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The hydrogen bond in K3H(SO4)2 was investigated using a neutron single crystal diffraction technique at room temperature and 2.3 K. The title compound crystallizes in the monoclinc space group A12/...

Impedance measurements were performed on single crystals of (NH4)3H(SO4)2 in the temperature range of 295–441 K. Anisotropy of protonic conductivity was found in the monoclinic room-temperature phase (space group C2/c) as well as in the rhombohedral high-temperature phase (space group R 3 ¯ m ). Above the high-temperature phase transition at 413 K, conductivity in the hexagonal (001) plane yields a value of ~ 10− 2 S cm− 1, which is typical for superionic conductors. A verification of a sample quality after the impedance measurements, using X-ray powder diffraction, showed minor additional peaks of (NH4)2(SO4). This suggests a beginning of thermal decomposition at high temperature. The possible pathways of proton conduction were discussed according to crystal structure analyses by single-crystal neutron diffraction method.

The temperature variation of the Co2SiO4structural parameters has been investigated by means of both neutron powder and single-crystal diffraction. Measurements were performed in a broad temperature range from 5 to 500 K. There is clear evidence of an anomalous thermal expansion related to the magnetic phase transition at 50 K. Below this temperature an abrupt elongation and contraction were observed along thecandbaxes (space groupPnma), respectively, whereas the lattice parameteraremains constant in the range between 5 and 50 K. The resulting volume of the unit cell is contracted below 50 K. This phenomenon is attributed to magnetostriction effects. A model based on the specific heat capacity data was applied for the quantitative description of the anomalous thermal expansion behaviour. On the other hand, no anomalies can be observed in the temperature dependency of the interatomic distances and angles.

A displacive, 2nd order structural phase transition at Ts=395 K from space group I`4 2 m below Ts to I 4/m c m above Ts has been discovered in the two-dimensional spin dimer compound SrCu2(BO3)2. The temperature evolution of the structure in both phases has been studied by X-ray diffraction and Raman scattering, supplemented by differential scanning calorimetry and SQUID magnetometry. The implications of this transition and of the observed phonon anomalies in Raman scattering for spin-phonon and interlayer coupling in this quantum spin system will be discussed., Comment: 13pages, 13 figures

In this paper we present a high temperature heating device, working under defined environmental conditions, for a Siemens D500 Bragg–Brentano powder diffractometer. The powder sample is prepared in a flat mould on a metal block consisting either of copper or of platinum depending on the temperature range selected for investigations. Although the heating cell can be used separately under ambient conditions up to sample temperatures of 1000 °C, it is possible to work under defined environmental conditions in the temperature range between 20 and 200 °C and up to a water vapour pressure of 1000 mbar. For that purpose a special cover for the in situ control of temperature and water vapour pressure has been constructed. It is important to note that the three sample conditions (sample temperature, gas temperature, and gas humidity) can be adjusted separately by the user. Current studies have shown that the described X-ray heating device is a powerful tool to study dehydration reactions in the frame of fundamental research as well as to understand industrially relevant processes concerning dehydration reactions and their mechanisms.

The structure of the spin-Peierls compound ${\mathrm{CuGeO}}_{3}$ has been analyzed by time-of-flight neutron powder diffraction as a function of external pressure. The structural changes allow us to explain the strong pressure dependence of the magnetic susceptibility most likely related to the pronounced pressure-induced increase of the spin-Peierls transition temperature. The structural changes induced by Si substitution are found to be opposite to those observed upon applying external pressure.

The structural phase transition in Gd 2− x Ce x CuO 4 ( x = 0, 0.12) characterized by the rotation of the CuO 4 -squares around the c axis has been studied by different diffraction techniques. The order parameter of this transition is found to increase continuously below 658(1) K for Gd 2 CuO 4 and below 707.2(7) K for Gd 1.88 Ce .12 CuO 4 with an unusual temperature dependence. The rotation of the CuO 4 -squares is coupled to a spontaneous strain along the c -axis. Structure analysis of the undistorted high temperature T′-phase in Gd 2 CuO 4 indicates persisting local distortions.

