Marcin Stachowicz

X-ray Diffraction, FT-IR, and 13C CP/MAS NMR Structural Studies of Solvated and Desolvated C-Methylcalix[4]resorcinarene

Solid C-methylcalix[4]resorcinarene solvated by acetonitrile and water (CAL-Me) and then modified by slow solvent evaporation (CAL-Me*) was studied using single-crystal and powder X-ray diffraction, FT-IR, and (13)C CP/MAS NMR. The CAL-Me solvate crystallizes in the monoclinic P2(1)/n space group with three CH(3)CN and two H(2)O molecules in the asymmetric part of the unit cell. The CAL-Me molecules adopt a typical crown conformation with all of the hydroxyl groups of the aryl rings oriented up and all of the methyl groups disposed down (the rccc isomeric form). The crystalline network is formed by resorcinarene, CH(3)CN, and H(2)O molecules and assembled by intermolecular hydrogen bonds and weak C-H...A or C-H...pi interactions. The desolvated CAL-Me* loses its crystalline character and becomes partly amorphous. It is devoid of CH(3)CN and deficient in water. However, the resorcinarene molecules still remain in the crown conformation supported by intramolecular hydrogen bonds, while intermolecular hydrogen bonds are considerably disintegrated. The work directs general attention to the problem of stability and polymorphism of resorcinarene solvates. It shows that the joint use of diffractometric and spectroscopic methods is advantageous in the structural studies of complex crystalline macromolecular systems. On the other hand, the solid-state IR and NMR spectroscopic analyses applied in tandem have been found highly beneficial to elucidate the disordered structure of poorly crystalline, desolvated resorcinarene.

Homodiselenacalix[4]arenes: Molecules with Unique Channelled Crystal Structures.

A synthetic route towards homodiselenacalix[4]arene macrocycles is presented, based on the dynamic covalent chemistry of diselenides. The calixarene inner rim is decorated with either alkoxy or tert-butyl ester groups. Single-crystal X-ray analysis of two THF solvates with methoxy and ethoxy substituents reveals the high similarity of their molecular structures and alterations on the supramolecular level. In both crystal structures, solvent channels are present and differ in both shape and capacity. Furthermore, the methoxy-substituted macrocycle undergoes a single-crystal-to-single-crystal transformation during which the molecular structure changes its conformation from 1,3-alternate (loaded with THF/water) to 1,2-alternate (apohost form). Molecular modelling techniques were applied to explore the conformational and energetic behaviour of the macrocycles.

Applications of Hirshfeld surfaces to mineralogy: An example of alumohydrocalcite, and the classification of the dundasite group minerals

Abstract The crystal structure of alumohydrocalcite was determined using synchrotron X-ray radiation. Alumohydrocalcite crystallizes in the triclinic P1̅ space group with unit-cell parameters: a = 5.71(5), b = 6.54(4), c = 14.6 (2) Å, α = 81.8(3)°, β = 83.9(3)°, γ = 86.5(7)°, and V = 537(7) Å3. This mineral has the formula CaAl2(CO3)2(OH)4·4H2O as opposed to the commonly accepted formula CaAl2(CO3)2(OH)4·3H2O. The fourth water molecule interacts with the strongly bonded polyhedral unit of the structure through hydrogen bonds and connects three adjacent units. This water molecule plays a major role in crystal stability. On heating the sample, this fourth water molecule escapes from the crystal structure as a first one at lower temperature (~105 °C) than the other water molecules in the crystal structure (~128 °C). Analysis and description of the alumohydrocalcite crystal structure and particularly of the intermolecular interactions, together with a comparison to the crystal structures of other minerals with the analog formula M2+M3+2 (CO3)2(OH)4·nH2O, suggests that this mineral is an extension of the dundasite group that should, we propose, be formed for all minerals with the above formula (dundasite, dresserite, strontiodresserite, petterdite, kochsándorite, hydrodresserite, and alumohydrocalcite). They all exhibit very similar patterns on Hirshfeld surfaces. Hirshfeld surfaces appear to be a very useful tool in the analysis of interactions, classification, and validation of mineral crystal structures.

