arXiv:cond-mat/0503203v1 [cond-mat.mtrl-sci] 9 Mar 2005 Spin-Peierls transition in TiOCl Mohammad Shaz, 1 Sander van Smaalen, 1, * Lukas Palatinus, 1, 2 Markus Hoinkis, 3 Matthias Klemm, 3 Siegfried Horn, 3 and Ralph Claessen 3 1 Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany 2 Institute of Physics of the Academy of Sciences of the Czech republic, Prag, Czech Republic 3 Experimentalphysik II, University of Augsburg, D-86135 Augsburg, Germany (Dated: February 2, 2008) Temperature-dependent x-ray diffraction of the low-dimensional spin 1/2 quantum magnet TiOCl shows that the phase transition at Tc2 = 90 K corresponds to a lowering of the lattice symmetry. Below Tc1 = 66 K a twofold superstructure develops, that indicates the formation of spin-singlet pairs via direct exchange between neighboring Ti atoms, while the role of superexchange is found to be negligible. TiOCl thus is identified as a spin-Peierls system of pure 1D chains of atoms. The first-order character of the transition at Tc1 is explained by the competition between the structurally deformed state below Tc2 and the spin-Peierls state below Tc1. PACS numbers: 61.50.Ks, 75.30.Kz, 75.30.Et, 61.66.Fn Low-dimensional S =1/2 quantum spin systems are of interest, because of the importance for understanding the mechanism of high-T c superconductivity [1], as well as for their potential applications in quantum computers. Fur- thermore, the relatively simple, yet complicated materi- als with 1-dimensional (1D) or 2D quantum spin systems offer a wide variety of ground states, that are accessi- ble by ab initio theory, and therefore might help towards the understanding of fundamental quantum mechanical properties of solids. The development of magnetic or- der at low temperatures may or may not be coupled to a change of the electronic structure, resulting in ground states with antiferromagnetic order or spin-density waves (SDW). The spin-Peierls state is defined by singlet pairs of localized electrons, that form because of an enhance- ment of exchange interactions between neighboring mag- netic atoms due to a dimerization of the crystal structure. CuGeO 3 is the only inorganic compound for which the spin-Peierls state below T c = 14 K has been unambigu- ously established [2, 3, 4, 5]. Initially NaV 2 O 5 was con- sidered to be a candidate spin-Peierls material, but more recent work showed that the 4-fold superstructure below T c = 34 K is related to a combination of charge-, orbital- and magnetic order [6]. Recently, Seidel et al. [7] pro- posed that TiOCl is a 1D S =1/2 quantum spin system, that transforms into a spin-Peierls state at low tempera- tures. TiOCl crystallizes in a layered structure [8] (Fig. 1), in which two different types of chains of Ti 3+ , d 1 (S =1/2) have been identified [7]. The chain along a allows interac- tions between the electrons via superexchange, whereas the chain along b supports direct exchange interactions. The latter type of chains has been proposed to be respon- sible for the quasi-1-dimensional (1D) character of the magnetic interactions, as evidenced by the magnetic sus- ceptibility, electron spin resonance (ESR), IR reflectivity, angle-resolved photoelectron spectroscopy (ARPES) and electronic band structure calculations [7, 9, 10, 11, 12]. Based on the temperature dependencies of the magnetic susceptibility [7], ESR [9] and NMR [13] a second-order phase transition was found at T c2 = 94 K, while a first- order transition takes place at T c1 = 67 K. The latter transition corresponds to a sudden development of mag- netic order, accompanied by a doubling of the lattice con- stant along b [7, 13]. The magnetic moments are zero below T c1 , and the size (E g = 430 K) of the spin-gap has been taken as an indication for a non-conventional spin- Peierls state at low temperatures [13]. Above T c2 up to T * = 135 K a pseudo spin-gap due to fluctuations has been found [10, 13]. In the present contribution we report the discovery of superlattice reflections in the x-ray diffraction of TiOCl below T c1 = 66 K as well as a complete structure deter- mination at T = 10 K. The latter shows that the 2-fold superstructure can be interpreted as a dimerization of the 1D chains of Ti atoms along b, while the interatomic distances and bond angles between chains are much less affected by the structural deformation. Our results thus indicate that – despite the unconventional magnetic be- havior observed at higher temperatures – the opening of the spin gap below 66 K is due to a spin-Peierls transi- tion, where a surprisingly simple dimerization of the 1D Ti atom chains is responsible for the formation of spin singlets. Single crystals of TiOCl were prepared by gas trans- port in evacuated quartz glass tubes, following published procedures [8]. Single-crystal x-ray diffraction with syn- chrotron radiation was performed at beam-line D3 of Ha- sylab (DESY, Hamburg), employing monochromatized radiation of wavelength 0.5000 ˚ A. A single crystal of di- mensions 0.05×0.11×0.01 mm 3 was mounted on a carbon fibre attached to a closed-cycle helium cryostat mounted on a Huber 4-circle diffractometer. X-ray diffraction was measured by a point detector. In a first experiment q-scans were performed at T = 10 K along the reciprocal lattice lines (h+ξ,k,l), (h, k +ξ,l),