NMR, magnetization, and heat capacity studies of the uniform spin-$\frac{1}{2}$ chain compound ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$

We report the local (NMR) and bulk (magnetization and heat capacity) properties of the vanadium-based $S=\frac{1}{2}$ uniform spin chain compound ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}\left({\mathrm{Bi}}_4{\mathrm{V}}_2{\mathrm{O}}_{10.66}\right)$. In the low-temperature $\alpha$ phase, the magnetic ions $\left({\mathrm{V}}^{4+}\right)$ are arranged in one-dimensional chains. The magnetic susceptibility shows a broad maximum around 50 K signifying a short-range magnetic order. Heat capacity measurements also reveal low-dimensional magnetism. The ${}^{51}\mathrm{V}$ magic angle spinning nuclear magnetic resonance measurements clearly show that the magnetic ${\mathrm{V}}^{4+}$ and nonmagnetic ${\mathrm{V}}^{5+}$ species are located on different crystallographic sites with no mixed occupation. The spin susceptibility calculated from the shift of the ${}^{51}\mathrm{V}$ NMR spectra reproduces the behavior observed in magnetic susceptibility and agrees well with the $S=\frac{1}{2}$ uniform spin chain model with $J=113\left(5\right)$ K.

Figure 1

Left: Possible interaction path between the magnetic ${\mathrm{V}}^{4+}$ ions in ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ mediated via nonmagnetic ${\mathrm{V}}^{5+}$ ions and oxygen. Right: Top view of the chains separated by Bi-O layers.

Figure 2

EDX spectrum (top) and SEM image (bottom) of polycrystalline ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$.

Figure 3

Experimental (black points) and calculated (red line) powder XRD patterns of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$. Positions of peaks are given by black ticks, and the difference plot is shown by the black line in the bottom part.

Figure 4

Temperature dependence of the mass change and the heat flow during heating of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ in the TGA experiment.

Figure 5

The temperature dependence of the susceptibility $\chi =M/H$ at $H=4.7$ T for ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$. Black open circles are $\chi -{\chi }_0$ data, the green line is the low-$T$ Curie-Weiss contribution and the blue solid diamonds show $\chi -{\chi }_0-{\chi }_{\mathrm{Curie}}$. The region of short-range magnetic ordering is indicated by the red arrow. The red line shows the high-$T$ Curie-Weiss fit in the temperature range of 190–300 K. The inset shows the variation of the position of maximum in the magnetic susceptibility curves with the change in applied field.

Figure 6

The experimental magnetization data of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ vs applied magnetic field at 1.8 K.

Figure 7

The temperature dependence of specific heat of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ in zero magnetic field; the blue points are the fit described in the text, and the red line is its extrapolation. The top inset displays the $C_p/T$ vs T plot at 0 T (black solid circles) and 9 T (red open diamonds) magnetic fields, respectively. The bottom inset shows the magnetic contribution to the heat capacity (black open circles). The magenta line is the magnetic heat capacity contribution for the 1D uniform Heisenberg chain; the green data points (right axis) show the change in entropy $\mathrm{\Delta }S$ with $T$.

Figure 8

${}^{51}\mathrm{V}$ MAS-NMR spectra of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ measured at different temperatures in 4.7-T fixed magnetic field. The main line at the isotropic value of the magnetic shift is highlighted in red, and the rest of the peaks are spinning sidebands at multiples of the spinning frequency (30 kHz) apart from the main line.

Figure 9

The top spectrum is the zoomed part of the central peak of the room-temperature (305 K) ${}^{51}\mathrm{V}$ NMR spectrum of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$. The bottom spectrum is the ${}^{51}\mathrm{V}$ MAS-NMR spectrum of ${\mathrm{Bi}}_4{\mathrm{V}}_2{\mathrm{O}}_{11}$. The black line is the experimental spectrum, which can be well fitted by several Lorentzian lines, as given by green lines; the red line is the sum of the components. The spectra were recorded on an AVANCE-III-800 spectrometer at ${}^{51}\mathrm{V}$ resonance frequency of 210.5 MHz, using a home-built MAS probe for 1.3-mm rotors at 50.1-kHz sample spinning speed.

Figure 10

Temperature dependence of the ${}^{51}\mathrm{V}$ NMR shift $K$ of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ (shown by black circles) measured using the cryoMAS technique. The black and green lines are the fittings with the susceptibility model using Eq. (1) for a $S=\frac{1}{2}$ uniform chain $\left(\alpha =1\right)$ and for an alternating chain $\left(\alpha =0.995\right)$, respectively. The inset shows the ${}^{51}\mathrm{V}$ MAS-NMR shift $K$ of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$ vs $\chi -{\chi }_0-{\chi }_{\mathrm{Curie}}$, where both $\chi \left(T\right)$ and $K$ are measured at 4.7 T, with temperature being an implicit parameter. The solid line is the linear fit.

Figure 11

Temperature dependence of the spin-lattice relaxation rate $\left(1/T_1\right)$ of ${\mathrm{Bi}}_6{\mathrm{V}}_3{\mathrm{O}}_{16}$. In the top inset the same plot is shown with temperature in the log scale and low-$T$ data measured on a static sample (red diamonds). The bottom inset shows the plot of $1/K{T}_1\mathrm{T}$ vs $T$.