Structure of Sr-Zr-bearing perrierite-(Ce) from the Burpala Massif, Russia

Abstract Sr- and Zr-bearing perrierite-(Ce) occurring in aegirinized syenite pegmatites of the Burpala massif, Russia, is compositionally intermediate between perrierite-(Ce) and hezuolinite and occupies a compositional gap in minerals of the chevkinite group. Its crystal structure has been determined using a single-crystal diffractometer fitted with a CCD detector and MoKa X-ray radiation. The mineral is monoclinic; a = 13.815(1), b = 5.668(1), c = 11.842(1) Å, β = 113.843(3)°, V = 848.18(4) Å3, space group C2/m, Z = 2. The crystal structure was refined with the occupancies [(Ce1.2La1.0Nd0.15) (Sr1.0Ca0.5Na0.15)]4(Zr0.5Fe0.3Mn0.2)(Ti1.3Fe0.7)2Ti2(Si2O7)2O8 on the basis of chemical composition although the allocation of cations to particular sites was performed on the basis of the number of refined electrons in each unique site. The dominance of Zr in the B site links the Burpala perrierite-(Ce) to more Sr-Zr-rich members of the chevkinite group, such as hezuolinite and rengeite. As in all of the perrierite members, there is a distortion of the D site octahedra, which is interpreted as due to the packing of the REE ions.

SBN60, strontium-barium niobate at 100 K

The title compound, Sr0.6Ba0.4Nb2O6 (strontium barium niobium oxide), belongs to the group of strontium–barium niobates with varying composition of Sr and Ba. Their general formula can be written as SrxBa1 - xNb2O6. Below the Curie temperature, T c, these materials indicate ferroelectric properties. The Curie temperature for SBN60 is equal to 346±0.5 K so the structure is in the ferroelectric phase at the measurement temperature of 100 K. Characteristic for this family of compounds is the packing along the z-axis. The NbO6 corner-sharing octahedra surround three types of vacancy tunnels with pentagonal, square and triangular shapes. The Sr2+ ions partially occupy two unique sites, the first one located inside the pentagon and the second one in the square tunnels. Consequently, they are situated on the mirror plane and the intersection of two glide planes, respectively. The site inside the pentagonal tunnel is additionally disordered so that the same position is shared by Ba2+ and Sr2+ ions whereas another part of the Ba2+ ion occupies a different position (relative occupancies 0.43:0.41:0.16). One of the NbV atoms and three of the O2− ions occupy general positions. The second NbV atom is located on the intersection of the mirror planes. Two remaining O2− ions are located on the same mirror plane. Only the NbV atom and one of the O2− ions which is located on the mirror plane are not disordered. Each of the remaining O2− ions is split between two sites, with relative occupancies of 0.52:0.48 (O2− ions in general positions) and 0.64:0.36 (O2− ion on the mirror plane).

