Positions of cations and molecules in zeolites with the mordenite-type framework: VIII Dehydrated sodium-exchanged mordenite

Natural mordenite was ion-exchanged in NaCl solution and dehydrated at 300°C [$\text{a}$ 17.92(1) $\text{b}$ 20.31(1) $\text{c}$ 7.480(7)Å at room temperature]. Dehydration reduced the symmetry from Cmcm to Pbcn. All diffractions violating the C-centering are diffuse, and in one crystal are split into doublets indicating a domain structure with $\text{a}$ (true cell) $\text{～5}\text{a}$ (pseudo-cell). The diffuse diffractions disappear upon rehydration. Diffraction intensities were collected using a wide scan for diffuse diffractions. Site occupancies are: I', 3.1Na; IV, 2.6Na; VI, 1.5Na. Electron microprobe analysis yielded Na_7.3K_0.2Ca_0.03Al_8.3Si_39.9O₉₆, and some cationic Al, or OH^?, or both, may be needed for charge balance. (Na(I') is displaced 0.53Å from its ideal position at (0,0.5,0), perhaps because of electrostatic repulsion between adjacent ions. An ordered model with 3.2Na and 0.8 vacancies in site I' provides a qualitative explanation of the domain structure. Na(IV) is displaced laterally from the center of an eight-ring. The effect of “decationization” on site occupancy and molecular sorption is discussed.     

Positions of cations and molecules in zeolites with the mordenite-type framework VI dehydrated barium mordenite

Natural mordenite was ion-exchanged with Ba(NO₃)₂ solution and dehydrated at 300°C. The crystal structure ($\text{a}$ 17.974(7) $\text{b}$ 20.320(8) $\text{c}$ 7.419(4)Å; Pbcn; electron microprobe analysis Ca_0.35Ba_3.3Al_8.6 Si_39pre9O_post96 has all the cations in the “side pocket” (site I, 0.3Ca; II, 1.9Ba; III, 0.3Ba; IV, 1.1Ba). No cations were located in the main channels, but the type IV cations lie in 8-rings of the channel walls. For charge balance, 1.2H are needed.     

High temperature form of Pb2WO5 and transformation phenomena to its low form

The phase transformation of Pb₂WO₅ was studied by means of high temperature x-ray diffractometry, differential scanning calorimetry ( DSC ) and visual observation of the crystal growth in the range between room temperature and to its melting point. Two forms were found to be stable in the range studied. The Pb₂WO₅, which had been reported by DeVries and Fleischer was confirmed to be low temperature form. The newly synthesized high temperature form is isomorphous with Pb₂MoO₅ and its cell parameters are; $\text{a}\text{=14.206 (2)}\text{A}\text{?}$, $\text{b}\text{=5.799 (1)}\text{A}\text{?}$, $\text{c}\text{=7.346 (1)}\text{A}\text{?}$ and β=113°54 (0.9)′. The transition temperature was determined by DSC measurements as 330±10°C. The transition from high to low form is remarkably affected by the crystal size of its high form. Large crystals easily transform to its low temperature form. However, small crystalline sample (?100 μm), the rate of transformation is quite low and the high form is easily retained to the room temperature. One of the previously reported x-ray diffraction pattern of Pb₂WO₅ was revealed to be the mixture of high and low form of this compound.     

Synthese et etude structurale de l'oxyde double Dy5Re2O12

The unit cell of Dy₅Re₂O₁₂ is monoclinic, space group P2_l/m, with $\text{a}\text{= 12.425(8),}\text{b}\text{= 7.511(5),}\text{c}\text{= 5.653(5)}\text{A}\text{?}\text{, γ = 107.8(2)°,}\text{Z}\text{= 2}$.The crystal structure has been determined from single crystal diffractometer data. However, the presence of a polysynthetic twinning prevents a correct refinement.This structure is built from infinite chains of octaedra ReO₆ along c axis which are bridged along b axis by isolated octaedra DyO₆. These two types of octaedra from sheets along (100) between them are located the other Dy atoms which assume the cohesion of this tridimensionnal network.     

