Lattice thermal vibration and its nonharmonic effect in Nd-doped rare-earth vanadates

摘要

The 0.5 at. percent Nd-doped gadolinium and yttrium orthovanadate, Nd:GdVO_4 (NGV) and Nd:YVO_4 (NYV), crystallize in the tetragonal space group I4_1/amd and are of a zircon-type structure with the lattice constants a = b = 0.7212(6) and c = 0.6348(3) nm for NGV and a = b = 0.7123(5) and c = 0.6292(5) nm for NYV. At high and room temperatures, the Raman spectra of NYV are much the same; however, the Raman spectra of NGV are different from those of NYV. The scattering intensity of NGV is largely stronger than that of NYV. Nonharmonic effect of the lattice thermal vibrations, including the thermal conductivity and expansion, is theoretically and experimentally discussed. The theoretical results roughly indicate that a crystal with the larger integrated intensities of the Raman-scattering peaks under the cutoff value of linear dependence of Debye frequency versus the temperature has the larger thermal conductivity. The experimental data of the thermal conductivity of the five samples show that the thermal conductivity of NGV is more outstanding than that of NYV in every direction. Compared the integrated intensity ratio I_G/I_Y between the NGV and NYV Raman-scattering peaks under the cutoff wavenumber of Debye frequency with the ratio K_G/K_Y, of their crystal thermal conductivity, it may be seen that the Raman results are basically in agreement with the conductivity measurements within our experimental error. The experimental datum of the thermal conductivity of NYAG (Nd-doped yttrium aluminum garnet) with the cubic symmetry and stable laser properties and thermodynamics parameters, which is often considered as a comparable standard, approximately approaches to the average value of the NGV thermal conductivity in the a and c directions. The thermal-expansion data of NGV exhibit its small line-expansion coefficients, which imply that the large thermal conductivity is needed and existential and that NGV is more capable to bear a large temperature gradient, therefore, is also more suitable as a solid-state laser material than NYV crystals.