V. V. Bakakin

Structural evolution of natrolite during over-hydration: a high-pressure neutron diffraction study

The crystal structure of deuterated natrolite, Na1.85Mg0.05Ca0.03[Al2.06Si 2.95O10]·nD2O, compressed in liquid D2O at 0.9 and 1.0 GPa has been determined from neutron powder diffraction data. At 0.9 GPa, the crystal structure is close to the original natrolite with the same space group Fdd 2 and 1 % smaller unit cell volume. New water positions are found in addition to the original ones indicating the early stage of natrolite over-hydration. The unit cell volume of high-pressure phase stable at 1.0 GPa is expanded by 5.4 % with respect to initial natrolite. According to structural investigations, HP phase contains 3.5 water molecules pfu. Higher degree of hydration is accompanied by the drastic rearrangement of extra-framework subsystem, water molecules occupying four independent positions. Three of them belong to Na+ coordination sphere and together with three framework O-atoms form a distorted octahedron. Water molecule in the fourth position (occupancy = 0.5) has no contact to the cations. The evolution of natrolite structure with increasing pressure is discussed in terms of framework flexibility and hydrogen bonding rearrangement.

High-temperature crystal structure of wairakite

Single crystal X-ray structure data have been obtained for wairakite (Wairakei, New Zealand), Ca 0.95 Na 0.06 [Al 1.96 Si 4.04 O 12 ]·2H 2 O, at temperatures of 20°C, 170°C, 210°C, 400°C, and 600°C. Heating of wairakite up to 200°C is accompanied by a significant increase in the unit cell volume. At 145°C, the initial monoclinic phase ( I2/a ) transforms into a tetragonal one ( I4 1 /acd ). The main features of this reversible phase transformation are the rearrangement and the length changes in various H 2 O-O contacts. Below the transition point the shortened H 2 O-O contacts exist and provide structure stabilization possibility through the formation of weak H bonds. Upon heating above 200°C the dehydration of wairakite begins and is accompanied by continuous contraction with no fundamental changes in the structure while retaining symmetry I4 1 /acd. The Ca 2+ cations remain in the vicinity of the original positions, but their coordination changes from octahedral [60] = O 4 (H 2 O) 2 , to semi-octahedral [5y] = O 4 (H 2 O), square-pyramidal [4n] = O 4 , and square-planar [4s] = O 4 .

A new mechanism of anionic substitution in fluoride borates

A comprehensive study of the BaF2-Ba3(BO 3)2 phase diagram has revealed a significant difference between the two intermediate phases Ba5(BO3)3F and Ba 7(BO3)4-y F2+3y. The latter exhibited (BO 3)3- ↔ 3F- anionic substitution which, unusually, strongly influences the solidus temperature. A comparison of the Ba5(BO3)3F and Ba7(BO3)4-y F2+3y crystal structures, along with consideration of other compounds demonstrating (BO3)3- ↔ 3F- isomorphism, allows for the disclosure of the mechanism of (BO3)3- ↔ 3F - heterovalent anionic substitution in fluoride borates via [(BO 3)F]4- tetrahedral groups being replaced by four fluoride anions. No exception to this mechanism has been discovered among all known phases with (BO3)3- ↔ 3F- substitution.

Phase formation in the BaB2O4–BaF2–BaO system and new non-centrosymmetric solid-solution series Ba7(BO3)4−xF2+3x

Detailed study of the BaB2O4–BaF2–BaO system resulted in the discovery of the new Ba7(BO3)4−xF2+3x solid solution belonging to the BaF2–Ba3(BO3)2 section. The distinguishing feature of the crystal structure of Ba7(BO3)4−xF2+3x phase is its extensive (BO3)3− ↔ 3F− anionic isomorphic substitution, confirmed by X-ray diffraction study of Ba7(BO3)3.51F3.47 (x = 0.49) single crystals (space group P63; a = 11.18241(11) A, c = 7.23720(8) A). The area of homogeneity for Ba7(BO3)4−xF2+3x solid solution spans between Ba7(BO3)3.35F3.95 and Ba7(BO3)3.79F2.63 compositions (0.21 < x < 0.65). Also, a new orthorhombic phase with a tentative composition of Ba5(BO3)3F has been identified in XRD powder patterns and indexed with cell parameters a = 7.605 A, b = 14.843 A and c = 10.291 A.

Ag-exchanged analcime: crystal structure and crystal chemistry

Abstract The crystal structure of Ag-substituted analcime (Ag 1.88 [Al 1.88 Si 4.12 O 12 ]·2H 2 O) prepared by ion exchange of Ag + for Na + was studied by single-crystal X-ray diffraction. The monoclinic unit cell parameters are a =19.369(3) A, b =13.752(2) A, c =19.385(2) A, γ =90.26(1)°, V =5163(1) A 3 , Z =16, space group: F 112/ d (No. 15). A comparison was made with the structure of the original Na-analcime. The Ag + cations are located in the vicinity of the former Na positions and octahedrally coordinated (O 4 (H 2 O) 2 ). The larger Ag + cations enter the rectangular O 4 -windows relatively easily. Only the Ag–OH 2 distances increase significantly. The correlation between the population of extra-framework positions and the Si/Al distribution is significantly better in the Ag-analcime than in the initial Na-analcime. Changes in the unit cell parameters and the symmetry are explained from crystal chemical considerations.

