High-resolution Synchrotron Powder Diffraction Data Research Articles

Li2[SeC2Se]·2NH3: A Crystalline Ammoniate with a –Se–C≡C–Se– Dianion

Reaction of Li2C2 with elemental selenium in a molar ratio of 1:2 in liquid ammonia led to the formation of the ammoniate Li2[SeC2Se]·2NH3. Its crystal structure was solved and refined from high-resolution synchrotron powder diffraction data (P21/c, Z = 4). It contains the -Se-C≡C-Se- anion, unprecedented in a crystalline material, whose existence was corroborated by IR/Raman spectra and electronic-structure theory, showing an almost perfect agreement with calculated spectra. Elaborated magnetic-bottle and velocity-map imaging photoelectron spectroscopic investigations show that the -Se-C≡C-Se• radical anion can be transferred to the gas phase, where it was analyzed by NIPE (Negative Ion Photoelectron) and VMI (Velocity-Map Imaging) spectra, which correlate nicely with simulated spectra based on 2Πu → 3Σg- and 2Πu → 1Σg+ transitions including spin-orbit couplings.

Irreversible Transition from GaO6 Octahedra to GaO4 Tetrahedra for Improved Electrochemical Stability in Ga-Doped Li(Ni0.9Co0.1)O2

Trace doping is an efficient way to improve the stability of nickel-rich layered cathodes for lithium-ion batteries, but the structural origin of such improvement, rather than a simple replacement, has been rarely explored. Herein, Ga dopants have been introduced into a nickel-rich host, LiNi0.9Co0.1O2, by a combination of coprecipitation and the solid-state sintering method. Structural analyses based on high-resolution synchrotron powder diffraction data and X-ray absorption spectra suggest that Ga preferentially sits in the NiO6 slabs, resulting in reduced Ni/Li cationic mixing and depressed lattice vibration. Due to the smaller dissociation energy of Ga-O bonds, some Ga3+ cations migrate first toward Li layers upon delithiation and form the GaO4 tetrahedral symmetry irreversibly during the electrochemical process, which acts as a pillar in the subsequent electrochemical processes. As a result, the doped material exhibits both improved cycling performance and rate capability under a high operating voltage (4.5 V) due to the stabilized structure in the lithiation/delithiation process. This study illustrates how a dopant enhances the electrochemical stability in views of both pristine and charged structure and suggests that a positive effect is expected from the dopant favoring the tetrahedral symmetry (e.g., Al).

Low temperature structures and magnetic interactions in the organic-based ferromagnetic and metamagnetic polymorphs of decamethylferrocenium 7,7,8,8-tetracyano-p-quinodimethanide, [FeCp*2]˙+[TCNQ]˙.

To identify the genesis of the differing magnetic behaviors for the ferro- (FO) and metamagnetic (MM) polymorphs of [FeCp*2][TCNQ] (Cp* = pentamethylcyclopentadienide; TCNQ = 7,7,8,8-tetracyano-p-quinodimethane) the low temperature (18 ± 1 K) structures of each polymorph were determined from high-resolution synchrotron powder diffraction data. Each polymorph possesses chains of alternating S = 1/2 [FeCp*2]˙+ cations and S = 1/2 [TCNQ]˙+, but with differing relative orientations. These as well as an additional paramagnetic polymorph do not thermally interconvert. In addition, the room and low (<70 ± 10 K) temperature structures of the MM polymorph, MMRT and MMLT, respectively, differ from that previously reported at 167 K (-106 °C) MM structure, and no evidence of either phase transition was previously noted even from the magnetic data. This transition temperature and enthalpy of this phase transition for MMRT⇌MM was determined to be 226.5 ± 0.4 K (-46.7 ± 0.4 °C) and 0.68 ± 0.04 kJ mol-1 upon warming, respectively, from differential calorimetry studies (DSC). All three MM phases are triclinic (P1[combining macron]) with the room temperature phase having a doubled unit cell relative to the other two. The lower temperature phase transition involves a small rearrangement of the molecular ions and shift in lattice parameters. These three MM and FO polymorphs have been characterized and form extended 1-D chains with alternating S = 1/2 [FeCp*2]˙+ cations, and S = 1/2 [TCNQ]˙- anions, whereas the fifth, paramagnetic (P) polymorph possesses S = 0 π-[TCNQ]22- dimers. At 18 ± 1 K the intrachain FeFe separations are 10.738(2) and 10.439(3) Å for the FO and MMLT polymorphs, respectively. The key structural differences between FO and MMLT at 18 ± 1 K are the 10% shorter interchain NN and the 2.8% shorter intrachain FeFe separation present for MMLT. Computational analysis of all nearest-neighbor spin couplings for the 18 K structures of FO and MMLT indicates that the intrachain [FeCp*2]˙+[TCNQ]˙- spin couplings (H = -2Si·Sj) are the strongest (4.95 and 6.5 cm-1 for FO and MMLT, respectively), as previously hypothesized, and are ferromagnetic due to their S = 1/2 spins residing in orthogonal orbitals. The change in relative [TCNQ]˙-[TCNQ]˙- orientations leads to a computed change from the ferromagnetic interaction (0.2 cm-1) for FO to an antiferromagnetic interaction (-0.1 cm-1) for MMLT in accord with its observed antiferromagnetic ground state. Hence, the magnetic ground state cannot be solely described by the dominant magnetic interactions.