The structure of the antiferroelectric phase in TlD2PO4 is presented. The phase diagrams of both TlH2PO4 and TD2PO4 are discussed from the point of view of a structural study by neutron diffraction on single crystals, carried out at different temperatures.

Les structures cristallines de TlH 2 PO 4 et TlD 2 PO 4 ont ete determinees a 373 K par diffraction de neutrons sur des echantillons monocristallins. L'acquisition de donnees est faite avec un diffractometre 4-cercles, en utilisant la longueur d'onde λ = 0.830 A. A cette temperature les deux composes ont la meme symetrie orthorhombique decrite dans le groupe d'espace Pbcn. Cette phase haute temperature apparait comme la phase mere du diagramme de phase obtenu pour des temperatures decroissantes pour chacun des deux composes TDP et DTDP.

We present the results of neutron-diffraction experiments on a series of La2−xSrxCuO4+δ single crystals at temperatures between 540 K and 22 K. There is an intrinsic misfit between the size of the LaO9 polyhedra and the CuO6 octahedra, which seems to cause the structural transition from the high-temperature tetragonal (HTT) to the low-temperature orthorhombic (LTO) phase. Within this misfit model it is possible to explain almost the entire phenomenology of the HTT-LTO transition. The electronic structure is related to this phase transition via the length of the CuO bonds. One of the effects of the transition on the structure is to lengthen these CuO bonds and hence to alter the electronic band structure. The introduction of additional charge by doping leads to a decrease of the equilibrium CuO distances, and hence to a reduction of the misfit. In this way the charge-carrier concentration determines the structural transition. We always observe strongly anisotropic thermal parameters, which confirm the instability of the La2−xSrxCuO4 structure against a rotation of the CuO6 octahedra around any axis. In the case of an undoped crystal (x = 0.0), we find a good agreement between the experimental mean square displacements and the results of a lattice dynamics model which is based on the known phonon dispersion curves. In a doped (x = 0.13) and superconducting crystal, the anharmonic effects related to the structural transition lead to significant changes in the temperature dependence of the thermal parameters.

The atomic displacement parameters of individual ions in SrO have been determined from the Rietveld analysis of high-resolution powder neutron diffraction data. As the neutron velocity is smaller than the sound velocity in SrO, the intensity data was not corrected for the effect of thermal diffuse scattering. The room temperature value of overall isotropic thermal displacement parameters wasB=0.57(2) Å2; which corresponds to Debye temperature, Θ of 242(4) K. The results are compared with experimental and theoretical estimates.

The crystal structure of the orthorhombic high-temperature phase II (708–949 K) of KLiSO4 was analyzed by single-crystal neutron diffraction. The ambiguity concerning the symmetry of this phase is solved and a disordered structure is established in the space-group Pcmn. Hence, KLiSO4 II is confirmed to be isostructural to the highest-temperature modification I of RbLiSO4.The investigated KLiSO4 crystals were pseudohexagonally twinned with three ferroelastic domain states according to the phase transition from P63 (phase III at room temperature) to Pcmn (phase II).

The ND3 group in Ni(ND3)6Br2 is orientationally disordered at room temperature: the D3 triangle gives rise to a density distribution with maxima on the corners of a square. We explain this observation as the consequence of a coupled rotational-translational motion of the molecule in a two-dimensional anharmonic single particle potential with tetragonal symmetry. Potential parameters are calculated from experimental data by a least squares procedure. From the total crystal potential of the D3 group we conclude that a combination of center of mass motion and rotation which is possible essentially without an energy barrier, is the basic process observed in quasielastic neutron scattering observations of the jump diffusion.

