High-resolution Synchrotron Powder Research Articles

Ionic conductivity in the Mg intercalated fullerene polymer Mg2C60

We report the identification of the new intercalated fullerene polymer Mg2C60. Mg intercalation was obtained either by solid state reaction between C60 and Mg, or by thermal decomposition of the metallorganic precursor Mg–Anthracene–(THF)3. High resolution powder synchrotron and neutron diffraction data have clearly shown that Mg2C60 is isostructural to the superionic conductor Li4C60, where fullerenes form a two-dimensional network connected either by four-membered carbon rings, or single C–C bonds. Because of its peculiar structural arrangement Mg2C60 behaves as a good ionic conductor by means of uncorrelated ionic hopping across very small energy barriers (ΔE less than 100meV), as found from DC and AC conductivity measurements, thus suggesting its possible use in future Mg-ion batteries.

The crystal structure of tin sulphate, SnSO4, and comparison with isostructural SrSO4, PbSO4, and BaSO4

The crystal structure of tin (II) sulphate, SnSO4, was obtained by Rietveld refinement using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. The structure was refined in space group Pbnm. The unit-cell parameters for SnSO4 are a = 7.12322(1), b = 8.81041(1), c = 5.32809(1) Å, and V = 334.383(1) Å3. The average 〈Sn–O〉 [12] distance is 2.9391(4) Å. However, the Sn2+cation has a pyramidal [3]-coordination to O atoms and the average 〈Sn–O〉 [3] = 2.271(1) Å. If Sn is considered as [12]-coordinated, SnSO4 has a structure similar to barite, BaSO4, and its structural parameters are intermediate between those of BaSO4 and PbSO4. The tetrahedral SO4 group has an average 〈S–O〉 [4] = 1.472(1) Å in SnSO4. Comparing SnSO4 with the isostructural SrSO4, PbSO4, and BaSO4, several well-defined trends are observed. The radii, rM, of the M2+(=Sr, Pb, Sn, and Ba) cations and average 〈S–O〉 distances vary linearly with V because of the effective size of the M2+cation. Based on the trend for the isostructural sulphates, the average 〈Sn–O〉 [12] distance is slightly longer than expected because of the lone pair of electrons on the Sn2+cation.

Structural trends for celestite (SrSO4), anglesite (PbSO4), and barite (BaSO4): Confirmation of expected variations within the SO4 groups

The crystal structures of the isostructural orthorhombic sulfates celestite (SrSO 4 ), anglesite (PbSO 4 ), and barite (BaSO 4 ) were refined by Rietveld methods using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data. Their structural model was refined in space group Pbnm . The unit-cell parameters are a = 6.87032(3), b = 8.36030(5), c = 5.34732(1) A, and V = 307.139(3) A 3 for SrSO 4 ; a = 6.95802(1), b = 8.48024(3), c = 5.39754(1) A, and V = 318.486(1) A 3 for PbSO 4 ; and a = 7.15505(1), b = 8.88101(3), c = 5.45447(1) A, and V = 346.599(1) A 3 for BaSO 4 . The average [12] distances are 2.827(1), 2.865(1), and 2.953(1) A for SrSO 4 , PbSO 4 , and BaSO 4 , respectively, and their corresponding average [4] distances are 1.480(1), 1.477(3), and 1.471(1) A. The geometrical features of the SO 4 and MO 12 polyhedra become more symmetrical from SrSO 4 to BaSO 4 . Across the series, the a , b , and c parameters vary non-linearly with increasing V . The radii of the M 2+ cations, r M , [12], and [4] distances vary linearly with V . These structural trends arise from the effective size of the M 2+ cation (r M : Sr

Acetylene and argon adsorption in a supramolecular organic zeolite

The adsorption properties of a new nanoporous organic zeolite with respect to acetylene and Ar were studied by volumetric adsorption analysis, microcalorimetric experiments, and synchrotron high-resolution X-ray powder diffraction. This allowed us to locate the guest molecules inside the host channels and characterize the host-guest interactions.

