Volume 209, Issue 4

Important physico-chemical properties of the regular, lamellar eutectic LiF–LiBaF 3 and its components have been investigated and the phase diagram LiF–BaF 2 was redetermined. These investigations have been carried out partially to prepare and develop experimental concepts for the directional solidification of the eutectic LiF–LiBaF 3 under microgravity conditions. Eutectic and off-eutectic LiF–LiBaF 3 , melts have been solidified under different growth conditions (growth velocity ν , temperature gradient G ) and the correlation between the G / ν ratio and the eutectic structure was determined. The correlation between the average phase spacings λ and the growth rate ν (quantitative image analysis data) resulted in the calculated relation λ 2.08 ν = constant (regions of exact eutectic compositions) which is in good agreement with the Zener criterion λ 2 ν = constant. The high melt viscosity leads to low Rayleigh numbers, Ra, which is orders of magnitude lower than the critical Rayleigh number, Ra c , and therefore a strong reduction of convective influences during the eutectic growth is expected. It can be concluded that the growth from eutectic LiF–LiBaF 3 , melts is mainly governed by diffusion and this confirms the growth theories by Jackson and Hunt. During eutectic growth the G / ν ratio plays an important role in respect to the eutectic structure. Low growth rates and high temperature gradients lead locally to a transition of the eutectic structure from lamellae to fibers. We found empirically a correlation between fibers, lamellae and eutectic cells. But at present no quantitative theory of modification in the eutectic structure has been formulated yet that a given temperature gradient with a given growth rate will cause lamellae to fiber transition in a given normal eutectic. Furthermore the reason for the observed deviations of the volume fractions in the eutectic structure from exact eutectic melt compositions was studied. The enrichment of the LiBaF 3 , component at the growth interface of a seed with eutectic composition in dependence of the soaking time of the eutectic melt could be attributed to the Soret effect (thermodiffusion) and is not primary a consequence of sedimentation effects (influence of gravity). Our experiments under microgravity conditions are expected to give unequivocal informations concerning the influence of gravity and Soret effect on the directional solidification. The directional solidification of hypo- and hyper-eutectic melts indicate that the LiF (phase with higher undercooling) acts as nucleus for LiBaF 3 , but not vice versa (‘non-reciprocal nucleation’). Hyper-eutectic melts have no detectable negative influence on the quality of the eutectic structure whereas the crystallization of hypo-eutectic melts leads to disturbances of the regularity in the lamellar structure.

In electrostatic point charge models with formal charges, lattice-site energies and crystal energies have been calculated for alkali feldspars with different degrees of Si/Al ordering. In sequence of increasing ordering, i.e. high sanidine – ordered orthoclase – low microcline, the crystal energies become more negative. The differences in crystal energy between the various polymorphs is a measure of the energy needed for the Al ordering. This ordering energy amounts to −134 kcal/mol, being the difference between an ordered and a disordered potassium feldspar. In a point charge model with lowered charges (K 1+ T 1.75+ O 8 − ) this energy decreases to −25 kcal/mol. Lattice-site energies are calculated for all ion sites of high sanidine, ordered orthoclase, low microcline and analbite. During the two-step ordering the Al concentrates first on both T1 sites. The lattice-site energy of the T1(0) site is almost linearly correlated with the degree of Si/Al ordering. For ordered orthoclase the T2 lattice-site energy has almost the same low value as the T1( m ) and both T2 sites of low microcline. In addition, the lattice-site energies have been computed in three differently ordered hypothetical models, respectively with Al concentrated on T1( m ), T2(0) and T2( m ), and in a fourth model, also hypothetical, with disordered Al and the unit cell geometry of low microcline. The calculations show that the occupation of a T site by Al instead of Si results in a lattice-site energy which is about 400 kcal/mol less negative due to the larger T–O distances. When this geometrical correction factor is taken into account, Al must be ordered in the T1(0) site, as in this way the lowest negative lattice-site energy is obtained. Lattice-site energies and crystal energy of analbite do not differ much from those of high sanidine. In contrast with a change in the charge distribution due to the Al ordering the crystal energies do not change significantly by the replacement of potassium by sodium ions.

