First‐Order Raman Scattering in NiS_2—xSe_x

Abstract

The Raman spectra of the system NiSz -&e, are investigated for z = 0. OO,0.10,0.20,0.30, and 0.55, at room temperature and at temperatures below the point where the z = 0.55 sample suffers a resistivity change from semiconducting ( T 2 120 K) to metallic ( ! I ' 5 120 K). No qualitative or anomalous quantitative changes in the spectrum of the sample are found that could be associated with the anomaly in its transport properties. A mode of A, symmetry appears in the alloys which is absent from the spectrum of pure Xis,. This mode is successfully explained in terms of vibrations of S-Se pairs. Other modes decrease in frequency with increasing 'selenium concentration, a trend easily understood considering the increase in the unit cell volume with increasing z. Die Ramen-Spektren des Systems NiSs-Jh, (z = 0,OO; 0,lO; 020; 0,30 und 0,55) werden bei Zimmertemperatur und bei Temperaturen unterhalb des Punktes untersucht, wo die Probe mit z = 0,55 eine Widerstandsiinderung von halbleitend (T 2 120 K) zu metallisch ( T 120 K) zeigt. Es wird keine qualitative oder anomale quantitative Anderung im Spektrum der Probe gefunden, die mit der h o m a l i e in ihren Transporteigenschaften verkniipft werden kann. EineMode der Ag-Symmetrie in den Legierungen tritt 8 U f , die im Spektrum des reinen NiS, fehlt. Diese Mode liiI3t sich erfolgreich mit Schwingungen der SSe-Paare erklzren. Die Frequenzen an-dererModen nehmen mit steigender Selenkonzentration ab ; ein Trend der sich durch Beriicksichtigung der VergraSerung des Volumens der Einheitszelle mit steigendem z leicht erklaren liiI3t.

Structural investigation of the insulator-metal transition in NiS$_{2-x}$Se$_x$ compounds

“…With the room-temperature Raman data (qualitatively consistent with the literature [25][26][27] ), the assignment of Raman-active phonons was based on their symmetry relations with respect to the light polarizations, the doping dependence of relative intensities, and the energy shift due to the change in the reduced masses of the dimers 20,[25][26][27] in two crystals (x = 0.45 and 0.23): A g and E g in c(bb)c and T g phonons in c(ab)c. Compared to x = 0.23, phonons in x = 0.45 showed decreased wavenumbers; this observation can be understood in terms of the increased distance between chalcogen ions. Also, the intensity of the S -Se relative to that of the S -S phonon was larger in the more highly doped crystal, owing to larger population of S -Se bonds [compare Fig.…”

“…However, strong A g (in-phase) and T g (out-of-phase) stretching modes of all three chalcogen bonds in NiS 2−x Se x compounds were detectable separately via the polarization measurements, as shown in Figs. 4(a-d); their phonon energies decreased in the order of S -S, S -Se and Se -Se, as the mass of the chalcogen bond became heavier 20,25 .…”

Local disorder investigation in NiS2−xSexusing Raman and Ni K-edge x-ray absorption spectroscopies

“…Room temperature Raman spectra of the four NiS 2−x Se x compounds (x = 0.00, 0.55, 0.60 and 1.20) are shown in figure 1. According to the space group analysis (space group: T 6 h ) five Raman active modes, A g + E g + 3T g , are expected [22,23]. As noted in the earlier studies [8,22], Raman spectroscopy is intrinsically particularly sensitive to the S/Se chemical substitution in NiS 2−x Se x .…”

Electronic Properties of NiS2-xSex Single Crystals: From Magnetic Mott−Hubbard Insulators to Normal Metals

“…Raman spectroscopy has been used , to document the presence of bond stretching vibrations assigned to S−S, S−Se, and Se−Se molecular units. This indicates that there is a quasi-random distribution of Se for S in the anionic pairs of the pyrite structure shown in Figure , along with the nearest neighbor configurations for both cations and anions.…”