Structural, Magnetic, and Electrical Characterization of New Polycrystalline Phases of Nickel- and Platinum-Doped [(DT-TTF)_n][Au(mnt)₂] (n = 1, 2)

Abstract

Doping of spin-ladder systems by isostructural paramagnetic complexes was attempted. Despite the close isostructural nature of the pure (DT-TTF)2[M(mnt)2] (M = Au, Ni, Pt) end-members, which present a ladder structure, doping of the spin-ladder (DT-TTF)2[Au(mnt)2] with either 5% or 25% [M(mnt)2]- (M = Ni, Pt) generates two (metrically) new phases. Their markedly different crystal structures have been determined using laboratory X-ray powder diffraction data. (DT-TTF)2[Au0.75Ni0.25(mnt)2] consists of a mixed-valence compound (of triclinic symmetry), which was only detected, pure or in a mixture of phases, when [Ni(mnt)2]- was used as a dopant. Differently, the stoichiometric 1:1 [DT-TTF][Au0.75Pt0.25(mnt)2] monoclinic phase was found when [Pt(mnt)2]- (in 5% and 25%) was employed as the doping agent. Remarkably, only in the 5% Pt doping experiment, the major component of the mixture was the ladder structure compound (DT-TTF)2[Au(mnt)2] doped with minor amounts of Pt. This 5% Pt-doped specimen shows an EPR signal (g = 2.0115, DeltaHpp = 114 G at 300 K) wider than the pure compound (DT-TTF)2[Au(mnt)2], denoting exchange between the donor spins and Pt(mnt)2- centers. The electrical transport properties of the 5% Pt-doped composition at high temperatures are comparable to those of (DT-TTF)2[Au(mnt)2] with room-temperature conductivity sigma300K = 13 S/cm and thermopower S300K = 46 microV/K, with a sharp transition at 223 K similar to that previously observed in the Cu analogue at 235 K.

Infrared investigation of the charge ordering pattern in the organic spin ladder candidate (DTTTF)2Cu(mnt)2

“…This system is particularly attractive because it belongs to a family of quasi-isostructural compounds, with tunable properties depending on the counterion (M = Pt, Cu, Au, Ni) [3,4]. That the cation stacks of (DTTTF) 2 Cu(mnt) 2 have a 1 4 -filled band with delocalized charge and spin at high temperatures, and are therefore described by an electronic extended Hubbard Hamiltonian, as opposed to the localized Heisenberg spin Hamiltonian, brings additional complexity to the field.Two different theoretical models, the rectangular [4,6,7,8] and the zigzag ladder [9], have been proposed for this family of coupled stack materials (Figs. 1(d) and (e)) Although determination of the wavevector dependence of the magnon bands [10] is the strongest test of spin ladder character and can in principle distinguish between the models, inelastic neutron scattering experiments on (DTTTF) 2 Cu(mnt) 2 are limited by sample quantity and isotopic decoration requirements [11].…”

“…Two different theoretical models, the rectangular [4,6,7,8] and the zigzag ladder [9], have been proposed for this family of coupled stack materials (Figs. 1(d) and (e)) Although determination of the wavevector dependence of the magnon bands [10] is the strongest test of spin ladder character and can in principle distinguish between the models, inelastic neutron scattering experiments on (DTTTF) 2 Cu(mnt) 2 are limited by sample quantity and isotopic decoration requirements [11].…”

“…Interestingly, application of pressure recovers the metallic state [12]. The structural, magnetic, and electrical properties of doped DTTTF-based materials were also studied, [7] but [4], (b) photograph of the (DTTTF) 2 Cu(mnt) 2 single crystal and the aperture used during our infrared measurements, (c) proposed low-temperature arrangement of charge supporting the rectangular spin ladder picture [4], (d) schematic of the bond distortion pattern within the rectangular ladder model [6], and (e) schematic charge ordering and bond distortion pattern within the zigzag ladder model. The grey circles in (d) denote site charge of 0.5 while the filled (unfilled) circles in (e) denote site charges greater than (less than) 0.5 [9].…”

“…Interestingly, application of pressure recovers the metallic state [12]. The structural, magnetic, and electrical properties of doped DTTTF-based materials were also studied, [7] but superconductivity was not observed.…”

Spin-ladder behaviour in molecular materials

“…The attempts to dope these spin-ladder compounds with diamagnetic anions, which could create holes in the ladders, has been so far unsuccessful, and the doping with paramagnetic anions ([Ni(mnt) 2 ] À ) and ([Pt(mnt) 2 ] À ) promoted the formation of completely different structures. 47…”