy gap with a reasonable magnitude [11]. More recently, it has been proposed, based on cluster-model calculations and resonant photoemission experiments, that the energy gap in NiS is a charge-transfer gap between the upper 3d and the S 3p bands [1,14–16]. In this work, we have performed optical reflectivity experiments of Ni12dS in wide ranges of temperature (8– 300 K) and photon energy (0.008–30 eV), to study changes in the electronic structures upon the metal–nonmetal transition. Around Tt we observe a sudden decrease in the reflectivity [R…v†] and the formation of an energy gap of ,0.2 eV in the optical conductivity [s…v†]. Although a gap formation in s…v† of NiS was reported previously by Barker and Remeika [8], their work was done in limited ranges of temperatures and photon energies for samples with Tt . 220 K only. In contrast, our present work provides more detailed temperature dependence of the energy gap, and a comparison between samples having different values of Tt (and d). We find that the energy gap becomes larger with decreasing temperature, and smaller with increasing d. In addition, we show that the overall spectrum in the LT phase can be explained in terms of a charge-transfer semiconductor, consistent with cluster-model calculations and photoemission experiments [14–16]. The samples used in this work were polycrystalline Ni12dS prepared as follows. Ni and S powders were mixed with molar ratios of [Ni]:[S] ˆ 1:1 and 0.98:1, and melted at 10008C in an evacuated quartz tube. Then they were annealed at 7008C for two days and at 5008C for one week, and quenched in iced water. The 1:1 mixture resulted in an ingot with Tt . 260 K, and the 0.98:1 mixture with Tt . 150 K. Comparing these Tt values with those in previous works [9,10] the vacancy contents in these samples are estimated as d , 0:002 (or Ni0:998S, Tt ˆ 260 K) and d , 0:02 (Ni0:98S, Tt ˆ 150 K). The ingots were cut into disk-shaped samples, and the surface was mechanically polished with alumina powders. Then the samples were annealed at 5008C for three days in an evacuated quartz tube, followed by a quench in iced water. This re-annealing process is necessary because the mechanical cutting and polishing suppress the phase transition in the sample surface [8]. After this annealing process, the sample surface appeared less shiny and the reflectivity decreased by 10– 25% depending on the wavelength. This was due to small roughness on the sample surface caused by the annealing. To correct for the roughness, we first measured the reflectivity of the annealed sample with respect to a flat mirror, then evaporated Au or Ag on the entire sample surface and measured the reflectivity again with respect to the same flat mirror. We divided the former spectrum by the latter to obtain a reflectivity spectrum corrected for the roughness. This method has been shown to be quite efficient in measuring the reflectivity of a rough surface [19]. A standard nearnormal incidence configuration was used for the reflectivity measurements, using a rapid-scan Fourier interferometer (Bruker Inc. IFS-66v) and conventional sources for measurements below # 2:5 eV, and using synchrotron radiation source at the beamline BL7B of the UVSOR Facility, Institute for Molecular Science [18] for measurements between 1.5 eV and 30 eV. The correction for roughness was done only for the Fourier interferometer data, to which the synchrotron data were smoothly connected. s…v† spectra H. Okamura et al. / Solid State Communications 112 (1999) 91–95 92