We study with the Raman scattering technique two types of phase transitions in the pyroxene compounds NaMSi2O6 (with M=Ti, V, and Cr). In the quasi one-dimensional S=1/2 system NaTiSi2O6 we observe anomalous high-temperature phonon broadening and large changes of the phonon energies and line-widths across the phase transition at 210 K. The phonon anomalies originate from an orbital order-disorder phase transition and these results --combined with theoretical considerations-- indicate that the high temperature dynamical Jahn-Teller phase of NaTiSi2O6 exhibits a spontaneous breaking of translational symmetry into a dimerized, Jahn-Teller distorted, orbital ordered state under the formation of spin valence bonds. In S=1 NaVSi2O6 orbital degrees of freedom are strongly suppressed and the magnetic excitations are well described within a Heisenberg model, indicating that at TN=19K this system orders antiferromagnetically.
@article{arxiv.cond-mat/0210191,
title = {Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi$_2$O$_6$},
author = {M. J. Konstantinović and J. van den Brink and Z. V. Popović and V. V. Moshchalkov and M. Isobe and Y. Ueda},
journal= {arXiv preprint arXiv:cond-mat/0210191},
year = {2007}
}