Niobium-rich chevkinite-(Ce) – structural investigations

The minerals belonging to the chevkinite group (CGM) are recognised as accessory phases in a wide range of igneous and metamorphic rocks [1, 2]. The geochemical importance of this group is associated with a high REE-concentration; the total REE2O3 contents are up to 50 wt%. They can be the dominant REE-bearing phases in any given rock. The REE are increasingly used in green technologies, such as the production of novel wind turbines, low-energy light bulbs also mobile phones. Additionally neodymium, one of the most common REE, is a key part of neodymium-iron-boron magnets used in hyperefficient motors and generators [3]. We will present the results of electron microprobe analyses and structural determination of a Nbbearing mineral from Biraya, Russia, which has the Nb2O5 content of 10.19 wt.% and the Ti/Nb ratio of 1.1:1. A special attention in the study has been paid to the role of Nb and its valency in the structure. The refinement of X-ray data based on microprobe analysis leads to the following formula for this phase: ( C e 2 L a 1 . 2 5 N d 0 . 4 P r 0 . 2 N a 0 . 1 5 ) (Fe0.7Ca0.15Sr0.15)(Fe1.3Mg0.15Nb0.55) (Ti1.2Nb0.55Al0.15V0.1) Si4O22. In order to determine which atoms occupy particular sites, the Bond Valence (BV) Model [4], together with an analysis of the ionic radii and volumes at each site within the first coordination sphere polyhedra, was used. There is some uncertainty in the literature as to which space group (or groups) does CGM belong to. P21/a was favoured in some publications ([5],[6],[7]), whereas in the other ones ([8],[9],[10], [11]) the C2/m space group was prefered. On the basis of our new structural data for niobian chevkinite-(Ce), one can rationalise a possible relationship between the P21/a and C2/m space groups. The same single crystal which was investigated using X-ray radiation was annealed at 750oC for 24 hours, and then rapidly cooled to room temperature (within 1 hour). After this process, we collected the X-ray scattering data on our single crystal X-ray diffractometer. The analysis of the reconstructed reciprocal lattice layers indicates a significant decrease of symmetry for the niobian chevkinite(Ce) from the C2/m to P21/a space group. The observed phase transition is in a good agreement with the group theory. The possibe space groups can be presented as the Bärnighausen tree [12]. This scheme is showing transition pathway from the supergroup (C2/m in this case) to one of the subgroups (P21/a). A possible explanation of the phase transition that occured for niobian chevkinite-(Ce) will be presented by authors.

Structural analysis of new mineral phases

Redikortsevite, NH4MgCl3 x6H2O, from the burned dumps of the Chelabinsk coal basin in Russia, described by Chesnokov et al. (1988), has not been submitted to IMA for approval and it is not recognized as a valid mineral species. The occurrence of this phase has also been noticed on burning waste dumps of one of the Upper Silesia coal mines in Poland. Redikortsevite forms there aggregates and fine crystals suitable for structural study. We have determined its 3D structure and performed chemical and mineralogical analysis necessary for the approval process. Alumohydrocalcite is hydrated calcium and aluminium carbonate. It is ascribed to have chemical formula CaAl2(CO3)2(OH)4 x3H2O and commonly forms compact fine-crystalline aggregates. This mineral still does not have reliable crystal structure although it has been known since 1926. The reason of such a situation is difficulty in finding single crystals of a good quality and adequate size. We have acquired a unique sample of this mineral from its classic occurrence site at Nowa Ruda, Sudetes Mts., Poland. It consists of spherulitic needle aggregates. Crystals are very well formed and large enough for X--ray structural investigations. They reach length even up to decimal parts of millimeter and are suitable for synchrotron sources rather than laboratory ones. For this mineral, powder diffraction and chemical analysis have been performed for the sake of its identification. The chevkinite group of minerals are found as accessory phases in a wide variety of parageneses, including igneous rocks ranging from gabbros to peralkaline granites, fenites, ore deposits, granulite facies gneisses and metacarbonates [1]. The composition of the majority of occurrences closely approaches the ideal formula A4BC2D2Si4O22, where A = REE, Ca, Sr, Th; B = Fe2+; C = Ti, Al, Fe2+, Fe3+, Mn, Mg, Zr, Nb; and D = Ti, but there is a wide range of compositionally different species. The geochemical importance of chevkinite group is that they are strong REE-concentrators; total REE2O3 contents are up to 50 wt%. They can be the dominant REEbearing phase in any given rock. The REE are being increasingly used in a host of green technologies, such as the production of novel wind turbines, low-energy light bulbs also mobile phones. Additionally neodymium, one of the most common REE, is a key part of neodymiumiron-boron magnets used in hyperefficient motors and generators. Published work has shown that compositional variations in the group are mirrored in the structure but there is still no consensus as to how. Further, several species within the group have not been structurally determined. We are attempting to define the complete compositionstructure relationships in the group.