Thursday, June 27, 2013

1306.6310 (J. R. L. Mardegan et al.)

Pressure induced amorphization and collapse of magnetic order in type-I
clathrate Eu8Ga16Ge30

J. R. L. Mardegan, G. Fabbris, L. S. I. Veiga, C. Adriano, M. A. Avila, D. Haskel, C. Giles
We investigate the low temperature structural and electronic properties of the type-I clathrate Eu8Ga16Ge30 under pressure using x-ray powder diffraction (XRD), x-ray absorption near-edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) techniques. The XRD measurements reveal a transition to an amorphous phase above 18 GPa. Unlike previous reports on other clathrate compounds, no volume-collapse is observed prior to the crystalline-amorphous phase transition which takes place when the unit cell volume is reduced to 81% of its ambient pressure value. Fits of the pressure-dependent relative volume to a Murnaghan equation of state (EOS) yield a bulk modulus B0 = 65+-3 GPa and a pressure derivative B'0 = 3.3+-0.5. The Eu L2-edge XMCD data shows quenching of the magnetic order at the crystalline-amorphous phase transition. The XANES spectra indicate the persistence of Eu2+ valency state up to 22 GPa, therefore the suppression of XMCD intensity is due to the loss of magnetic order as a result of frustrated exchange interactions in the amorphous phase, and not due to quenching of local moments. When compared with other clathrates, the results point to the importance of guest ion-cage interactions in determining the mechanical stability of the framework structure and the critical pressure for amorphization. Finally, the crystalline structure is not found to recover after pressure release, resulting in a novel amorphous material that is at least metastable at ambient pressure and temperature.
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