Investigation of hydrothermal routes to mixed-metal cerium titanium oxides and metal oxidation state assignment using XANES

The reaction between TiF(3) or TiO(2) and Ce(3+) in sodium hydroxide solutions yields highly crystalline NaCeTi(2)O(6) at room temperature and under mild hydrothermal conditions (T < or = 240 degrees C). There is no evidence for the formation of ternary Ce-Ti-O materials by this method, and the use of bases other than NaOH always produces poorly crystalline materials. The material NaCeTi(2)O(6) has a distorted perovskite structure with sodium and cerium ions randomly occupying the A sites: Pnma, a = 5.4517(8) A, b = 7.7292(6) A, c =5.4573(3) A. XANES spectroscopy at the Ti K edge and Ce L(III) edge, with reference to crystalline model compounds, reveals that cerium is found solely as Ce(III) and titanium as Ti(IV) in NaCeTi(2)O(6). Isomorphous substitution of Ce(3+) by Nd(3+) or Ti(4+) by V(4+) is found to be very facile under hydrothermal conditions (at a temperature of 240 degrees C), by addition of appropriate amounts of metal salts to the hydrothermal reaction mixtures. The series NaCe(1-x)Nd(x)Ti2O6 (0 < or = x < or = 1) and NaCeTi(2-x)V(x)O6 (0 相似文献   

An open-framework thorium sulfate hydrate with 11.5 A voids

We report the synthesis of a thorium sulfate hydrate with 11.5 A open channels that propagate through the structure. The compound crystallizes in the tetragonal space group P4(2)/nmc, a = b = 25.890(4) A, c = 9.080(2) A, Z = 8, V = 6086.3(2) A(3). The thermal stability of the compound was investigated using thermogravimetric analysis and high-energy X-ray scattering (HEXS) revealing that the compound begins to undergo decomposition near 200 degrees C with an accompanied loss in crystallinity. The immediate coordination environment about the thorium atoms remains intact through heating to 500 degrees C as demonstrated by HEXS. Further heating reveals the formation of at least two crystalline phases, Th(SO4)2 and ThO2, which ultimately decompose to ThO2.     

Toward an intimate understanding of the structural properties and conformational preference of oxoesters and thioesters: gas and crystal structure and conformational analysis of dimethyl monothiocarbonate, CH3OC(O)SCH3

[structure: see text] The molecular structure and conformational properties of dimethyl monothiocarbonate, CH3OC(O)SCH3, have been studied in the gas phase by gas electron diffraction (GED) and vibrational spectroscopy and in the solid state by X-ray crystallography. The experimental investigations were supplemented by quantum chemical calculations at the B3LYP/6-311++G(3df,2p) and MP2/6-311++G(2df,p) levels of approximation. The gaseous molecule exhibits only one conformation having Cs symmetry with synperiplanar orientation of both the C-S and the C-O single bonds relative to the C=O double bond. The following skeletal geometric parameters were derived from the GED analysis (r(hl) values with 3sigma uncertainties): C=O = 1.203(4) A, C(sp(2))-O = 1.335(5) A, C(sp(3))-O = 1.437(5) A, C(sp(2))-S = 1.763(5) A, and C(sp(3))-S = 1.803(5) A; O=C-O = 125.9(8) degrees , O=C-S = 125.7(7) degrees , O-C-S = 108.4(9) degrees , and C-O-C = 113.4(15) degrees . The structure of a single crystal, grown by a miniature zone-melting procedure, was determined by X-ray diffraction analysis at a low temperature. The crystalline solid [monoclinic, P2(1)/n, a = 12.6409(9) A, b = 4.1678(3) A, and c = 19.940(1) A, beta = 98.164(1) degrees ] exists exclusively as molecules in the synperiplanar conformation and with geometrical parameters that agree with those of the molecule in the gas phase. The results are discussed in terms of anomeric and mesomeric effects and in terms of a natural bond orbital analysis.     

Low-temperature synthesis of uranium tetraboride by solid-state metathesis reactions

A novel synthesis of uranium tetraboride (UB(4)) by solid-state metathesis reaction is demonstrated. This approach significantly lowers the temperature required to synthesize this material to < or = 850 degrees C. When UCl(4) is reacted with 2 equiv of MgB(2) at 850 degrees C, crystalline UB(4) is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl(4) to UCl(3) as the initial step in the reaction. The UB(4) product is purified by washing with water.     