The crystal structure of paranatrolite

Paranatrolite from the Khibiny massif, Kola Peninsula, Russia, Na 1.88 K 0.22 Ca 0.06 [Al 2.24 Si 2.76 O 10 ].3.1H 2 O, is monoclinic (space group Cc, Z = 4). For better comparison of this mineral with the related structure of natrolite, we have selected a pseudoorthorhombic setting in F1d1 ( a = 18.971(4), b = 19.204(3), c = 6.5952(12) A, β = 91.601(18)°, Z = 8). The dominant Na+ cations are situated near the sodium positions in the natrolite structure. Additional positions occupied by K+ are located in the eight-membered rings. H 2 O molecules are located in four independent positions, two being occupied statistically. The Na-polyhedra correspond to distorted NaO 3 (H 2 O) 3 octahedra forming chains along the c axis by sharing common H 2 O-H 2 O edges and H 2 O vertices. The availability of potassium in the structure results in two configurations for the coordination environment of Na+.

Synthesis and crystal structure of silver–gold sulfide AgAuS. Four-fold interpenetrated three-dimensional [(Au,Ag)10S8]-networks

Crystals of the silver(I) thioaurate(I), AgAuS, were obtained by fusing the elements in stoichiometric amounts. The X-ray diffraction analysis showed that the synthesized phase AgAuS represents a new structural type. The unit cell parameters are a = 13.4235(3), c = 9.0873(4) A, V = 1418.07(6) A3 and Z = 24, and the space group is Rm. The sublattice of sulfur atoms has a strongly distorted body-centered cubic packing. All Au atoms are in a typical linear coordination with sulfur ones. Three-quarters of the silver atoms are surrounded by distorted tetrahedra of sulfur atoms, while the rest of the silver atoms are involved in a linear motif typical of Au atoms as well. The resulting motif is composed of four-fold interpenetrating networks of linearly coordinated metals with 2- and 4-connected sulfur atoms: [Au8[2]Ag2[2]S6(2)S2(4)]. The formula of AgAuS can be represented as Ag24[4][Au8[2]Ag2[2]S8]4. The features of the AgAuS structure suggest the possibility of wide variation in the Au : Ag ratio.

The reversibility of the paranatrolitete-tranatrolite transformation

The problem of the reversibility of paranatrolite-tetranatrolite transformation is a key problem in understanding the paragenesis of the natrolite group zeolites. Two paranatrolite samples of different chemical composition were studied by X-ray powder diffraction and thermogravimetry. It was found that the existence of a particular phase depends on the water vapor pressure in an ambient atmosphere. High-potassium paranatrolite from the Khibiny massif, Kola Peninsula, Na 1.90 K 0.22 Ca 0.06 [Al 2.24 Si 2.76 O 10 ]·3.1H 2 O, is stable at 25 °C and air humidity of about 70 %. Upon heating, the sample loses some of the water content and transforms into tetranatrolite. At 38 °C it consists of a pure tetranatrolite phase. The reverse tetranatrolite-paranatrolite transformation occurs upon cooling the sample to room temperature. The recovery of the paranatrolite phase proceeds even after heating to 300 °C with a 60 % water loss. A high-calcium sample from Mont Saint-Hilaire, Quebec, with approximate formula Na 1.59 Ca 0.32 Sr 0.02 [Al 2.35 Si 2.65 O 10 ]·nH 2 O, had significantly lower stability. It consisted of a mixture of paranatrolite and tetranatrolite in ambient conditions. Upon heating, the sample already consisted of a pure tetranatrolite at 31 °C. After keeping the sample for one day under normal conditions a two-phase mixture close to the initial sample was restored. The sample wetted by water immediately transformed into paranatrolite. A lower stability of high-calcium paranatrolite as compared with the high-potassium sample may be explained by the difference in the configuration of ionic-molecular assemblage and, presumably, by a higher water content.

Synthesis and crystal structure of gold–silver sulfoselenides: morphotropy in the Ag3Au(Se,S)2 series

Gold–silver sulfoselenides of Ag3Au(Se,S)2 series—Ag3AuSe1.5S0.5, Ag3AuSeS, and Ag3AuSe0.5S1.5—have been synthesized by fusing the elements in the required stoichiometric amounts in evacuated quartz ampoules. The single crystal X-ray diffraction data indicate the existence of two solid-solution series: petzite-type cubic Ag3AuSe2—Ag3AuSeS (space group I4132) and trigonal Ag3AuSe0.5S1.5—Ag3AuS2 (space group