The crystal structure of a biogenic aragonite from the nacre of an ammonite shell

The crystal structure of a biogenic aragonite from the nacre of an ammonite shell was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. The well-preserved ammonite sample is from Alberta, Canada, and is from the Cretaceous period. The aragonite structure was refined in space groupPmcn, Z = 4, and the cell parameters obtained are a = 4.96265(2), b = 7.97016(4), c = 5.74474(3) A, and V = 227.222(2) A3. The chemical analyses indicate a formula of [Ca0.995Sr0.004Ba0.001]∑=1.0(CO3). The average and distances are 2.5281(3) and 1.2871(6) A, respectively, and the average angle is 119.94(8)°. The CO3 groups are non-planar. Based on crystal-structure data for biogenic and non-biogenic aragonite samples, aragonite from ammonite nacre has minimal structural distortions and is very similar to non-biogenic aragonite, in particular, a sample from Spain.

Crystal-structure analysis of four mineral samples of anhydrite, CaSO4, using synchrotron high-resolution powder X-ray diffraction data

The crystal structures of four samples of anhydrite, CaSO4, were obtained by Rietveld refinements using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and space group Amma. As an example, for one sample of anhydrite from Hants County, Nova Scotia, the unit-cell parameters are a = 7.00032(2), b = 6.99234(1), c = 6.24097(1) Å, and V = 305.487(1) Å3 with a > b. The eight-coordinated Ca atom has an average <Ca-O> distance of 2.4667(4) Å. The tetrahedral SO4 group has two independent S-O distances of 1.484(1) to O1 and 1.478(1) Å to O2 and an average <S-O> distance of 1.4810(5) Å. The three independent O-S-O angles [108.99(8) × 1, 110.38(3) × 4, 106.34(9)° × 1; average <O-S-O> [6] = 109.47(2)°] and S-O distances indicate that the geometry of the SO4 group is quite distorted in anhydrite. The four anhydrite samples have structural trends where the a, b, and c unit-cell parameters increase linearly with increasing unit-cell volume, V, and their average <Ca-O> and <S-O> distances are nearly constant. The grand mean <Ca-O> = 2.4660(2) Å, and grand mean <S-O> = 1.4848(3) Å, the latter is longer than 1.480(1) Å in celestite, SrSO4, as expected.

The structures of marialite (Me6) and meionite (Me93) in space groupsP42/nandI4/m, and the absence of phase transitions in the scapolite series

The crystal structures of marialite (Me6) from Badakhshan, Afghanistan and meionite (Me93) from Mt. Vesuvius, Italy were obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements. Their structures were refined in space groupsI4/mandP42/n, and similar results were obtained. TheMe6sample has a formula Ca0.24Na3.37K0.24[Al3.16Si8.84O24]Cl0.84(CO3)0.15, and its unit-cell parameters area=12.047555(7),c=7.563210(6) Å, andV=1097.751(1) Å3. The average ⟨T1-O⟩ distances are 1.599(1) Å inI4/mand 1.600(2) Å inP42/n, indicating that the T1 site contains only Si atoms. InP42/n, the average distances of ⟨T2-O⟩=1.655(2) and ⟨T3-O⟩=1.664(2) Å are distinct and are not equal to each other. However, the mean ⟨T2,3-O⟩=1.659(2) Å inP42/nand is identical to the ⟨T2′-O⟩=1.659(1) Å inI4/m. The ⟨M-O⟩ [7]=2.754(1) Å (M site is coordinated to seven framework O atoms) and M-A=2.914(1) Å; these distances are identical in both space groups. TheMe93sample has a formula of Na0.29Ca3.76[Al5.54Si6.46O24]Cl0.05(SO4)0.02(CO3)0.93, and its unit-cell parameters area=12.19882(1),c=7.576954(8) Å, andV=1127.535(2) Å3. A similar examination of theMe93sample also shows that both space groups give similar results; however, the C–O distance is more reasonable inP42/nthan inI4/m. Refining the scapolite structure nearMe0orMe100inI4/mforces the T2 and T3 sites (both with multiplicity 8 inP42/n) to be equivalent and form the T2′ site (with multiplicity 16 inI4/m), but ⟨T2-O⟩ is not equal to ⟨T3-O⟩ inP42/n. Using different space groups for different regions across the series implies phase transitions, which do not occur in the scapolite series.