Metastable crystalline lamella of cetylpyridinium chloride in the Krafft transition

The metastable crystalline lamella was found in the Krafft transition of aqueous cetylpyridinium chloride (CPC) solutions. Temperature-dependent profiles of small-angle X-ray scattering (SAXS) for the CPC solution incubated for 10 min at 5 degrees C exhibited the metastable lamella structure with a lattice spacing of dL = 3.19 nm at temperatures below 12 degrees C and the stable lamella structure with a lattice spacing of dL = 2.85 nm at temperatures between 12 and 19 degrees C. The former lamella structure, however, was not observed in the temperature scanning SAXS profiles of the CPC solution incubated for 24 h at 5 degrees C. The latter lamella structure observed in the SAXS profile mentioned above started melting at 18 degrees C. The electric conductance change of the CPC solution with a time elapsed after dropping the temperature showed the existence of the temperature-dependent induction period in the Krafft transition, indicating high activation energies for the transition. In the differential scanning calorimetry measurements over temperatures ranging from 5 to 30 degrees C, a single endothermic enthalpy peak at 19 degrees C observed for the CPC solution incubated at 5 degrees C for a longer period than 6 h was split into double peaks at 14 and 19 degrees C when the same solution was incubated at 5 degrees C for a shorter period than 6 h. The observed calorimetric behavior is explained by the existence of the metastable crystalline state that grows faster and melts at a lower melting temperature than the stable crystalline state.     

Preparation and properties of methoxycarbonylsulfenyl isocyanate, CH(3)OC(O)SNCO

Pure methoxycarbonylsulfenyl isocyanate, CH3OC(O)SNCO, is quantitatively prepared by the metathesis reaction between CH3OC(O)SCl and AgNCO. This novel species has been obtained in its pure form and characterized by 1H and 13C NMR, UV-vis, FTIR, and FT-Raman spectroscopy. The conformational properties of the gaseous molecule have been studied by vibrational spectroscopy and quantum chemical calculations (B3LYP and MP2 methods). The compound exhibits a conformational equilibrium at room temperature having the most stable form CS symmetry with the C=O double bond synperiplanar with respect to the S-N single bond. A second form was observed in the IR spectrum and corresponds to a conformer possessing the C-S bond antiperiplanar with respect to the N=C double bond of the isocyanate group. The structure of a single crystal of CH3OC(O)SNCO was determined by X-ray diffraction analysis at low temperature using a miniature zone melting procedure. The crystalline solid (triclinic, P1, a = 8.292(6) A, b = 9.839(7) A, c = 11.865(8) A, alpha = 67.290(2) degrees , beta = 71.5570(10) degrees , gamma = 83.4850(10) degrees and Z = 6) shows the presence of molecules having exclusively a synperiplanar conformation with respect to the three phi(CO-C=O), phi(O=C-SN), and phi(CS-N=C) dihedral angles.     

High-pressure bulk synthesis of crystalline C(6)N(9)H(3).HCl: a novel c(3)n(4) graphitic derivative

A novel carbon nitride compound, structurally related to the proposed graphitic phase of C(3)N(4), has been synthesized in a bulk well-crystallized form. The new material, with stoichiometry C(6)N(9)H(4)Cl, was prepared through a solid-state reaction of 2,4,6-triamino-1,3,5-triazine with 2,4,6-trichloro-1,3,5-triazine at 1.0-1.5 GPa and 500-550 degrees C and also through a self-reaction of 2-amino-4,6-dichloro-1,3,5-triazine at similar conditions. X-ray and electron diffraction measurements on the yellowish compound indicate a hexagonal space group (P6(3)/m) with cell parameters of a = 8.4379(10) A and c = 6.4296(2) A. This new compound possesses a two-dimensional C(6)N(9)H(3) framework that is structurally related to the hypothetical P6m2 graphitic phase of C(3)N(4), but with an ordered arrangement of C(3)N(3) voids. The large voids in the graphene sheets are occupied by chloride ions with an equivalent number of nitrogen atoms on the framework protonated for charge balance. The composition of the sample was determined by bulk chemical analysis and confirmed by electron energy loss (EELS) spectroscopy. The chemical and structural model is consistent with bulk density measurements and with the infrared and (13)C NMR spectra. This work represents the first bulk synthesis of a well-characterized and highly crystalline material containing a continuous network of alternating carbon and nitrogen atoms.     

