Disorder in Ho2Ti2−xZrxO7: pyrochlore to defect fluorite solid solution series

Pyrochlore (A₂B₂O₇) is an important, isometric structure-type because of its large variety of compositions and structural derivatives that are generally related to different disordering mechanisms at various spatial scales. The disordering is key to understanding variations in properties, such as magnetic behavior or ionic conduction. Neutron and X-ray total scattering methods were used to investigate the degree of structural disorder in the Ho₂Ti_2−xZr_xO₇ (x = 0.0–2.0, Δx = 0.25) solid solution series as a function of the Zr-content, x. Ordered pyrochlores (Fd$\bar{3}$m) disorder to defect fluorite (Fm$\bar{3}$m) via cation and anion disordering. Total scattering experiments with sensitivity to the cation and anion sublattices provide unique insight into the underlying atomic processes. Using simultaneous Rietveld refinement (long-range structure) and small-box refinement PDF analysis (short-range structure), we show that the series undergoes a rapid transformation from pyrochlore to defect fluorite at x ≈ 1.2, while the short-range structure exhibits a linear increase in a local weberite-type phase, C222₁, over the entire composition range. Enthalpies of formation from the oxides determined using high temperature oxide melt solution calorimetry support the structural data and provide insight into the effect of local ordering on the energetics of disorder. The measured enthalpies of mixing are negative and are fit by a regular solution parameter of W = −31.8 ± 3.7 kJ mol⁻¹. However, the extensive short-range ordering determined from the structural analysis strongly suggests that the entropies of mixing must be far less positive than implied by the random mixing of a regular solution. We propose a local disordering scheme involving the pyrochlore 48f to 8a site oxygen Frenkel defect that creates 7-coordinated Zr sites contained within local weberite-type coherent nanodomains. Thus, the solid solution is best described as a mixture of two phases, with the weberite-type nanodomains triggering the long-range structural transformation to defect fluorite after accumulation above a critical concentration (50% Ti replaced by Zr).

Combined neutron and X-ray total scattering with calorimetric measurements of the solid solution series Ho₂Ti_2−xZr_xO₇ reveals a complex order–disorder transition across short, intermediate, and long length scales induced by chemical substitution.     

Ptosis secondary to anterior segment surgery and its repair in a two-year follow-up study

OBJECTIVE: Aponeurotic blepharoptosis is a postoperative complication of anterior segment surgery with a reported incidence of 1-2% and a variable aetiology. In this 2-year follow-up study, we investigated the incidence of this postoperative complication in our experience of anterior segment surgery and propose a modified technique of aponeurosis advancement for its repair. METHODS: 200 consecutive patients undergoing anterior segment surgery in our eye clinic were enrolled in the study. Patients who developed any other operative or postoperative complication were excluded from the study. In all patients, the following upper lid parameters were calculated to determine whether postoperative blepharoptosis had occurred: margin-reflex distance, upper eyelid crease, use of frontalis muscle and levator function. A questionnaire was submitted to all blepharoptosis patients investigating mainly their subjective judgement of the impact of blepharoptosis on their quality of life and if they had been informed accurately about the incidence of this postoperative complication. RESULTS: 163 patients were included in our study. 11 had postoperative blepharoptosis (6.7%). 9 patients wanted ptosis repair and were operated on with our modified technique. None of the 11 ptosis patients had been informed about the possible occurrence of the blepharoptosis as postoperative complication. Our modified technique shows good, long-lasting results. CONCLUSIONS: Postoperative blepharoptosis is a well-known postoperative complication of anterior segment surgery. It can be successfully treated surgically by aponeurosis advancement. It is our opinion that all patients should be informed of the possibility of postoperative blepharoptosis when consenting for anterior segment surgery.     

