Å. Kvick

The structure of dinitrogen tetroxide N2O4: Neutron diffraction study at 100, 60, and 20 K and ab initio theoretical calculations

Single crystal neutron diffraction studies are reported for cubic dinitrogen tetroxide (space group Im3; Z = 6). The crystals were grown from NO2 vapor in situ on the diffractometer by precise cryostatic control. The lattice parameter, measured at seven temperatures, increases from 7.6937(6) A at 20 K to 7.7925(6) A at 140 K. The nuclear positional and thermal parameters were refined using diffraction data (sin ϑ/λ⩽0.79 A−1) measured at 20, 60, and 100 K. Final fit indices R(F2) are 0.028, 0.034, 0.037, respectively. The observed N–N bond length and O–N–O angle are invariant between 20 and 100 K at values 1.7562(±4) A and 134.46(±6)° with individual e.s.d.’s of 0.001 A and 0.1°. The observed N–O bond length increases linearly from 1.1855(9) A at 100 K to 1.1893(5) A at 20 K; the extrapolated zero‐point value is 1.191 A. Ab initio self‐consistent field calculations using a two‐configuration wave function which allows partial occupation of the σ* MO (antibonding with respect to the two nitrogens) yields a r...

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.

A neutron diffraction study of the zeolite edingtonite

A neutron diffraction study at 294 K of a single crystal of edingtonite (Ba2Al4Si6O20 ⋅ 7H2O; a 9.537(3) b 9.651(2) c 6.509(2) A; P21212) utilized 1876 diffraction intensities from the Brookhaven National Laboratory high‐flux beam reactor. The agreement factor R(F2)=0.055 for conventional anisotropic refinement was reduced to 0.045 for a Gram–Charlier expansion up to fourth order for the thermal factors of the water atoms. The Si–O and Al–O distances correlate inversely with the Si–O–Al angle as in scolecite. There is no indication of substitutional disorder. The barium atom is coordinated to three pairs of framework oxygens (2.89, 2.96, and 3.04 A) and two pairs of water oxygens (2.79 and 2.79 A). Two framework oxygens have weak hydrogen bonds to both water molecules [O(4)–OW(1) 2.87, −OW(2) 2.96; O(5) −OW(1) 3.02, −OW(2) 3.02 A] and the other three framework oxygens are each bonded to a Ba atom. The OW–H ⋅⋅⋅ O angles (163.5°, 165.1°, 173.9°, and 178.0°) are fairly close to 180°, the H ⋅⋅⋅ O distances ar...

Neutron diffraction structure refinement of the zeolite gismondine at 15 K

Abstract A single crystal neutron diffraction study was carried out at 15 K on gismondine (Ca 3.91 Al 7.77 Si 8.22 O 32 17.6H 2 O; a=10.011(3), b=10.614(6), c=9.853(3) A , β = 93.11(2)°; P2 1 /c) . The final agreement factor for the anisotropic refinement is R ( F 2 )=0.081 ( R w ( F 2 )=0.099) using 1941 diffraction intensities collected at the Brookhaven National Laboratory High Flux Beam Reactor (HFBR). Refinement of the neutron scattering lengths for tetrahedral atoms and mean tetrahedral TO distances are consistent with an essentially ordered distribution of Si and Al and the possibility of a minor substitution of excess Si in the Al(1) site. The extraframework cation site has full Ca occupancy and a coordination involving two framework oxygens and 4.3 water oxygen atoms in two geometrical configurations. One configuration involves three fully occupied water molecules (OW1, OW2, OW3) and one (OW4), with a statistical occurrence of 70%; the second configuration shows two sites (OW5, OW6) replacing OW4 with an occurrence of 30%. Proton positions were located for all the water molecules; disordered hydrogen sites were found for OW3 and OW4. The hydrogen bonding configuration is related to the hydrogen—hydrogen and hydrogen—calcium electrostatic repulsion.