The preparation,crystal structures and properties of the potassium vanadium sulfides KXV5S8 (0.5≦x≦0.7)

The crystal structures of the potassium vanadium sulphides K_0.7V₅S₈ and K_0.5V₅S₈ (=KV₁₀S₁₆) have been determined. K_0.7V₅S₈ has a C-centered monoclinic unit cell of dimensions $\text{a}\text{=17.499(3)}\text{A}\text{?}$, $\text{b}\text{=3.2986(6)}\text{A}\text{?}$, $\text{c}\text{=8.489(1)}\text{A}\text{?}$, ß=103.98(1)°, spacegroup C2/m; isomorphous with TIV₅S₈; K_0.5V₅S₈ has essentially the same structure, but due to ordering of the K atoms, the monoclinic b-axis is doubled, thus forming a superstructure. The cell parameters are: $\text{a}\text{=17.462(4)}\text{A}\text{?}$, $\text{b}\text{=6.556(2)}\text{A}\text{?}$, $\text{c}\text{=8.4595(9)}\text{A}\text{?}$, ß=103.86(1)°, spacegroup P2. The structures are characterized by a three-dimensional framework of VS₆ octahedra with channels in which the K atoms are situated. Both compounds exhibit metallic behaviour.     

Positions of cations and molecules in zeolites with the mordenite-type framework X dehydrated calcium hydrogen mordenite

Prolonged acid treatment of natural mordenite (2M Hc1, 90°C, 2 months) did not remove 1.4Ca atoms which persisted in site I of the final product after dehydration at 300°C. Electron microprobe analysis showed no detectable reduction of Al/Si ratio, but the cell parameters at room temperature ($\text{a}$ 18.058(3) $\text{b}$ 20.297(3) $\text{c}$ 7.484(2)Å) are lower than for the dehydrated-deammoniated variety, and the mean T-O distance is lower (1.606 vs. 1.617Å). The O-T-O and T-O-T angles are closer generally to those of dehydrated Ca-mordenite than to dehydrated-deammoniated mordenite. Population refinement produced no evidence in favor of vacant tetrahedral sites, but technical considerations cause complications. The present data would not be inconsistent with earlier suggestions that Al is replaced by Si in tetrahedral sites.     

Structure cristalline et proprietes magnetiques de la variete basse temperature TlFeBr3β

DTA and X-Ray studies of TlFeBr₃ show a transition at 384°C. The low temperature phase β-TlFeBr₃ is related to NH₄CdCl₃ and crystallizes in the orthorhombic system with $\text{a}\text{= 9.279(4)}\text{A}\text{?}$, $\text{b}\text{= 3.984(3)}\text{A}\text{?}$, $\text{c}\text{= 15.070(7)}\text{A}\text{?}$, Z = 4. The crystal structure has been determined from 564 independent reflexions. The structure contains FeBr₆ octahedra in which each Fe atom is coordinated to six Br atoms. The FeBr₆ octahedra share vertices to form infinite double chains along the b? axis. This compound behaves paramagnetically above 40 K, with a Curie-Weiss temperature of θ = 12 K and a magnetic moment of μ = 5.59 μB, but becomes antiferromagnetic below 14 K.     

Positions of cations and molecules in zeolites with the mordenite framework IX dehydrated H-mordenite via acid exchange

Nearly complete removal of cations was accomplished for natural mordenite by preliminary Na-exchange followed by exposure to 2M HCl for 1 month at 90°C. The electron microprobe analysis (atomic Si/Al 4.7) indicates no detectable extraction of Al. However the cell dimensions after dehydration at 300°C followed by cooling to room temperature ($\text{a}$ 18.178(7) $\text{b}$ 20.394(6) $\text{c}$ 7.488(4)Å) are lower than for “hydrogen-mordenite” prepared via an ammonium-exchanged intermediary ($\text{a}$ 18.223(7) $\text{b}$ 20.465(9) $\text{c}$ 7.531(4)Å). Furthermore the tetrahedral distances are smaller (T4 down 0.017?, T3 0.010, T1 0.008, T2 0.002). Diffractions from the HCl-treated mordenite are sharp, and the positional and population parameters are close to those for the de-ammoniated mordenite. The simplest interpretation is that treatment with HCl results in extraction of Al from tetrahedral sites into other positions in the crystal coupled with migration of Si so that a crystalline structure is retained: however, many subtle complications require cautious interpretation.     