Unravelling the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ mixed ionic electronic conductors.

Mixed ionic electronic conducting ceramics Nd6-y WO12-δ (δ is the oxygen deficiency) provide excellent stability in harsh environments containing strongly reactive gases such as CO2, CO, H2, H2O or H2S. Due to this chemical stability, they are promising and cost-efficient candidate materials for gas separation, catalytic membrane reactors and protonic ceramic fuel cell technologies. As in La6-y WO12-δ, the ionic/electronic transport mechanism in Nd6-y WO12-δ is expected to be largely controlled by the crystal structure, the conclusive determination of which is still lacking. This work presents a crystallographic study of Nd5.8WO12-δ and molybdenum-substituted Nd5.7W0.75Mo0.25O12-δ prepared by the citrate complexation route. High-resolution synchrotron and neutron powder diffraction data were used in combined Rietveld refinements to unravel the crystal structure of Nd5.8WO12-δ and Nd5.7W0.75Mo0.25O12-δ. Both investigated samples crystallize in a defect fluorite crystal structure with space group Fm 3 m and doubled unit-cell parameter due to cation ordering. Mo replaces W at both Wyckoff sites 4a and 48h and is evenly distributed, in contrast with La6-y WO12-δ. X-ray absorption spectroscopy as a function of partial pressure pO2 in the near-edge regions excludes oxidation state changes of Nd (Nd3+) and W (W6+) in reducing conditions: the enhanced hydrogen permeation, i.e. ambipolar conduction, observed in Mo-substituted Nd6-y WO12-δ is therefore explained by the higher Mo reducibility and the creation of additional - disordered - oxygen vacancies.

Ferrimagnetic Ordering and Anomalous Stoichiometry Observed for the Cubic, Extended 3D Prussian Blue Analogues (NEt3 Me)2 MnII 5 (CN)12 and (NEt2 Me2 )2 MnII 5 (CN)12 : A Cation-Adaptive Structure.

The reactions of MnII (O2 CCH3 )2 with NEt3 Me+ CN- and NEt2 Me2 + CN- form (NEt3 Me)2 MnII 5 (CN)12 (1) and (NEt2 Me2 )2 MnII 5 (CN)12 (2), respectively. Structure model-building and Rietveld refinement of high-resolution synchrotron powder diffraction data revealed a cubic [a=24.0093 Å (1), 23.8804 Å (2)] 3D extended structural motif with adjacent tetrahedral and octahedral MnII sites in a 3:2 ratio. Each tetrahedral MnII site is surrounded by four low-spin octahedral MnII sites, and each octahedral MnII site is surrounded by six high-spin tetrahedral MnII sites; adjacent sites are antiferromagnetically coupled in 3D. Compensation does not occur, and magnetic ordering as a ferrimagnet is observed at Tc =13 K for 2 based on the temperature at which remnant magnetization, Mr (T)→0. The hysteresis has an unusual constricted shape with inflection points around 50 and 1.2 kOe with a 5 K coercivity of 16 Oe and remnant magnetization, Mr , of 2050 emuOe mol-1 . The unusual structure and stoichiometry are attributed to the very ionic nature of the high-spin N-bonded MnII ion, which enables the maximization of the attractive van der Waals interactions through minimization of void space via a reduced ∠ MnNC. This results in an additional example of the Ax MnII y (CN)x+2y (x=0, y=1; x=1, y=3; x=2, y=1; x=2, y=2; x=2, y=3; x=3, y=5; and x=4, y=1) family of compounds possessing an unprecedented stoichiometry and lattice motif that are cation adaptive structured materials.

