Orbital disorder and ordering in NaTiSi2O6 : Si29 and Na23 NMR study

Abstract

${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$ is an exemplary compound, showing an orbital assisted spin-Peierls phase transition at ${T}_{c}=210$ K. We present the results of $^{29}\mathrm{Si}$ and $^{23}\mathrm{Na}$ nuclear magnetic resonance (NMR) measurements of ${\mathrm{NaTiSi}}_{2}{\mathrm{O}}_{6}$. The use of magic angle spinning (MAS) techniques unambiguously shows that only one dynamically averaged silicon site can be seen at $T$ $>\phantom{\rule{4pt}{0ex}}{T}_{c}$. At cooling, the $^{29}\mathrm{Si}$ MAS NMR spectrum shows interesting changes. Immediately below ${T}_{c}$, the spectrum gets very broad. Cooling further, it shows two broad lines of unequal intensities which become narrower as the temperature decreases. Below 70 K, two narrow lines have chemical shifts that are typical for diamagnetic silicates. The hyperfine couplings for the two sites are $^{29}\mathrm{H}$ ${}_{\text{hf}}=7.4$ kOe/${\ensuremath{\mu}}_{B}$ and 4.9 kOe/${\ensuremath{\mu}}_{B}$. In the paramagnetic state at high temperature, the spin-lattice relaxation of $^{29}\mathrm{Si}$ was found to be weakly temperature dependent. Below ${T}_{c}$ the Arrhenius-type temperature dependence of the relaxation rate indicates an energy gap $\mathrm{\ensuremath{\Delta}}$/${\mathrm{k}}_{B}=1000(50)$ K. In the temperature region from 120 to 300 K, the relaxation rate was strongly frequency dependent. At room temperature, we found a power law dependence ${T}_{1}^{\ensuremath{-}1}\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\propto}\phantom{\rule{3.33333pt}{0ex}}{\ensuremath{\omega}}_{L}^{\ensuremath{-}0.65(5)}$. For 70 K $<$ $T$ $<$ 120 K, the relaxation appeared to be nonexponential, which we assigned to a relaxation due to fixed paramagnetic centers. Simulation of the magnetization recovery curve showed activation type temperature dependence of the concentration of these centers. The NMR spectrum of $^{23}\mathrm{Na}$ shows the line with typical shape for the central transition of a quadrupolar nucleus. A small frequency shift of $^{23}\mathrm{Na}$ resonance corresponds to a very small hyperfine coupling $^{23}\mathrm{H}$ ${}_{\text{hf}}=0.32$ kOe/${\ensuremath{\mu}}_{B}$. In addition, at $T$ $>\phantom{\rule{4pt}{0ex}}{T}_{c}$ the $^{23}\mathrm{Na}$ spectrum shows another Lorentzian shaped resonance which we attribute to the Na sites where the quadrupolar coupling is partly averaged by ionic motion.