The structure of the low temperature phase Mg2NiH4(LT)

The crystal structure of a low temperature modification of Mg₂NiH₄(LT), stable below 235°C, has been determined from Guinier-Hägg powder diffraction data. The unit cell dimensions are $\text{a}\text{= 6.497(2)}\text{A}\text{?}$, $\text{b}\text{= 6.414(1)}\text{A}\text{?}$, $\text{c}\text{= 6.601(2)}\text{A}\text{?}$ and β = 93.23(2)°. The structure has been refined by profile analysis from a sample also containing MgH₂ and small amounts of two other phases, viz. the high temperature modification Mg₂NiH_3.9(HT) and Mg₂NiH_x(LT). It is indicated that the phase transformation of Mg₂NiH_3.9 (HT) at 235°C is eutectoid, giving mainly Mg₂NiH₄(LT) but also small amounts of a less hydrogen containing phase Mg₂NiH_x(LT) (x≈2).     

Nouvelles familles de phases MIMII2Nb3O10 a feuillets “perovskites”

A new series of phases M^ICa₂Nb₃O₁₀ (M^I = Li, Na, K, Rb, Cs, NH₄, Tl) has been prepared and characterized. Their unit cells are tetragonal. The structures consist of treble perovskite sheets interleaved with M^I sheets. According to the M^I nature, the relative displacement of adjacent treble perovskite sheets, parallel to (001) is O, $\text{b}\text{4}$ or $\text{(}\text{a}\text{+}\text{b}\text{)}\text{4}$     

Positions of cations and molecules in zeolites with the chabazite framework III. Dehydrated Na-exchanged chabazite

Chabazite from Bozen, Tyrol was ion-exchanged with NaC? solution to Na_15.2A?_15.2Si_32.8O₉₆.nH₂O. After vacuum dehydration at 320°C, the crystal structure was determined at room temperature. Reduction of the ideal rhombohedral symmetry to monoclinic (C2/m or lower; $\text{a}$ 19.319(3) $\text{b}$ 13.833(2) $\text{c}$ 11.849(2)Å β 11.348(3)°) is interpreted to result from near-elliptical distortion of most 8-rings to accommodate 57% of the Na⁺ cations. All these cations have 4 framework oxygens as nearest neighbors, but positional disorder complicates interpretation of the distances which range from 2.34 to 2.92Å. Most other Na⁺ cations (38%) are displaced into the large cavity from 6-rings with 3 framework oxygens as nearest neighbors at 2.32 to 2.49Å. Diameters of three types of 8-rings range from 4.5 to 9.4Å, but the fourth type with very low occupancy of Na⁺ is less distorted with diameters ranging from 6.0 to 7.0Å. Correlation between T-O distances and T-O-T angles is semi-quantitatively similar to that in mordenite.     

X-ray diffraction studies of potassium polytungstates with high WO3 content

The potassium polytungstates in the composition region between K₂W₄O₁₃ ($\text{n}\text{= 4}$ in K₂O·$\text{n}$WO₃) and WO₃ have been studied by x-ray diffraction methods. The existence of a (pseudo-) phase with a homogeneity region extending from the hexatungstate (n = 6) to the octatungstate ($\text{n}\text{= 8}$) composition has been confirmed. The crystal structure is basically the same as that of hexagonal tungsten bronze (HTB), but two types of superstructures occur, both with lattices which can be derived from the orthohexagonal representation of the HTB cell ($\text{a}\text{= 7.30}\text{A}\text{?}$, $\text{b}\text{= 12.71}\text{A}\text{?}$, $\text{c}\text{= 7.63}\text{A}\text{?}$). One of these has $\text{b}\text{= 2}\text{b}{}_{\mathrm{HTB}}$ and forms for $\text{n}\text{< 6.25}$ while the other has $\text{b}\text{= 5}\text{b}{}_{\mathrm{HTB}}$ and is obtained when $\text{n}\text{> 6.25}$. Powder patterns for these phases are given. The substructure has been refined from single crystal data but the super-structures have not been determined.     