Nonadiabatic coupling of the dynamical structure to the superconductivity in YSr2Cu2.75Mo0.25O7.54 and Sr2CuO3.3

A crucial issue in cuprates is the extent and mechanism of the coupling of the lattice to the electrons and the superconductivity. Here we report Cu K edge extended X-ray absorption fine structure measurements elucidating the internal quantum tunneling polaron (iqtp) component of the dynamical structure in two heavily overdoped superconducting cuprate compounds, tetragonal YSr₂Cu_2.75Mo_0.25O_7.54 with superconducting critical temperature, T_c = 84 K and hole density p = 0.3 to 0.5 per planar Cu, and the tetragonal phase of Sr₂CuO_3.3 with T_c = 95 K and p = 0.6. In YSr₂Cu_2.75Mo_0.25O_7.54 changes in the Cu-apical O two-site distribution reflect a sequential renormalization of the double-well potential of this site beginning at T_c, with the energy difference between the two minima increasing by ∼6 meV between T_c and 52 K. Sr₂CuO_3.3 undergoes a radically larger transformation at T_c, >1-Å displacements of the apical O atoms. The principal feature of the dynamical structure underlying these transformations is the strongly anharmonic oscillation of the apical O atoms in a double-well potential that results in the observation of two distinct O sites whose Cu–O distances indicate different bonding modes and valence-charge distributions. The coupling of the superconductivity to the iqtp that originates in this nonadiabatic coupling between the electrons and lattice demonstrates an important role for the dynamical structure whereby pairing occurs even in a system where displacements of the atoms that are part of the transition are sufficiently large to alter the Fermi surface. The synchronization and dynamic coherence of the iqtps resulting from the strong interactions within a crystal would be expected to influence this process.

