Maria Orlova

4-Aminoquinaldine monohydrate polymorphism: prediction and impurity aided discovery of a difficult to access stable form

Crystal structure prediction studies indicated the existence of an unknown high density monohydrate structure (Hy1B°) as global energy minimum for 4-aminoquinaldine (4-AQ). We thus performed an interdisciplinary experimental and computational study elucidating the crystal structures, solid form inter-relationships, kinetic and thermodynamic stabilities of the stable anhydrate (AH I°), the kinetic monohydrate (Hy1A ) and this novel monohydrate polymorph (Hy1B°) of 4-AQ. The crystal structure of Hy1B° was determined by combining laboratory powder X-ray diffraction data and ab initio calculations. Dehydration studies with differential scanning calorimetry and solubility measurements confirmed the result of the lattice energy calculations, which identified Hy1B° as the thermodynamically most stable hydrate form. At 25 °C the equilibrium of the 4-AQ hydrate/anhydrate system was observed at an aw (water activity) of 0.14. The finding of Hy1B° was complicated by the fact that the metastable but kinetically stable Hy1A shows a higher nucleation and growth rate. The presence of an impurity in an available 4-AQ sample facilitated the nucleation of Hy1B°, whose crystallisation is favored under hydrothermal conditions. The value of combining experimental with theoretical studies in hydrate screening and characterisation, as well as the reasons for hydrate formation in 4-AQ, are discussed.

Creatine: Polymorphs Predicted and Found

Hydrate and anhydrate crystal structure prediction (CSP) of creatine (CTN), a heavily used, poorly water-soluble, zwitterionic compound, has enabled the finding and characterization of its anhydrate polymorphs, including the thermodynamic room temperature form. Crystal structures of the novel forms were determined by combining laboratory powder X-ray diffraction data and ab initio generated structures. The computational method not only revealed all experimental forms but also predicted the correct stability order, which was experimentally confirmed by measurements of the heat of hydration.

Structure Refinement of Cadmium Cerium(IV) Phosphate Cd{sub 0.5}Ce{sub 2}(PO{sub 4}){sub 3}

Cadmium cerium orthophosphate Cd0.5Ce2(PO4)3 is synthesized by precipitation from aqueous solutions. The structure refinement from powder X-ray diffraction data is preceded by the sample preparation and structure solution. The refinement is carried out by the Rietveld method (ADP-2 diffractometer, CuKα radiation, Ni filter, 15° < 2θ < 120°, 2θ-scan step 0.02°, counting time 10 s per step). All calculations are carried out using the WYRIET program (version 3.3) within the sp. gr. P21/n. The structure is refined with anisotropic displacement parameters for cations and isotropic displacement parameters for oxygen atoms.

Investigations on alunogen under Mars-relevant temperature conditions: An example for a single-crystal-to-single-crystal phase transition

Abstract The low-temperature (LT) dependent behavior of a synthetic alunogen sample with composition Al2(SO4)3·16.61H2O has been studied in the overall temperature range from -100 to 23 °C by DSC measurements, in situ powder and single-crystal X‑ray diffraction as well as Raman spectroscopy. Cooling/heating experiments using the different techniques prove that alunogen undergoes a reversible, sluggish phase transition somewhere between -30 and -50 °C from the triclinic room-temperature (RT) form to a previously unknown LT-polymorph. A significant hysteresis for the transition was observed with all three methods and the transition temperatures were found to depend on the employed cooling/ heating rates. The crystal structure of the LT-modification has been studied at -100 °C using single crystals, which have been grown from an aqueous solution. Basic crystallographic data are as follows: monoclinic symmetry, space group type P21, a = 7.4125(3), b = 26.8337(16), c = 6.0775(3) Å, β = 97.312(4)°, V = 1199.01(10) Å3, and Z = 2. Structure analysis revealed that LT-alunogen corresponds to a non-stoichiometric hydrate with 16.61 water moieties pfu. Notably, the first-order transition results in a single-crystal-to-single-crystal transformation. In the asymmetric unit there are 2 Al-atoms, 3 [SO4]-tetrahedra, and 17 crystallographically independent sites for water molecules, whose hydrogen positions could be all located by difference-Fourier calculations. According to site-population refinements only one water position (Ow5) shows a partial occupancy. A comfortable way to rationalize the crystal structure of the LT-modification of alunogen is based on a subdivision of the whole structure into two different slabs parallel to (010). The first type of slab (type A) is about 9 Å thick and located at y ≈ 0 and y ≈ ½, respectively. It contains the Al(H2O)6-octahedra as well as the sulfate groups centered by S1 and S2. Type B at y ≈ ¼ and y ≈ ¾ comprises the remaining tetrahedra about S3 and a total of five additional “zeolitic” water sites (Ow1-Ow5), which are not a part of a coordination polyhedron. Within slab-type A alternating chains of (unconnected) octahedra and tetrahedra can be identified, which are running parallel to [100]. In addition to electrostatic interactions between the Al(H2O)63+- and the (SO4)2--units, hydrogen bonds are also essential for the stability of these slabs. A detailed comparison between both modifications including a derivation from a hypothetical aristotype based on grouptheoretical concepts is presented. Since alunogen has been postulated to occur in martian soils the new findings may help in the identification of the LT-form by X‑ray diffraction using the Curiosity Rover’s ChemMin instrument or by Raman spectroscopy.

