L. V. Interrante

The Spin-Peierls Transition

In a spin-Peierls transition, a spin-lattice system consisting of one-dimensional antiferromagnetic linear chains in a 3-D lattice progressively dimerizes and thereby becomes nonmagnetic at T = 0. Like the usual Peierls transition, this is a soft-mode transition associated with a “fermi-surface-driven” instability (in a pseudo-fermion representation). We discuss the character of the transition and make predictions concerning the dynamic structure factor.

Design and Synthesis of Metal-Organic Precursors to Aluminosilicates

Diketonate aluminum alkoxides were prepared and allowed to react with acetoxyalkylsilanes. The conversion of the aluminosiloxanes thus obtained to aluminosilicates at 450°C was demonstrated. These aluminosiloxane ceramic precursors are glassy, oligomeric materials which readily dissolve in organic solvents to give viscous solutions ideal for casting films. Films of the metal-organic compounds prepared in this manner yield monolithic, crack-free aluminosilicate films directly on thermal curing, with a thickness limit of 3000A for single crystal silicon wafer substrates. These aluminosilicate films have been found to be effective anticorrosion barriers for various metal substrates. The chemistry of this organoaluminosilane system and the nature of the aluminosilicate films obtained has been investigated using a variety of chemical and physical methods. The results of this investigation will be described and the possible advantages of this direct conversion process over the sol-gel method as a means of obtaining aluminosilicate films will be discussed.

Magnetic properties of donor‐acceptor compounds of tetrathiofulvalene with bis‐Dithiolene metal complexes

Donor‐acceptor compounds of tetrathiofulvalene (TTF) with bis‐dithiolene (BDT) metal complexes are studied as flexible analogues to (TTF) ‐tetracyano‐quinodimethane. For BDT complexes, MS4C4X4, the X=CF3 derivatives with M=Ni and Pt, and the X=H derivative with M=Ni, have been examined magnetically for susceptibility and magnetization behavior. The 1:1 (TTF) (BDT) PtCF3 compound follows a Curie‐Weiss law with Θ=16K for dominant ferromagnetic interactions, but the susceptibility peaks near 12K indicating antiferromagnetic coupling. Its ferromagnetic subsystems are probably chain‐or layer‐like. The μeff value of (2.24μB) is inconsistent with two S=1/2 spins per formula. The 1:1 NiCF3 compound has a similar μeff (2.27 μB), but exhitibs dominant antiferromagnetic interactions with ϑ=−18K down to 40K. At lower temperature its Curie constant is reduced to that of a single spin with an indicated ferromagnetic intercept ϑ?+5K. Below 10K the magnetization‐field behavior tends toward saturation and a Curie point near 2.18K is suggested. A model of two nearly independent magnetic systems, analogous to CuSO4⋅5H2O is offered. The 2:1 X=H derivative (TTF)2(NiS4C4H4) is essentially paramagnetic (‖ϑ‖<2K) with one unpaired S=1/2 spin per formula.Donor‐acceptor compounds of tetrathiofulvalene (TTF) with bis‐dithiolene (BDT) metal complexes are studied as flexible analogues to (TTF) ‐tetracyano‐quinodimethane. For BDT complexes, MS4C4X4, the X=CF3 derivatives with M=Ni and Pt, and the X=H derivative with M=Ni, have been examined magnetically for susceptibility and magnetization behavior. The 1:1 (TTF) (BDT) PtCF3 compound follows a Curie‐Weiss law with Θ=16K for dominant ferromagnetic interactions, but the susceptibility peaks near 12K indicating antiferromagnetic coupling. Its ferromagnetic subsystems are probably chain‐or layer‐like. The μeff value of (2.24μB) is inconsistent with two S=1/2 spins per formula. The 1:1 NiCF3 compound has a similar μeff (2.27 μB), but exhitibs dominant antiferromagnetic interactions with ϑ=−18K down to 40K. At lower temperature its Curie constant is reduced to that of a single spin with an indicated ferromagnetic intercept ϑ?+5K. Below 10K the magnetization‐field behavior tends toward saturation and a Curie point ne...

The spin-peierls transition and other aspects of the magnetic behavior of the compounds TTF · MS 4 C 4 (CF 3 ) 4 (M = Pt, Cu, Au)

This paper reviews work on the isostructural TTF·MS4C4(CF3)4 (M = Pt, Cu, Au) and TTF-CuSe4C (CF )4 compounds, members of a new class of quasi-one dimensional (1-D magnetic systems. These materials (with M = Cu and Au) have provided the first unambiguous experimental demonstration of the spin-Peierls transition, the magnetic analog of the Peierls transition in the 1-D metals. The current status of the experimental work on these compounds is summarized and used to derive information regarding the magnetic exchange interactions occurring in the solid state.

New High-Field Phenomena in Spin-Peierls Systems

The spin-Peierls (SP) transition in zero field is a spin-lattice dimeri-zation phenomenon which occurs in a quasi-1-D Heisenberg S= 1/2 antiferromag-netic chains. The chains are coupled to the 3-D phonon field of the lattice, and a magneto-elastic transition results which is the insulating analogue of the well-known Peierls transition in a quasi-1-D conductor. For T<Tsp, the magnetic chains have a dimerized (alternating) character, resulting from an underlying progressive lattice distortion, with a nonmagnetic singlet ground state and an excitation energy gap to magnetic excited states. The character of an alternating as opposed to uniform chain lattice, together with the corresponding magnetic excitations, is shown schematically in Fig. 1.

X-Ray and Shubnikov-De Haas Studies of C8n AsF5(n= 1,2,3)

(0,0,l)-X-ray-reflection studies of single crystal and HOPG graphites intercalated with AsF5 have provided maps of the c-axis charge density distribution of these intercalates. Analysis of this distribution allows an estimate of the ratio of AsF6 -:AsF5 in the intercalant layers. This in turn yields the charge transfer f (how far the reaction 3 AsF5 + 2 e- → 2 AsF6 - + AsF3 has proceded to the right). Presence of significant AsF5 shows that in stage 2, f ≃ 0.4 − 0.45, considerably below the maximum value of 2/3.