Cs2Zn(CN)4: a first example of a non-cyano spinel of composition A2M(CN)4 with A=alkali metal and M=group 12 metal

Abstract The crystal structure of monoclinic Cs2Zn(CN)4 (C2/c, Z = 4) was solved and refined from high-resolution synchrotron powder diffraction data (ESRF: Swiss-Norwegian beamline). In contrast to all other known cyanides of composition A2M(CN)4 with A = alkali metal and M = group 12 metal, which crystallize in cubic or rhombohedrally distorted spinel variants and thus with A+ in an octahedral coordination, the Cs+ cation in Cs2Zn(CN)4 shows an eight-fold coordination by CN− anions of the [Zn(CN)4]2− tetrahedra. Upon heating, no phase transition is observed. Instead, a reversible melting at approx. T = 380°C occurs.

Metal-Organic Frameworks as Hosts for Fluorinated Azobenzenes: A Path towards Quantitative Photoswitching with Visible Light.

Fifteen new photochromic hybrid materials were synthesized by gas phase loading of fluorinated azobenzenes, namely ortho-tetrafluoroazobenzene (tF-AZB), 4H,4H'-octafluoroazobenzene (oF-AZB), and perfluoroazobenzene (pF-AZB), into the pores of the well-known metal-organic frameworks MOF-5, MIL-53(Al), MIL-53(Ga), MIL-68(Ga), and MIL-68(In). Their composition was analysed by elemental (CHNS) and DSC/TGA. For pF-AZB0.34 @MIL-53(Al), a structural model based on high-resolution synchrotron powder diffraction data was developed and the host-guest and guest-guest interactions were elucidated from this model. These interactions of O-H⋅⋅⋅F and π⋅⋅⋅π type were confirmed by significant shifts of the O-H frequencies in loaded and unloaded MOFs of the MIL-53 and MIL-68 series. Most remarkably, all of the synthesized F-AZB@MOF systems can be switched with visible light, and some of them show almost quantitative (>95 %) photo-isomerization between its E and Z forms with no significant fatigue after repeated switching cycles.

Anomalous Stoichiometry and Antiferromagnetic Ordering for the Extended Hydroxymanganese(II) Cubes/Hexacyanometalate-Based 3D-Structured [MnII 4 (OH)4 ][MnII (CN)6 ](OH2 )6 ⋅H2 O.

The reaction of MnII (O2 CMe)2 and NaCN or LiCN in water forms a light green insoluble material. Structural solution and Rietveld refinement of high-resolution synchrotron powder diffraction data for this unprecedented, complicated compound of previously unknown composition revealed a new alkali-free ordered structural motif with [MnII 4 (μ3 -OH)4 ]4+ cubes and octahedral [MnII (CN)6 ]4- ions interconnected in 3D by MnII -N≡C-MnII linkages. The composition is {[MnII (OH2 )3 ][MnII (OH2 )]3 }(μ3 -OH)4 ][MnII (μ-CN)2 (CN)4 ]⋅H2 O=[MnII 4 (μ3 -OH)4 (OH2 )6 ][MnII (μ-CN)2 (CN)4 ]⋅H2 O, which is further simplified to [Mn4 (OH)4 ][Mn(CN)6 ](OH2 )7 (1). 1 has four high-spin (S=5/2) MnII sites that are antiferromagnetically coupled within the cube and are antiferromagnetically coupled to six low-spin (S=1/2) octahedral [MnII (CN)6 ]4- ions. Above 40 K the magnetic susceptibility, χ(T), can be fitted to the Curie-Weiss expression, χ ∝(T-θ)-1 , with θ=-13.4 K, indicative of significant antiferromagnetic coupling and 1 orders as an antiferromagnet at Tc =7.8 K.

Ca12 [Mn19 N23 ] and Ca133 [Mn216 N260 ]: Structural Complexity by 2D Intergrowth.

Two new calcium nitridomanganates, Ca12 [Mn19 N23 ] (P3, a=11.81341(3) Å, c=5.58975(2) Å, Z=1) and Ca133 [Mn216 N260 ] (P3‾ , a=39.477(1) Å, c=5.5974(2) Å, Z=1), were obtained by a gas-solid reaction of Ca3 N2 and Mn with N2 at 1273 K and 1223 K, respectively. The crystal structure of Ca12 [Mn19 N23 ] was determined from high-resolution X-ray synchrotron powder diffraction data, whereas single-crystal X-ray diffraction was employed to establish the crystal structure of the Ca133 [Mn216 N260 ] phase, which classifies as a complex metallic alloy (CMA). Both crystal structures have 2D nitridomanganate layers containing similar building blocks but of different levels of structural complexity. Bonding analysis as well as magnetic susceptibility and electron spin resonance measurements revealed that only a fraction of the Mn atoms in both structures carries a localized magnetic moment, while for most Mn species the magnetism is quenched as a result of metal-metal bond formation.