Synthesis of aluminium borate-boron oxide and binary titanium-boron and zirconium-boron oxides from metal alkoxides and (MeO)3B3O3 in non-aqueous solvents

The reaction of metal alkoxides M(OR)4 (M = Ti, Zr; R = organyl) with (MeO)3B3O3 (1 : 0.67) in dry propan-2-one at room temperature led to gels which when dried and calcined in air for 24 h at 500-1000 degrees C afforded bi-phased mixed-oxide materials formulated as 4TiO2 x 3B2O3 and ZrO2 x B2O3 in high ceramic yields and purity; the B2O3 phases of these materials were amorphous. The materials remained amorphous upon calcination at lower temperatures. The TiO2 phase of the 4TiO2 x 3B2O3 was crystalline when calcined at higher temperatures with either anatase (600 degrees C) or rutile (>800 degrees C) being obtained. The ZrO2 phase of the ZrO2 x B2O3 was crystalline when calcined at higher temperatures and was obtained as a metastable tetragonal phase (<700 degrees C) or baddeleylite (>800 degrees C). In a similar reaction, Al(O(i)Pr)3 (2 : 1) gave a bi-phased aluminium borate-boron oxide (Al18B4O(33).7B2O3) after calcination at >700 degrees C. The dried gels and oxide materials were all characterized by elemental analysis, TGA-DSC, and powder XRD.     

Tri-alpha-naphthylbenzene as a crystalline or glassy matrix for matrix-assisted laser desorption/ionization: a model system for the study of effects of dispersion of polymer samples at a molecular level

Tri-alpha-naphthylbenzene (TalphaNB) can exist as either a crystalline or glassy solid at ambient temperatures, making it a unique matrix in matrix-assisted laser desorption/ionization (MALDI) spectroscopy. Electrosprayed TalphaNB is crystalline and has a melting point of 180 +/- 2 degrees C, as measured by differential scanning calorimetry (DSC). A glass of TalphaNB is obtained upon heating above the crystalline melting point with a glass transition temperature of 68 +/- 2 degrees C having no remaining crystallinity. MALDI samples containing mass fraction 1% polystyrene (PS) are run in both the crystalline and amorphous states. In the crystalline state, there is a strong spectrum typical of PS, but upon melting and quenching to the glassy state, the MALDI signal disappears. If the transparent, amorphous sample is treated with 1-butanol, it becomes white, and the MALDI signal returns. DSC shows that the 1-butanol treatment leads to the return of some of the crystallinity. Small angle neutron scattering (SANS) shows that the crystalline state has large aggregations of PS while the amorphous state has molecularly dispersed PS molecules. MALDI gives strong signals only when there are large aggregations of polymer molecules, with individually dispersed molecules producing no signal.     

Thermal behavior and film formation from an organogermanium polymer/nanoparticle precursor

In situ high-resolution transmission electron microscopy (HRTEM) was used to investigate the effect of heating on an organo-Ge polymer/nanoparticle composite material containing 4-8 nm diameter alkyl-terminated Ge nanoparticles. The product was obtained from the reduction of GeCl4 with Na(naphthalide) with subsequent capping of the -Cl surface with n-butyl Grignard reagent. The in situ HRTEM micrographs show that the product undergoes significant changes upon heating from room temperature to 600 degrees C. Two pronounced effects were observed: (i) Ge nanoparticles coalesce and remain crystalline throughout the entire temperature range, and (ii) the organo-Ge polymer acts as a source for the in situ formation of additional Ge nanoparticles. The in situ-formed Ge nanoparticles are approximately 2-3 nm in diameter. These in situ-formed nanoparticles (2-3 nm) are so dense that, together with the original ones, they build up an almost continuous crystalline film in the temperatures between 300 and 500 degrees C. Above 480 degrees C, melting of the in situ formed Ge nanoparticles (2-3 nm) is observed, while nanoparticles greater than 5 nm remain crystalline. After cooling to room temperature, the 2-3 nm Ge nanoparticles recrystallized.     

