Saturation molalities m(sat) in H2O(l) have been measured for the substances 2′-deoxyadenosine·H2O(cr), 2′-deoxycytidine·H2O(cr), 2′-deoxyguanosine·H2O(cr), 2′-deoxyinosine(cr), and 2′-deoxyuridine(cr) by using h.p.l.c. The states of hydration were established by performing Karl Fischer analyses on samples of these substances, which had been allowed to equilibrate with their respective aqueous saturated solutions for several days at T≈298 K and then dried with air at T≈296 K for ≈24 h. The crystalline forms of the substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Also, calorimetric molar enthalpies of solution ΔsolHm(cal) for these substances were measured by using an isoperibol solution calorimeter. A self-association (stacking) model was used to estimate values of the activity coefficients γ and relative apparent molar enthalpies Lϕ for these substances. These γ and Lϕ values were used to adjust the measured values of m(sat) and ΔsolHm to the standard state and thus obtain values of the standard molar Gibbs free energy ΔsolGm∘ and enthalpy changes ΔsolHm∘ for the dissolution reactions of these substances. The values of the pKs and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of ΔsolGm∘andΔsolHm∘.

Saturation molalities m(sat) in H2O(l) have been measured for the substances cytidine(cr), hypoxanthine(cr), thymidine(cr), thymine(cr), uridine(cr), and xanthine(cr) by using h.p.l.c. The states of hydration were established by performing Karl-Fischer analyses on samples of these substances, which had been allowed to equilibrate with their respective aqueous saturated solutions for several days at T≈298 K and then dried with air at T≈296 K for ≈24 h. The crystalline forms of the substances were identified by comparison of the results of X-ray diffraction measurements with results from the literature. Also, molar enthalpies of solution ΔsolHm(cal) for these substances were measured by using an isoperibol solution calorimeter. A self-association (stacking) model was used to estimate values of the activity coefficients γ and relative apparent molar enthalpies Lφ for these substances. These γ and Lφ values were used to adjust the measured values of m(sat) and ΔsolHm(cal) to the standard state and thus obtain values of the standard molar Gibbs free energy ΔsolG∘m and enthalpy changes ΔsolHm∘ for the dissolution reactions of these substances. The values of the pKs and of the standard molar enthalpies of the ionization reactions were also used to account for speciation of the substances in the calculations of ΔsolGm∘ and ΔsolHm∘. Values of standard molar enthalpies of formation ΔfHm∘, standard molar Gibbs free energies of formation ΔfGm∘, and standard partial molar entropies S2,m∘ for the aqueous species of hypoxanthine and xanthine were calculated. A detailed summary and comparison of thermodynamic results from the literature for these substances is presented.

A lattice metric singularity occurs when unit cells defining two (or more) lattices yield the identical set of unique calculated d-spacings. The existence of such singularities, therefore, has a practical and theoretical impact on the indexing of powder patterns. For example, in experimental practice an indexing program may find only the lower symmetry member of a singularity. Obviously, it is important to recognize such cases and know how to proceed. Recently, we described: a binary singularity involving a monoclinic and a rhombohedral lattice in a subcell-supercell relationship anda second type of singularity-a ternary singularity-in which two of the three lattices are in a derivative composite relationship. In this work, we describe a ternary lattice metric singularity involving a cubic P, a tetragonal P, and an orthorhombic C lattice. Furthermore, there is a binary singularity, involving a hexagonal P and orthorhombic P lattice, which is characterized by a set of unique d-spacings very close to that of the ternary singularity. The existence of such singularities is more common than once thought and requires a paradigm shift in experimental practice. In addition singularities provide opportunities in material design as they point to highly specialized lattices that may be associated with unusual physical properties.

