Orbital Dimerization and Dynamic Jahn-Teller Effect in NaTiSi$_2$O$_6$

TL;DR

This study reveals that NaTiSi$_2$O$_6$ undergoes a phase transition involving orbital order-disorder and Jahn-Teller effects, leading to a dimerized, orbital-ordered state, while NaVSi$_2$O$_6$ exhibits conventional antiferromagnetic order.

Contribution

It provides experimental evidence of orbital order-disorder transition and Jahn-Teller effects in NaTiSi$_2$O$_6$, highlighting the role of orbital degrees of freedom in phase transitions.

Findings

NaTiSi$_2$O$_6$ shows phonon anomalies indicating orbital order-disorder transition.

NaTiSi$_2$O$_6$ exhibits a spontaneous symmetry breaking into a dimerized Jahn-Teller distorted state.

NaVSi$_2$O$_6$ orders antiferromagnetically with suppressed orbital degrees of freedom.

Abstract

We study with the Raman scattering technique two types of phase transitions in the pyroxene compounds NaMSi$_2$O$_6$ (with M=Ti, V, and Cr). In the quasi one-dimensional S=1/2 system NaTiSi$_2$O$_6$ 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 NaTiSi$_2$O$_6$ 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 NaVSi$_2$O$_6$ orbital degrees of freedom are strongly suppressed and the magnetic excitations are well described within a Heisenberg model, indicating that at T$_N$=19K this system orders antiferromagnetically.