Kenny Ståhl

On the Non-Stoichiometry in Rutile-Type »SbVO4

Heating equimolar mixtures of Sb{sub 2}O{sub 3} and V{sub 2}O{sub 5} at 800{degrees}C in flowing gas with varying O{sub 2}/N{sub 2} ratios produces a continuous nonstoichiometric series of rutile type, i.e., Sb{sub 0.9}V{sub 0.9+x}{open_square}{sub 0.2-x}O{sub 4}, Sb{sub 0.9}{sub 0.9}{open_square}{sub 0.2}O{sub 4}, a = 4.63, c = 3.03 {angstrom} (X-ray powder data, XRD), is formed in pure oxygen and exhibits a modulated structure with an approximate supercell: 2{radical}2a, 2{radical}2b, 4c (electron diffraction, (ED)). In pure nitrogen, reduced Sb{sub 0.9}V{sub 1.1}O{sub 4}, a = 4.60, c = 3.08 {angstrom} (XRD), with the supercell {radical}2a, {radical}2b, 2c (ED), is produced. Heating at intermediate partial pressures of oxygen give phases with the basic rutile cell a = b, c (XRD, ED). The formulation of this series is supported by data obtained by Fourier transform infrared spectroscopy. Under reducing conditions (in pure nitrogen), a solid solution series of Sb{sub 0.9}V{sub 1.1}O{sub 4} and VO{sub 2} is observed, i.e., Sb{sub 0.9-y}V{sub 1.1+y}O{sub 4}, 0 < y < 0.7. Vanadium-rich Sb{sub 0.2}V{sub 1.8}O{sub 4}, with a = 4.56, c = 2.99 {angstrom} (XRD), exhibits a basic rutile lattice with diffuse intensity between Bragg spots (ED).

One-dimensional water chain in the zeolite bikitaite: Neutron diffraction study at 13 and 295 K

The crystal structure of the natural zeolite bikitaite, Li2Al2Si4O12·2H2O from Bikita, Zimbabwe, has been determined by neutron diffraction at 295 and 13 K. Space group P1, Z = 1, a = 8.6071(9), b = 4.9540(5), c = 7.5972(7) A, α = 89.900(7), β = 114.437(8), γ = 89.988(8)°, V = 294.9(1) A3 at 295 K, and a = 8.5971(8), b = 4.9395(4), c = 7.6121(7) A, α = 89.850(7), β = 114.520(7), γ = 90.004(7)°, V = 294.10(4) A3 at 13 K. λ = 1.0505(1) A, μ = 0.650 cm−1, Dx = 2.28 g/cm3. Final R(F) = 0.027 and 0.026 for 1845 and 2083 unique reflections at 295 and 13 K, respectively. Refined values of SiAl site occupancies show a high degree of ordering, which results in a triclinic distortion from the monoclinic symmetry reported by X-ray studies. The H2O molecules are hydrogen bonded only to each other, H···O = 1.949(3) and 1.955(3) A, and with H···O (framework) distances in the range 2.544(4)-2.946(4) A. The infinite water chains, parallel to b, are linked to the zeolite framework by Li+···Ow coordination only. The Li+ ions are each tetrahedrally coordinated by one water and three SiOAl oxygens. The Li coordination causes a strong decrease in the SiOAl angles. The significant structural changes when going from 295 to 13 K are stronger hydrogen bonding and a general decrease in the SiOSiAl angles.

Fully hydrated laumontite: A structure study by flat-plate and capillary powder diffraction techniques☆

Abstract The structure refinement of laumontite [Ca4Si16Al48·O48·18 H2O; Z = 1, CuKα radiation; T = 295 K; cell a = 14.845(9), b = 13.167(2), c = 7.5414 (8) A ; β = 110.34(2)°, C2/m, Dx = 2.306, Mr = 1917.80] was completed from X-ray powder data obtained with two different techniques using a fully hydrous sample. Data collection was performed using (a) a mylar-sealed flat-plate holder with the sample submerged into water, Bragg-Brentano geometry, and a conventional two-circle Philips goniometer, and (b) the sample in a water-filled glass capillary, Debye-Scherrer geometry, and an INEL CPS120 position sensitive detector. The results obtained by Rietveld refinement of the two independent data sets are compared and found to be in full agreement. The water content of fully hydrated laumontite is shown to be 18.0 water molecules per cell. The excess water molecules present in laumontite with respect to leonhardite completely stabilize a 7-coordinated geometry for the Ca atoms in the channels. This study shows that reliable structural information not accessible with other techniques can be extracted by Rietveld refinement of powder data collected with unconventional experimental geometries and sample preparation techniques.