Synthesis of platinum nanoparticles by reaction of filamentous cyanobacteria with platinum(IV)-chloride complex

Interaction of cyanobacteria (Plectonema boryanum UTEX 485) with aqueous platinum(IV)-chloride (PtCl(4) degrees ) has been investigated at 25-100 degrees C for up to 28 days, and 180 degrees C for 1 day. The addition of PtCl(4) degrees to the cyanobacteria culture initially promoted the precipitation of Pt(II)-organic material as amorphous spherical nanoparticles (< or =0.3 microm) in solutions and dispersed nanoparticles within bacterial cells. The spherical Pt(II)-organic nanoparticles were connected into long beadlike chains by a continuous coating of organic material derived from the cyanobacterial cells, and aged to nanoparticles of crystalline platinum metal with increase in temperature and reaction time. The stepwise reduction for the formation of platinum nanoparticles in the presence of cyanobacteria was deduced to be Pt(IV) [PtCl(4) degrees ] --> Pt(II) [Pt(II)-organics] --> Pt(0). Spherical platinum-bearing nanoparticles were not present in abiotic PtCl(4) degrees experiments conducted under similar conditions and duration.     

Coupling sol-gel synthesis and microwave-assisted techniques: a new route from amorphous to crystalline high-surface-area aluminium fluoride

A non-aqueous sol-gel Al-based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 degrees C) X-ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 degrees C) the metastable form beta-AlF3 was obtained with a very large surface area of 125 m2 g(-1). The mechanism of the amorphous=crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 degrees C) a multi-component material mixture exhibiting a huge surface area of 525 m2 g(-1) has been obtained and further investigated after mild post-treatment fluorination using F2 gas. The resulting aluminium-based fluoride still possesses a high-surface-area of 330 m2 g(-1). HRTEM revealed that the solid is built from large particles (50 nm) identified as alpha-AlF3, and small ones (10 nm), relative to an unidentified phase. This new high-surface-area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al-based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites.     

Rare-earth amidate complexes. Easily accessed initiators for epsilon-caprolactone ring-opening polymerization

The direct synthesis of yttrium amidate complexes using the simple reaction of amide proligands and Y(N(SiMe3)2)3 results in the high-yielding preparation and isolation of crystalline, monomeric materials. The complex, tris(N-2',6'-diisopropylphenyl(naphthyl)amidate)yttrium mono(tetrahydrofuran) (4), was structurally determined to be a 7-coordinate C1 symmetric structure, maintaining one bound tetrahydrofuran molecule. Compound 4 (C12H17[NCO]C10H7)3Y(C4H8O) crystallized in the monoclinic space group P2(1)/c with a = 13.7820(11) A, b = 33.598(3) A, c = 16.0575(12) A, alpha = 90 degrees, beta = 98.762(3) degrees, gamma = 90 degrees, Z = 4. Solution phase NMR spectroscopic characterization of this same complex showed a highly symmetric species, consistent with a fluxional coordination environment for these compounds. Preliminary studies into the initiation of epsilon-caprolactone ring-opening polymerization using these complexes indicate high activity, producing high molecular weight polymer.     

Novel titanium compounds for metal-organic chemical vapor deposition of titanium dioxide films with an ultrahigh deposition rate

