Impurity effects of the 3d transition metal atoms on the first-order magnetic and electrical transition in NiS

Abstract

In the NiAs-type Nil _ x Mx S (M: the 3d transition metal impurities), the solubility limits XL are determined to be 0.077 ± 0.005 for M:Ti, 0.04 ± 0.01 for M:Cr, 0.001 ± 0.002 for M:Mn, 0.060 ± 0.005 for M:Co, and 0.015 ± 0.005 for M:Cu. The first-order transition temperature T, (between the antiferromagnetic less-conductive and the Pauli-paramagnetic high-conductive states) is investigated as a function of X for M:Ti, Cr, Co, and Cu (as described on M:V in our previous paper). The lattice parameters c and a are also investigated. The larger x gives the lower T, for these M, except in the lower x for M:V or Cr. The following features are found on M:Ti (similar to V and Cr): a Curie-Weiss-type susceptibility is superimposed on the constant Pauliparamagnetic one in temperatures above T,; the sign of dO (T < T t )I dT (0: thermoelectric motive force) changes from plus to minus at a critical concentration. The ratio c/ a hardly depends on x in the cases of more than half filJed M (Co and CuI, while it increases in x in the less than half filled M (Ti, V, and Cr). Correlations between these experimental results and the nuclear charges of M atoms or the exchange splitting energies of M(3s) bands in MS compounds are discussed on the basis of the Friedel-type dilute-alloy model. It is additionally noted that Nio.96 CO O . 04 S exhibits an anomalous temperature behavior of susceptibility just above T,. This behavior indicates the presence of an intermediate state, between the above two magnetic states, in the high x region for M:Co.

Revealing the Role of the Tetragonal Distortion in the Metal–Insulator Transition of Co- and Fe-Doped NiS

“…Nevertheless, compared to other representative MIT chalcogenides, the mechanism associated with the MIT of NiS was not yet clear. It is interesting to note that Fe substitution of Ni is, to date, the only way to elevate the T MIT of NiS, while a reduction in T MIT was always observed for the case of other elementary substitution. , Although the lattice shrinkage (expansion) was expected to dominate the MIT of NiS, since it well explains the high-pressure regulation in its T MIT , such theory fails to explain the adjustment in T MIT upon elementary substitutions . Alternatively, the reduction in T MIT of the Co- or Se-substituted NiS was ascribed to the significant Ni 3 d –Co 3 d hybridization or the less-electronegative Se atoms that reduces the energy band gap. , This unveils the complexity in the mechanisms associated with the MIT of NiS, as the regulation in T MIT could be determined by both perspectives of the structures and orbital configurations that are worthy of further exploration.…”

“…This differs to the situations for reducing the T MIT of NiS via Co (or Se, Ti, and Rh) substitutions, where no significant changes in the crystal structure were observed and the regulation in MIT is dominated by the bandwidth related to the hybridizations. 10,11,16 Furthermore, compared to the bandwidth and hybridization regulations, the structural regulation results in less abrupt MIT properties from the perspective of both electrical and thermal transportations across T MIT . As illustrated in Figure 1a, the previous understanding in regulating the T MIT of NiS mainly focus on the perspective of orbital configuration, since substituting Ni with Co was expected to allow charge transfer between the Co and Ni sites and enlarge the Ni 3d bandwidth, while Fe substitution was expected to prohibit electron transfer between the Fe and Ni sites.…”

“…19−21 Substituting Ni with Co results in the reduction of T MIT while replacing Ni with Fe can elevate T MIT , which is consistent with the previous literature. 10 In Figure 2a, the T MIT of the Fe-and Co-substituted NiS were plotted as a function of the respective substitution concentration (n dopant ) and further compared to the previous reports. 9,10,13,16 For most of the substitution elements (e.g., Co, Se, Ti, and V) either substituting the Ni-or S-site reduces the T MIT of NiS with the only exception of Fe, in which situation Fe can substitute Ni at a much higher concentration and elevate T MIT .…”

“…It is interesting to note that Fe substitution of Ni is, to date, the only way to elevate the T MIT of NiS, while a reduction in T MIT was always observed for the case of other elementary substitution. 9,10 Although the lattice shrinkage (expansion) was expected to dominate the MIT of NiS, since it well explains the high-pressure regulation in its T MIT , such theory fails to explain the adjustment in T MIT upon elementary substitutions. 15 Alternatively, the reduction in T MIT of the Co-or Se-substituted NiS was ascribed to the significant Ni 3d−Co 3d hybridization or the less-electronegative Se atoms that reduces the energy band gap.…”

“…Among the presently known MIT chalcogenides, the NiS exhibits the MIT behavior mostly approaching room temperature, while a wide adjustment in its critical temperature ( T MIT ) can be achieved via elementary substitutions. Typically, substituting the S site by Se and Te, or substituting the Ni site by Co, Ti, V, Cr, and Rh, − can effectively reduce the T MIT of NiS, and its T MIT can be elevated beyond room temperature by Fe substitution of Ni . In addition to the resistive change similar to other typical MIT materials, it is also worth noticing the abrupt variation in the thermal properties of NiS across T MIT , as its thermal conductivity elevates for ∼120% when NiS transits from the insulating to metallic phase, as accompanied by high magnitudes of the latent heat (e.g., 15.5 J/g) .…”