Jan Fábry

Phase Transition in K3Na(MoO4)2 and Determination of the Twinned Structures of K3Na(MoO4)2 and K2.5Na1.5(MoO4)2 at Room Temperature

The room-temperature phases of sodium potassium molybdates K 3 Na(MoO 4 ) 2 and K 2.5 Na 1.5 (MoO 4 ) 2 are isostructural with the monoclinic low-temperature phases of K 3 Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 , which are twinned distorted glaserite structures. In the molybdates there are two crystallographically independent potassiums and their environment slightly differs from those in K3Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 . The excessive Na in K 2.5 Na 1.5 (MoO 4 ) 2 occupies the position of the more firmly bound potassium. A reversible phase transition at 513 K was discovered in K 3 Na(MoO 4 ) 2 by DSC (differential scanning calorimetry), but no such transition in K 2.5 Na 1.5 (MoO 4 ) 2 was detected. Both samples used in the diffractometer experiment were found to be composed of six domains being related by twinning operations of the point group 6. The twinning may be considered as a combination of a merohedral and a pseudo-merohedral twinning with two- and threefold rotations as twinning operations, respectively. However, a reversible domain switching, which is observable in the related ferroelastic crystals of K 3 Na(SeO 4 ) 2 and K 3 Na(CrO 4 ) 2 , was not observed either in K 3 Na(MoO 4 ) 2 or in K 2.5 Na 1.5 (MoO 4 ) 2 , either due to semitransparency of the samples or high ferroelastic distortion. This distortion is manifested by the values of the atomic displacement vectors which are about twice as large as those in the selenate or the chromate.

Structure determination of KLaS2, KPrS2, KEuS2, KGdS2, KLuS2, KYS2, RbYS2, NaLaS2 and crystal-chemical analysis of the group 1 and thallium(I) rare-earth sulfide series.

One of the purposes of this work is to provide a crystallographic review of group 1 and thallium rare-earth ternary sulfides M(+)Ln(3+)S2. We have therefore determined crystal structures of KLaS2, KPrS2, KEuS2, KGdS2, KLuS2, KYS2, RbYS2, which belong to the α-NaFeO2 structural family (R3m), as well as NaLaS2, which is derived from the disordered NaCl structural type (Fm3m). The determined structures were compared with known members of the group 1 as well as thallium(I) rare-earth sulfides by the standard tools of crystal-chemical analysis such as comparison of bond-valences, analysis of interatomic distances and comparison of the unit-cell parameters. The results indicate why the cubic structural type is limited to Li(+) and Na(+) members of the series only. The analysis has also revealed frequent problems in the reported crystal structures, especially in the determination of the K(+) compounds, probably due to severe absorption and different accuracy and sensitivity of various instruments. Intense diffuse scattering has been discovered in NaLaS2, which will be the subject of further investigation. The newly determined as well as already known structures are summarized, together with critical comments about possible errors in the previous structure determinations.

Location of Mn sites in ferromagnetic Ga1-xMnxAs studied by means of X-ray diffuse scattering holography

A three-dimensional image of the local neighbourhood of Mn atoms in a Ga 1-x Mn x As (x = 0.02) layer has been obtained by using X-ray diffuse scattering holography. The first and second nearest neighbours of the Mn atoms correspond to the local structure around Ga atoms in the zinc-blende GaAs structure. Accordingly, the Mn atoms are situated in substitutional positions.

Ferroelastic structures of n-pentyl-, n- hexyl- and n-nonylammonium dihydrogenphosphate crystals.

This study reports the structure redeterminations of C(5)H(11)NH(3)(+).H(2)PO(4)(-) (n-pentylammonium dihydrogenphosphate, C5ADP), C(6)H(13)NH(3)(+).H(2)PO(4)(-) (n-hexylammonium dihydrogenphosphate, C6ADP) and C(9)H(19)NH(3)(+).H(2)PO(4)(-) (n-nonylammonium dihydrogenphosphate, C9ADP). The structures are monoclinic (P2(1)/n), belonging to the series of previously studied structures C2ADP-C8ADP and C10ADP. The structures exhibit reproducible ferroelastic switching. There are hydrogen bonds between the dihydrogenphosphates and the n-alkylammonium groups. Among them there are two hydrogen bonds with hydrogens which hop from the donor to the acceptor oxygens during the ferroelastic switching. C5ADP as well as C3ADP differ from the other members of the series by packing of the double layers of the dihydrogenphosphates. Moreover, the packing of n-alkylammonium molecules in all these structures depends on the parity of the number of atoms in the n-alkylammonium chains. All the samples contained two domains and their structures were refined as twins.