Bi5AgNb4O18 is a new phase, which was discovered during the phase equilibrium study of the Bi2O3–Ag2O–Nb2O5 system. Bi5AgNb4O18 was prepared at 750°C and is stable in air up to its melting temperature of 1160.1±5.0°C (standard error of estimate). Results of a Rietveld refinement using neutron powder diffraction confirmed that Bi5AgNb4O18 is isostructural with Bi3TiNbO9, Bi5NaNb4O18, and Bi5KNb4O18. The structure was refined in the orthorhombic space group A21am, Z=2, and the lattice parameters are a=5.4915(2) Å, b=5.4752(2) Å, c=24.9282(8) Å, and V=749.52(4) Å3. The structure can be described as the m=2 member of the Aurivillius family, (Bi2O2)2+ (Am−1BmO3m+1)2− (where A=Bi and B=Ag, Nb), which is characterized by perovskite-like (Am−1BmO3m+1)2− slabs regularly interleaved with (Bi2O2)2+ layers. The octahedral [NbO6] units are distorted with Nb–O distances ranging from 1.856(4) to 2.161(2) Å and the O–Nb–O angles ranging from 82.6(3)° to 98.5(3)°. These octahedra are tilted about the a- and c-axis by about 10.3° and 12.4°, respectively. Ag was found to substitute exclusively into the Bi-site that is located in the layer between the two distorted [NbO6] units. Although the Ag substitutes into the Bi-site with the Bi:Ag ratio of 1:1, the existence of a superlattice was not detected using electron diffraction. A comparison of (Bi2O2)2+(Am−1NbmO3m+1)2− structures (where A=Ag, Na, and K) revealed a relation between the pervoskite tolerance factor, t, and structural distortion. The reference pattern for Bi5AgNb4O18 has been submitted to the International Centre for Diffraction Data (ICDD) for inclusion in the Powder Diffraction File.

In the selection of a centered cell in the monoclinic system, it is recommended that the experimentalist select an I-centered cell for those cases in which it is the conventional cell - cases in which a and c are coincident with the shortest two translations in the net perpendicular to b (b-axis unique). The common practice of selecting a non-conventional C-centered cell in such cases should be discontinued.

Lattice-matching techniques have proved to be extremely effective for the identification of unknown crystalline materials. A commonly employed lattice-matching strategy is based on matching the reduced cell of an unknown against a database of known materials represented by their respective standard reduced cells. The success of the method relies on the fact that the lattice or the lattice plus chemical information (e.g., element types) is highly characteristic of a material—like a fingerprint. Because of its intrinsic power, the procedure has many and diverse applications—in materials characterization, in nano-technology, in epitaxial growth, in materials design, etc. An especially fruitful role for the method is in the journal publication process as the quality of the scientific literature can be enhanced. The focus herein is on the major role that lattice matching can play in the prevention of inadvertent duplicate publications of the same structure and in the determination of key cross-references.

In 1969, a seminal section on reduced forms and conventional cells was published in the International Tables for X-Ray Crystallography. The section contains a table that gives a metric classification of the 44 reduced forms. In 2001, this table with appropriate revisions was republished in the Journal of Research of the National Institute of Standards and Technology. An especially valuable feature of the table is that it defines and allows the user to determine a standard conventional cell. Since 1969, there has been an evolution toward acceptance and widespread use of such conventional cells. An inspection of the articles in key crystallographic journals reveals that most cells follow the conventions. However, one major exception remains-the centered monoclinic lattices. In approximately one-third of these cases, non-conventional C-centered cells are used, apparently to avoid the use of I-centered cells. It is recommended that the crystallographic community routinely use the I-centered conventional cell in such cases.

In theory, physical crystals can be represented by idealized mathematical lattices. Under appropriate conditions, these representations can be used for a variety of purposes such as identifying, classifying, and understanding the physical properties of materials. Critical to these applications is the ability to construct a unique representation of the lattice. The vital link that enabled this theory to be realized in practice was provided by the 1970 paper on the determination of reduced cells. This seminal paper led to a mathematical approach to lattice analysis initially based on systematic reduction procedures and the use of standard cells. Subsequently, the process evolved to a matrix approach based on group theory and linear algebra that offered a more abstract and powerful way to look at lattices and their properties. Application of the reduced cell to both database work and laboratory research at NIST was immediately successful. Currently, this cell and/or procedures based on reduction are widely and routinely used by the general scientific community: (i) for calculating standard cells for the reporting of crystalline materials, (ii) for classifying materials, (iii) in crystallographic database work (iv) in routine x-ray and neutron diffractometry, and (v) in general crystallographic research. Especially important is its use in symmetry determination and in identification. The focus herein is on the role of the reduced cell in lattice symmetry determination.

The Research Associateship program of the Joint Committee on Powder Diffraction-International Centre for Diffraction Data (JCPDS-ICDD, now known as the ICDD) at NBS/NIST was a long standing (over 35 years) successful industry-government cooperation. The main mission of the Associateship was to publish high quality x-ray reference patterns to be included in the Powder Diffraction File (PDF). The PDF is a continuing compilation of patterns gathered from many sources, compiled and published by the ICDD. As a result of this collaboration, more than 1500 high quality powder diffraction patterns, which have had a significant impact on the scientific community, were reported. In addition, various research collaborations with NBS/NIST also led to the development of several standard reference materials (SRMs) for instrument calibration and quantitative analyses, and computer software for data collection, calibration, reduction, for the editorial process of powder pattern publication, analysis of powder data, and for quantitative analyses. This article summarizes information concerning the JCPDS-ICDD organization, the Powder Diffraction File (PDF), history and accomplishments of the JCPDS-ICDD Research Associateship.

