Temperature dependent structural, vibrational and magnetic properties of K₃Gd₅(PO₄)₆

Herein we report the evolution of the crystal structure of K₃Gd₅(PO₄)₆ in the temperature range from 20 K to 1073 K, as observed from variable temperature X-ray diffraction and Raman spectroscopic studies. K₃Gd₅(PO₄)₆ has an open tunnel containing a three dimensional structure built by [Gd₅(PO₄)₆]^3- ions which in turn are formed of PO₄ tetrahedra and GdO_n (n = 8 and 9) polyhedra. The empty tunnels in the structure are occupied by K⁺ ions and maintain charge neutrality in the lattice. Evolution of unit cell parameters with temperature shows a systematic increase with temperature. The average axial thermal expansion coefficients between 20 K and 1073 K are: α_a = 10.6 × 10^-6 K^-1, α_b = 5.5 × 10^-6 K^-1 and α_c = 16.4 × 10^-6 K^-1. The evolution of distortion indices of the various coordination polyhedra with temperature indicates a gradual decrease with increasing temperature, while those of Gd2O₉ and K2O₈ polyhedra show opposite trends. The overall anisotropy of the lattice thermal expansion is found to be controlled largely by the effect of temperature on GdO_n polyhedra and their linkages. Temperature dependent Raman spectroscopic studies indicated that the intensities and wavenumbers of most of the Raman modes decrease continuously with increasing temperature. Anharmonic analyses of Raman modes indicated that the lattice, rigid translation and librational modes have larger contributions towards thermal expansion of K₃Gd₅(PO₄)₆ compared to high frequency internal modes. The temperature and field dependent magnetic measurements indicated no long range ordering down to 2 K and the observed effective magnetic moment per Gd³⁺ ion and the Weiss constant are 7.91 μ_B and 0.38 K, respectively.