Thermal Stability of High Performance Thermoelectric β‐Zn4Sb3 in Argon

AbstractThe stability of the high performance thermoelectric β‐Zn4Sb3 is investigated by multi‐temperature high resolution synchrotron powder X‐ray diffraction. Contrary to previous claims β‐Zn4Sb3 is not stable when heated in argon and degradation is observed already at 500 K. Nevertheless, the thermal stability of β‐Zn4Sb3 is much improved in argon (ca. 5 wt.‐% degradation after three heating cycles to 625 K) relative to heating in atmospheric air (ca. 60 wt.‐% degradation). Based on identification of the decomposition products two specific degradation processes are proposed, and the excellent stability of β‐Zn4Sb3 synthesized by zone melting (equilibrium method) is discussed in relation to the poor stability of material synthesized by thermal quenching (non‐equilibrium process). The room temperature peak widths and unit cell parameters change significantly after three heating cycles to 625 K suggesting that the β‐Zn4Sb3 phase diagram could be more complex than currently believed.

COMPLETE Al-Si ORDER IN SCAPOLITE Me37.5, IDEALLY Ca3Na5[Al8Si16O48]Cl(CO3), AND IMPLICATIONS FOR ANTIPHASE DOMAIN BOUNDARIES (APBs)

The structure of an intermediate scapolite (Me{sub 36.6}) from Lake Clear, Ontario, was obtained using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement in space group P4{sub 2}/n. The chemical formula obtained by electron microprobe is Na{sub 2.19}Ca{sub 1.35}K{sub 0.16}[Al{sub 3.95}Si{sub 8.05}O{sub 24}]Cl{sub 0.55}(CO{sub 3}){sub 0.41}(SO{sub 4}){sub 0.04}, equivalent to Me{sub 36.6}. The unit-cell parameters are a 12.07899(1), c 7.583467(9) {angstrom}, and V 1106.443(2) {angstrom}{sup 3}. The average distances are = 1.617(1) {angstrom}, = 1.744(1) {angstrom}, and = 1.601(1) {angstrom}. Therefore, the T1 and T3 sites contain only Si atoms, and the T2 site contains only Al atoms, so the Al and Si atoms are completely ordered. Complete Al-Si order was predicted for Me{sub 37.5}, ideally Ca{sub 3}Na{sub 5}[Al{sub 8}Si{sub 16}O{sub 48}]Cl(CO{sub 3}), and is confirmed in this study. Antiphase domain boundaries (APBs) in scapolite cannot arise from Al-Si order because the average distances indicate complete Al-Si order in Me{sub 36.6}. If APBs were to arise from Al-Si order, switching of the T sites across the APBs will occur, and complete Al-Si order cannot be observed. Therefore, Al-Si order, which is present to various extents across the scapolite series, can be ruled outmore » as the cause for the APBs. Order involving Cl and CO{sub 3} is the cause for the APBs in scapolite.« less

