A.V. Irodova

Lattice structure and phase transitions of hydrogen in intermetallic compounds

Abstract Structural aspects of the behaviour of hydrogen in intermetallic compounds are discussed. A similar pattern of hydrogen behaviour is shown to occur in Laves phase intermetallic compounds and metals of groups IV and V: a change in the hydrogen coordination, the occurrence of short-range order in the arrangement of interstitial atoms, the formation of superstructures etc. Some special features which produce a complex structure of the metallic matrix are discussed.

Crystal structure and volume effects in the hydrides of d metals

Abstract We suggest, as the result of a generalization of structural data on the hydrides of d transition metals, a simple model, within the framework of which the structural characteristics of the metal hydrides (hydrogen coordination, volume change and compressibility) are connected with electronic parameters (Fermi energy and density of states at the Fermi level). Volume effects have been considered by using a rigid d-band model. Phase transformations with changing hydrogen coordination have been predicted for a number of hydrides of d metals.

Compressibility of dihydrides of transition metals

Abstract The crystal structures and equations of state of the transition metal dihydrides VD 1.9 , NbD 1.9 , TiD 2 , ScD 2 and YD 2 have been studied by neutron diffraction in the pressure range up to 6–13 GPa. It has been established that the structure of the deuterides (CaF 2 type) is stable within the whole pressure range. An increase in tetragonal splitting under pressure was observed in TiD 2 . The compressibilities of vanadium and niobium deuterides are close to those of the pure metals but the compressibilities of titanium, scandium and yttrium deuterides are less than those of the metals. These results can be explained in terms of the electronic structures of the dihydrides.

Hydrogen ordering in the cubic laves phase HfV2

Abstract We investigated the crystal structure of HfV2D4 using a neutron diffraction technique. The high temperature phase is shown to be a disordered solid solution in which deuterium atoms occupy two types of tetrahedral interstices in the cubic lattice of HfV2; moreover, there is a short-range “blocking-type” order in the mutual arrangement of the interstitial deuterium atoms. Below 278 K an order-disorder phase transition occurs, resulting in the formation of a superstructure belonging to space group I4 1 a . We suggest, within the framework of the statistical thermodynamic theory of order-disorder transitions developed for this case, an explanation for the super-structure formation and we reach some conclusions about the nature of regularities of hydrogen behaviour which are common to metals and intermetallic compounds.

Neutron diffraction study of crystal structure and equation of state AID3 up to the pressure of 7.2 GPa

Abstract The phase α-AID 3 has been studied by neutron diffraction up to pressures of 7.2 GPa using a sapphire anvil cell. The lattice contracts mainly in the basal plane as pressures increases. Strong changes of the deuterium sublattice were found, resulting in the formation of a close packing structure. The structural data obtained suggest that the bonding is mainly ionic in the aluminium hydride.

Hydrogen Caused Ordering in PdAg Alloy

Hydrides of PdAg alloy synthesized at high hydrogen pressures have been studied by neutron diffraction. At temperatures of about 470K saturation with hydrogen causes ordering of the metal atoms. At higher temperatures of about 640K no ordering occurs. The results have been explained by the theory taking into account an influence of interstitial atoms on the ordering temperature of binary alloys. It is expected that hydrogénation and subsequent dehydrogenation could be a perspective technique for manufacturing ordered alloys in the cases when the ordering cannot be made by the usual means.

Evolution of hydrogen superstructure with k=(1/2 1/2 1/2) in ZrV2D2+δ, −0.8<δ<0.2

Abstract We have performed a neutron-diffraction study of the superstructure with k =(1/2 1/2 1/2) in the hydrogen solid solution ZrV 2 –D and have followed its evolution with hydrogen content at 100 K. The superstructure belongs to the space group Cc and can be characterized by one of the two stoichiometric compositions, either ZrV 2 D 1 or ZrV 2 D 3 . Its real range of existence is limited from top by the composition ZrV 2 D 2 owing to the short-range hydrogen–hydrogen interaction. An indication of temperature evolution of the superstructure has been found.

A neutron diffraction study of ZrD2 up to the pressure of 10 GPa

Abstract A neutron diffraction study of the effect of pressure on the lattice parameters of ZrD2 has been carried out at room temperature using a diamondanvil cell up to a pressure of 10 GPa. It was found that ZrD2 is compressed less than zirconium metal, and the change in the axial ratio c a in ZrD2 is linear with the pressure. These facts are explained within the framework of the electronic structure of the dideuteride. Some possible phase transitions in ZrD2 at higher pressures are considered.

The ZrV2D6 crystal structure

Abstract We have studied by neutron powder diffraction the crystal structure of the maximum deuteride ZrV 2 D 6 based on the cubic Laves phase ZrV 2 . The structure is orthorhombic, space group Pnma (No. 62), cell parameters: a =5.5738(3) A, b =5.6996(3) A, c =7.9942(3) A. The arrangement of the metal atoms, Zr and V, is the same as in the initial ZrV 2 . Three D-atoms are ordered in the tetrahedral interstices Zr+3V, and other three D-atoms are ordered in the tetrahedral interstices 2Zr+2V. Distances between the nearest-neighbour D-atoms are ∼2.1 A. The ZrV 2 D 6 structure can be considered as a derivative of the Spinel-type structure found earlier in ZrTi 2 D 4 . Unlike the Spinel-type structure, where four D-atoms ought to be located in Zr+3V interstices, in ZrV 2 D 6 one of these atoms is replaced by three atoms located in the nearest 2Zr+2V interstices. The replacement differs for adjacent cells and is directed by the propagation vector k =(001).

Phase transition between hydrogen superstructures with k=(001) and k=0 via incommensurate phase in ZrV2Dx (2.8≤x≤3.9)

Abstract We have studied, by means of neutron diffraction, the hydrogen-concentration evolution of an incommensurate phase located between the superstructures with k =(001) and k =0 in hydrogen solid solution ZrV 2 D x , 2.8≤ x ≤3.9, at a fixed temperature T =90 K. The superstructure with k =(001) is found for x =2.8, and its complete structure determination is performed. At x =2.9, the superstructure transforms into the incommensurate phase with k =(0, 0, 0.8). With further increasing the hydrogen content, the propagation vector k is reduced to (0, 0, 0.5) and, near x =3.4, it drops abruptly down to (0, 0, 0). Between x =3.4 and x =3.7, both superstructures, with k =(0, 0, 0.5) and k =0, coexist. At higher hydrogen content, x =3.9, the superstructure with k =0 alone exists. Its complete structure determination is performed, and a genetic relation between it and the superstructure with k =(001) is found. In addition to the results on the evolution of the incommensurate phase with composition at the fixed temperature, the first results are presented for the evolution of the incommensurate phase as a function of the temperature for two compositions, x =2.9 and x =3.3, limiting its homogeneity range. A structure model for the incommensurate phase is proposed. It is shown that this phase may be considered as unstable with respect to a formation of antiphase domains. Its existence is caused by the short-range interaction between hydrogen atoms.