Synthesis and symmetry transformation in the perovskite compounds PbHfO3 and CdHfO3

From high temperature X-ray diffraction experiments the perovskite compounds PbHfO₃ and CdHfO₃ have been shown to undergo a series of structural phase transformations. At 298°K PbHfO₃ is orthorhombic with cell dimensions $\text{a}\text{= 5.8572(5)}\text{A}\text{?}$, $\text{b}\text{= 11.689(1)}\text{A}\text{?}$, $\text{c}\text{= 4.0971(4)}\text{A}\text{?}$ and most probable space group Pnam. At 450°K it is rhombohedral with hexagonal cell dimensions $\text{a}\text{= 5.854(1)}\text{A}\text{?}$ and $\text{c}\text{= 7.145(2)}\text{A}\text{?}$ and at 520°K PbHfO₃ is cubic perovskite with $\text{a}\text{= 4.1354(4)}\text{A}\text{?}$. CdHfO₃ is also orthorhombic at 298°K with cell dimensions $\text{a}\text{= 5.5014(8)}\text{A}\text{?}$, $\text{b}\text{= 5.6607(8)}\text{A}\text{?}$, $\text{c}\text{= 7.969(1)}\text{A}\text{?}$, and space group Pbnm. At 1075°K CdHfO₃ is rhombohedral with hexagonal cell dimensions $\text{a}\text{= 5.747(4)}\text{A}\text{?}$ and $\text{c}\text{= 13.49(1)}\text{A}\text{?}$     

Comments on the system Ag2SIn2S3

The high temperature polymorph of AgInS₂ has been found to be orthorhombic with $\text{a}\text{=7.001}$, $\text{b}\text{=8.278}$, $\text{c}\text{=6.698}\text{A}\text{?}$, space group probably Pna2₁ with a distorted wurtzite structure. The phase transition temperature was found to be 620 ± 10°C and the melting point 880 ± 10°C. A new cubic spinel type phase was found at the composition AgIn₅S₈ with $\text{a}\text{=10.827}\text{A}\text{?}$.     

Crystal structure of K4[H2J2O10].8H20

Crystals of K₄ [H₂J₂O₁₀] 8H₂O belong to the triclinic system, space group P 1 with a = 7.161 (2) $\text{A}\text{?}\text{, b}\text{= 10,553}$ (5) $\text{A}\text{?}\text{, c}\text{= 7,081}$ (2) $\text{A}\text{?}\text{, α = 98°1′}$, β = 117°8′, γ = 90°6′ and Z = 1. The crystal structure has been determined on the basis of photographic data from 877 independent reflections, with the final R value of 6,6%. The iodine atoms are surrounded by a distorted octahedra consisting of five oxygen atoms and one OH group. The average J-O distance is 2.03 Å. There are 2 independent K atoms in the structure. K(1) has a coordination number of eight, while K(2) is surrounded by 6 (5 water molecules + one OH group) nearest neighbours.     

Structure cristalline de Mn0, 4Er4, 6S7

The crystal structure of Mn₀, ₄Er₄, ₆S₇ ($\text{a}\text{= 12,573 (4)}\text{A}\text{?}$, $\text{b}\text{= 11,390 (4)}\text{A}\text{?}$, $\text{c}\text{= 3,777 (4)}\text{A}\text{?}$, γ = 105,45°, space group B2/m, Z = 2) has been refined by a least square method to a final R = 0,045, with 1431 independant reflections. The octahedral positions are occupied either by Er and Mn atoms or by Er atoms only and the prismatic sites by Er atoms.     

The crystal structure of Ni5TiB2O10

The crystal structure of Ni₅TiB₂O₁₀ was determined by single crystal X-ray analysis. The compound has the same structure as the mineral ludwigite. The orthorhombic celldimensions and space group are $\text{a}\text{=9.206(7)}\text{A}\text{?}$, $\text{b}\text{=12.224(9)}\text{A}\text{?}$, $\text{c}\text{=2.994(2)}\text{A}\text{?}$, Pbam z=2. Ti and Ni are disordered on one equipoint.     

