Band 216, Heft 2

Based on quantum mechanical calculations we predict the elastic constants of kyanite at 0 K. The reliability of the prediction has been evaluated by computing the elastic constants of andalusite and of sillimanite and comparing them to experimentally determined values. The computed bulk moduli of andalusite (145 GPa) and of sillimanite (159 GPa) are constistent with experimental values. Only two of the computed elastic contants c 13 of andalusite and c 23 of sillimanite differ from the experimental values by more than 11%. As the parameter-free model is transferable, the predictions for the bulk modulus, B = 178 GPa, and the elastic constants of kyanite are believed to be equally reliable. In contrast to the promising results of our quantum mechanical calculations, the agreement with experimental values is poor for elastic properties derived from a transferable empirical core-shell model.

The recent progress of the accurate charge density studies by the Maximum Entropy Method(MEM) utilizing X-ray powder diffraction is reviewed with some examples. Results for PrBCO (PrBa 2 Cu 3 O 7-δ ), YBCO (YBa 2 Cu 3 O 7-δ ), C 60 superconductors (Rb 2 CsC 60 , K 2 RbC 60 , Na 2 RbC 60 ) and the layered manganite, NdSr 2 Mn 2 O 7 , which is well known as colossal magnetoresistive (CMR)-related material, are given. For non-super conductor, PrBCO, it is found in the MEM charge density that there exist directional robes of the charge density from Pr atom toward the O atoms in the CuO 2 conduction planes. On the other hand, for a very well known high Tc super conductor, YBCO, appreciable charge densities in the interatomic region around the Y atom is not recognized in the MEM charge density. The distinct difference of the charge densities between PrBCO and YBCO presents clear experimental evidence of the hybridization between Pr(4f)–O(2p π ) orbitals which supports the idea that the hole trapping by the hybridizes states suppresses the superconductivity in PrBCO. The MEM charge densities of the fullerene superconductors, Rb 2 CsC 60 , K 2 RbC 60 and Na 2 RbC 60 , show distinct structural differences from that of non-superconductors, C 60 and Li 2 CsC 60 , reflecting the superconducting properties. And the charge deficiencies of the doped metal atoms, which should be associated with charge transfer from the metal atoms to the C 60 molecule seems to have strong correlation to the superconducting transition temperature, Tc. As the bigger the charge charge trnsfer, the higher the Tc. In the accurate MEM charge density of antiferromagnetic manganite, NdSr 2 Mn 2 O 7 , the direct imaging of spontaneous ordering of the dx 2 - y 2 orbital indicating anisotropic exchange couplings between the local-spins on the Mn sites, which causes an unique layered-type spin order. The theoretical background of the MEM is also mentioned in some detail.

Structure determination or refinement by diffraction methods on solid solutions containing more than one type of central ion in certain ployhedra of their crystal structures, e.g. (M 1-x M x ′)O 6 octahedra, yield crystal-averaged mean distances R̅(M 1-x M x ′)-O depending on the site fraction x M′i of the substituting metal ion M′, which are equal or close to the interpolations between the mean distances in the respective and member polyhedra. However, such results represent virtual crystal approximations as they imply that no structural relaxation occurs to balance the different sizes of M and M′. Another extreme would be the hard sphere model, implying full structrual relaxation. In this case, R̅ M-O and R̅ M′-O retain the end member values independent of x M′i , and the results of the virtual crystal approximation obtained by diffraction methods, are to be interpreted as fractional distance sums x M′ *R̅ M′-O +(1- x M′ )*R̅ M-O . The real local situation is expected between these extremes, though the crystal averaging diffraction methods of structure research do not see that. This paper shows that the required local polyhedral distances in solid solutions in the case of substituitive 3d N transition metal ions, may be obtained by evaluating their crystal field parameters 10 Dq as extracted from their dd-transitons in electronic absorption spectra of the respective crystals. For this purpose, 10 Dq x M′ at a site fraction x M′i is to be extracted from measured spectra and reference values of (10 Dq M′i ) x=1 and (R̅ (M′-O)i ) x=1 in the M′-bearing end member of the solid solution series have to be known or measured. The evaluation of single crystal electronic spectra as well as of refinements of data from X-ray diffraction on crystals of a number of different 3d N -ion bearing solid solution series as viridine, piemonite, (Ni,Mg)-olivine, rubbies, (Fe,Mg)-chloritoid, prove that in all these cases R̅ (M′-O)i strongly deviates from the virtual crystal approximation towards the fully relaxed situation of the real crystal. The lattice relaxations for infinite dilution, i.e. lim x M′i → 0, in corundum, andalusite, Mg-chloritoid and olivine are appreciable but do not strongly differ from each other, 0.52 < ε < 0.76. That the relaxation in corundum (ruby) is also in this range indicates that there is no strong compression of Cr 3+ -ions substituting for Al in this "hard structure".

