H.W. Brinks

Accurate structure of LiAlD4 studied by combined powder neutron and X-ray diffraction

The accurate structure of LiAlD4 was determined from combined Rietveld-type profile refinements of powder X-ray and neutron diffraction data at 295 K. In addition, neutron diffraction was used for structure analyses at 8 K. The space group is P21/c with a=482.54(1), b=780.40(1), c=789.68(1) pm and β=112.268(1)° at 295 K. The slight distortion of the AlD4 tetrahedra increases at low temperatures. The Al–D distances are in the range 160.3(7)–163.3(5) pm at 295 K and 159.6(6)–164.5(5) pm at 8 K. The lithium atoms are surrounded by a trigonal bipyramid of five deuterium atoms from five neighbouring [AlD4] tetrahedra.

The decomposition of LiAlD4 studied by in-situ X-ray and neutron diffraction

Abstract The decomposition of LiAlD 4 was studied by thermal desorption spectroscopy and neutron and synchrotron X-ray in-situ diffraction. The first two reaction stages (3LiAlD 4 =Li 3 AlD 6 +2Al+3D 2 and Li 3 AlD 6 =3LiD+Al+ 3 2 D 2 ) were confirmed. No other intermediate phases were observed. Both reactions are very heat-rate dependent. The lowest temperature measured for complete reactions were 112 and 127xa0°C, respectively. Based on refinements of the unit-cell dimensions, neither LiAlD 4 nor Li 3 AlD 6 lose any D 2 prior to their thermal decompositions.

Hexagonal LaNiSnD2 with a filled ZrBeSi-type structure

Abstract LaNiSnD 2 dideuteride was synthesised and characterised by powder synchrotron X-ray and neutron diffraction and thermal desorption spectroscopy. Transformation from orthorhombic TiNiSi into the hexagonal ZrBeSi type proceeds in the metal sublattice upon deuteration. In LaNiSnD 2 (space group P6 3 /mmc ; a =4.42249(4), c =8.67405(9) A) deuterium atoms completely occupy one type of La 3 Ni tetrahedra, sharing common vertexes and edges and forming a spatial framework. The shortest interatomic distances in the structure are: La–D, 2.6118(4) A; Ni–D, 1.619(2) A; Sn–D, 2.718(2) A; and D–D, 2.780(2) A. Reversible formation of LaNiSn from the deuteride proceeds in vacuum at 670 K, with a peak of D evolution at 530–610 K.

Crystal structure of TbNiSiD1.78

Abstract A hydride of TbNiSi has been synthesized and studied by neutron and synchrotron X-ray diffraction. TbNiSiD 1.78 (space group P 6 3 / mmc , a =4.03078(10) A, c =7.97563(11) A) takes the ZrBeSi-type structure with D in 4 f sites ( 1 3 , 2 3 , 0.0466(3)), surrounded by Tb 3 Ni in tetrahedral coordination. The metal atoms of TbNiSiD 1.78 occupy 2 a , 2 c and 2 d sites, respectively. Each D-filled Tb 3 Ni tetrahedron shares three edges (via Tb–Tb bonds) and one corner (via Ni) with equivalent Tb 3 Ni tetrahedra. During deuteration a displacive transition of the metal lattice take place, which increases the symmetry from orthorhombic (TiNiSi-type structure) to hexagonal. TbNiSiD 1.78 has the geometry of an AA stacking of close-packed Tb layers. The same transition has earlier been observed for LaNiSn deuteride.

In situ powder neutron diffraction study of LaNiInD1.63 with short D…D distances

Abstract The recent powder neutron diffraction study of the crystal structure of LaNiInD 1.22 [J. Alloys Comp. 330–322 (2002) 132] concluded on the formation of a D…D pair with an unusually short interatomic distance of 1.63 A. Hydrogen atoms in LaNiInD 1.22 occupy a single crystallographic site and are coordinated by face-sharing La 3 Ni tetrahedra (92% occupancy). PCT measurements show that hydrogen storage capacity of LaNiIn, 1.63 at.H/formula unit, exceeds the limit of 1.33 at.H/LaNiIn when the La 3 Ni sites are completely occupied. In the present work, in situ powder neutron diffraction data were collected under D 2 pressure of 4.6 bar in order to study the deuteride with a maximum D content in the metal matrix. In the hexagonal structure of LaNiInD 1.63 (space group P 6 2 m ; a =7.3874(4); c =4.6816(2) A) Rietveld refinements showed that deuterium atoms occupy 36% of the available distorted La 3 NiIn 2 octahedra, in addition to the 96% filled La 3 Ni sites. The structure of LaNiInD 1.63 represents the first example of a deuteride containing direct In–D bonds (2.346(2) A). H bonding to the La 3 NiIn 2 sites is rather weak and a desorption from these sites takes place at room temperature and hydrogen pressures below 1 bar.