Crystal chemistry of cerium titanates,tantalates and niobates

Cerium dioxide has been found to react with other oxides at high temperatures in an open air environment with the formation of Ce⁺³, Ce⁺⁴ or mixed valence phases. The compound Ce⁺⁴Ti₂O₆ forms with the brannerite structure $\text{a}\text{=9.804,}\text{b}\text{=3.758,}\text{c}\text{=6.914, β=119° 8.7′}$, however, with the addition of sodium, the black pseudocubic perovskite compound NaCe⁺³Ti₂O₆ is formed, $\text{a}\text{=3.864}\text{A}$. Single crystals of the Ce⁺³Ta₇O₁₉ reveal that this compound is hexagonal, P6₃/mcm, $\text{a}\text{=6.232,}\text{c}\text{=19.985}\text{A}$. Single crystals of the compound Ce⁺³TaO₄ are light green of the LaTaO₄-type, P2₁/c with $\text{a}\text{=7.618,}\text{b}\text{=5.531,}\text{c}\text{=7.767}\text{A}\text{, β=100° 56.3}$. On oxidizing at low temperature, ≈600°C, the crystals turn black and change to monoclinic, P2/m, $\text{a}\text{=7.617,}\text{b}\text{=5.491,}\text{c}\text{=3.851}\text{A}\text{, β=102° 30.5'}$, with a corresponding change to ≈CeTaO_4.174. Another phase which is also light yellow is formed by oxidizing at 350°C for long periods of time and corresponds to CeTaO_4.50. The compound CeNb₅O₁₄ is orthorhombic Pmnb, $\text{a}\text{=20.12,}\text{b}\text{=12.474,}\text{c}\text{=7.744}\text{A}$. Ce⁺³NbO₄ has a fergusonite unit cell when quenched to room temperature $\text{a}\text{=5.544,}\text{b}\text{=11.434,}\text{c}\text{=5.177, β=94° 41.8'}$. On oxidation in air the cell size is $\text{a}\text{=5.364,}\text{b}\text{=11.424,}\text{c}\text{=5.129}\text{A}\text{, β93° 22.7'}$ at room temperature and corresponds to ≈CeNbO_4.25.     

Positions of cations and molecules in zeolites with the mordenite-type framework VII dehydrated cesium mordenite

Natural mordenite was ion-exchanged with CsCl solution and dehydrated at 300°C. The crystal structure [$\text{a}$ 18.194(19) $\text{b}$ 20.470(17) $\text{c}$ 7.506(9): Cs₇₄Al～₈Si～₄₀O₉₆] probably belongs to space group P2₁cn but was refined in the average space group Cmcm. Site occupancies for Cs are site II 3.8, IV 1.9, VI 1.8 atoms. Combined occupancies of sites IV and VI are less than 4 in dK-, dRb- and dCs- varieties. Cations larger than 1.3Å have not been found in site I. Cations in site VI block diffusion of large molecules in the main channel, and conversion of Na-mordenite from small-port to large-port variety by “decationization” towards H-mordenite might be explained by emptying sites IV and VI while up to 4 Na could remain in site I.     

Hochdruckreaktionen von Oxometallaten,VI: Hochdrucksynthese gemischtvalenter verbindungen VO2−δ (OH)δ mit CaCl2-struktur

Mixed valence compounds VO_2?δ(OH)_δ (0.13 < δ < 0.37) were synthesized by high pressure - high temperature decomposition of ammonium metavanadate, NH₄VO₃, at p > 20 kbar and T = 700–800°C. The blue-black substances crystallize orthorhombic, space group Pnnm. Single crystal data for VO_1.7(OH)_0.3 ($\text{a}\text{= 4.5113(7)}\text{A}\text{?}$, $\text{b}\text{= 4.6303(8)}\text{A}\text{?}$, $\text{c}\text{= 2.8652(5)}\text{A}\text{?}$) confirmed the structure to be CaCl₂ type, a distorted variant of the rutile structure. On heating in air to about 250°C, VO_2?δ(OH)_δ is oxidized to VO₂, which can conveniently be prepared from NH₄VO₃ in this way.