Dependences of Space-Group Type Incidences of Organic and Metal-Organic Compounds on Reduced Unit-Cell Volumes

The dependences of occurrences of the most frequent space-group types P, P2 1 , P2 1 /c, C2/c, P2 1 2 1 2 1 and Pbca on reduced (primitive) unit-cell volumes up to 8000 Å3 were investigated for organic and metal-organic compounds with different number of residues (1–3), i.e. for the compounds with different number of molecular constituents. The dependences for these space-group types are similar, single-peaked; their maxima are proportional to the number of asymmetric units (a compact symmetry-independent part of the unit cell) affected by Z′ (number of formula units per asymmetric unit) in the pertinent space-group types. The dependences of Z′ < 1, Z′ = 1, Z′ > 1 within each space group type on reduced unit-cell volumes are also similar in shape. From these dependences it can be inferred that ability for crystallization of organic and metal-organic molecules ceases for the structures with the reduced unit-cell volumes above 8000 Å 3 . This volume corresponds roughly to 450 non-hydrogen atoms.

4-Amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one (metamitron) and 4-amino-6-methyl-3-phenyl-1,2,4-triazin-5(4H)-one (isometamitron).

The title structures, both C(10)H(10)N(4)O, are substitutional isomers. The N-N bond lengths are longer and the C=N bond lengths are shorter by ca 0.025 A than the respective average values in the C=N-N=C group of asymmetric triazines; the assessed respective bond orders are 1.3 and 1.7. There are N-H...O and N-H...N hydrogen bonds in both structures, with 4-amino-3-methyl-6-phenyl-1,2,4-triazin-5(4H)-one containing a rare bifurcated N-H...N,N hydrogen bond. The structures differ in their molecular stacking and the hydrogen-bonding patterns.

A simple method of shielding area detectors from unwanted Bragg diffraction

Shielding of unwanted Bragg diffraction is performed by tiny permanent magnets made from SmCo5. The magnets are situated on a non-absorbing foil that is fixed tightly in a metal frame situated between the sample and the detector.

Revision of Ferroelastic Structures of n-Heptyl- and n-Octylammonium Dihydrogen Phosphate Crystals

This study deals with the structure determination of C 7 H 15 NH 3 + .H 2 PO 4 - (C7ADP) and C 8 H 17 NH 3 + .H 2 PO 4 - (C8ADP). The samples used in this study were not subjected to a phase transition after they had been crystallized. Unlike a previous structure determination, weak reflections, now with indices h = 2n + 1, were included. This means that both structures are described in unit cells with the lattice parameters a twice as long as given previously. Both structures are quite similar; two double layers of dihydrogen phosphates, which are interconnected by hydrogen bonds (2.52-2.62 A), pass through each unit cell. Alkylammonium groups interact with these dihydrogen phosphates via longer hydrogen bonds (>2.75 A), while the rest of the aliphatic chains interact via van der Waals contacts. All H atoms were localized and no disorder of the H atoms was detected. Both structures described in the space group P12 1 /n1 exhibit a reproducible ferroelastic switching. The hypothetical prototypic phase is orthorhombic with the space group number 60 P2/b2 1 /n2 1 /a. All atoms except two hydrogen species exist in pairs linked by the lost symmetry operations derived from the prototypic space group and are brought close to each other - up to 0.25 A - under the action of them. Each of these two different hydrogens is involved in an asymmetric hydrogen bond between an oxygen pair. Under the action of a lost symmetry operation each of these hydrogens is displaced from one oxygen towards the other. Therefore, it is asssumed that during the ferroelastic switching the jumps of these two hydrogen species take place between the pertinent hydrogen-bond acceptor and donor oxygens. Hence, these oxygens reverse their role as hydrogen-bond donors and acceptors during the ferroelastic switching.

Structure redetermination of SrS2O6.4H2O

The present structure determination of SrS 2 O 6 .4H 2 O has been performed in a unit cell which is four times larger than assumed in previous structure determinations, i.e. relatively faint reflections with h or k odd have now been included in the calculation. The disordered structural model in the small (incorrect) unit cell is compatible with eight different ordered arrangements in the large unit cell; diffraction data clearly favour one of these arrangements. The structure is trigonal, being minutely deviated from hexagonal symmetry. There are weak hydrogen bonds between water molecules, as well as between water and dithionate molecules. Each Sr atom is coordinated by four water molecules and four dithionate oxygens

Modelling of cation displacements in SrTiO3 by means of multi-energy anomalous X-ray diffuse scattering

X-ray diffuse scattering of SrTiO3 has been measured at two photon energies, the first just below the absorption edge and the second far from the K absorption edge of strontium, in order to vary the atomic scattering factor of the strontium cations. It is shown that two different models of cation displacement comply with the single-energy diffuse scattering patterns, because single-energy diffuse scattering provides only ambiguous information on the directions of displacement of the Sr2+ and Ti4+ cations. However, the application of multi-energy anomalous diffuse scattering determines unambiguously that the Sr2+ cations are moved from their ideal positions in the [100] direction and the Ti4+ cations are shifted in {111} directions.