Crystal and magnetic structure of Eu₄Ga₈Ge₁₆

The antiferromagnetic ordering and crystal structure of the clathrate compound Eu4Ga8Ge16 was investigated using multitemperature neutron and synchrotron x-ray powder diffraction. High-resolution low-Q neutron data were measured at long wavelength (lambda=4.2 Angstrom) 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 Angstrom 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) Angstrom at 1.5 K, is significantly shorter than the distance observed in the ferromagnetic clathrates beta-Eu8Ga16Ge30 (5.23 Angstrom) and alpha-Eu8Ga16Ge30 (5.56 Angstrom).

Antiferromagnetic coupling to the nearest and next-nearest chains at distances of 5.99 and 6.98 Angstrom, respectively, leads to an overall antiferromagnetic structure. A fit to a power law of the temperature dependence of the ordered Eu2+ magnetic moment results in a moment of 7.01(7)mu(B) at 0 K in agreement with the 7mu(B) for the free ion value of Eu2+.

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 Theta(E)=82(3) K for the guest atom and Theta(D)=266(4) K for the framework atoms.

Based on Theta(D) the lattice contribution to thermal conductivity is estimated to be 0.0125 W/cm K.