Characterization of the anion-ordering transition in „TMTTF… 2 ReO 4 by x-ray absorption and photoemission spectroscopies G. Subías Instituto de Ciencia de Materiales de Aragón (ICMA), Departamento de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, Pza. San Francisco s/n, E-50009 Zaragoza, Spain T. Abbaz and J. M. Fabre Institut Charles Gerhardt, UMR CNRS 5253, AM2N, ENSCM, 8 rue de l’ École Normale, F-34296 Montpellier cedex 5, France J. Fraxedas* Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, E-08193 Bellaterra, Spain Received 4 May 2007; revised manuscript received 18 June 2007; published 3 August 2007 We have characterized the electronic structure of single crystals of the organic quasi-one-dimensional salt TMTTF 2 ReO 4 TMTTF = tetramethyl-tetrathiafulvalene by x-ray-absorption near-edge spectroscopy and x-ray photoemission spectroscopy using synchrotron radiation under low-intensity conditions single-bunch ring operation mode in order to strongly reduce detrimental beam-induced damage. The observed differences in spectra taken at T =116 K and T = 207 K are ascribed to the anion-ordering transition, which takes place at 155 K. It is proposed that the ordering transition driven by the anions perturbs the structural organization of the organic stacks, increasing the overlap of the -type orbitals through dimerization. DOI: 10.1103/PhysRevB.76.085103 PACS numbers: 71.20.-b, 71.30.+h, 78.70.Dm, 79.60.-i I. INTRODUCTION The family of the isostructural quasi-one-dimensional or- ganic mixed-valence radical cation salts TMTTF 2 X and TMTSF 2 X based on the  donors TMTTF tetramethyl- tetrathiafulvalene and TMTSF tetramethyl-tetraselenaful- valene, where X stands for a monovalent anion PF 6 - , ClO 4 - , ReO 4 - , etc., exhibits complex and extremely rich phase diagrams with a variety of ground states metallic, Mott-Hubbard, spin-Peierls, antiferromagnetic, spin and charge density wave, and superconducting. Such unusual wealth of competing electronic and structural instabilities, which gives rise to a fascinating low-dimensional physics, continues to attract materials chemists and physicists after more than 25 years, these salts, known as the Bechgaard- Fabre salts, becoming intensively studied reference sys- tems. 1–8 The crystal structure of such salts consists of nearly uni- form stacks of TMTTF or TMTSF molecules, aligned along the crystallographic a axis, the conduction axis, ordered in sheets separated by anion sheets, which can be regarded as organic and inorganic superlattices. When grown by constant low dc oxidation of an organic solution of the corresponding neutral donor molecule and tetrabutyl ammonium salt of the anion as electrolyte, 9 the Bechgaard-Fabre salts crystallize in the triclinic P1 ¯ space group. 10,11 However, they have been found to order in the monoclinic C2/ c space group when prepared under different experimental conditions, i.e., by confined electrocrystallization. 12 Figure 1 shows the room- temperature structure of the triclinic phase of the title com- pound TMTTF 2 ReO 4 . 13 The monovalent anions can be either centrosymmetric or noncentrosymmetric. Those salts with noncentrosymmetric anions, such as, e.g., ClO 4 - and ReO 4 - , exhibit anion order- ing below a given temperature. Above the transition tempera- ture T AO , the anions exhibit random orientations but for suf- ficiently low temperatures they become ordered. The anion- ordering transition has important consequences on the low- temperature properties and ground states of these materials. When the period along a is doubled, the anion potential can open an energy gap at the Fermi level E F , thus inducing a metal-insulator transition. 14 TMTTF salts with centrosymmetric anions exhibit charge-ordering phase transitions. 15,16 In this case no struc- tural modifications have been observed for these salts along the transition, hence deserving the term structureless. Upon charge ordering the electronic equivalence of the TMTTF molecules is removed below a critical temperature, making the charge disproportionate. Let us now concentrate on the title compound. For TMTTF 2 ReO 4 , where ReO 4 - has a tetrahedral sym- metry, the room-temperature conductivity is about 25  -1 cm -1 . 13,17,18 The temperature dependences of the electri- cal resistance and of the thermopower show two anomalies near 225 K and 160 K. 19 In addition, the dielectric constant measured at 6.5 GHz presents also two anomalies near the same temperatures. 20 The 160 K feature is due to anion or- dering, as revealed by x-ray studies, showing a superstruc- ture represented by the reduced wave vector  1 2 1 2 1 2  , leading to the opening of an energy gap at E F . 21 The transition near 225 K is due to charge ordering with a ferroelectric charac- ter, as suggested by low-frequency conductivity and dielec- tric permittivity measurements. 18,22 Here we study the electronic structure of triclinic TMTTF 2 ReO 4 by means of x-ray-absorption near-edge spectroscopy XANES and x-ray photoemission spectros- copy XPS with synchrotron radiation with special emphasis on the anion-ordering transition. XANES yields information on unoccupied states, and being sensitive to bond-angles, linearly polarized x rays are best suited for molecules pos- PHYSICAL REVIEW B 76, 085103 2007 1098-0121/2007/768/0851037 ©2007 The American Physical Society 085103-1