A TWO-PHASE INTERGROWTH IN GENTHELVITE FROM MONT SAINT-HILAIRE, QUEBEC

Synchrotron high-resolution powder X-ray-diffraction (HRPXRD) data and Rietveld structure refinement were used to examine a two-phase intergrowth of genthelvite, ideally Zn{sub 8}[Be{sub 6}Si{sub 6}O{sub 24}]S{sub 2}, from the alkaline intrusive complex at Mont Saint-Hilaire, Quebec, where genthelvite occurs in nepheline syenite pegmatites. The structural parameters obtained for the two phases are slightly different from each other. The unit-cell parameters are 8.119190(7) {angstrom} (51% phase 1) and 8.128914(9) {angstrom} (49% phase 2). The refinement gives the chemical formulae and interstitial M cation in terms of Zn and Mn (the Fe content is practically zero) for the Mn-poor genthelvite phase 1 as (Zn{sub 7.8}Mn{sub 0.2}){Sigma}{sub 8}[Be{sub 6}Si{sub 6}O{sub 24}]S{sub 2}, and the Mn-rich genthelvite phase 2 as (Zn{sub 7.0}Mn{sub 1.0}){Sigma}{sub 8}[Be{sub 6}Si{sub 6}O{sub 24}]S{sub 2}. These formulae are comparable to the Mn-poor and Mn-rich phases obtained by electron-microprobe analysis. The intergrowth indicates that diffusion is absent among the interstitial M cations of similar size (Zn{sup 2+}, Fe{sup 2+} and Mn{sup 2+}). Such intergrowths may form under distinct f(O{sub 2}) conditions and probably low temperature of crystallization that inhibits diffusion of M cations.

Synthesis and structural characterization of the Ca2MnReO6 double perovskite

The Ca2Mn1Re1O6 double perovskite has been prepared in polycrystalline form by using the encapsulated quartz tube method. The partial oxygen pressure inside the quartz tube revealed this to be a crucial synthesis parameter for the production of a single structural phase sample. This parameter was controlled using the ratio between ReO2 and ReO3 content and the filling factor parameter (ratio between mass and total inner volume of the quartz tube). The morphology and chemical composition was investigated by scanning electron microscopy and energy dispersive X-ray spectroscopy. The crystal structure parameters were determined by analysis of the synchrotron high-resolution X-ray powder diffraction pattern. The analysis indicates that the sample is an ideal single-phase compound with a monoclinic crystal structure (space group P2(1)/n) with a = 5.44445(2) Å; b = 5.63957(3) Å; c = 7.77524(3) Å; and β = 90.18(1)º. Computer simulations were performed considering two cation valence configurations, namely, (i) Mn2+Re6+ or (ii) Mn3+Re5+, for the Ca2Mn1Re1O6 compound. XANES analysis measurements indicated +2.3 for the average valence of Mn (a mixture of Mn2+ and Mn3+) and +5.7 for the effective valence of Re (an intermediate valence between Re4+ (ReO2) and Re6+ (ReO3)). As a summary, we concluded there is a mixed valence configuration for Mn and Re in Ca2Mn1Re1O6 , taken into account the oxygen content of 6.0±0.1.

X-ray diffraction studies of Pompeian wall paintings using synchrotron radiation and dedicated laboratory made systems

The full identification of artwork materials requires not only elemental analysis but also structural information of the compounds as provided by X-ray diffraction (XRD). This is easily done when taking samples (or micro-samples) from artworks. However, there is an increasing interest in performing non-destructive studies that require adapted XRD systems. Comparative study of synchrotron high-resolution X-ray powder diffraction (SR-HRPD) and laboratory non-destructive systems (portable XRD and micro-XRD) is the main objective of this work. There are no qualitative differences among the three systems as for detected phases in the Pompeian wall paintings that were studied, except in the case of minority phases which only were detected by SR-HRPD. The identified pigments were goethite, hematite, cinnabar, glauconite, Pompeian blue, together with calcite, dolomite and aragonite. Synchrotron XRD diagrams show better resolution than the others. In general, the peak widths in the diagrams obtained with the portable XRD system are similar to those obtained by micro-diffraction equipment. Factors such as residual divergence of X-ray sources, incidence angle and slit or collimator size are discussed in relation with the quality of XRD diagrams.