We describe investigations of growth of cubic zirconium dioxide single crystals doped with rare-earth ions. The cubic phase is stabilized down to room temperature by adding 12 mole% Y 2 O 3 . The crystals are grown using the skull-melting technique. For colouring they are doped with Nd 2 O 3 , and Er 2 O 3 . We investigate the effective distribution coefficient of these dopants for crystals grown with growth velocities in the range of 4 to 18 mm/h. From optical absorption data the dopant concentration is calculated. The use of the Burton-Prim-Slichter (BPS) equation allows to estimate the equilibrium distribution coefficient of Nd 3+ ( k 0 = 0.43) and Er 3+ ( k 0 = 1.11) in cubic zirconium dioxide.

Applying the videographic reconstruction and simulation method, the real domain structure configuration of the Cu 3 Au binary alloy is established at various temperatures. Starting from the partially-ordered state and varying the domain size (and shape), domain structures, interface type and the domain distribution in a disordered matrix, good agreement between the diffraction patterns (X-ray- and electron diffraction) and the videographic calculations is obtained. Both the reconstruction and the simulation results indicate that the gradual variation of the intensity distribution around the superlattice reflection positions arises from a domain-size distribution in a disordered matrix. The real structure of Cu 3 Au at 678 K, e.g., just above T C , is comprised of small 2-d domains with a certain size distribution (approx. 1–8 cells, small size preferred) and different shapes. The domains are almost randomly distributed in a disordered matrix and occasionally form an anti-phase relationship. They are separated by the disordered matrix and are not fully-ordered (short-range order). The matrix contains regions either deviating considerably from the average structure or resembling the average Cu 3 Au structure. The results obtained from the reconstruction using the random phase random amplitude model indicate that this model can be generally used as a starting model to “recover” the real structure in similar cases in which order/disorder phase transitions are present.

High temperature X-ray powder diffraction measurements of rubidiumpolyphosphate, Rb 2 { ∞ 1 } [P 2 O 6 ] were carried out with a position sensitive detector (PSD) and analysed by the Rietveld method. Three new high temperature phases were identified. The monoclinic [ P 2 1 / n , a = 12.1249(3) Å, b = 4.2279(1) Å, c = 6.4761(2) Å, β = 96.006(2)° room-temperature modification RbPO 3 -T, transforms at T c = 661 K to orthorhombic RbPO 3 -H ( Pbnm ) with the lattice constants a = 13.1674(4) Å, b = 4.5657(1) å and c = 6.1183(2) å (750 K). A continuous transition at T c = 914 K leads to RbPO 3 -HT ( Bbmm , 940 K) with a = 13.3517(4) Å, b = 4.5917(1) Å and c = 6.1665(2) Å, for which the empirical power law behaviour of the order parameter Q is Q ∼ ( T − T c ) β with β near 0.2 for T c > T > 0.8 T c . There are saturation effects below 0.8 T C . The T→H transition shows a hysteresis of ca. 110 K and on cooling an intermediate phase, RbPO 3 -Z, occurs at 591 K, with space group P 2 1 / n , a = 14.3476(5) Å, b = 4.5730(1) å, c = 12.0839(4) Å and β = 65.604(2)[unk], which transforms back to the RbPO 3 -T state at 550 K.

The structure of β -Si 3 N 4 was reinvestigated with X-ray powder diffraction and Rietveld refinement to determine if the space group should be centrosymmetric, P 6 3 / m , or acentric, P 6 3 . Displacement for one sort of the nitrogen atoms N(1) away from the high-symmetry positions are found to be even larger than reported before and this behaviour is retained to higher temperatures. R -factor ratio tests favour P 6 3 at a 95% significance level. No exceptional axial thermal expansion was observed. Above 900°C a significant growth of a disordered high-cristobalite phase was observed. It is shown that structure refinements with the X-ray powder technique can be done even at high temperature with high confidence. A newly developed X-ray mirror furnace reaching temperatures up to 1360°C is described.

A quantitative evaluation of the influence of the amount of the crystallographically unoriented grains of YBa 2 Cu 3 O 7− x epitaxial films on the major high temperature superconducting (HTS) properties such as microwave surface resistance R s and critical current J c has been analyzed. The correlation is quite complex but it is clear that a very high percentage of 〈100〉 basal plane orientation, as determined by the X-ray (108) phi-scan, is essential to obtain the optimal HTS properties. The role of (005) rocking curves width and percentage of crystallographic orientation of YBa 2 Cu 3 O 7 − x grains decomposed into Y 2 BaCuO 5 and BaCuO 2 are also discussed. The optimal deposition temperature for thin films is analyzed in terms of the structural and HTS properties.

