$^{29}$Si NMR study of polycrystalline NaTiSi$_{2}$O$_{6}$

NaTiSi$_{2}$O$_{6}$ (NTSO) structure consists of quasi 1-D zig-zag chains of edge-sharing slightly distorted TiO$_{6}$ octahedrons. The chains are separated by SiO$_{4}$ tetrahedrons. At high T the distance between magnetic spin-1/2 Ti$^{3+}$ ions in the chain is equal. At T$_c=$210 K the compound undergoes orbital-Peierls transition. As a result, below 210 K TiO$_{6}$ chain becomes dimerized having diamagnetic singlet ground state. Neutron spectroscopy provided singlet-triplet gap value 615(35) K [*], $\mu$SR 2$\Delta=$700(100) K. Our $^{29}$Si magic angle spinning NMR spectra show in paramagnetic region one single resonance with paramagnetic shift $K=$713 ppm at 300 K. The shift slightly increases with decreasing T and has maximum $K=$796 ppm at T= 213 K. Below T$_c$ the resonance transforms into two lines with different paramagnetic shifts. At T = 56 K the spectrum shows 2 sharp lines with diamagnetic chemical shifts -84 and -101 ppm corresponding to 2 different Si sites in the low-T unit cell. T-dependence of $^{29}$Si spin-lattice relaxation $T_{1}$ in 70 K $<$ T $<$ 140 K follows activation type T-behavior with $E_{a}$=300(20) K, which we ascribe to the splitting between the 2 lowest $d$-orbital energy levels.\\ $^{*}$ H. J. Silverstein et al., PRB 90, 140402(R) (2014).