Anomalous Thermal Expansion of HoCo0.5Cr0.5O3 Probed by X-ray Synchrotron Powder Diffraction

Mixed holmium cobaltite-chromite HoCo0.5Cr0.5O3 with orthorhombic perovskite structure (structure type GdFeO3, space group Pbnm) was obtained by solid state reaction of corresponding oxides in air at 1373 K. Room- and high-temperature structural parameters were derived from high-resolution X-ray synchrotron powder diffraction data collected in situ in the temperature range of 300–1140 K. Analysis of the results obtained revealed anomalous thermal expansion of HoCo0.5Cr0.5O3, which is reflected in a sigmoidal temperature dependence of the unit cell parameters and in abnormal increase of the thermal expansion coefficients with a broad maxima near 900 K. Pronounced anomalies are also observed for interatomic distances and angles within Co/CrO6 octahedra, tilt angles of octahedra and atomic displacement parameters. The observed anomalies are associated with the changes of spin state of Co3+ ions and insulator-metal transition occurring in HoCo0.5Cr0.5O3.

Silver Indium Telluride Semiconductors and Their Solid Solutions with Cadmium Indium Telluride: Structure and Physical Properties.

Ag0.8In2.4Te4 (= AgIn3Te5) and Ag0.5In2.5Te4 (= AgIn5Te8) form solid solutions with CdIn2Te4, which are interesting as materials for photovoltaics or with respect to their thermoelectric properties. The corresponding crystal structures are related to the chalcopyrite type. Rietveld refinements of high-resolution synchrotron powder diffraction data measured at K-absorption edges of Cd, Ag, In, and Te and electron diffraction reveal the symmetry as well as the element and vacancy distribution in Ag0.8In2.4Te4 (= AgIn3Te5)/Ag0.5In2.5Te4 (= AgIn5Te8) mixed crystals such as Ag0.25Cd0.5In2.25Te4 and Ag0.2Cd0.75In2.1Te4. All compounds of the solid solution series (CdIn2Te4)x(Ag0.5In2.5Te4)1-x exhibit the HgCu2I4 structure type (space group I4̅2m) with completely ordered vacancies but disordered cations. The uniform cation distribution and thus the local charge balance are comparable to that of CdIn2Te4. In contrast, Ag0.8In2.4Te4 (= AgIn3Te5) crystallizes in the space group P4̅2c with disordered cations and partially ordered vacancies. This is corroborated by bond-valence sum calculations and the fact that there is a Vegard-like behavior for compounds with 0.5 < x in the pseudobinary system (CdIn2Te4)x(Ag0.8In2.4Te4)1-x. Owing to the different structures, there is no complete solid solution series between CdIn2Te4 and AgIn3Te5. All compounds in this work are n-type semiconductors with a low electrical conductivity (∼1 S/m) and rather high absolute Seebeck coefficients (up to -750 μV/mK; 225 °C). Electrical band gaps (Eg) determined from the Seebeck coefficients as well as (more reliably) from the electrical conductivity range between 0.19 and 1.13 eV.

Structural properties and phase diagram of the La(Fe1−xRux)AsO system

Structural refinement, lattice micro-strain and spontaneous strain analyses have been carried out on selected members of the La(Fe1−xRux)AsO system using high-resolution neutron and synchrotron powder diffraction data. The obtained results indicate that the character of the tetragonal to orthorhombic structural transition changes from first order for x = 0.10, possibly to tricritical for x = 0.20, up to second order for x = 0.30; for x ≥ 0.40 symmetry breaking is suppressed, even though a notable increase of the lattice micro-strain develops at low temperature. By combining structural findings with previous muon spin rotation data, a phase diagram of the La(Fe1−xRux)AsO system has been drawn. Long-range ordered magnetism occurs within the orthorhombic phase (x ≤ 0.30), whereas short-range magnetism appears to be confined within the lattice strained region of the tetragonal phase up to x < 0.60. Direct comparison between the magnetic and structural properties indicates that the magnetic transition is always associated with structural symmetry breaking, although confined to a local scale at high Ru contents.