More than 30 y after the discovery of unconventional superconductivity in cuprates (1) and subsequently in analogous materials its underlying mechanism and in particular the role of the lattice are still under debate. Proposed microscopic theories range from purely electronic Mott–Hubbard and t-J approaches at one extreme to Bose–Einstein condensates of bipolarons at the other (2–4). Experimentally, however, anomalous isotope effects (5), resonant ultrasound (6), angle-resolved photoemission spectroscopy (7–9), femtosecond optical pump terahertz (10)/megaelectron-volt transmission electron microscopy probe (11), infrared pump (12), and so on have demonstrated that specific phonons not only couple to the superconductivity but correlate directly with the gap energy and may even transiently induce it well above the superconducting critical temperature, T_c. Cuprates also exhibit a plethora of superstructures indicative of strong electron–lattice coupling, stripes that have been proposed to stabilize the superconductivity (13), and charge-density waves (14, 15) and the pseudogap (PG) (16) that compete with it. Another possibility is mechanisms that boost T_c from a low value expected within a conventional Bardeen-Cooper-Schrieffer (BCS) scheme. That this question remains unanswered suggests considering more unconventional approaches (4). One candidate is the dynamical structures of cuprates, S(Q, E) or experimentally S(Q, t = 0), specifically their internal quantum tunneling polarons (iqtp). An iqtp is a set of atoms oscillating between two structures that possess different geometries, energy levels, and charge distributions (17–19). A chemist would describe these endpoints as separate species, adapting this term that applies more intuitively to solutions to the atoms in crystalline solids. Neutron scattering and X-ray absorption fine structure (XAFS) measurements identified O-centered iqtps and their correlation with the superconductivity 30 y ago (17–25). We now present Cu K edge extended XAFS (EXAFS) results from “overdoped” YSr₂Cu_2.75Mo_0.25O_7.54 (YSCO-Mo) that is isostructural with YBa₂Cu₃O₇ (Fig. 1A and SI Appendix, Fig. S1), T_c = 84 K (26) and hole doping p (excess charge on the planar Cu2 site) = 0.3 to 0.5 (27) and Sr₂CuO_3.3 (SCO) that is structurally analogous to La₂CuO₄ (Fig. 1B and SI Appendix, Fig. S1), T_c = 95 K (28, 29), and p = 0.6, both synthesized via high-pressure oxygenation (HPO) (30, 31). In YSCO-Mo the Cu2-apical O (Oap) double-well potential is degenerate in the normal state but renormalizes below T_c with the energy difference between its two minima increasing with decreasing temperature by ∼6 meV. SCO is already unique among cuprates in not having intact CuO₂ planes (32, 33). Its Cu EXAFS demonstrate that it is unique among superconductors in that its Oap shift by >1 Å at its superconducting transition, challenging our conception of superconductivity as an electronic transition that is incompatible with structural transformations.Open in a separate windowFig. 1.Structures and modulus and real components of the Fourier transforms of the EXAFS spectra, χ(R), of YSCO-Mo and SCO across temperature ranges bracketing their superconducting transitions. (A) Structure representation of YSCO-Mo. The CuO₂ planes are turqoise (Cu2) and magenta (Opl), Cu-O chains are blue (Cu1) and gold (Och), Oap is red, and Sr is green. In the actual structure one-fourth of Cu1 are substituted by Mo. The orientation is shown underneath. (B) The same as A for SCO, except a significant number of Oap and half of the O sites in the a direction in the CuO₂ planes are vacant. The CuO₂ planes are blue (Cu) and gold (O). For the χ(R) spectra the blue traces denote the lowest temperatures, then green to yellow, purple, and red-orange to brown at the highest ones. (C) YSCO-Mo spectra for E of the X-ray probe beam in the aa plane, with the modulus peaks labeled with their principal sources. The first temperature above T_c is red. (D) YSCO-Mo spectra for E||c, with the Cu1- and Cu2-Oap contributions overlapping at R = 1.6 Å. The peaks at higher R are a combination of direct, two-leg path contributions from more distant neighbor atom shells and ordered multiple scattering paths. (E) SCO spectra for E||H used for the orientation that is assigned to the a direction of the orthorhombic O sublattice. T_c is red and double width. (F–I) SCO over the designated temperature ranges for E⊥H spectra that will be the contributions in the bc plane defined by the orthorhombic O sublattice. (F) The extent of the change in the spectra, and by inference in their originating structures, across the superconducting transition. The features appearing at R = 2 to 2.5 Å below T_c result from the ∼2 Å shift of the O depicted in Fig. 3 B–D. In G the first temperature above T_c is orange and double width.We have recently shown that HPO cuprates are described by their own phase diagram (34). The principal feature of the well-known one for non-HPO cuprates is the superconducting “dome” that begins at p ∼0.06, peaks at p ∼ 0.16, and ends at p ∼ 0.27. Subsequent augmentations with the microstrain in the planes (35) and hole density on the O atoms (36) explain some of the material specificity but do not modify this overall pattern. For HPO compounds the superconductivity may begin at p < 0.06 and continues to increase beyond p = 0.27 with possible flattening but no reduction in T_c. Although we have found that the excess O in YSCO-Mo is mostly taken up by domains enriched in octahedral Mo(VI) substituting in the Cu(1) chains, much of the extra charge resides in the CuO₂ planes (27) and some of the carriers constitute a normal Fermi liquid that coexists with the superconductivity (26). The inherent inhomogeneity (37) in YSCO-Mo and SCO was probed by EXAFS, which is arguably the most incisive experimental method for characterizing short-range order and is especially sensitive to its changes. Diffraction patterns originate in the long-range average structure of a material and provide precise information on the symmetry and symmetry-constrained locations of the atoms that dominate the Bragg peaks. In contrast, EXAFS—and pair distribution function (pdf) analysis—are sensitive to local order separate from the crystallographic symmetry. The element selectivity of EXAFS provides further advantages by separating the atom pairs comprising the distribution function. Especially important for this study, EXAFS measures the instantaneous structure factor, S(Q, t = 0), that incorporates the dynamic structure components, S(Q, E), observed with inelastic scattering. EXAFS therefore accesses time and energy scales corresponding to collective dynamical phenomena (25, 38, 39). Dynamical structures such as the iqtp are demonstrated when S(Q, E/t = 0) gives locations for atoms that differ from those (19, 38) obtained from diffraction and elastic scattering measurements (20–22, 40). This complementarity was the basis for the original identification of the Cu-Oap two-site distribution (41) and its assignment to the double-well potential of the iqtp (17, 19, 42).     