The ND3 group is in the high temperature phase of the title compound dynamically disordered. Fourier maps derived from the analysis of neutron single crystal Bragg data with phases based on a split atom and a Frenkel model show uniquely that each D3 group has four density maxima occupying the corners of a square. Neither one nor three dimensional models of purely rotational disorder can describe the observations adequately.

The structure of La2NiO4.13 has been studied as a function of temperature (20–673 K) with single crystal neutron diffraction. In order to achieve a high resolution, data sets were measured up to 1.05 A −1 . Structure refinements were carried out in this space group F4/mmm. Due to the incorporated oxygen on interstitial lattice sites the apical oxygen becomes disordered and the NiO6 octahedra are partly tilted around the [110] direction. The disorder is still present at temperatures above the high temperature orthorhombic/tetragonal phase transition of stoichiometric La2NiO4.00 at 670 K.

A combined X-ray and neutron diffraction study on powder samples and single crystals revealed the following sequence of polymorphic phases: Fdd 2 ( I ) ← 147 K → I 4 2 d ( II )—357 ± 7 K → P 2 1 / a ( III ) ← 380 ± 3 K → P 2 1 / m ( IV ) . The low temperature phases I and II are iso-structural to those of KDP. The H-ordering in RDP is almost complete at Tc − 1 K, which favours a first-order character of this ferroelectric phase transition.

Neutron and high resolution X-ray diffraction investigations on perfect single crystals of RbH2PO4 (RDP), a hydrogen bonded ferroelectric of KDP type are reported. The results of crystal structure analysis from diffraction data, below and above the paraelectric - ferroelectric phase transition, support a disorder - order character of [PO4H2]--groups. The tetragonal symmetry of the paraelectric phase with the double well potential of the hydrogen atoms obtained by diffraction, results simply from a time-space average of orthorhombic symmetry. According to the group - subgroup relation between the tetragonal space group I42d and the orthorhombic Fdd2 a short range order of ferroelectric clusters in the tetragonal phase is observed. With decreasing temperature the ferroelectric clusters increase and the long range interaction between their local polarisation vectors leads to the formation of lamellar ferroelectric domains with alternating polarisation directions at TC = 147 K. From the high resolution X-ray data it is concluded that below TC the ferroelastic strain in the (a,b)-plane leads to micro-angle grain boundaries at the domain walls. The tilt angle is enhanced by an applied electric field parallel to the ferroelectric axis. The resulting dislocations at the domain walls persist in the paraelectric phase leading to a memory effect for the arrangement of twin lamellae. With increased electric field the phase transition temperature TC is decreased. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dielectric spectroscopy experiments are carried out in the ferroelectric phase of rubidium dihydrogen phosphate $({\mathrm{RbH}}_{2}{\mathrm{PO}}_{4}),$ within the frequency range $0.1 \mathrm{Hz}lfl1 \mathrm{MHz}.$ A strong dielectric time decay (ageing) dominates after thermal equilibration. However, spectra taken after long dwelling times reveal a significant dispersion below the fundamental piezoelectric resonance frequency ${f}_{\mathrm{res}}.$ The permittivity of the well-aged sample is found to decrease linearly with the logarithm of the frequency. Not detectable above the paraelectric-ferroelectric phase transition temperature ${T}_{c},$ the dispersion is observed both in the range ${T}_{c}gTg{T}_{f}\ensuremath{\approx}117 \mathrm{K}$ of the anomalously high domain wall contribution and in the low-temperature range $Tl{T}_{f}$ corresponding to the frozen-in domain-wall response. The result indicates that the concept of weak pinning in the context of the theory of elastic interfaces in disordered media may be applicable to describe the interaction between the ferroelectric/ferroelastic domain walls in ${\mathrm{RbH}}_{2}{\mathrm{PO}}_{4}$ and randomly distributed impurities.

