Analysis of the phonon spectrum in the titanium oxyhalide TiOBr

Abstract

We present the electrodynamic response of TiOBr, which undergoes a transition to a spinsinglet ground state for temperatures below T C1 = 28 K. The temperature evolution of the phonon anomalies indicates, like in TiOCl, an extended fluctuation regime, extending well above T C1 . At low frequencies, the spectral weight is progressively suppressed by decreasing the temperature, suggesting the formation of a spin-gap and a considerable electron-phonon coupling. A comparison of the two oxyhalides shows a weaker interplane coupling and reduced dimensionality for TiOBr that enhances fluctuations and shifts the ordering temperatures to lower values. In the fluctuation regime the temperature dependencies of some phonon parameters track the temperature evolution of the magnetic susceptibility. 71.36.+c

Symmetry disquisition on theTiOXphase diagram(X=Br,Cl)Fausti1, Lummen2, Angelescu3 et al. 2007Phys. Rev. B274271View full textAdd to dashboardCite

“…Here, the infrared active modes are taken from the literature, 7,9 and for the Raman modes, the temperatures chosen for the two compounds are 300 K for TiOCl and 100 K for TiOBr. The observed Raman frequencies agree well with previous reports.…”

“…7,12,15 The temperature dependence of the Raman active modes for TiOBr between 3 and 50 K is depicted in Fig. 4.…”

“…7 Only recently, infrared spectroscopy supported by cluster calculations excluded a ground state degeneracy of the Ti d orbitals for TiOCl, hence suggesting that orbital fluctuations cannot play an important role in the formation of the anomalous incommensurate phase. 27,28 Since the agreement between the previous cluster calculations and the experimental results is not quantitative, the energy of the lowest 3d excited level is not accurately known, not allowing us to discard the possibility of an almost degenerate ground state.…”

“…5 Only recently TiOX emerged in a totally new light, namely, as a one-dimensional antiferromagnet and as the second example of an inorganic spin-Peierls compound ͑the first being CuGeO 3 ͒. 6,7 The TiO bilayers constituting the TiOX lattice are candidates for various exotic electronic configurations, such as orbital ordered, 3 spin-Peierls, 6 and resonating-valence-bond states. 8 In the case of the TiOX family, the degeneracy of the d orbitals is completely removed by the crystal field splitting, so that the only d electron present, mainly localized on the Ti site, occupies a nondegenerate energy orbital.…”

Incommensurate interactions and nonconventional spin-Peierls transition in TiOBr

“…13,14 Physical properties of both compounds are similar, with the two transition temperatures scaled down towards T c1 = 27 K and T c2 = 47 K in TiOBr. 9,15,16,17,18 The two-fold superstructures below T c1 are similar in TiOCl and TiOBr, suggesting a spin-Peierls state for TiOBr too. 19 In the present contribution we report the discovery of incommensurate satellite reflections in xray diffraction of TiOBr at temperatures T with T c1 < T < T c2 .…”