Low-temperature Debye–Scherrer powder diffraction on Beamline I11 at Diamond

A bespoke capillary sample holder is described that attaches to the cold head of a commercially manufactured (PheniX) closed-cycle helium cryostat originally intended for flat-plate geometry. The new holder allows high-resolution synchrotron powder diffraction data to be collected from samples in Debye–Scherrer geometry over the temperature range 11–295 K. To demonstrate that high-quality powder data can be obtained using this new sample holder, structural refinement (Rietveld) and thermal expansion results measured from reference samples (Si and Al) are presented.

Metal–Organic Frameworks as Hosts for Photochromic Guest Molecules

Several metal-organic framework compounds (MOF-5, MIL-68(Ga), MIL-68(In), MIL-53(Al)) were loaded with azobenzene (AZB), as confirmed by XRPD measurements and elemental analysis. By IR spectroscopy, it was shown that the light-induced trans/cis isomerization of AZB in these hybrid host-guest compounds is improved compared to that of solid AZB. A population of the excited cis state up to 30% has been obtained for AZB0.66@MIL-68(In). However, no light-induced trans/cis isomerization was observed for AZB0.5@MIL-53(Al). Structural models obtained from high-resolution synchrotron powder diffraction data show that AZB molecules are densely packed within the channels of MIL-53(Al) so that no trans/cis isomerization can occur. A different situation was observed for AZB in the larger channels of MIL-68(Ga). Thus, this investigation shows the influence of the host material on the switching behavior of the embedded AZB molecules.

Plimerite from Krásno near Horní Slavkov ore district, Czech Republic

The Zn-dominant analogue of rockbridgeite, recently described mineral plimerite, was found in samples from the Huber open pit, the Krasno near Horn Slavkov ore district, western Bohemia, Czech Republic. Plimerite occurs there in five morphologically different types, as black-green to black semi-spherical to spheroidal radiating aggregates (type 1), olive to grey-green aggregates formed by minute tabular crystals (type 2), dark green crystalline aggregates composed of tiny tabular crystals (type 3) and two types (4 and 5) of microscopic aggregates. All plimerite types were found in cavities of strongly altered original triplite accumulations; fluorapatite, isokite, kolbeckite, leucophosphite, beraunite, whitmoreite, kunatite and other rare phosphates were found in the association. Detailed mineralogical data (EPMA, TG, IR, Raman) are given for plimerite type 1. Plimerite from Krasno is orthorhombic, space group Bbmm, with unit-cell parameters a 13.850(1), b 16.798(1), c 5.1581(3) angstrom and V 1200.1(2) angstrom3, Z = 4. The crystal structure was refined by the Rietveld method from the high-resolution synchrotron powder diffraction data to indices Rp 13.57%, Rwp 18.59% and R(Bragg) 13.92% for 140 observed reflections. The refined crystal structure of plimerite from Krasno is related with those of rockbridgeite and of plimerite from Australia; the main differences are connected with the site-occupancies, derived from the site-symmetries and bond-valence parameters.

Structural Studies on the Basic Ionic Liquid 1-Ethyl-1,4-diazabicyclo[2.2.2]octanium Bis(trifluoromethylsulfonyl)imide and Its Bromide Precursor

The structure of 1-ethyl-1,4-diazabicyclo[2.2.2]octanium bis(trifluoromethylsulfonyl)imide ([C2DABCO][NTf2]) was determined by single crystal X-ray diffraction, and its behavior with temperature was examined by synchrotron powder diffraction, differential scanning calorimetry (DSC), and Raman spectroscopy. [C2DABCO][NTf2] undergoes a solid–solid phase transition at 309 K and a solid–liquid (melting) transition at 350 K, as reported previously in the literature. The crystal structure of [C2DABCO][NTf2] was determined at temperatures below the solid–solid transition (solid-I phase), whereas in situ synchrotron powder diffraction enabled the unit cell of the solid phase observed above the solid–solid transition temperature (solid-II phase) to be indexed. High-resolution synchrotron powder diffraction data indicated the coexistence of a metastable phase with the solid-I and solid-II phases around the temperature of the solid–solid phase transition. A variable temperature Raman study suggested rotational movem...