A new titanium diolate compound Ti(mpd)(mdop)(2) (1) containing a beta-ketoester and a dimeric derivative [Ti(mpd)(mdop)(mu-OMe)](2) [2; mpd = 2-methyl-2,4-pentanediolate, mdop = (CH(3))(3)CC(O)C(-)HCOOCH(3)] have been synthesized and characterized by FT-IR, (1)H NMR, (13)C NMR, mass spectroscopy, and elemental analysis. Complex 2 was further characterized by X-ray structural analysis. Both 1 and 2 are fairly stable in air and in solvents such as tetrahydrofuran and toluene. They are also thermally stable and do not leave any residue during flash evaporation at around 280 degrees C. The new titanium complexes were used as precursors for the deposition of TiO(2) thin films by liquid-source metal-organic chemical vapor deposition. Compared with commercial Ti precursors, such as Ti(mpd)(tmhd)(2) (tmhd = 2,2,6,6-tetramethylheptanedionate) and Ti(OPr(i)())(2)(tmhd)(2), the new titanium complexes demonstrated a much higher deposition rate of TiO(2) film growth (3-6 times) at 400-475 degrees C. The deposited TiO(2) film from complex 2 was found to be in a crystalline anatase phase with a smooth surface morphology and low carbon content. Crystal data for 2: 233(2) K, a = 12.570(4) A, b = 13.817(4) A, c = 11.157(3) A, beta = 101.059(5) degrees, monoclinic, space group P2(1)/c, Z = 2.     

In situ studies of phase transitions in thin discotic films

The crystalline to liquid crystalline (Cr-LC) phase transition in thin films of zone-cast hexa-peri-hexabenzocoronene sixfold substituted with dodecyl side chains (HBC-C12H25) has been studied in detail using grazing incidence X-ray diffraction (GID), electron diffraction (ED), and variable angle spectroscopic ellipsometry (VASE), When heating the material, a first minor transition is observed around 42 degrees C. This change is attributed to alterations of the crystalline alkyl chain packing, which only slightly changes the electronic properties of the material. At higher temperatures of about 90 degrees C, but still significantly below the previously reported transition temperature in bulk, the Cr-LC transition is observed. An accompanying large increase in optical anisotropy is compatible with the X-ray data, showing a transition from the as-cast herringbone-like crystalline state to a highly ordered discotic hexagonal columnar LC phase. The structural transition has the macroscopic effect of increasing the film thickness. The high structural order of the as-cast low-temperature phase is only partly recovered after cooling, and the phase transition exhibits a large hysteresis. From the ellipsometry data, the dielectric tensor of HBC-C12H25 was refined to unprecedented detail.     

Melem (2,5,8-triamino-tri-s-triazine), an important intermediate during condensation of melamine rings to graphitic carbon nitride: synthesis,structure determination by X-ray powder diffractometry,solid-state NMR,and theoretical studies

Single-phase melem (2,5,8-triamino-tri-s-triazine) C(6)N(7)(NH(2))(3) was obtained as a crystalline powder by thermal treatment of different less condensed C-N-H compounds (e.g., melamine C(3)N(3)(NH(2))(3), dicyandiamide H(4)C(2)N(4), ammonium dicyanamide NH(4)[N(CN)(2)], or cyanamide H(2)CN(2), respectively) at temperatures up to 450 degrees C in sealed glass ampules. The crystal structure was determined ab initio by X-ray powder diffractometry (Cu K alpha(1): P2(1)/c (No. 14), a = 739.92(1) pm, b = 865.28(3) pm, c = 1338.16(4) pm, beta = 99.912(2) degrees, and Z = 4). In the solid, melem consists of nearly planar C(6)N(7)(NH(2))(3) molecules which are arranged into parallel layers with an interplanar distance of 327 pm. Detailed (13)C and (15)N MAS NMR investigations were performed. The presence of the triamino form instead of other possible tautomers was confirmed by a CPPI (cross-polarization combined with polarization inversion) experiment. Furthermore, the compound was characterized using mass spectrometry, vibrational (IR, Raman), and photoluminescence spectroscopy. The structural and vibrational properties of molecular melem were theoretically studied on both the B3LYP and the MP2 level. A structural optimization in the extended state was performed employing density functional methods utilizing LDA and GGA. A good agreement was found between the observed and calculated structural parameters and also for the vibrational frequencies of melem. According to temperature-dependent X-ray powder diffractometry investigations above 560 degrees C, melem transforms into a graphite-like C-N material.     