In this work, we present a structure investigation on K3H(SO4)2 by single crystal neutron diffraction. Letovicite with a chemical composition (NH4)3H(SO4)2 belongs to a large family of M3(H,D)(XO4)2 compounds, where M = K+, Rb+, (NH4)+, Cs+, Tl+ and X = Se6+ and S6+. This compound crystallizes in the monoclinic space group A2/a with a = 9.789(7) Å, b = 5.6815(9) Å, c = 14.703(2) Å and β = 103.03(4)0at 300K. At 2.3K, the lattice parameters are a = 9.687(20) Å, b = 5.648(13) Å, c = 14.613(9) Å and β = 103.23(14)0. Data at 2.3K were measured up to (sinθ/λ) = 0.807Å-1 with the single crystal neutron diffractormeter HEiDi at the FRM-II, Germany. H/D shows a dynamic disorder at high temperature, which can be related to very high proton conductivity. In letovicite, two types of disorder related with hydrogen atoms are reported [1]. Although letovicite shows various phase transitions owing to the proton ordering at low temperature, K3H(SO4)2, without the possibility of an orientational disorder of NH4+, undergoes no phase transition at low temperature. At room temperature, the title compound is isostructural to lectovicite, and has an inversion center in the middle of the SO4-H-SO4 dimer. The bond length, 2.483(3) Å, and bond angle, 1800, support the hypothesis that the disordered proton shows a double-well potential, if the distance between the oxygen atoms of the hydrogen bond Ro-o are longer than a critical bond length rc(2.47 Å for protons and 2.40 Å for deuterons) [2]. However, it is not easy to determine if the hydrogen bond is a low-barrier hydrogen bond (LBHB) or centered hydrogen bond (centered HB). Based on an analysis of the anisotropic parameters, the bond lengths and elongation of the hydrogen atom toward the two oxygen atoms by neutron single crystal diffraction experiments at 300K and 2.3K, it seems that the hydrogen bond in the title compound can be classified as a centered hydrogen bond or intermediate form between a cigar-like shape and the disk-like shape [3].

Direct structural evidence of the periodic ordering of neutral and ionic planes is reported for 2,6-dimethyltetrathiafulvalene-p-chloranil, detected by high-resolution x-ray diffraction experiments. Simultaneous to the neutral-ionic layered ordering, theoretically predicted a long time ago to occur in stages, an antiparallel dimerization ordering takes place, leading to a ``ferrielectric'' structure. Both intramolecular deformations and intermolecular contacts, related to the charge-transfer ordering and to the dimerization process, are discussed and compared with results obtained for the prototype compound of the neutral to ionic transition, tetrathiafulvalene-p-chloranil. Thus essential insights are provided for an understanding of the mechanism of this unusual electronic-structural instability, and important physical features are discussed such as the interstack interactions, the coupled order parameters, and the interplay between the staging and condensation mechanisms.

A crystal structure analysis of the mixed crystal Rb 0.51 Tl 0.49 H 2 PO 4 was conducted at ambient temperature by neutron diffraction analysis using a high-quality single crystal grown from an aqueous solution. RbH 2 PO 4 (RDP) and TlH 2 PO 4 (TDP) exhibit clearly different crystal structures from each other, although Rb + and Tl + have almost the same ionic radii. The crystal structure of the title compound, i.e., Rb 0.51 Tl 0.49 H 2 PO 4 , is isotypic to TlH 2 PO 4 , having partially disordered H atoms in strong symmetric O…H…O hydrogen bonds. The lone-pair electrons of Tl + ions are considered to have an effect on the stabilization of the two-dimensional network of hydrogen-bonded PO 4 groups. Although the difference in chemical composition is very subtle, in contrast to the previous case of Rb 0.46 Tl 0.54 H 2 PO 4 , the distribution of the lone-pair electrons seems to change and the hydrogen bond angle, especially for disordered hydrogen atoms, also shows significant changes.