The magnetic structure of TbNiAlD1.1

Abstract The magnetic structure of orthorhombic TbNiAlD 1.1 was determined by powder neutron diffraction at 3.8 and 10 K, below the antiferromagnetic ordering temperature T N =11 K. The unit cell is doubled in b and c directions compared to the nuclear unit cell (space group Amm 2; at 3.8 K a =3.6554; b =12.3954; c =7.6316 A), and the magnetic moments are confined to the bc plane with main component along c . The magnetic structure is different from TbNiAl and TbNiAlD 0.3 which both have ordered magnetic moments along the corresponding a axis. The magnetic moments of Tb1 and Tb2 at 3.8 K are 6.8 and 5.7 μ B , respectively. On heating to 10 K, the type of magnetic structure is not changed, however, μ (Tb) are decreased to 2.9 μ B (Tb1) and 5.4 μ B (Tb2).

On the ferromagnetism of AuMnSn

Abstract The ferromagnetic ordering of the Heusler-related alloy AuMnSn has been verified by powder neutron diffraction. The cubic AlLiSi-type crystal structure of AuMnSn [ a =634.12(11) pm at 298 K, Au in 4 c , Mn in 4 b , Sn in 4 a of F 43 m ] has been confirmed. Contrary to the indications from the preceding single-crystal X-ray diffraction study of AuMnSn the powder neutron diffraction data gave no evidence for intermixing of Mn and Sn on the 4 b and 4 a sites. The ordered magnetic moments are confined to the Mn atoms and their size at 298 K is μ F,Mn =3.62(7) μ B .

Structure and magnetic properties of TbNiAl-based deuterides

Abstract This work concerns the study of the crystal and magnetic structure of TbNiAlD x compounds ( x =0.54, 1.23 and 1.33) by neutron diffraction. TbNiAlD 0.54 is isostructural with TbNiAl. It undergoes two magnetic transitions at T 1 =16 K and T N =36 K, with magnetic structure similar to that of TbNiAl below T 1 =23 and T N =45 K. TbNiAlD 1.23 is in crystallography and magnetism comparable to TbNiAlD 1.1 . When the Tb 2 NiAl positions are occupied, an orthorhombic distortion is induced. This completely changes the magnetic ordering and could be explained by the easy magnetisation directions being perpendicular to the shortest Tb–Tb distances.

Crystallographic and magnetic structure of Pr6Fe13AuD13

Neutron diffraction performed on deuterated powder samples of Pr 6 Fe 13 Au shows that the crystal structure of the parent compound is retained upon charging with deuterium. The refined composition of the deuteride is Pr 6 Fe 13 AuD 13 . The deuterium atoms occupy various types of interstitial sites bounded by Pr and Fe atoms. Each of these sites is surrounded by at least two Pr atoms. A collinear antiferromagnetic ordering of the four Fe and the two Pr sublattices was reported previously for the uncharged Pr 6 Fe 13 Au compound. By contrast, a collinear ferromagnetic structure has been derived from the present neutron data of the deuteride. The easy moment direction is the same in Pr 6 Fe 13 AuD 13 and Pr 6 Fe 13 Au, being perpendicular to the c-axis. For the deuteride this result was derived from a complementary synchrotron radiation X-ray study made on magnetically aligned particles of the ferromagnetic deuteride.

The magnetic structure of TbNiSiD1.78

Abstract TbNiSiD 1.78 has been studied by powder neutron diffraction below 100 K. The compound takes the hexagonal room temperature structure at 100 and 50 K ( P 6 3 / mmc ). At 2 K, below the antiferromagnetic ordering temperature of 10 K, there is a small orthorhombic distortion of the lattice. The refined unit-cell dimensions at 2 K (space group Pnma ) are a =7.9505(2), b =4.02502(14), c =6.9823(2) A. The magnetic moments of Tb are 8.71(6) μ B , and are ordered antiferromagnetically along a .