Nervous tissue thiamine metabolism in vivo. II. Thiamine and its phosphoesters dynamics in different brain regions and sciatic nerve of the rat

Different steps of the metabolism of thiamine (T), thiamine mono- (TMP), pyro- (TPP) and triphosphate (TTP) in the cerebellum, brainstem, cerebral cortex and the sciatic nerve were evaluated in the rat in vivo. The radioactivity of T and its phosphoesters was determined at fixed time intervals (0.5–240 h) after an intraperitoneal injection of [¹⁴C]T (30μg:1.25 μCi), under steady state conditions. The dynamics of thiamine compounds was evaluated using a compartmental mathematical model that allowed the fractional rate constants (FRC), turnover rates (TR) and turnover times to be calculated. The phosphorylation of T to TPP and the dephosphorylations of TPP to TMP and TMP to T could be estimated in all the structures investigated. Their turnover rates were found to be ordered in the sequence: cerebellum > brainstem > cerebral cortex > sciatic nerve. The transphosphorylation of TPP to TTP was so small that it could not be determined in a reliable way. Regional differences were found both in the rate and in the composition of T and TMP mixture released from nervous structures. The shortest turnover time of TPP was found in the cerebellum, while the sciatic nerve exhibited the fastest renewal of T and TMP. In all the structures investigated TPP had a rather short turnover time, suggesting that its function might be associated to a rapid conversion into chemically different forms.The possible relationships between the rates of turnover of T compounds are the sensitivity of the nervous structures to T deficiency are discussed.     

Thiamine Pyrophosphate-Dependent and Thiamine Metabolizing Enzymes in the Deafferented Cerebellum and in the Intact Cerebral Cortex of Rat

The effects of chemical (CD) and surgical (SD) deafferentation of the cerebellum on thiamine pyrophosphate (ThPP)- dependent enzymes (transketolase, TK; pyruvate-, PDH, and -ketoglutarate, KGDH, dehydrogenases) and thiamine (Th) metabolizing enzymes (thiamine pyrophosphokinase, ThPK; thiamine mono-, ThMPase, and pyrophosphatases, ThPPase) were evaluated in vitro in rats in steady state conditions. The enzymes were also determined in the intact cerebral cortex of the same animals. CD was carried out by i.p. injection of 3-acetylpyridine, followed by harmaline and niacinamide. SD was carried out by complete dissection of the peduncles of the left cerebellar hemisphere. Chemical and surgical cerebellar deafferentations significantly lowered the contents of both Th-metabolizing and ThPP-dependent enzymes in the cerebellum without modifying those of the cerebral cortex. ThPK and TK, which are particularly concentrated in neurons, were the most affected enzymes. The decrease in ThPP-dependent enzymes shown here is associated with the slowing down of the Th phosphorylation-dephosphorylation cycle and with no modification in the ThPP content in the cerebellum, both of which were found in a previous study. Cerebellar deafferentation seems to only affect the apoenzyme moiety of the enzymes.     

Thiamine content and turnover rates of some rat nervous regions, using labeled thiamine as a tracer

The content of total thiamine radioactivity in some nervous structures and liver of the rat was determined in a steady state condition, using [thiazole-2-¹⁴C]thiamine as a tracer. The contents were analyzed by a mamillary type compartmental model which enabled us to calculate the influx and efflux fractional rate constants, turnover times, turnover rates and relative accuracy. Total thiamine turnover rates of the central nervous system regions were found to be ordered in the following sequence: cerebellum (0.55 μg/g·h) > medulla and pons > spinal cord and hypothalamus > midbrain (plus thalamic area) and corpus striatum > cerebral cortex (0.16 μg/g·h). Sciatic nerve turnover rate was 0.58 μg/g·h. The turnover times were mainly between 5 and 10 h (range 2.4–16.4 h). The influx rate constants could be ordered as follows: cerebellum > hypothalamus, pons and medulla > corpus striatum, spinal cord, midbrain (plus thalamic area) and sciatic nerve > cerebral cortex. The results show in general a good agreement between turnover rate values and brain regional sensitivity to thiamine deficiency, the most vulnerable areas to thiamine depletion being those with the highest turnover rates.