The oxycarbonate phase YBa2Cu2.85(CO3)(0.15)O-6.73 has been analyzed by neutron powder diffraction. The phase crystallizes with the symmetry of space group P4/mmm and with unit cell parameters a = 3.8717(3), c = 11.60(1) Angstrom. Rietveld refinements show that the basic structure of the oxycarbonate is the same as that of the 193 superconductor YBa2Cu3O6+w. The CO32- ions are located on the basal plane of the unit cell, with the carbon atoms replacing an equal number of "chain" copper atoms. The presence of the CO32- defects explains why the oxygen stoichiometry of the oxycarbonate can he larger than seven atoms per formula unit. (C) 2000 Elsevier Science B.V. All rights reserved.

A lattice metric singularity occurs when unit cells defining two (or more) lattices yield the identical set of unique calculated d-spacings. The existence of such singularities, therefore, has a practical impact on the indexing of powder patterns. For example, when experimental data from ζ-LiBO2 were indexed, two solutions (a rhombohedral and a monoclinic lattice) with approximately the same figure of merit were found. These two lattices yield the same set of unique d-spacings even though they are characterized by different reduced cells with cell volumes in the ratio 2 to 1. From the indexing point of view, both answers are correct. A singularity of this type is common and not a mathematical rarity. In fact, any rhombohedral cell of this kind has a derivative monoclinic subcell, each of which gives the same set of unique calculated d-spacings. In actual cases like this, one can run into a trap. Due to experimental error and input parameters, an indexing program may determine only one of the cells with a high figure of merit. When this happens, it is critical to recognize that another solution exists, especially if one has determined the lower symmetry lattice.

Compounds of composition RBa{sub 2}Fe{sub 3}O{sub 8+w} (R = La, Nd, Sm, Gd, Dy, Er, Yb, Lu, and Y) with variable oxygen content have been synthesized using the liquid mixing technique and have been analyzed by powder X-ray and neutron diffraction methods. A triple perovskite-type structure with ordered Ba and R cations and having the symmetry of space group P4/mmm was obtained only for R = Y, Dy, and Er, whereas the larger R atoms gave the atomic arrangement of disordered, defective perovskites with average symmetry Pm{bar 3}m. No perovskite-type phases were obtained when Yb and Lu were tried. The oxygen content of the oxygen-saturated phases was found to increase with increasing size of the cation R from w = 0.07 for Er to w = 0.83 for La. The Neel temperature ({approximately}650 K) and the magnitude of the Fe magnetic moment ({approximately}3.3 {mu}{sub B} at room temperature) are highest when trivalent iron is involved (w = 0), but these quantities are rather insensitive to the nature of R and to slight variations of oxygen content in the triple perovskite-type structure of the Y, Dy, and Er compounds. In the case of the cubic structures, however, both quantities depend stronglymore » on the oxygen stoichiometry. In the magnetically ordered state, nearest-neighbor iron moments are coupled antiferromagnetically along the three crystallographic directions in all samples, resulting in magnetic structures with symmetry Imm{prime}m and magnetic unit cells related to those of the corresponding nuclear structures by the transformation matrix (1{bar 1}0/110/002).« less

Compounds of compositionRBa2Fe3O8+w(R=La, Nd, Sm, Gd, Dy, Er, Yb, Lu, and Y) with variable oxygen content have been synthesized using the liquid mixing technique and have been analyzed by powder X-ray and neutron diffraction methods. A triple perovskite-type structure with ordered Ba andRcations and having the symmetry of space groupP4/mmmwas obtained only forR=Y, Dy, and Er, whereas the largerRatoms gave the atomic arrangement of disordered, defective perovskites with average symmetryPm3m. No perovskite-type phases were obtained when Yb and Lu were tried. The oxygen content of the oxygen-saturated phases was found to increase with increasing size of the cationRfromw=0.07 for Er tow=0.83 for La. The Néel temperature (∼650 K) and the magnitude of the Fe magnetic moment (∼3.3μBat room temperature) are highest when trivalent iron is involved (w=0), but these quantities are rather insensitive to the nature ofRand to slight variations of oxygen content in the triple perovskite-type structure of the Y, Dy, and Er compounds. In the case of the cubic structures, however, both quantities depend strongly on the oxygen stoichiometry. In the magnetically ordered state, nearest-neighbor iron moments are coupled antiferromagnetically along the three crystallographic directions in all samples, resulting in magnetic structures with symmetryImm′mand magnetic unit cells related to those of the corresponding nuclear structures by the transformation matrix (110/110/002).