The crystal structure of SrGa 12 0 19 has been refined at 473 K, 298 K and 160 K to R -values of 0.025, 0.023 and 0.030, respectively. The temperature dependence of the Debye-Waller factors shows that five-fold coordinated Ga 3+ is randomly displaced from the center of its trigonal bipyramidal coordination polyhedron along positive and negative directions of the c -axis. The magnitude of displacement increases with the temperature. Although the Madelung part of lattice energy favours central location, this is prevented by repulsion of the three equatorial oxygen atoms.

The crystal stucture of (NH 4 ) 1.4 [Cu(NH 3 ) 2 ] 0.3 · Br 2 is composed from a NH 4 Br structure where part of NH 4 + ions was replaced with NH 3 groups. The Cu atoms statistically occupy the centre of a cubic unit cell, with Br − ions in its apexes resulting in the creation of linear [H 3 N–Cu–NH 3 ] 2+ units. The statistical weight of the cationic units is 0.13(1). The Cu–N and Cu–Br − bond distances are 2.0293(4) (2 ×) and 2.8699(5) (4 ×) Å, respectively. Microcrystalline preparations having the composition of (NH 4 ) 2(1 − x ) [Cu(NH 3 ) 2 ] x Br 2 ( x = 0.0 to 0.92) show continuous logarithmic normal x vs. density dependence. The formula for calculation of x from density ( D m ) of crystalline and microcrystalline preparations was derived.

Iron aluminium phosphate, FeAlPO 5 , is monoclinic with space group P 2 1 / c , a = 7.141(3) Å, b = 10.549(4) Å, c = 5.493(3) Å, β = 98.19(9)°, Z = 4 and D calc. = 3.14 g/cm 3 . Single crystals were prepared synthetically. The structure was refined to R (unweighted) = 0.056 and R (weighted) = 0.039 using 1240 non-equivalent observed reflections. The structure contains Fe 2+ in fivefold coordination by oxygen in the form of a distorted trigonal bipyramid.

K 2 Eu 4 {1B, 1[unk]}[ 6 Si 8 O 20 ]F 2 , prepared by flux-melting of Eu 2 O 3 , SiO 2 , and Si in KF under reducing conditions crystallizes with the monoclinic cell, P 2 1 / n , a = 1149.9(3) pm, b = 848.8(2) pm, c = 1162.6(4) pm, β = 112.10(1)[unk], Z = 4. The structure determination led to R = 7.3% ( R w = 3.7%). The new silicate-fluoride contains strongly folded silicate sheets of connected four-membered rings, separated by interlayers of europium(II) and fluoride. The potassium ions are located in the “half-pipes” that are formed by the silicate sheet. The structure of the silicate anion is related to that of apophyllite, KCa4{uB, [unk]}[ 4 Si 4 O 10 ] 2 (F, OH) · 8 H 2 O. The isotypic compound K 2 Sr 4 {1B, [unk]}[ 6 Si 8 O 20 ]F 2 was also synthesized. It has slightly smaller lattice constants.

The crystal structure of bis(3-methyl-orotato)dimanganese(II)hexahydrate, [Mn 2 (C 6 H 4 N 2 O 4 ) 2 (H 2 O) 6 ] has been determined by single crystal X-ray diffraction. C 12 H 20 Mn 2 N 4 O 14 , M r = 554.19, triclinic, P [unk], a = 6.575(1) Å, b = 8.577(1) Å, c = 9.608(2) Å; α = 64.54(1)[unk], β = 83.01(1)[unk], γ = 89.97(1), V = 484.76 Å 3 , Z = 1, d c = 1.89 Mgm −3 , λ (Mo K α = 0.71073 Å, μ = 1.300 mm −1 , F (000) = 282, T = 293 K, R = 0.037 for 2548 observed data with F o > 3 σ ( F o ), 2719 unique reflections. The deprotonated 3-methyl orotato ligand coordinates to Mn(II) by the N(1) nitrogen atom and by the oxygen atom of the adjacent carboxylato group. The crystal structure consists of centrosymmetric dimeric units in which two 3-methyl-orotato ligands bridge the two metal centers by carboxylato groups. Three water molecules per metal complete the octahedral geometry.