Structure of a new high-pressure–high-temperature modification of antimony(III) oxide, γ-Sb2O3, from high-resolution synchrotron powder diffraction data

A quenchable new high-pressure-high-temperature modification of antimony(III) oxide, γ-Sb(2)O(3), has been obtained at hydrostatic pressures of 9-11 GPa and temperatures of 573-773 K. Its crystal structure has been determined from high-resolution synchrotron powder diffraction data. γ-Sb(2)O(3) consists of three-dimensionally cross-linked infinite chains of SbO(3)E units (E = lone pair) with the chains forming tetragonal rod-packing. The underlying topology of γ-Sb(2)O(3) (3,3T8) is found very rarely in inorganic structures; it is realised only for the polyanion [Si(4)O(4)N(6)](10-) that occurs in the Ce(4)(Si(4)O(4)N(6))O structure type. The structural relation to the two previously known polymorphs of Sb(2)O(3) at ambient pressure, valentinite and senarmontite is discussed.

Determination of the crystal structure of nifedipine form C by synchrotron powder diffraction

The crystal structure of the metastable form C polymorph of nifedipine [C(17)H(18)N(2)O(6), 3,5-dimethyl 2,6-dimethyl-4-(2-nitrophenyl)-1,4-dihydropyridine-3,5-dicarboxylate] was determined by means of direct-space techniques applied to high-resolution synchrotron powder diffraction data. The polymorph crystallizes in the space group P1 and exhibits a molecular packing significantly different from that of the stable modification, with molecules aligned in an orthogonal configuration inside the unit cell. The molecular conformation, on the other hand, remains substantially unmodified between the two polymorphs. Additionally, in situ thermal characterization of nifedipine crystallization behaviour was performed, confirming the nucleation of another metastable polymorph (form B) prior to the complete crystallization of the stable modification. A complete structural characterization of form B was not possible owing to its very limited stability interval.

Unprecedented Zeolite-Like Framework Topology Constructed from Cages with 3-Rings in a Barium Oxonitridophosphate

A novel oxonitridophosphate, Ba(19)P(36)O(6+x)N(66-x)Cl(8+x) (x ≈ 4.54), has been synthesized by heating a multicomponent reactant mixture consisting of phosphoryl triamide OP(NH(2))(3), thiophosphoryl triamide SP(NH(2))(3), BaS, and NH(4)Cl enclosed in an evacuated and sealed silica glass ampule up to 750 °C. Despite the presence of side phases, the crystal structure was elucidated ab initio from high-resolution synchrotron powder diffraction data (λ = 39.998 pm) applying the charge flipping algorithm supported by independent symmetry information derived from electron diffraction (ED) and scanning transmission electron microscopy (STEM). The compound crystallizes in the cubic space group Fm ̅3c (no. 226) with a = 2685.41(3) pm and Z = 8. As confirmed by Rietveld refinement, the structure comprises all-side vertex sharing P(O,N)(4) tetrahedra forming slightly distorted 3(8)4(6)8(12) cages representing a novel composite building unit (CBU). Interlinked through their 4-rings and additional 3-rings, the cages build up a 3D network with a framework density FD = 14.87 T/1000 Å(3) and a 3D 8-ring channel system. Ba(2+) and Cl(-) as extra-framework ions are located within the cages and channels of the framework. The structural model is corroborated by (31)P double-quantum (DQ) /single-quantum (SQ) and triple-quantum (TQ) /single-quantum (SQ) 2D correlation MAS NMR spectroscopy. According to (31)P{(1)H} C-REDOR NMR measurements, the H content is less than one H atom per unit cell.

A re-examination of the structure of the germanosilicate zeolite SSZ-77

The zeolite framework structure of the germanosilicate SSZ-77 ([Si 45.7Ge 10.3O 112], C2/ m, a = 13.2090(3) Å, b = 13.3238(3) Å, c = 21.5551(5) Å, β = 123(2)°) has been re-examined using high-resolution synchrotron powder diffraction data collected with better counting statistics at higher angles. In the original Rietveld refinement, the geometry was strained and not all of the Ge atoms could be located. With the better data, both of these problems could be resolved. The powder charge-flipping algorithm in the program Superflip was used to determine that the space group should be reduced from C2/ m to C2. With this symmetry reduction, the Ge atoms could be located more correctly, and the geometry became less strained. In the final structure, one of the 14 T sites (tetrahedrally coordinated atoms) is pure Ge, four are mixed Ge/Si positions, and the remaining nine are pure Si. Diffuse scattering under the Bragg peaks is clearly evident in the background correction. This is consistent with a Si/Ge substitutional disorder in which there is a positive correlation between neighboring unit cells. That is, adjacent unit cells are more likely to have the same ordering, so the crystal contains domains of similar unit cells rather than a completely random ordering from unit cell to unit cell.