Raman and crystal field studies of Tb-O bonds in $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$

We have studied the temperature dependence of $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ Raman-active phonons and its $\mathrm{T}{\mathrm{b}}^{3+}$ crystal field (CF) excitations. Interestingly, the Raman intensities of some phonons are significantly reduced below $\sim 180\mathrm{K}$. Such behaviors are also observed in $\mathrm{HoM}{\mathrm{n}}_2{\mathrm{O}}_5$ and $\mathrm{YM}{\mathrm{n}}_2{\mathrm{O}}_5$ systems. A connection between the Raman intensities and the nearest-neighbor mean-square relative displacement ${\sigma }^2$ is established. Also, some of the $\mathrm{T}{\mathrm{b}}^{3+}$ and $\mathrm{H}{\mathrm{o}}^{3+}$ CF excitations become broader below $\sim 180\mathrm{K}$. These results are discussed in terms of the disorder induced by the Tb-O bond splitting.

Figure 1

(a) X-ray diffraction pattern at room temperature of $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ single crystal. (b) An example of reciprocal space mappings for the (102) reflection.

Figure 2

(a) Temperature dependence of zero-field-cooled (ZFC) magnetization of $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ under a magnetic field of 500 Oe. (b) The inverse susceptibility as a function of temperature fitted to the Curie-Weiss law.

Figure 3

The temperature dependence of the Raman spectra of (a) $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$, (b) $\mathrm{HoM}{\mathrm{n}}_2{\mathrm{O}}_5$, and (c) $\mathrm{YM}{\mathrm{n}}_2{\mathrm{O}}_5$ in the $zz$ scattering configuration between 297 and 5 K.

Figure 4

(a) Thermal evolution of the Raman frequency of the $\sim 327\mathrm{c}{\mathrm{m}}^{-1}$ mode in $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5,\mathrm{HoM}{\mathrm{n}}_2{\mathrm{O}}_5$, and $\mathrm{YM}{\mathrm{n}}_2{\mathrm{O}}_5$. Temperature dependence of the Raman intensities of the (b) $\sim 220\mathrm{c}{\mathrm{m}}^{-1}$ and (c) $\sim 700\mathrm{c}{\mathrm{m}}^{-1}$ modes of $R\mathrm{M}{\mathrm{n}}_2{\mathrm{O}}_5$ ($R=\mathrm{Tb}$, Ho, and Y) in the $zz$ scattering configuration. CP and WP indicate cooling and warming processes, respectively.

Figure 5

${\sigma }^2$ of the Tb-O bonds in $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ as a function of temperature. The squares and triangles are the ${\sigma }_{\mathrm{EXAFS}}^2$ experimental data obtained by Tyson et al. [19], as reported with their corresponding error bars. The solid circles are the results of the calculation of ${\sigma }_R^2$ (Tb-O) with the Raman data.

Figure 6

Transmission spectra of $\mathrm{T}{\mathrm{b}}^{3+} F_6\to F_5,F_4,F_3,F_2$, and $F_1$ CF transitions in $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ at 4.2 K. The inset shows the reflectivity spectrum of $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ at 4.2 K. The $\mathrm{T}{\mathrm{b}}^{3+}$ CF excitations at 75, 120, and $143\mathrm{c}{\mathrm{m}}^{-1}$ are observed as inflections in the reflectivity spectrum (gray arrows).

Figure 7

(a) Transmission maps of $\mathrm{T}{\mathrm{b}}^{3+} F_6\to F_4$ CF transitions, where the solid black circles indicate the CF levels of the ${}^{7}F_{J=4}$ multiplets and the open black circles trace the excited levels of the ground state ($J=6$) thermally populated. (b)–(d) show the temperature dependence of the full width at half maximum (FWHM) of the 3697, 3453, and $3393\mathrm{c}{\mathrm{m}}^{-1}$ CF excitations.

Figure 8

Transmission spectra of $\mathrm{H}{\mathrm{o}}^{3+} I_8\to I_7$ CF transitions in $\mathrm{HoM}{\mathrm{n}}_2{\mathrm{O}}_5$ at 4.2 K. The insets show the temperature dependence of the FWHM of the 5220 and $5340\mathrm{c}{\mathrm{m}}^{-1}$ CF excitations.