The title complex (C 5 H 25 Cu NO 13 S)has been synthesized and characterized by single crystal X-ray analysis ( R = 0.059 for 5934 observed hkl ; Mo K α data). The complex crystallizes in the triclinic space group P [unk] with a = 8.491(1), b = 8.942(2), c = 12.144(2) Å, α = 101.17(2), β = 96.69(1), γ = 114.04(1)°, and Z = 2. The Cu(II) atom is in a distorted square pyramidal coordination environment, being surrounded by a carboxy oxygen atom of the betaine ligand [1.944(5) Å and three aqua oxygen atoms [Cu–O = 1.921(5) Å ∼ 1.968(5) Å] in the basal plane, with another aqua oxygen atom occupying the apical position [2.249(2) Å]. Hydrogen bonds are formed by the aqua ligands, the lattice water molecules, the uncoordinated oxygen atom of the carboxy group, and the sulfate ion to generate a three-dimensional network.

The title compound (C 37 H 38 O) belongs to the family 1-hetero-2,5-cyclohexadienes. The structure is triclinic, space group P [unk], with a = 11.576(5), b = 12.229(5), c = 12.305(5) Å, α = 68.52(3), β = 71.50(3), γ = 77.54(3)° and Z = 2. Three-dimensional intensity data collected on the SYNTEX P2 1 diffractometer using Cu K α radiation at room temperature used for the structure determination and refinement. The final R value is 0.082 for 3713 observed reflections. Both tert-butyl groups are disordered as a result of their rotation around the C(phenyl)-C(tertbutyl) bond.

C 30 H 22 N 2 O 4 Cl 2 , M r 545.41, triclinic, P [unk], a = 12.127(2) Å, b = 12.759(2) Å, c = 8.879(1) Å, α = 100.40(1)°, β = 110.89(1)°, γ = 91.06(1)°, V = 1257.3(3) Å 3 , Z = 2, D x = 1.438 g cm −3 , λ (Mo K α = 0.7107 Å, μ = 2.957 cm −1 , F (000) = 564, T = 296 K, R = 0.036 for 2352 unique reflections with I > 4 σ ( I ). The configuration of the stereocentres allows to infer the mechanism of the rearrangement of the cycloadduct, formed under kinetic conditions, of the 1.3-dipolar cycloaddition between oxoindolin-3-ylidene dipolarophile and azomethine ylide acting as 1,3-dipole. The stereocentres feature was confirmed by the crystal structure study.

The structure of the semisynthetic ergot derivate – lisuride hydrogen maleate (C 20 H 27 N 4 O) + · (C 4 H 3 O 4 ) − was solved by direct methods and refined anisotropically to an R value of 0.053 for 1618 unique observed reflections. The title compound crystallizes in the monoclinic space group P 2 1 with lattice parameters a = 11.968(3) Å, b = 5.754(2) Å, c = 17.311 (5) Å, β = 107.15(2)°, Z = 2. The lisuride molecule exhibits a great flexibility; the indole moiety of A and B rings of an ergoline fragment is slightly puckered only. Ring C has a 1 E envelope conformation and ring D has a 1 H 2 half-chair one. Lisuride-maleate interaction is mediated both through electrostatic forces and H-bridges.

6,9-dimethyl-6,9-diisopropyl-[2.2] (1,4)phenanthrenoparacyclophane 1 , C 26 H 24 , crystallises in P 2 1 / n , with a = 13.963(7) Å, b = 7.745(3) Å, c = 16.867(6) Å, β = 94.03(4)[unk], Z = 4, R ( F ) = 0.044. 6,9-diisopropyl-[2.2] (1,4)phenanthrenoparacyclophane 2 , C 30 H 32 , crystallises in P 2 1 / n , with a = 10.779(4) Å, b = 11.402(4) Å, c = 18.245(6) Å, β = 90.85(3)[unk], Z = 4, R ( F ) = 0.046. The steric compression from the substituents at C9 and C10 causes the phenanthrene systems of both compounds to be markedly helical. Absolute torsion angles in the bay region are as large as 32°. The paracyclophane moieties show the usual distortions at the bridgehead carbons, which lie ca. 0.15 Å out of the plane of the other four atoms.