The nuclear and magnetic structures of (Y1-xCax)Ba2Fe3O8+delta with x = 0.05, 0.10, and 0.20 have been investigated by neutron-powder diffraction at room temperature. The compound crystallizes with the symmetry of space group P4/mmm and has a structure similar to that of the superconductor YBa2Cu3O7 (1:2:3). There are two types of Fe atoms in the unit cell, one having fivefold pyramidal coordination and the other octahedral coordination. The Ca and Y atoms are randomly distributed over the 1/2,1/2,1/2 position, which are exclusively occupied by Y in the 1:2:3 superconductor. Since the oxygen sites on the basal plane of the structure are fully occupied to achieve octahedral coordination for the iron atoms on this plane, the oxygen content is eight atoms per formula unit, the barium atoms are twelvefold coordinated, and the coordination polyhedron is a cuboctahedron. Extra oxygen atoms corresponding to delta = 0.08 and 0.05 are located in the Y/Ca layer. The presence of these atoms produces disorder in the structure resulting in a shift of the oxygen atoms located near the plane of the pyramidal iron atoms. The charge compensation required by the substitution of Y3+ by Ca2+ is achieved in these materials by elimination of the extra oxygen, rather than oxidation of the iron atoms. The magnetic structure is based on a unit cell related to that of the nuclear structure by the transformation of axes (1,-1,0/1,1,0/0,0,2). The magnetic origin of some of the observed diffraction peaks was established by polarized neutron diffraction measurements. As in YBa2Fe3O8, the iron moments are coupled antiferromagnetically within each (FeO2) layer as well as along the c axis of the structure, and they lie in the planes perpendicular to c. The magnetic moments of the two iron atoms are practically identical and have values at room temperature of (mu) = 3.50(3)mu(B), 3.52(2)mu(B), and 3.62(2)mu(B) for the three compositions x = 0.05, 0.10, and 0.20, respectively.

The nuclear and magnetic structures of the oxygen-deficient perovskite YBaCuCoO5 have been determined by neutron powder diffraction at room temperature. The nuclear structure has the symmetry of space group P 4/mmm and lattice parameters a = 3.8679(1), c = 7.5674(2) Å. Copper and cobalt atoms are completely disordered in this compound and the oxygen vacancies are located on the layer of the yttrium atoms. As a consequence of this configuration, the Co/Cu atoms have fivefold, pyramidal coordination. The pyramids share corners in the (Co/Cu)O2 planes, as well as in the direction of the c axis, thus forming slices of two pyramids with opposite orientation separated by layers of yttrium atoms. The Ba atoms, on the other hand, have 12-fold coordination as in nondefective perovskites, and the Y atoms have eightfold prismatic coordination as in the superconductor YBa2Cu3O6+δ. The magnetic structure is based on a unit cell related to that of the nuclear structure by an axis transformation of matrix (1, -1, 0/1, 1, 0/0, 0, 2). The magnetic origin of the extra reflections observed experimentally was established by polarized neutron diffraction measurements. The reflection conditions of the magnetic intensities are consistent with a model in which magnetic moments of equal modulus have alternately ferromagnetic and antiferromagnetic orientations along the c axis and antiferromagnetic ordering in the planes perpendicular to c. If we also assume that the magnetic structure has tetragonal symmetry, then the magnetic moments have to assume an orientation parallel to the c axis of the unit cell.

A review of the symmetry of proteins has revealed that in many cases the metric symmetry exceeds the reported crystal symmetry. Regardless of the reason, this observation has important implications for experimental protein crystallography. Standard laboratory procedure should always include a direct determination of the lattice metric symmetry. With full knowledge of the highest possible symmetry, the experimentalist is then able to determine in a logical and accurate manner the Laue group and the space group. For those proteins in which it has been proved that the metric symmetry exceeds the crystal symmetry, the protein crystallographer must proceed with caution (e.g. in relating multiple sets of data, positional parameters etc. on the same or related crystals), because a given lattice will have metrically similar unit cells that are not symmetrically equivalent.