V. A. Somenkov

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.

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.

Polyamorphic transition in amorphous fullerites

Samples of amorphous fullerites have been prepared by mechanoactivation (grinding in a ball mill), and their structure has been studied using neutron diffraction. It has been found that the amorphous fullerites subjected to high-temperature (600–1700°C) annealing undergo a polyamorphic transition from the molecular glass to the atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles.

Structure of (Ti,Zr)–Mn–V nonstoichiometric Laves phases and (Ti0.9Zr0.1)(Mn0.75V0.15Ti0.1)2D2.8 deuteride

Abstract The structure of several Laves phase alloys and deuteride was studied using methods of X-ray analysis and time-of-flight neutron diffraction. The refinement of diffraction profiles was performed using the Rietveld method. Introduction of hydrogen does not change the metal matrix structure and the hydriding is accompanied by isotropic increase of cell volume by 20%. The minimisation of the R -factor showed that the best fit is achieved for the model of deuterium location in three sites, 24(l) , 12(k) 1 and 6(h) 1 . However, at low temperature approximately 0.11 deuterium atoms relocate from the 24(l) to the 12(k) 1 position. The analysis of alloy and deuteride structure performed in this work as well as the reference data allow to draw a conclusion, that the set of sites in hexagonal Laves phase structure which are occupied by deuterium is a stable one and does not depend on temperature or alloy composition.

Neutron diffraction study of the high-temperature superconductor HgBa2CaCu2O6.3 under high pressure

Abstract The pressure dependence of the crystal structure of a powder sample of HgBa2CaCu2O6.3 (Tc=123 K) was studied in the pressure range of 0 to 3.6 GPa, using the DN-12 time-of-flight diffractometer at the IBR-2 pulsed reactor and a sapphire anvil cell chamber. At P=O the results are in a good accordance with the published data. At P=3.0 and 3.6 GPa the best fit of the experimental data was achieved using the structural model with apical oxygen disordered in the (a, b) plane. The compressibility values of the lattice constants and selected bonds were calculated from the structural data. The highest absolute value of compressibility was found for the distance between the Ba and O atom layers.

Experimental study of the vibrational spectrum and structure variations in NH4Cl under high pressure

Abstract In this paper application of the sapphire anvils technique to the study of vibrational spectra of hydrogen-containing systems by inelastic neutron Scattering under high pressure is described. The experiments were performed with the DN-12 time-of-flight spectrometer at the IBR-2 pulsed reactor in Dubna. The structural changes and pressure dependence of the libration and lattice modes in NH4Cl were studied up to 40 kbar. The results give some evidence of the NH4 + ion instability.

Neutron Scattering Investigations of Structure and Dynamics of Materials Under High Pressure at IBR-2 Pulsed Reactor

High-pressure research plays an important role in modern condensed matter physics and is attracting the growing interest of researchers. Pressure-induced changes of interatomic and magnetic interactions in materials often lead to structural and magnetic phase transitions and consequent drastic changes in their macroscopic properties. Neutron diffraction and inelastic neutron scattering investigations provide an invaluable insight into the nature of the different pressure-induced phenomena in materials. Due to a relatively small intensity of neutron sources, typical sample volumes required for a neutron scattering experiment are about cm3 and the attainable pressure range has been restricted to only about 03 GPa for a long time. Since the first successful experiments performed using diamond anvil cells in the 0-31GPa range

Structural study of ND4I at high pressures and low temperatures

Abstract Structure, positional, and thermal parameters of ND4I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND4I(II) into a recently discovered high pressure phase ND4I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.

Neutron studies of structural changes in amorphous C60 fullerite under temperature, pressure, and thermobaric effects

The structural behavior of amorphous fullerites obtained as a result of mechanical activation and thermal, baric, and thermobaric effects is studied via neutron diffraction. It is shown that the phase transition between the molecular crystal (fullerite) and atomic crystal (graphite) phases in the nanoscale state occurs through intermediate amorphous phases.