Roland Tellgren

Investigation of the structure of the relaxor ferroelectric Pb(Fe1/2Nb1/2)O3 by neutron powder diffraction

The complex perovskite lead iron niobate, Pb(Fe1/2 Nb1/2 )O3 (PFN), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. As expected, a straightforward unit cell of symmetry R 3m was obtained for the 300 K structure, with the same symmetry and a similar unit cell also obtained at low temperature. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic alignment. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFN are discussed by comparison with the related system of Pb(Mg1/3 Nb2/3 )O3 .

Hydrogen and deuterium in transition metal-p element compounds: Crystal chemical aspects of interstitial solid solubility and hydride phase formation☆

Abstract Compounds of transition metals with p elements of groups IIIb–VIIb which have been reported to exhibit hydrogen-absorbing properties are surveyed. Hydrogen is taken up only by those materials that have metallic properties or contain metal clusters with delocalized bonding. Almost all p elements can be constituents of these materials, and at least one transition metal component which is itself capable of dissolving hydrogen or forming hydride phases must be present. The crystallography of the hydride phases is presented and discussed with particular emphasis on results obtained by neutron diffraction methods. The hydrogen atoms are accommodated either in interstitial solid solutions with only minor changes in the host lattices or in new phases formed by major rearrangements of both transition metal and p element atoms. The distribution of the hydrogen atoms on the interstitial sites in the structures appears to be governed by two empirical rules: the hydrogen atoms are always more than 2 A apart and they preferentially occupy those sites that are most distant from the p element atoms.

A neutron powder diffraction study of the ferroelectric relaxor Pb(Fe1/2Ta1/2)O3

The complex perovskite lead iron niobate, Pb(Fe1/2Ta1/2)O3 (PFT), has been studied by neutron powder diffraction. Following collection of diffraction data at 300 K and at 10 K, structural refinements have been carried out by means of the Rietveld method. A straightforward unit cell of symmetry R3m was obtained for the 300 K structure, with the same symmetry, and a similar unit cell also obtained at the low temperature. In both cases, the iron and titanium ions were found to be disordered over the perovskite B-sites. Furthermore, in order to obtain a good agreement with experiment at 10 K, it was necessary to assign non-zero magnetic moments to the iron ions, these being in a collinear, antiferromagnetic arrangement. This magnetic structure can be described with reference to doubled unit cell axes. The factors governing the observed structures of PFT are discussed by comparison with the related system of Pb(Fe1/2Nb1/2)O3.

A Neutron Powder Diffraction Study of Pd3MnDx

The crystal structures of pure and deuterated PdjMn have been studied using the Rietveld profile refinement technique on neutron powder intensity data. PdjMn forms an ordered structure of the Al3 Zr-type below a temperature of approximately 800 K. The space-group is I4lmmm (No. 139) with the unit cell dimensions a=3.8984(3) Â, c=l5.602(2) Â and Z=4. The positions and occupancies for the dissolved deuterium atoms were determined at 298 and 100 kPa deuterium pressure. Under these conditions deuterium dissolved to an extent corresponding to the formula Pd,MnD061. The deuterium atoms mainly occupy octahedral sites with six palladium nearest neighbour atoms and to a lesser extent octahedral sites with one manganese and five palladium nearest neighbour atoms.

Hydrogen Bond Studies. XXXXVI. The Crystal Structures of Normal and Deuterated Sodium Hydrogen Oxalate Monohydrate NaHC2O4·H2O and NaDC2O4·D2O

The crystal structures of NaHC2O4·H2O and NaDC2O4·D2O have been determined from three‐dimensional single crystal x‐ray diffractometer data obtained at room temperature. Two formula units crystallize in a triclinic unit cell with the dimensions: a = 6.503, b = 6.673, c = 5.698 A, α = 85.04, β = 110.00, γ = 105.02° for the hydrogen compound and a = 6.501, b = 6.671, c = 5.716 A, α = 84.91 β = 109.93 γ = 105.00° for the deuterated compund. The space group is P1. The hydrogen oxalate ions are linked end to end in infinite chains by hydrogen bonds (2.571 A). The chains are cross linked to form layers by both O–H···O bonds from the water molecules (2.808, 2.826 A) and by ionic bonds Na+···O. These layers are in turn held together by Na+···O bonds. The oxalate group is nonplanar with an angle of twist about the C–C bond of 12.9°. The only significant difference between the bond distances in the normal and deuterated compound occurs in the shortest hydrogen bond, which is 0.022 A longer in the deuterated case.

