The crystal structure of rutile-type magnesium deuteride, α-MgD2, has been studied by in situ neutron powder diffraction at room temperature and pressures up to 9.3 GPa. The bulk modulus B
0 = 49(2) GPa is about half, the linear compressibilities (0.0063/GPa along a and 0.0041/GPa along c) are about twice and the variation of the free positional parameter x about six times the values for the isostructural magnesium fluoride, indicating the strong polarizability of the deuteride anion. Under pressure the MgD6 octahedra are more compressed (four Mg–D distances at 199(1), two at 173(1) pm) than at ambient pressure (195.1(1), 194.8(2) pm) and the structural arrangement becomes more CaCl2 like, although no structural phase transition has been observed.
Among the many problems associated with high-pressure X-ray diffraction from polycrystalline samples in the diamond-anvil cell are strain and preferred orientation. A method is presented for efficiently reducing preferred orientation of powder samples compressed in diamond-anvil cells to pressures in excess of 20 GPa. This method may be successfully applied to samples of yield strength higher than alkalihalides. In addition, the problem of strain is discussed using ice-VII as an example and as an illustration of the importance of laser heating as a method of minimizing strain.
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