V. P. Glazkov

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.

High pressure effects on the crystal and magnetic structure of Pr1?xSrxMnO3 manganites (x = 0.5?0.56)

The crystal and magnetic structures of the manganites Pr1−xSrxMnO3 (x = 0.5, 0.56) have been studied by means of powder neutron diffraction at high external pressures up to 4.8 GPa. At ambient pressure both Pr0.44Sr0.56MnO3 and Pr0.5Sr0.5MnO3 have a tetragonal structure (space group I4/mcm). At K in Pr0.44Sr0.56MnO3 the onset of A-type antiferromagnetic (AFM) state occurs, which is accompanied by a structural phase transformation to the orthorhombic structure (space group Fmmm). Pr0.5Sr0.5MnO3 at 175 K<T< TC = 265 K exhibits an intermediate tetragonal ferromagnetic (FM) phase followed by the onset of the orthorhombic A-type AFM phase at K. Under high pressure, in Pr0.44Sr0.56MnO3 a tetragonal C-type AFM phase ( K) appears and the phase separated state is formed, consisting of its mixture with the initial orthorhombic A-type AFM phase ( K). In Pr0.5Sr0.5MnO3 the application of high pressure leads to a noticeable increase of the FM–A-type AFM transition temperature from up to 230 K and formation of a phase separated state below 150 K, consisting of a mixture of orthorhombic A-type AFM phase and tetragonal phase without long range magnetic order. The stability of different magnetic states of Pr0.44Sr0.56MnO3 and Pr0.5Sr0.5MnO3 under high pressure is discussed.

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.

High-pressure effect on the crystal and magnetic structures of the frustrated antiferromagnet YMnO3

The high-pressure (to 5 GPa) effect on the crystal and magnetic structures of the hexagonal manganite YMnO3 is studied by neutron diffraction in the temperature range 10–295 K. A spin-liquid state due to magnetic frustration on the triangular lattice formed by Mn ions is observed in this compound at normal pressure and T > TN = 70 K, and an ordered triangular antiferromagnetic state with the symmetry of the irreducible representation Γ1 arises at T < TN. The high-pressure effect leads to a spin reorientation of Mn magnetic moments and a change in the symmetry of the antiferromagnetic structure, which can be described by a combination of the irreducible representations Γ1 and Γ2. In addition, it is observed that the ordered magnetic moment of Mn ions decreases from 3.27 μB (5 GPa) to 1.52 μB (5 GPa) at T = 10 K and diffuse scattering is enhanced at temperatures close to TN. These effects can be explained within the model of the coexistence of the ordered antiferromagnetic phase and the spin-liquid state, whose volume fraction increases with pressure due to the enhancement of frustration effects.

Neutron diffraction investigation of the magnetic structure of the Mn2Sb pnictide at high pressures

This paper reports on the results of investigations into the influence of high pressures (up to 5.3 GPa) at T=300 K on the magnetic structure of the Mn2Sb pnictide. The crystal and magnetic structures of the Mn2Sb compound are studied using a direct neutron diffraction method. It is demonstrated that the magnetic ferrimagnet-antiferromagnet phase transition, which is observed in a number of Mn2Sb-based substitutional solid solutions upon chemical compression of the Mn2Sb crystal lattice, does not occur in the Mn2Sb compound under high pressures in the aforementioned range due to an anisotropic pressure-stimulated strain in the Mn2Sb lattice. At pressures P≥2.8 GPa, the Mn2Sb compound is characterized by a spin reorientation with respect to the tetragonal axis and the basal plane of the crystal lattice.

The influence of high pressure on the crystal and magnetic structures of the La0.7Sr0.3CoO3 cobaltite

The atomic and magnetic structures of the La0.7Sr0.3CoO3 cobaltite are investigated using neutron diffraction at high pressures up to 4.2 GPa in the temperature range 16–300 K. The structural parameters are determined as functions of the pressure. It is revealed that the effect of high pressure leads to a decrease in the Curie temperature: dTC/dP = −8.3(8) K/GPa. It is established that, under high pressure, the decrease in the Curie temperature TC is associated with the transition of a number of Co3+ ions from the intermediate-spin state (t2g5eg1, S = 1) to the low-spin state (t2g6, S = 0).

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.

Pressure-induced antiferromagnetism in La0.75Ca0.25MnO3 manganite

The crystal and magnetic structures of La0.75Ca0.25MnO3 manganite are studied under high pressures up to 4.5 GPa in the temperature range 12–300 K by the neutron diffraction method. At normal pressure and temperature TC = 240 K, a ferromagnetic state is formed in La0.75Ca0.25MnO3. At high pressures P ≥ 1.5 GPa and at temperatures T < TN ≈ 150 K, a new A-type antiferromagnetic state appears. A further increase in pressure leads to an increase in the volume fraction of the antiferromagnetic phase, which coexists with the initial ferromagnetic phase. The effect of high pressure causes a considerable increase in TC with the slope dTC/dP ≈ 12 K/GPa. Calculations performed in the framework of the double exchange model with allowance for the electron-phonon interaction make it possible to explain this pressure dependence of TC on the basis of experimental data.