The reciprocal lattices of a range of rare earth titanate and zirconate pyrochlores have recently been shown to exhibit an extremely characteristic diffuse intensity distribution identical in form to that characteristic of β -cristobalite implying the existence of β -cristobalite-like orientational disorder of essentially rigid, (O2)(R 3+ ) 4 tetrahedra within the pyrochlore structure type. A single crystal X-ray diffraction study of La 2 Zr 2 O 7 has been carried out utilizing synchrotron radiation in order to investigate this proposed orientational disorder and to determine an unknown oxygen atom fractional co-ordinate. An annular distribution of residual electron density is found around the average La ion positions in {111} planes perpendicular to the local O2–La–O2 directions and interpreted as implying that the La ions never in fact occupy the average 16d positions but rather are dynamically disordered around an annulus perpendicular to that direction in an exactly analogous manner to that which occurs in β -cristobalite.

The structures of a number of complex inorganic systems, including the positions of the hydrogen atoms, have been refined using ab initio density functional modeling of interatomic interactions. The calculated O–H, N–H, P–H bond lengths in various structures, as well as the geometry of various hydrogen bonds, satisfy basic crystal chemistry requirements and represent a substantial improvement over the experimentally measured ones.

The thermal stability of completely Zn 2+ exchanged zeolites A and Y during calcination is studied from R.T. to 1350°C. The phase formation processes are analysed by in situ X-ray diffraction experiments as well as by quenched samples analysed and quantified by Rietveld refinements. The framework structures of the exchanged zeolites A and Y are observed to collapse above 850°C. At higher temperatures the crystallisation of Zn- β -quartz, mullite, and gahnite is initiated from the amorphous phase. Above 1050°C quartz appears and Zn- β -quartz disappears. The crystallisation of willemite is only observed for ZnA, mullite is not stable above 1150°C in both systems. Further increase in temperature leads to the crystallisation of cristobalite accompanied by the disappearance of quartz. The final crystalline components at 1350°C are, for both zeolite precursors, gahnite and cristobalite. Differences between the zeolite precursors are the amounts of amorphous phase formed, indicating a suppression of the formation of a crystalline SiO 2 phase in the ZnY samples.

The crystal and molecular structure of 1-(2,4,6-tri-tert-butylphenyl)-4,4-bis(dimethylamino)-1-aza-2,3λ 3 -phosphabutadiene-1,3, Ttb-N=P-P=C(NMe 2 ) 2 , has been determined (crystal data of C 23 H 41 N 3 P, C 2/ c ; Z =8, monoclinic, a = 17.252(7), b = 9.014(3), c = 32.095(8) Å, β = 94.00(3)). The main geometrical parameters of 1 are: N=P 1.582(5), P-P 2.130(3), P=C 1.808(6) Å, CNP 123.8(4), NPP 111.6(2), PPC 94.1(2), CNPP -0.7, NPPC 176.8 o . Peculiarities of the molecular structure 1 (in particular, significant shortening of P-P single bond and realization of C-N=P-P bonds system in sterically unfavourable cis -form) are discussed on the basis of ab initio calculations of the model phosphabutadiene systems.

Crystal structure solution of organic compounds without heavy atoms by powder diffraction method is still a challenging task, particularly when there is not a rigid group in the compound, or when there are more than one molecule in the asymmetric unit. In this paper the crystal structure solution of the complex of “proton sponge” 1,8-bis(dimethylamino)naphthalene (DMAN) and p -nitrosophenol: C 14 H 19 N 2 + ‧ C 6 H 4 (NO)O – ‧ C 6 H 5 (NO)OH is presented. This structure, with 25 atoms and three independent molecules in the asymmetric unit, was solved from powder data by “pseudo-atom” method. The space group Pnma , Z =4, a =12.2125(5) Å, b =10.7524(7) Å, c =18.6199(14) Å, R wp and R F are 12.8 and 16.0%, respectively.

The structure determination of 5-carboxy-2-thiouracil shows it to exist in the lactam-thione form. Extensive hydrogen bonding involving all possible donors and acceptors leads to the formation of interconnected 8- (x2), 10-, 16- and 20-membered rings and a two-dimensional layer structure; no evidence for base stacking was found.