Structure of a LiH 2 PO 4 (LDP) single crystal has been studied at room temperature and at 100 K by means of neutron diffraction. The LiO 4 tetrahedra in LDP are linked by vertices forming chains along the a -axis, and a three-dimensional network is formed by PO 4 groups connected by two different hydrogen bonds. The Li + ions in this structure were found to be quite immobile in spite of their very light mass; The very high proton conductivity in the system may have directly to do with the relatively weak hydrogen bonds and the relatively long interoxygen distances facilitating reorientation of the proton acceptor complexes, rather than with the Li + ionic motions.

Crystal structure analysis of a mixed crystal Rb 1- x Tl x H 2 PO 4 ( x = 0.5) was performed at room temperature by neutron diffraction on a high-quality single crystal grown from aqueous solution. RbH 2 PO 4 (RDP) and TlH 2 PO 4 (TDP) show distinct crystal structures despite the same ionic radii of Rb and Tl. The crystal structure of Rb 0.5 Tl 0.5 H 2 PO 4 is isotypic to TDP showing disordered H atoms in the O … H … O hydrogen bonds. The lone-pair electrons of Tl + ions are believed to stabilize the two-dimensional network of hydrogen bonded PO 4 -groups. In contrast to the case of TDP, Rb 0.5 Tl 0.5 H 2 PO 4 shows no phase transition at low temperatures despite the disordered character, with a strengthening of two hydrogen bonds as indicated by shorter O…O distances and greater O … H … O bond angles.

In order to obtain superconductivity in RE 2 CuO 4 (RE rare earth: Pr, Nd, Sm, Eu) all compounds have to be doped with Ce substituting the RE, additionally it is necessary to anneal the samples at high temperature in a reducing atmosphere. Single crystal neutron and X-ray diffraction was used to study the reduction effects in Nd 2− x Ce x CuO 4± y (with x = 0 and 0.15). The study of as grown, reduced and oxygenated samples indicate different structural changes in doped and undoped samples. The oxygen loss in the CuO 2 planes of Nd 2 CuO 4− y ( y = 0.056(8)) and of Nd 1.85 Ce 0.15 CuO 4− y ( y = 0.000(8)) is obtained from structural refinements of the same single crystals before and after reduction, and after reoxidation. The second oxygen site remains unchanged with respect to thermal parameters and occupancy. The strong changes in extinction effects of the samples were observed to be reversible with oxidation/reduction treatments.

The analysis of the density distribution of protons of methyl groups obtained by neutron diffraction on single crystals of 4-methylpyridine N-oxide and 2,6-dimethylpyrazine is reported.

The structural deformation of the T' phase has been studied by means of neutron, x-ray and electron diffraction. A complete data collection of Bragg intensities on a single crystal of Gd 2 CuO 4 (GdCO) allows to determine the nature of the distorted phase. The oxygen squares surrounding the Cu-sites are rotated around the c-axis (by 5.2° in GdCO at room temperature). The same structural distortion has also been observed in Nd 0.38 Tb 1.62 CuO 4 (NdTbCO) and in (Nd 1− x Tb x ) 1.85 Ce .15 CuO 4 for x > 0.5. Therefore, one may conclude that the distortion results from a reduced average ionic radius of the RE. The phase diagram of Nd 1.85− x Tb x Ce .15 CuO 4 concerning superconductivity and the structural distortion has been examined.

The electrochemical oxidation of La 2 CuO 4 was studied at room temperature in an aqueous alcali electrolyte by in situ neutron diffraction on polycrystalline electrodes. Structure analysis was undertaken for several phases with different oxidation levels at different temperatures. For the maximum oxidized phase with a T c of 44 K an increased orthorhombicity is observed. For this phase the superstructure reflections related to the tilting of the CuO 6 octahedra were no longer observed even on cooling to 4 K. The structure refinements of the 44 K phase imply an average structure in the space group Fmmm with a partial lost of the 3D-order of the CuO 6 octahedra.

