Metal-insulator transitions inBaCo1−xNi

Abstract

We report on the electrical and magnetic properties of the layered ternary sulfide system BaCoi_ x Ni x S2-y . Ni doped samples of BaCoS2 that contain sulfur vacancies exhibit a first-order phase transition from a high temperature antiferromagnetic, semiconducting phase to a low temperature metallic phase. Increasing the sulfur vacancy concentration increases the Neel temperature of the two-dimensional antiferromagnetic phase and decreases the resistivity in the metallic phase. As the Ni content is increased, the system undergoes a smooth transition to a paramagnetic metal at all temperatures.PACS numbers: 71.27.-fa, 71.30.+h, 75.50.Ee Ternary sulfides of alkaline earth metals with transition metals provide a class of materials that might be expected [1,2] to have several common features with highly correlated systems such as the intermediate valence compounds and the copper oxide superconductors. We have investigated the BaCoi_ x Ni a; S2-2/ system, and in this Letter we report the discovery of a discontinuous phase transition in BaCoo.9Nio.iS2-2, for 0.05 < y < 0.20 from semiconducting to metallic behavior with decreasing temperature. Above the transition temperature the materials are Mott-Hubbard insulators with localized states near the Fermi energy arising from the substitution of Ni for Co. There is also evidence of two-dimensional antiferromagnetism above the transition. The metallic phase below the transition is similar to that of the highly correlated paramagnetic metal BaNiS2 where there is mixed valency associated with the Ni and S ions. This first-order phase transition shows substantial hysteresis and involves a structural transformation to a commensurate superlattice state for the low temperature phase.We present resistivity and magnetic susceptibility data for single-phase polycrystalline BaCoi_ x Ni x S2-y samples all having the BaNiS2 crystal structure except at temperatures below the phase transition. The BaNiS2 [3] structure is shown in Fig. 1. In this tetragonal structure the transition metal ions are pentacoordinated to sulfur in a square-pyramidal environment. The apical sulfurs of the edge sharing pyramids alternate above and below the plane formed by the bases of the pyramids. These apical sulfurs together with the Ba atoms form slightly distorted rock salt layers between the Ni2S2 sheets. Within the sheets the transition metal ions alternate above and below the plane containing the sulfurs.In the BaNiS2 structure, the distance between neighboring Ni atoms is less than half the distance between the sheets. Hence it is expected that the Ni2S2 sheets determine the electric and magnetic properties much as the Cu02~2 sheets determine these properties in La2CuC>4. Apart from the net charge on the sheets, these two systems primarily differ by the additional Ni per unit cell of the sheet and the quadratic pyramidal symmetry of the crystal field at the Ni sites in BaNiS2 versus the octahedral symmetry at the Cu sites in La2CuC>4. However, the important nearest-neighbor Ni-S coordination in the she...

Novel Dynamical Effects and Glassy Response in a Strongly Correlated Electronic System

“…Striking feature is that a first-order phase transi- tion at T c ∼ 140K from insulator to metal with hysteresis occurs in the fourth round of 220K → 5K → 220K. It is similar to the AFI-PMM transition observed in the sample with large sulphur deficiency [15]. In the fifth round from 220K → 5K → 400K, resistivity shows similar behavior to that in the fourth round except that the transition is sharper with a larger reduction of resistivity.…”

“…Such quasi-two dimensional system BaCoS 2 with CoS conducting planes separated by insulating BaS rocksalt sheets is structurally analogous to the high-T c cuprates [15]. Substitution of Ni for Co leads to a first-order transition from an antiferromagnetic insulator (AFI) to a paramagnetic metal (PMM) upon cooling at T c in the layered BaCo 0.9 N i 0.1 S 2−y system for 0.05 ≤ y ≤ 0.20 [15]. Such AFI-PMM phase transition is associated with a structural change from hightemperature tetragonal (HTT) phase to low-temperature monoclinic (LTM) phase [16].…”

Anomalous Metal-Insulator Transition in BaCo1?xNixS2?y as a Triggered Phase Transition

“…the metallic state and dT MI /dp < 0 fall in line with the Philips assumptions [2][3][4]. However, the model does not comment on the puzzling neutron diffraction result that the magnetic state appears to be random or at best short-range ordered in the metallic state [6] while there is antiferromagnetic order in the semiconducting state ( [7][8][9] and Fig. 2a).…”

Structural and magnetic ordering in the VxNb1+yS2 system

“…Layered transition metal chalcogenides, for example, Ba(Co,Ni)S 2Ày [1] and Tl(Co,Ni) 2 S 2 [2], exhibit both metallic conductivity and magnetic ordering and have been investigated from the point of view of comparison with high-T c superconductors and other strongly correlated electron systems. NbS 2 is a typical two-dimensional conductor that exhibits superconductivity at 6.2 K [3].…”