NEPHELINE: STRUCTURE OF THREE SAMPLES FROM THE BANCROFT AREA, ONTARIO, OBTAINED USING SYNCHROTRON HIGH-RESOLUTION POWDER X-RAY DIFFRACTION

The crystal structure of three samples of nepheline (ideally, K{sub 2}Na{sub 6}[Al{sub 8}Si{sub 8}O{sub 32}]) from the Bancroft area of Ontario (1a, b: Egan Chute, 2: Nephton, and 3: Davis Hill), each with different types of superstructure reflections, has been studied using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. The samples have different origins. The structure was refined in space group P6{sub 3}. The R{sub F}{sup 2} index, number of unique observed reflections, pseudohexagonal subcell parameters, and site-occupancy factor (sof) for the K site are as follows: Sample 1b: R{sub F}{sup 2} = 0.0433, N{sub obs} = 1399, a = 9.99567(1), c = 8.37777(1) {angstrom}, V = 724.907(2) {angstrom}{sub 3}, and K (sof) = 0.716(1). Sample 2: R{sub F}{sup 2} = 0.0669, N{sub obs} = 1589, a = 10.00215(1), c = 8.38742(1) {angstrom}, V = 726.684(1) {angstrom}{sub 3}, and K (sof) = 0.920(1). Sample 3: R{sub F}{sup 2} = 0.0804, N{sub obs} = 1615, a = 9.99567(1), c = 8.37873(1) {angstrom}, V = 724.991(1) {angstrom}{sub 3}, and K (sof) = 0.778(2). Sample 2 has the largest sof for K and the largest volume. The satellite reflections in the three nepheline samples were observed in the HRPXRDmore » traces and give rise to different incommensurate superstructures. The Al and Si atoms in the T{sub 3} and T{sub 4} sites are ordered differently in the three samples, which may indicate the presence of a domain structure based on Al-Si order. The positions for the Al and Si atoms were interchanged in two samples because of the resulting distances. The slight excess of Si over Al atoms, characteristically encountered in well-analyzed samples of nepheline, is reflected in the distances.« less

THE ORTHORHOMBIC STRUCTURE OF CaCO3, SrCO3, PbCO3 AND BaCO3: LINEAR STRUCTURAL TRENDS

The crystal structures of four isostructural orthorhombic carbonates, CaCO{sub 3} (aragonite), SrCO{sub 3} (strontianite), PbCO{sub 3} (cerussite), and BaCO{sub 3} (witherite), were obtained by Rietveld refinements using data acquired by synchrotron high-resolution powder X-ray diffraction (HRPXRD). For BaCO{sub 3}, powder neutron-diffraction data were obtained and refined by the Rietveld method. For aragonite, we also carried out a refinement of the structure by single-crystal X-ray diffraction. These carbonates belong to the space group Pmcn, with Z = 4. The CO{sub 3} group is slightly non-planar, and the two independent C-O distances are slightly different. The CO{sub 3} group becomes more symmetrical and less aplanar from CaCO{sub 3} to BaCO{sub 3} (M{sub radii}{sup 2+}: Ca and distances, average angle, and aplanarity. These linear trends are the result of the effective size of the divalent ionic radius of the M cations thatmore » are coordinated to nine oxygen atoms. The geometrical features of the CO{sub 3} group can be obtained reliably only by using neutron-diffraction data, especially in the presence of other heavy atoms.« less

Spin-driven phase transitions inZnCr2Se4andZnCr2S4probed by high-resolution synchrotron x-ray and neutron powder diffraction

