Crystal and magnetic structure ofEu4Ga8Ge16

Abstract

The antiferromagnetic ordering and crystal structure of the clathrate compound ${\mathrm{Eu}}_{4}{\mathrm{Ga}}_{8}{\mathrm{Ge}}_{16}$ was investigated using multitemperature neutron and synchrotron x-ray powder diffraction. High-resolution low-Q neutron data were measured at long wavelength $(\ensuremath{\lambda}=4.2\AA{})$ between 1.5 and 15 K for an accurate description of the magnetic structure, whereas high-Q diffraction patterns were collected using neutrons of wavelength 1.9 \AA{} at the same temperatures to determine the nuclear structure precisely. The structure orders antiferromagnetically at about 8 K with ferromagnetic chains parallel to the a axis. The intrachain Eu-Eu distance, 4.1216(1) \AA{} at 1.5 K, is significantly shorter than the distance observed in the ferromagnetic clathrates $\ensuremath{\beta}\ensuremath{-}{\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ (5.23 \AA{}) and $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Eu}}_{8}{\mathrm{Ga}}_{16}{\mathrm{Ge}}_{30}$ (5.56 \AA{}). Antiferromagnetic coupling to the nearest and next-nearest chains at distances of 5.99 and 6.98 \AA{}, respectively, leads to an overall antiferromagnetic structure. A fit to a power law of the temperature dependence of the ordered ${\mathrm{Eu}}^{2+}$ magnetic moment results in a moment of $7.01(7){\ensuremath{\mu}}_{B}$ at 0 K in agreement with the $7{\ensuremath{\mu}}_{B}$ for the free ion value of ${\mathrm{Eu}}^{2+}.$ The temperature dependence of the crystal structure was investigated from 11 K to room temperature using synchrotron x-ray powder diffraction. Analysis of the atomic displacement parameters with Einstein and Debye models gives ${\ensuremath{\Theta}}_{E}=82(3)\mathrm{K}$ for the guest atom and ${\ensuremath{\Theta}}_{D}=266(4)\mathrm{K}$ for the framework atoms. Based on ${\ensuremath{\Theta}}_{D}$ the lattice contribution to thermal conductivity is estimated to be 0.0125 W/cm K.