A neutron powder diffraction study of Ta2C and W2C

Abstract Samples of Ta2C and W2C have been prepared by arc melting. Their crystal structures were studied by neutron powder diffraction. Ta2C crystallizes in the anti-CdI2-type structure ( P 3 m1 , No. 164) while W2C crystallizes in the ϵ-Fe2N-type structure ( P 3 1m , No. 162). The tungsten carbide was carbon deficient (W2C0.84) and partly disordered. The carbon atoms partially occupy three different atomic positions.

Hydrogen bond studies. 54. A neutron diffraction study of the ferroelectric lithium trihydrogen selenite, LiH3(SeO3)2

Abstract The crystal structure of the ferroelectric LiH 3 (SeO 3 ) 2 has been studied using three-dimensional neutron diffraction data. The position of the heavy atoms are in agreement with earlier investigations, whereas the hydrogen bond network differs considerably. The present study reveals three almost linear OH · O hydrogen bonds of length 2.518, 2.552, and 2.646 A with OH · O angles 176.6, 173.3, and 172.3°, respectively. The shortest bond has an OH distance of 1.14 A, which is longer than expected. The lithium ion is surrounded by six oxygen atoms forming an octahedron with LiO distances in the range 2.13–2.22 A.

Experimental electron density of urea–phosphoric acid (1/1) at 100 K

The deformation electron density of the urea-phosphoric acid adduct has been studied from 100 K X-ray and neutron diffraction experiments. Data were interpreted according to the Hirshfeld model. The long hydrogen bonds show characteristics of electrostatic interaction. Deformation density maps on the short hydrogen bond shows hydrogen more strongly bonded to urea than to phosphoric acid, and peak maxima at almost midway between the two O-H bonds.

Hydrogen bond studies 78. A neutron diffraction study of lithium formate monohydrate, LiHCOO·H2O

The crystal structure of lithium formate monohydrate has been determined from three-dimensional neuiron diffraction data. Two different hydrogen bonds occur in the crystal: an O(W)-H … O(W), 2.896(2) A bond linking together the water molecules to produce infinite chains, and an O(W)-H … O, 2.714(2) A bond connecting the water chains to the formate ions. The O-H … O angles are 166.6(3)° and 173.6(3)°, respectively. The water molecules have O-H distances 0.965(4) and 0.976(3) A, and an H-O-H angle of 107.8(3)°.

Magnetic Ordering in Polycrystalline NixZn1—xO Solid Solutions

In order to examine the influence of the dilution of magnetic ions on the crystal and magnetic structure of Ni x Zn 1-x O (x = 1, 0.90, 0.80, 0.70) solid solutions we have performed neutron diffraction experiments on each sample at seven temperatures between 10 and 295 K. To determine the Neel temperatures, the magnetic susceptibility measurements were done in the temperature region between room temperature and 600 K. Starting from the known rhombohedral distortion the crystal structure has been refined in the trigonal space group R3m. In this space group the cations occupy octahedral 3a position (with the local symmetry 3m), while the oxygen anion is placed in the 3b position (with the local symmetry 3m). The magnetic structure for all concentrations x is found to be antiferromagnetic and the magnetic cell is the doubled nuclear cell along the c-axis. The magnetic atoms are arranged in the planes (003) in respect to the nuclear cell. The magnetic moments of two Ni 2+ ions in adjacent sheets are oriented antiparallely and the magnetic vector makes an angle of 66° with the c-axis. The magnetic structure is preserved upon random site dilution of the magnetic ions. This is additionally confirmed from the Brillouin-type dependence of the magnetization on temperature as well as from the t = T N (x)/T N (1) versus x variation obtained from the magnetic susceptibility measurements. Concentration dependence of the Neel temperature also shows that Ni x Zn 1-x O behaves like a Heisenberg antiferromagnet with the dominant nearest-neighbor interaction, in agreement with the similar (Ni, Co) x Mg 1-x O solid solution.