Synthesis and structure of new thorium phosphate CaGdTh(PO4)3

Phosphate CaGdTh(PO4)3 was prepared by thermal treatment of a mixture of oxides. The final temperature was 1400°C. The phosphate was characterized by powder X-ray diffraction analysis and IR spectroscopy. The crystal structure was studied by the Rietveld method. The compound crystallizes in the monazite structure type (sp. gr. P21/n). A comparative analysis of the structures of this phosphate and cerium orthophosphate CePO4 was carried out.

Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO4, Ga(Ta,Nb)O4, and GaTaO4

Single crystals as well as polycrystalline samples of GaNbO4, Ga(Ta,Nb)O4, and GaTaO4 were grown from the melt and by solid-state reactions, respectively, at various temperatures between 1698 and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive electron microprobe analysis, and the crystal structures were determined by single-crystal X-ray diffraction. In addition, a high-P-T synthesis of GaNbO4 was performed at a pressure of 2 GPa and a temperature of 1273 K. Raman spectroscopy of all compounds as well as Rietveld refinement analysis of the powder X-ray diffraction pattern of GaNbO4 were carried out to complement the structural investigations. Density functional theory (DFT) calculations enabled the assignment of the Raman bands to specific vibrational modes within the structure of GaNbO4. To determine the hardness (H) and elastic moduli (E) of the compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load-displacement curves resulted in a high hardness of H = 11.9 ± 0.6 GPa and a reduced elastic modulus of Er = 202 ± 9 GPa for GaTaO4. GaNbO4 showed a lower hardness of H = 9.6 ± 0.5 GPa and a reduced elastic modulus of Er = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa0.5Nb0.5O4 ceramic sample was employed for the determination of the optical constants n and k. GaTa0.5Nb0.5O4 exhibits a high average refractive index of nD = 2.20, at λ = 589 nm. Furthermore, in situ high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO4 and GaNbO4, as well as the ability to relate them with structural features.

Low temperature phase transition and structure of CsMgPO4

CsMgPO4 doped in radioisotopes is a promising compound for usage as a radioactive medical source. However, a low temperature phase transition at the temperatures close to ambient conditions (-37°C) was observed. Information about structural changes is important in order to understand whether it can cause any problem for medical use of this compound. Structural changes have been investigated in detail using synchrotron powder diffraction methods, Raman spectroscopy and DFT calculations. The structure undergoes transformation from orthorhombic modification, sp. gr. Pnma (RT phase) to monoclinic modification, sp.gr P21/n (LT phase). New LT modification adopts similar to RT but slightly distorted unit cell: a=9.58199(2)Å, b=8.95501(1) Å, c=5.50344(2)Å, β=90.68583(1)°, V=472.198(3) Å3. The framework is made up of alternating magnesia and phosphate tetrahedra sharing vertices with caesium counter cations located in the channels formed. Upon the transformation a combined rotation of PO4 and MgO4 tetrahedral takes place. A comparison with other phase transition in ABW-type framework class compounds is given.