Conformational analysis and crystal structure of {[1-(3-chloro-4-fluorobenzoyl)-4-fluoropiperidin-4yl]methyl}[(5-methylpyridin-2-yl)methyl]amine, fumaric acid salt

{[1-(3-Chloro-4-fluorobenzoyl)-4-fluoropiperidin-4yl]methyl}[(5-methylpyridin-2-yl)methyl]amine, fumaric acid salt (C(20)H(22)ClF(2)N(3)O, C(4)H(4)O(4)) (1) was synthesized and characterized by the complete (1)H, (13)C and (19)F NMR analyses. The conformation of the piperidin ring, in the solution state, was particularly studied from the coupling constants determined by recording a double-quantum filtered COSY experiment in phase-sensitive mode. (1)H NMR line-shape analysis was used, at temperatures varying between -5 and +60 degrees C, to determine the enthalpy of activation of the rotational barrier around the CN bond. Compound 1 crystallizes in the triclinic space group P1 with a=8.517(3) Angstrom, b=12.384(2) Angstrom, c=12.472(3) Angstrom, alpha=70.88(2) degrees, beta=82.04(2) degrees, gamma=83.58(2) degrees. The results strongly indicate that the solid and solution conformations are similar. Thermal stability and phases transitions were investigated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). Furthermore polymorphism screening was studied from recrystallization of 1 performed in seven solvents and by slurry conversion in water. The X-ray powder diffraction (XRPD) and differential scanning calorimetry results suggested that 1 crystallizes into one crystalline form which melts at 157 degrees C (DeltaH=132 J g(-1)).     

Decomposition of ascorbic acid in the presence of cadmium ions leads to formation of a polymeric cadmium oxalate species with peculiar structural features

Slow decomposition of L-ascorbic acid, carried out under aerobic conditions and in the presence of cadmium ions, results in formation of a crystalline product that is highly insoluble in water. This compound has been identified as a cadmium oxalate polymeric species with formula Cd(C(2)O(4)).3H(2)O. The crystal structure of this compound is described. Relevant crystal data are the following: C(4)H(12)O(14)Cd(2), fw = 508.94; triclinic; space group P1 (No. 1); a = 6.010(1) A, b = 6.668(1) A, c = 8.498(1) A; alpha = 74.64(1) degrees, beta = 74.25(1) degrees, gamma = 80.91(1) degrees; V = 314.7(5) A(3); Z = 1.     

Crystal structures of synthetic analgetics. IV. Dextropropoxyphene.

The molecular and cystal structure of dextropropoxyphene has been determined by X-ray methods. The crystals are monoclinic, space group P21, with unit cell dimensions a=9.257(2) A; b=9.048(3) A; c=12.074(7) A; beta=93.01(4)degrees. The phase problem was solved by direct methods and the model refined to an R-value of 0.038 for 1799 observed reflections. E.s.d's are, in average, 0.004 A and O.3 degrees in interatomic distances and angles, respectively. The propylamine chain in nearly fully extended, the dihedral angel C4-C5-C7-N being -174.2 degrees. The conformation of this side chain is similar to that in the hydrochloride of the title compound. Thus the proposed bioactive conformation is not preferred by propoxyphene in the crystalline state, as was the case for the free base of methadone.     

Synthesis of nickel sulfides in aqueous solutions using sodium dithionite

Reaction of nickel chloride with sodium dithionite in aqueous solutions at ambient temperature has been investigated systematically to obtain nickel sulfides. The products are characterized by X-ray diffraction, thermogravimetric analysis, and electrical resistivity and magnetic susceptibility measurements. It is found that the compositions and structures of the products are controlled by the reaction pH and the amount of the reactants. While reactions under highly acidic (pH < or = 2) and basic (pH > or = 7) conditions yield crystalline sulfur and amorphous or poorly crystalline NiySx, respectively, those at intermediate 3 < or = pH < or = 6 give crystalline NiySx. Both crystalline Ni3S2 (heazlewoodite structure) and Ni3S4 (spinel structure) have been obtained at room temperature. Additionally, NiS (millerite structure) is obtained by carefully heating Ni3S4 at 200 degrees C in a mixture of 90% Ar and 10% H2. Ni3S4 is found to be metastable, and it begins to disproportionate above 100 degrees C. Both Ni3S2 and Ni3S4 show metallic behavior. While Ni3S2 exhibits temperature-independent magnetic susceptibility, Ni3S4 shows ferrimagnetic ordering below 20 K.