Neutron diffraction study of the zeolite thomsonite

Abstract A neutron study of a single crystal of thomsonite (Na(Ca,Sr) 2 Al 5 Si 5 O 20 .6H 2 O at 293K; a 13.088(2) b 13.052(2) c 13.229(2) A; Pncn) yielded a final agreement factor of R ( F 2 ) = 0.048 for conventional anisotropic refinement using 4626 diffraction intensities from the Brookhaven National Laboratory high-flux beam reactor. The SiO and AlO distances correlate inversely with SiOAl angle as in scolecite and edingtonite, and the scattering lengths for the tetrahedral nodes are consistent with alternation of Si and Al. Both sites for extra-framework cations are associated with elliptical 8-rings. The Na,Ca site contains 0.5 Na and 0.5 Ca, and is surrounded by a square antiprism containing 4 water-oxygens in one square (2.45, 2.50, 2.59, 2.60 A). Each square of water-oxygens is shared with an adjacent antiprism and one edge between framework-oxygens is shared with a second antiprism to give an infinite chain parallel to c. There is no evidence for long-range alternations of Na and Ca atoms. The Ca site is split into two positions displaced 0.28 A from the centroid of 6 framework-oxygens and 2 water-oxygens, and is coordinated to two framework-oxygens (2.44, 2.46 A) and two water-oxygens (2.34, 2.35 A) in a square and to two framework-oxygens (2.61, 2.65 A) on one side of the square. Some Sr may substitute in the Ca site. There is no evidence for ordered occupancy of Ca sites. All protons are in positions corresponding to electrostatic equilibrium. Those associated with Na,Ca lie close to the shared square, while those associated with Ca are almost coplanar with the cation and the water-oxygen. The distances from the proton to the water- and framework-oxygens are negatively correlated for edingtonite and thomsonite.

Neutron diffraction structural study of pyroelectric Li2SO4⋅H2O at 293, 80, and 20 K

Lithium sulfate monohydrate has been studied at 293, 80, and 20 K by neutron diffraction using the same ground spherical crystal. Li2SO4⋅H2O is monoclinic, space group P21, with lattice dimensions at 293 K of a=5.450(3), b=4.872(3), c=8.164(4) A, β=107.31(3)°; at 80 K: a=5.449(2), b=4.832(2), c=8.141(2) A, β=107.21(1)°; and at 20 K: a=5.449(2), b=4.832(2), c=8.137(2) A, β=107.19(1)°. There are two formula units in the unit cell. Least‐squares refinement based on 1749 (293 K), 1712 (80 K), and 880 (20 K) reflections gave final R(F) values of 0.022, 0.016, and 0.014, respectively. All structure factors were corrected for thermal diffuse scattering. A Gram–Charlier expansion of the probability density function was used to describe the thermal vibrations of the water molecule which at 293 K are consistent with a previously postulated flipping motion. The O(W)⋅⋅⋅O(W) hydrogen bond distance is 0.091(3) A shorter and the O(W)⋅⋅⋅O(W)⋅⋅⋅O(W) angle is 3.9(1)° larger at 20 K than at 293 K: the O(W)⋅⋅⋅O(1) distance t...

Single-crystal neutron diffraction study of partially dehydrated laumontite at 15 K☆

Abstract The structure of laumontite [(K0.10Na0.30Ca3.60)Si16.40Al7.60O48.n H2O; Z = 1] was refined from low-temperature neutron diffraction data collected at the BNL HFBR. Two crystals were used. One (A) was cooled to 15 K from its equilibrium condition at room temperature and humidity; the other (B) had been soaked in water for a few hours and then sealed in a quartz capillary before cooling, in an attempt to obtain the fully hydrated variety laumontite. The structure refinements, however, proved both crystals to be partially hydrated with 13.4 and 14.2 water molecules per unit cell, respectively. The refinements (A:a = 14.690(4), b = 13.061(3), c = 7.574(2) A , β = 112.01(2)°; B:a = 14.686(10), b = 13.133(8), c = 7.533 (5) A , β = 111.20(4)°; C 2/m) converged to final agreement factors of R(F2) = 0.084 (A, 2136 reflections) and R(F2) = 0.138 (B, 1784 reflections). An essentially ordered Si,Al distribution on framework tetrahedral sites was inferred from the tetrahedral distances with a minor amount of Si in the Al site in agreement with the chemical analysis. One well-defined Ca site in the cavity was found, in spite of a remarkably high degree of disorder of the water molecules. Eight positions for water oxygens were located in A and 11 in B. Most water molecules are bonded to the Ca atom and are directed toward the center of the channel in a circular arrangement of partially occupied sites. Some of the water molecules are not bonded to the extraframework cations but lie in the center of the major structural channel. They bridge opposite Ca coordination spheres across the cavity and framework oxygens through a complex system of weak hydrogen bonds. The ease of dehydration in laumontite is related to the existence of water molecules not coordinated to the extraframework cations and to the tolerance of the Ca atom to a varying number of coordinated water molecules.