The influence of the rare earth (RE) on the structure of the T′ phase has been investigated by single crystal neutron diffraction. There is an intrinsic instability of the T' phase against a deformation of the CuO 2 planes which is characterised by the rotation of the O-squares around the c-axis. As it can be seen from the atomic displacement parameters of the in plane oxygen, this instability increases with decreasing ionic radius of the RE. The evolution of structural parameters as a function of temperature has been measured for REPr and Nd in order to analyse statical contributions to the Debye-Waller factors. The oxygen loss in the Cu-planes of Nd 2 CuO 4−δ is derived ( δ =0.046(13)) from the structural parameters obtained for the same single crystal before and after reduction. The other oxygen site remains unchanged with respect to thermal parameters and occupancy. The existance of an extra apical oxygen claimed in the literature has not been observed.

We present results of single crystal neutron diffraction experiments on La 2− x Sr x CuO 4− δ . The analysis of the temperature dependence of the LaO bond lengths shows that almost the entire thermal contraction of La 2 CuO 4 between 540 K an 50 K is due to the decrease of the LaO 9 polyhedron, the CuO bond distances remain almost constant. Doping of La 2 CuO 4 with Sr (or additional O) decreases both CuO bond lengths. The large thermal parameters especially of the apical O indicate the instability of all examined crystals against a second phase transition.

We employed transmission electron microscopy (TEM) and single crystal X-ray diffraction techniques, supplemented by differential thermal and thermo-gravimetric analysis (DTA, TG) and high temperature powder X-ray diffraction, to study microtwinning in orthorhombic crystals of the high-T c superconductor YBa2Cu3O8−x (x≅1.0). This twinning is associated with a structural phase transition at 750°C from a tetragonal high temperature phase (s.g.P4/mmm) to the orthorhombic ambient temperature phase (s.g.Pmmm) and seems to be inherent to virtually all orthorhombic crystals of YBa2Cu3O8−x . The domain size ranges from typically 100 A to 1000 A. All our observations are compatible with a twin law where the tetragonal (110)-mirror plane is the twin element.

The crystal structure of the low temperature phase of anilinium bromide, C6H5NH3 ⊕Br⊖, was studied by neutron diffraction at T = 100 K. The refinement supports an ordered structure. The structures of the low and high temperature phases are compared and the mechanism of the phase transformation is discussed.

The crystal structure of strontianite (a = 5.1039(3), b = 8.4022(4), c = 6.0220(4) Å ; Z = 4 ; space group : Pmcn) was refined with 520 independent neutron diffraction data to RW = 0.011 . The aplanarity of the CO3 group, d = 0.0218(5) Å, is smaller by 0.006 Å than that in aragonite ; the average C-O bond length turns out to be greater by 0.003 Å than in aragonite., La structure de la strontianite ((a = 5,1039(3), b = 8,4022(4), c = 6.0220(4) Å ; Z = 4 ; groupe spatial : Pmcn) a été affinée en utilisant 520 données indépendantes de diffraction de neutrons, RW = 0,011. L'écart à la planéité du groupement CO3, d = 0,0218(5) Å, est réduit de 0,006 Å par rapport à l'aragonite ; la longueur moyenne de liaison C-O est plus grande de 0,003 Å que dans l'aragonite., Jarosch Dietmar, Heger Gernot. Neutron diffraction investigation of strontianite, SrCO3. In: Bulletin de Minéralogie, volume 111, 2, 1988. pp. 139-142.

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal- and defect-structure of YBa2(Cu0.93Fe0.05)3O7. Crystals of this compound are superconducting (T c ≈80 K) and appeart to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu3O7 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site too, leading to the approximate formula YBa2(Cu0.97Fe0.03)2(Cu0.86Fe0.1□0.04)O7. High resolution electron microscopy reveals the existence of orthorhombic micro-domains, 20–30 A in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.

Combined information from single crystal X-ray as well as powder and single crystal neutron diffraction on YBa 2 Cu 3 O 6 and YBa 2 Cu 3 O 7 samples strongly suggests that a considerable amount of copper deficiency (several percent), presumably due to vacancies, exists on the Cu(1) site in these compounds. The observation that even O 7 -samples containing as much as 9% vacancies on the Cu(1) site are still superconducting at liquid nitrogen temperature sheds a new light on the importance of the Cu(1)-O(1) chains in the 1:2:3 superconductors.