The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction

Abstract The dehydration and rehydration process in the natural zeolite mesolite, Na5.33Ca5.33Al16Si24O80·⋒H2O [structure type NAT, space group Fdd2, Z = 3, n(H2O) = 21.33, a = 18.4071(4), b = 56.668(1), c = 6.5464(1) A] has been studied by powder diffraction using CuKα, and synchrotron radiation [λ = 1.1999(1) A] and Rietveld analysis. The samples were dehydrated for 1 h at 458, 473, 523, 573, and 598 K, respectively, and sealed in capillaries prior to data collection at room temperature. After partial loss of OW(4), equaltorially coordinated to Ca, after dehydration at 458 K, mesolite goes through an order/disorder transition on dehydration at 473 K. The cations becomes randomly distributed over the Ca and Na sites and OW(4) is expelled completely. Reflections having k ≠ 3n are lost and the resulting crystal structure, metamesolite, is very close to that of natrolite, the Na site being equally occupied by Na, Ca, and vacancies, and n(H2O varying between 14.7(3) (T = 473 K), and 11.1(2) (T = 573 K). T = 523 K: Metamesolite; Fdd2, Z = 1, n(H2O) = 11.3(1), a = 18.11287(B), b = 18.63331(8), c = 6.56618(3) A. Dehydration at 598 K destroys the crystalline structure and the material becomes amorphous. Rehydration of metamesolite restores the original water content, but the random Na Ca distribution is retained and a new, partially occupied OW(4′) site coordinated to Ca appears: cation-disordered mesolite; Fdd2, Z = 1, n(H 2 O) = 23.2(4), a = 18.6180(9), b = 19.0312(9), c = 6.5421(3) A .

The dehydration process in the zeolite laumontite: a real-time synchrotron X-ray powder diffraction study

The dehydration process of the natural zeolite laumontite Ca4Si16Al8O48 · 18 H2O has been studied in situ by means of powder diffraction and X-ray synchrotron radiation. Powder diffraction profiles suitable for Rietveld refinements were accumulated in time intervals of 5 minutes using a position sensitive detector (CPS-120 by INEL), while the temperature increased in steps of about 5 K. The synchronization of accumulation time and temperature plateau allowed collection of 62 temperature-resolved powder patterns in the range 310–584 K, whose analysis produced a dynamic picture of the laumontite structure response to dehydration. The first zeolitic water molecules diffusing out of the channels are those not bonded to the Ca cations and located in the W(1) site, whose occupancy drops smoothly to 10% during heating to 349 K, while the sample in the capillary is still submerged in water. The remaining W(1) and 60% of W(5) water molecules are expelled rather sharply at about 370 K. At this temperature all remaining water submerging the powder crystallites is lost, the structure contains about 13 water molecules/cell, and the crystal structure is that of leonhardite. On continued heating 80% of the water molecules from the W(2) site are lost between 420 and 480 K, while a small amount of the diffusing water is reinserted in the W(5) site. The occupancy factor of the W(8) site decreases starting at 480 K, and reaches a maximum loss of 20% at 584 K. The combined occupancy of the Ca-coordinated W (2) and W (8) water sites never falls much below two, so that the Ca cations in the channels, which are bonded to four framework oxygen atoms, are nearly six-coordinated in the explored temperature range. The water loss is accompanied by large changes in the unit cell dimensions. Except at 367 K, where the excess surrounding water is leaving, all changes in cell dimensions are gradual. The loss of the hydrogen bonded W(1) and W(5) water molecules is related to most of the unit cell volume reduction below 370 K, as shown by the contraction of the a-, b- and c-axes and the increase in the monoclinic angle. Loss of the Ca-coordinated W(2) and W(8) water molecules has a small effect on the unit cell volume as the continued contraction of the a- and c-axes is counter-balanced by a large expansion in the b-axis and a decrease in the monoclinic β angle.

The crystal structure of NaIO3 at 293 K

Abstract The crystal structure of NaIO3 at 293 K has been refined from MoKα data in space group Pbnm, Z = 4, Mr 197.89, a = 5.7500(3), b = 6.3953(4), c = 8.1280(4), A, V = 298.89(5), A3. Three thousand eighty-five (857 unique) reflections refined to R(F) = 0.018. The iodate group forms a pyramid with I as one apex; the IO bond distances are 1.8016(8) and 1.8112(7) A, and the OIO angles are 96.43(4)° and 102.60(3)°. Including next-nearest neighbors at distances 2.95–3.30 A the IO coordination is described as a two-capped trigonal prism. NaIO3 is of the anti-cementite type, with Na occupying octahedral interstices (empty in cementite) and with the I lone pair approximately in the cementite C position.