Structural temperature dependence in α‐lithium iodate: Neutron and x‐ray study between 20 and 500 K

The crystal structure of hexagonal α‐LiIO3 has been determined by neutron and x‐ray diffraction at ten temperatures from 20 to 500 K. Four sets (1248 to 1622 reflections per set) of neutron data at λ=1.0467 A were measured at the Brookhaven High Flux Beam Reactor at 20, 100, 200, and 295 K. The integrated intensities of a further 13 data sets (1706 to 2963 reflections per set) were measured using MoKα radiation on a CAD‐4 x‐ray diffractometer. The structure at each temperature was refined by least squares in space group P63. The final agreement factors fell between 0.023 and 0.050. The unit cell dimensions at 295 K are a=5.481 27(6), c=5.171 65(4) A, with nonlinear thermal expansions. The principal change in structure with temperature is in the atomic positions of Li and O relative to I: The positions are given by O(z)T=−0.8435(3) A+62(4)×10−9 T2 A K−2 and Li(z)T=0.3808(10) A+167 (18)×10−9 T2 A K−2. The iodate ion has a constant I–O bond length of 1.8081(4) A, but the O–I–O angle increases from 99.97(2)° ...

NMR Study of the Motion of Water Molecules in the Natural Zeolite Bikitaite

Abstract Molecular mobility of water molecules as well as the possibilities for proton transport along the hydrogen bonded chains of water molecules have been investigated in the natural zeolite bikitaite (Li 2 Al 2 Si 4 O 12 ·2H 2 O) by 1 H NMR. Spin-lattice relaxation times in the laboratory and rotating frames ( T 1 and T 1 p ) and dipolar relaxation times ( T 1 D ) have been measured as a function of temperature for a polycrystalline sample. One dynamical process has been clearly identified as responsible for the relaxation in the temperature range studied (224–418 K): a 180° flip motion of the H 2 O molecule about its quasi-twofold axis. The activation energy and the pre-exponential factor in the Arrhenius relation for the correlation time of this motion are 30 kJ mol −1 and 8 × 10 −15 s, respectively. No evidence of any other dynamical process contributing to the proton relaxation is observed.

Neutron diffraction study of zoisite at 15 Κ and X-ray study at room temperature

The crystal structure of zoisite, Ca2Al3Si30i20H; Pnma, Ζ = 4, was determined for crystals of the tanzanite gem variety at 15 Κ (neutron diffraction, a = 16.218(3), b = 5.5406(14), c = 10.033(3) Ä) and room temperature, ~ 295 Κ (X-ray diffraction, a = 16.1909(15), b = 5.5466(5), c = 10.0323(6) A). Only one proton position was located with positional coordinates 0.2690(1), 0.25, 0.9753(2) and population factor 0.98(1) at 15 K. It forms a hydroxyl group with 0(10) at 0.986(2) A and a hydrogen bond with 0(4) at 1.757(2) A. There is no diffraction evidence of a second proton position to explain the weak infrared absorption at 2160 cm 1 at room temperature for both natural and synthetic zoisites; however, the detection level of 0.01 atom at the 2 σ level might be insufficient. The possibility of hydrogen bonding to two 0(2) at 2.96 A from O(IO) is discussed. Because the displacement ellipsoid of the proton at 15 Κ is oblate and smaller than the ones in staurolite, it is concluded that there is only a single center of motion of the proton. The wide ranges of Ca—Ο, Al—Ο and Si—Ο distances are consistent with valence balancing in which short * Joint appointment with the Intense Pulsed Neutron Source Division, Argonne National Laboratory, Argonne, IL 60439, USA. 306 J. V. Smith, J . J . Pluth, J . W. Richardson, Jr. and Ä. Kvick distances go to oxygen atoms with low values of formal bond strength. The displacement ellipsoids at 295 Κ can be explained qualitatively by anisotropic thermal motion constrained by the bonding to the nearest neighbors. There is an additional overall anisotropy at 15 Κ which is unexplained. However, values of Biia are consistent with zero-point motion, and there is no evidence for positional disorder.