High-resolution thermal expansion measurements have been performed for exploring the mysterious "structureless transition" in (TMTTF)2X (X=PF(6) and AsF6), where charge ordering at T(CO) coincides with the onset of ferroelectric order. Particularly distinct lattice effects are found at T(CO) in the uniaxial expansivity along the interstack c direction. We propose a scheme involving a charge modulation along the TMTTF stacks and its coupling to displacements of the counteranions X-. These anion shifts, which lift the inversion symmetry enabling ferroelectric order to develop, determine the 3D charge pattern without ambiguity. Evidence is found for another anomaly for both materials at T(int) approximately 0.6T(CO) indicative of a phase transition related to the charge ordering.
The 3d(1) system BaVS3 undergoes a series of remarkable electronic phase transitions. We show that the metal-insulator transition at T(MI)=70 K is associated with a structural transition announced by a huge regime of one-dimensional (1D) lattice fluctuations, detected up to 170 K. These 1D fluctuations correspond to a 2k(F)=c(*)/2 charge-density wave (CDW) instability of the d(z(2)) electron gas. We discuss the formation below T(MI) of an unconventional CDW state involving the condensation of the other V4+ 3d(1) electrons of the quasidegenerate e(t(2g)) orbitals. This study stresses the role of the orbital degrees of freedom in the physics of BaVS3 and reveals the inadequacy of current first principle band calculations to describe its electronic ground state.
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