A neutron diffraction study of hydrogen positions at 13 K, domain model, and chemical composition of staurolite

Abstract Comparison of new neutron and old X-ray diffraction data for single crystals of staurolite from Pizzo Forno yielded unique answers to some, but not all, outstanding structural questions. Neutron data were collected at 13(1) K for a crystal with assumed composition Li0.07Mg0.87Ti0.14V0.01Cr0.01Mn0.04Fe2+3.00 Fe3+0.06Co0.01Zn0.05Al17.69Si7.67O48H3.41F0.01, Mw = 1671, a = 7.8639(10), b = 16.625(2), c = 5.651(2) A, β = 90.015(14)°, C2 m , Z = 1, D x = 3.75 g cm −3 ; 1874 (1024 unique) reflections, λ = 1.1598(1) A, R(F) = 3.3%. The diffraction evidence is consistent with full occupancy of the Si, Al(1), Al(2), and Al(3) sites, but not of the other ones. Detailed assignment of atoms is based on diffraction evidence and crystal-chemical arguments, but some uncertainties remain; thus exchange of (Li + Fe) by two Mg would have little effect on diffraction data. A structural model with three types of domains is proposed: ∼63% type 1, (Fe, etc.) + H(1); ∼22% type 2, (Mg, etc.) + H(2); ∼15% type 3, (Fe, etc.) without hydrogen. For the orthorhombic pseudostructure, the occupancies of the two hydrogen sites place strong restrictions on the other site occupancies. The 25(4)% observed occupancy of H(2) limits the occupancy of the nearby (Fe, etc.) site to a maximum of 75(4)%. To explain the neutron scattering, the Fe site must be occupied mainly by Fe; Li, Mn, Zn, and Mg may also occupy this site. A good ionic balance is attained for the type 2 domain if the U site from the old X-ray work is occupied simultaneously with H(2). To match the neutron data, assignment of 21(2)% Mg to the U site is plausible, but other substituents are possible. H(2) lies directly between two O(1) atoms at ∼0.9 and 2.3 A, and H(1) is displaced away from the Fe site so that it is bonded to one O(1) at 1.01 A and one O(3) at 2.07 A. Four-fifths of the Fe atoms should be displaced from z = 0.25 because of electrostatic repulsion from H(1), and one-fifth should not be. This is consistent with the threefold distortion of the electron-density peak for the Fe site and the intensity ratio for the doublets in the Mossbauer spectrum. Assignment of 0.3 Al to the eight Si sites explains the low neutron scattering. Some Mg atoms, together with Ti and Fe3+ ones, are placed in the Al sites, but assignments among the three sites are uncertain. A plausible structural formula is: [Si 7.67, Al 0.33]8[Fe2+ 3.00, Li 0.07, Mn 0.04, Zn 0.02, Co 0.01]3.14[Al(1) site: Al 7.94, Fe3+ 0.06]8[Al(2) site: Al 7.52, Mg 0.35, Ti 0.11, V 0.01, Cr 0.01]8[Al(3) site: Al 1.88, Mg 0.09, Ti 0.03]2[U site: Mg 0.41, Zn 0.02]0.43[H(1)]2.54[H(2)]0.86. For the monoclinic structure, the A and B subcomponents are less different for the crystal used for neutron diffraction than for the smaller one used for X-ray diffraction. Maximum deviation from the orthorhombic superstructure would be generated by the following occupancies: Al(3A) 100%, H(1A) 63%, H(2A) and Mg(A) 22%. Atoms essentially unaffected by chemical substitutions have displacement factors consistent with zero-point motions, but some (especially Fe, O(1), O(3), H(2)) have large B values indicative of more than one center-of-motion. All bond lengths and angles are reasonable when chemical substitutions are considered.

Cation ordering waves in trirutiles. When X-ray crystallography fails?

Trirutiles are AB2O6 crystal structures with a tripled c-axis repeat compared to the rutile type, owing to ABBABBA metal ordering. Partially disordered trirutiles, space group P42/mnm, are here described by addition of a sinusoidal scattering density wave along c to the basic rutile structure. As a result of this approach, an infinite number of crystal structures can be envisaged, which pairwise exhibit identical X-ray diffraction patterns, i.e. this is a case where the expected one-to-one relationship between crystal structure and diffraction intensity distribution is replaced by a two-to-one relationship