The crystal structure of non-superconducting, tetragonal YBa2Cu3O6+ɛ (ɛ close to 0) (s.g.P4/mmm) was studied at room temperature by means of single crystal X-ray diffraction, electron microscopy and electron diffraction. Crystals may easily be obtained by heating YBa2Cu3O7 samples under vacuum. For ɛ≈0, the tetragonal single crystals are chemically and crystallographically homogeneous — without any twin domains. The only defects immediately apparent are stacking faults alongc. The structure is very closely related to that of the high-T c superconductor YBa2Cu3O7. The most important difference is, that the oxygen on theb-axis (O(1)), which together with Cu(1) forms Cu−O−Cu−O-chains alongb in YBa2Cu3O7, is vacant in YBa2Cu3O6. Some crystals show a\(2\sqrt {2a} \times 2\sqrt {2a} \) superstructure with superstructure reflections which are sharp in thea*−a*-plane but form diffuse streaks alongc*. We propose a model which relates these superstructure reflections to a 2-dimensional ordering of residual oxygen atoms on the O(1) site (corresponding to ɛ>0 in the chemical formula YBa2Cu3O6+ɛ).

The investigation of the orthorhombic superconductor YBa/sub 2/Cu/sub 3/O/sub 7//sub +- //sub epsilon/ by high-resolution electron microscopy (HREM) shows the existence of two sorts of domains: twinned domains and oriented domains forming slices perpendicular to the c axis. The interpretation of these domains and of their boundaries is given, based on the simulated HREM images previously made. It is shown that the orientation of the (CuO/sub 2/)/sub infinity/ planes formed by the CuO/sub 4/ square planar groups plays an important role. A phenomenon of misorientation of those domains is also observed and interpreted.

Properties of a series of Zn-doped YBa2Cu3O7-samples prepared under identical conditions have been studied by powder neutron diffraction (20 K, 270 K), powder X-ray and electron diffraction (290 K), by measurements of specific heat Cp (1.4 K–300 K) and Meissner effect (4 K–100 K). Tc decreases linearly at about 10 K /at% Zn. The Meissner signal is still present for high Zn contents. In contrast to this, the transition in Cp vanishes at x≈0.03. Zn is quantitatively dissolved even at x=0.1. The Cu(1)-(chain-) site incorporates about 50% of the total Zn content, while the two equivalent Cu(2)-(plane-) sites contain 25% each. The occupancy of the O(1)-(chain-) site decreases with increasing Zn content according to the approximate formula YBa2(Cu1-xZnx)3O7-x/2. This leads to an almost constant average copper oxidation state, independently confirmed by iodometric titration. Our findings are discussed in terms of a transition to gapless superconductivity upon Zn-doping.

Single crystal- and powder X-ray diffraction, powder neutron diffraction at room temperature and 3 K, high resolution electron microscopy and electron diffraction were used to study the crystal and defect-structure of YBa 2 (Cu 0.95 Fe 0.05 ) 3 O 7 , Crystals of this compound are superconducting (T c 80 K) and appear to be tetragonal down to at least 3 K. The structure resembles that of the undoped YBa2Cu307 phase with the oxygen content being very close to 7.0 and the Fe atoms preferentially occupying the Cu(1) site. A copper deficiency is shown to be present on this site. too leading to the approximate formula YBa 2 (Cu 0.97 Fe 0.03 ) 2 (Cu 0.86 Fe ·1 [1 0.04 )O 7 . High resolution electron microscopy reveals the existence of micro-domains, 20–30 d in diameter, probably due to short range ordering of oxygen atoms on the O(1) site. In view of these results the structure has to be regarded as being tetragonal only in a statistical sense.