Volume 229, Issue 11 - Special Issue: Russian-German Research Collaboration / Guest Editors: Elena V. Boldyreva and Wolfgang W. Schmahl

Powder α-VOSO 4 was prepared by dehydration of VOSO 4 ·3H 2 O. β-VOSO 4 was synthesized by boiling of V 2 O 5 in H 2 SO 4 . Thermal behaviour of VOSO 4 ·3H 2 O, α- and β-VOSO 4 modifications is studied by high-temperature powder X-ray diffraction and thermal analysis, including two-step dehydration of VOSO 4 ·3H 2 O, formation of α-VOSO 4 , thermal expansion and decomposition of both modifications into V 2 O 5 . Higher anisotropy of thermal expansion of the tetragonal α-modification α c = 39(2)·10 –6 K –1 along the vanadyl ion and α a = 2.4(6)·10 –6 K –1 in the perpendicular direction) comparing to the orthorhombic β-modification (α a = 20.2(7), α b = 2.8(8), α c = 17.8(4)·10 –6 K –1 ) is explained from a crystal chemical point of view.

The crystal structure of complex oxide ferroelectrics Ba 4 Sm 2 Fe 2 Nb 8 O 30 and Ba 4 Gd 2 Fe 2 Nb 8 O 30 have been studied by means of energy-dispersive X-ray diffraction method at high pressure up to 4 GPa at room temperature. At ambient pressure, the crystal structure of both compounds is described by the tetragonal symmetry with space group P4 / mbm . At pressure P∼ 1.8 GPa in Ba 4 Sm 2 Fe 2 Nb 8 O 30 and at P∼ 2.2 GPa in Ba 4 Gd 2 Fe 2 Nb 8 O 30 the structure phase transition have been observed. The orthorhombic structure model with Pba 2 space group for high-pressure phase of those compounds have been proposed. The baric dependences of lattice parameters and unit cell volumes of Ba 4 Sm 2 Fe 2 Nb 8 O 30 and Ba 4 Gd 2 Fe 2 Nb 8 O 30 compounds have been obtained.

A giant quasi-spherical supramolecule [{Cp*Fe(η 5 -P 5 )} 12 (CuBr) 18.8 ] encapsulating o -carborane was prepared by the reaction between CuBr and [Cp*Fe(η 5 -P 5 )] (Cp* = pentamethylcyclopentadienyl) and structurally characterized. The crystal structure can be treated as solid solution of various [(Cp*Fe(η 5 -P 5 )) 12 (CuBr) n ] supramolecules with n = 18–20. Two crystals obtained from different syntheses differ in the location of vacant CuBr positions and solvent molecules. Surprisingly, the supramolecules are packed following the low-density reo (ReO 3 ) packing.

The structural behavior of the non-linear optical, nanoporous compound CsLiB 6 O 10 was studied in a wide temperature range. Both the highly anisotropic thermal expansion of the borate with temperature variation and the deterioration of its crystals were investigated. The electron density distribution was analyzed in detail. A reversible, partial transition to a new modification, β -CsLiB 6 O 10 , was observed during cooling crystalline and moist samples of CsLiB 6 O 10 to –25 °C. The crystal structure of the new phase is described.

In this contribution, we briefly review the results of collaborative research between crystallography groups in St. Petersburg and Kiel, and report on the synthesis and crystal structure of Tl 16 (SiO 4 ) 2 (SO 4 )(OH) 6 ( 1 ), a new and unusual compound obtained from highly-alkaline TlOH solution. Its structure [monoclinic, C 2/ m , a = 28.7140(18), b = 7.3686(5), c = 7.4426(5) Å, β = 104.653(2)°, V = 1523.50(17) Å 3 , R 1 = 0.066 for 1528 independent observed reflections] can be described as an alternation of electroneutral [Tl 5 (SiO 4 )(OH)] and [Tl 3 (SO 4 ) 0.5 (OH) 2 ] layers. The [Tl 5 (SiO 4 )(OH)] layer consists of Tl(1)O 4 tetragonal pyramids sharing common corners with the SiO 4 tetrahedra to form pseudotetragonal [Tl(SiO 4 )] 3- layers capped by OH(1)-centered [(OH)Tl 4 ] square pyramids with the Tl(2) and Tl(3) atoms in their apices. The [Tl 3 (SO 4 ) 0.5 (OH) 2 ] layers contain OH(2)- and OH(3)-centered [(OH)Tl 4 ] square pyramids sharing common edges to form large (∼ 7.4 × 7.4 Å) cavities occupied by disordered (SO 4 ) tetrahedra. The electroneutral character of the [Tl 5 (SiO 4 )(OH)] and [Tl 6 (SO 4 )(OH) 4 ] layers allows us to suggest that the phases with these chemical compositions may form as independent compounds in highly alkaline Tl-bearing systems.

The high-pressure crystal chemistry of coesite was studied by means of single crystal X-ray diffraction in the pressure interval ∼2–34 GPa and at ambient temperature. We compressed the samples using diamond-anvil cells loaded with neon as pressure-transmitting medium and collected X-ray diffraction data using synchrotron radiation. The thermodynamically stable coesite – coesite-I – was observed up to ∼20 GPa, with the following unit-cell parameters: a = 6.6533(12) Å, b = 11.9018(10) Å, c = 6.9336(10) Å, β = 121.250(20)° and V = 469.38(15) Å 3 . The volume-pressure data of coesite-I are described by means of a third-order Birch-Murnhagan EoS with parameters V 0 = 547.26(66) Å 3 , K T 0 = 96(4) GPa, K′To${K'_{To}}$ = 4.1(4). Above such pressure we witness the formation of a well crystallized coesite-II, previously observed only by spectroscopic studies. The structure of the novel high-P polymorph was determined and refined at ∼28 and ∼31 GPa with final R indices of 8% and 12%, respectively. Coesite-II has P 2 1 /n symmetry and a unit cell that is “doubled” along the b -axis with respect to that of the initial coesite-I: a = 6.5591(10) Å, b = 23.2276(14) Å, c = 6.7953(9) Å, β = 121.062(19)° and V = 886.84(19) Å 3 at ∼28 GPa. All Si atoms are in tetrahedral coordination. The displacive phase transition I->II is likely driven by the extreme shortening (0.05 Å or 3.2%) of the shortest and the most compressible Si1-O1 bond, related to the stiff 180° Si1-O1-Si1 angle. Under compression the linear angle bends, resulting in two independent angles, one of which, however, retains almost linear geometry (∼178°). The requirement of this angle to be close to linear likely causes further Si-O compression down to an extremely short distance of ∼1.52 Å which prompts subsequent structural changes, with the formation of a triclinic phase at ∼31 GPa, coesite-III.