Orbital and lattice dynamics in pyroxenes

We have studied with infrared reflectivity and Raman scattering techniques the orbital and lattice dynamics, and the origin of the phase transitions in the pyroxene compounds NaMSi2O6 (with M ¼ Ti, V, and Cr). In the quasi one-dimensional S ¼ 1/2 system NaTiSi2O6 we observe the anomalous high-temperature phonon broadenings 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 type of the phase transition, which we describe as an orbital analogue of the spin-Peierls phase transition. In the S ¼ 1 NaVSi2O6 system orbital degrees of freedom are strongly suppressed since both of the active t2g orbitals are occupied and the magnetic excitations are well described within the Heisenberg model, indicating that at TN ¼ 19K this system orders antiferromagnetically. The NaCrSi2O6 has a fully polarized t2g core, no orbital degrees of freedom, and no anomalous phonon broadening in the Raman spectra.