The crystal and magnetic structures of the spinel compounds ${\text{ZnCr}}_{2}{\text{S}}_{4}$ and ${\text{ZnCr}}_{2}{\text{Se}}_{4}$ were investigated by high-resolution powder synchrotron and neutron diffraction. ${\text{ZnCr}}_{2}{\text{Se}}_{4}$ exhibits a first-order phase transition at ${T}_{N}=21\text{ }\text{K}$ into an incommensurate helical magnetic structure. Magnetic fluctuations above ${T}_{N}$ are coupled to the crystal lattice as manifested by negative thermal expansion. Both the complex magnetic structure and the anomalous structural behavior can be related to magnetic frustration. Application of an external magnetic field shifts the ordering temperature and the regime of negative thermal expansion toward lower temperatures. Thereby, the spin ordering changes into a conical structure. ${\text{ZnCr}}_{2}{\text{S}}_{4}$ shows two magnetic transitions at ${T}_{N1}=15\text{ }\text{K}$ and ${T}_{N2}=8\text{ }\text{K}$ that are accompanied by structural phase transitions. The crystal structure transforms from the cubic spinel-like (space group $Fd\overline{3}m$) at high temperatures in the paramagnetic state, via a tetragonally distorted intermediate phase (space group $I{4}_{1}/amd$) for ${T}_{N2}lTl{T}_{N1}$ into a low-temperature orthorhombic phase (space group $Imma$) for $Tl{T}_{N2}$. The cooperative displacement of sulfur ions by exchange striction is the origin of these structural phase transitions. The low-temperature structure of ${\text{ZnCr}}_{2}{\text{S}}_{4}$ is identical to the orthorhombic structure of magnetite below the Verwey transition. When applying a magnetic field of 5 T the system shows an induced negative thermal expansion in the intermediate magnetic phase as observed in ${\text{ZnCr}}_{2}{\text{Se}}_{4}$.

INCREASE IN Al-Si AND Na-Ca DISORDER WITH TEMPERATURE IN SCAPOLITE Me32.9

Scapolite Me 32.9 (marialite) from Monmouth Township, Ontario, was studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinement of data obtained by heating and cooling from 26 to 902°C. The structure was refined in space group P 4 2 /n . A room-temperature structure, obtained with synchrotron high-resolution powder X-ray-diffraction (HRPXRD) data, has unit-cell parameters a 12.06503(1), c 7.58360(1) A, and V 1103.906(2) A 3 . Moreover, the average bond-distances T 1–O>, T 2–O> and T 3–O> are equal to 1.606(2), 1.726(2) and 1.612(2) A, respectively, and correspond to occupancies T 1 = [Al 0 Si 1 ], T 2 = [Al 0.87 Si 0.13 ], and T 3 = [Al 0.02 Si 0.98 ]. At 902°C, T 1–O>, T 2–O> and T 3–O> are equal to 1.591(5), 1.683(8) and 1.686(7) A, respectively, and correspond to T 1 = [Al 0 Si 1 ], T 2 = [Al 0.55 Si 0.45 ], and T 3 = [Al 0.57 Si 0.43 ]. At 902°C, the T 1 site remains fully occupied by Si atoms, whereas the disorder at the T 2 and T 3 sites is close to being complete. Several structural parameters show a break at 200°C, which corresponds to disorder of the Na and Ca (= M ) atoms. On cooling, the structure does not go back to the initial structural state; some T 2– T 3 and Na–Ca disorder are quenched in. The type- b reflections that give rise to antiphase-domain boundaries (APBs) are present at all temperatures; they arise from Cl–CO 3 order, instead of Al–Si order. At 902°C, except for the anions, there is no difference between the structure of Me 32.9 in space group P 4 2 /n and that of Me 80 in space group I 4/ m .

STATE-OF-THE-ART HIGH-RESOLUTION POWDER X-RAY DIFFRACTION (HRPXRD) ILLUSTRATED WITH RIETVELD STRUCTURE REFINEMENT OF QUARTZ, SODALITE, TREMOLITE, AND MEIONITE

Synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinements were used to examine the crystal structures of quartz, sodalite, tremolite, and meionite Me79.6, and compare them with those obtained by single-crystal diffraction (SXTL). The purpose is to illustrate that crystal structures obtained by HRPXRD are comparable to those obtained by SXTL. The comparisons indicate important and significant differences between the structures obtained by the two methods. The cell parameters obtained by Rietveld refinements using HRPXRD data appear of superior quality to those obtained by SXTL. The distances in pure SiO4 tetrahedron in quartz, sodalite, and tremolite are 1.6081(3), 1.6100(2), and 1.620(1) A, respectively. These values are affected by interstitial cations. In meionite Me79.6, the average and distances are 1.647(1) and 1.670(1) A, respectively, and they indicate that the occupancies are (Al0.28Si0.72) for T 1 where the atoms are partially ordered and (Al0.45Si0.55) for T 2 site where the atoms are nearly disordered, based on sodalite Si–O and Al–O distances of 1.6100(2) and 1.7435(2) A, respectively.

Unusual Al-Si ordering in calcic scapolite, Me79.6, with increasing temperature

A scapolite sample, Me79.6, from Slyudyanka, Siberia, Russia, has been studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinements on heating from 26 to 900 °C and on cooling to about 300 °C. The structure was modeled and refined in space group I4/m. An accurate room-temperature structure was also obtained by using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. From HRPXRD, the cell parameters are a = 12.16711(2), c = 7.575466(5) Å, and V = 1121.461(3) Å 3 ; <T1-O> and <T2-O> are 1.643(1) and 1.672(1), respectively, so the T1 (Al 0.25 Si 0.75 ) and T2 (Al 0.46 Si 0.54 ) sites are partially ordered at room temperature. On heating, the <T-O> distances indicate that the T1 and T2 sites become more Si- and Al-rich, respectively, and therefore, ordering increases unusually with increasing temperature. This increase in Al-Si ordering occurs from 892 to 900 °C. At 900 °C, the T1 site becomes fully ordered with only Si atoms, while the T2 site contains Al 0.51 Si 0.49 and therefore, is fully disordered. On cooling, the sample does not fully revert back to the original partially ordered state. At 300 °C, all the cell parameters are smaller because of the increased Al-Si ordering that is quenched in.

Structures of mono-unsaturated triacylglycerols. III. The β-2 polymorphs of trans-mono-unsaturated triacylglycerols and related fully saturated triacylglycerols

The beta-2 crystal structures of a series of saturated and trans-mono-unsaturated triacylglycerols (TAGs) have been solved from high-resolution powder synchrotron diffraction data. The series comprises symmetric as well as asymmetric even-numbered TAGs and the trans-mono-unsaturated ones all have a single elaidoyl chain. The structures have been solved with the direct-space parallel-tempering program FOX and refined with the Rietveld program GSAS. The beta-2 structures all crystallized in the space group P\bar 1 with the same molecular conformation. Within the resolution of the data no significant difference in packing or conformation is observed between trans-mono-unsaturated TAGs and saturated (stearoyl or palmitoyl) chain-containing analogues, in spite of the lower melting points of the former. An analysis of the position of the stepped methyl end-plane in the various subgroups of TAGs confirms most but not all suppositions found in the literature.

GAUDEFROYITE, Ca8Mn3+6[(BO3)6(CO3)2O6]: HIGH-TEMPERATURE CRYSTAL STRUCTURE

The structural behavior of gaudefroyite, Ca8Mn3+6[(BO3)6(CO3)2O6], was determined by using Rietveld refinements based on synchrotron powder X-ray-diffraction data from 25 to 486°C. The structure was also refined at 25°C using synchrotron high-resolution powder X-ray-diffraction (HRPXRD) data that gave the pseudohexagonal unit-cell parameters a 10.60791(2), c 5.88603(1) A, and V 573.605(2) A3 at room temperature. The expansion of the gaudefroyite structure is nearly isotropic and is caused mainly by an increase in the Ca1–O3 distance. The volume change from 25° to 486°C is 2.2(2)%. Beyond 486°C, significant changes were observed in the X-ray-diffraction traces; they indicate the formation of a minor second phase that exsolved from the host gaudefroyite.