Comprehensive characterization of the structure of Zr-based metallic glasses.

Structure of metallic glasses fascinates as the generic amorphous structural template for ubiquitous systems. Its specification necessitates determination of the complete hierarchical structure, starting from short-range-order (SRO) → medium-range-order (MRO) → bulk structure and free volume (FV) distribution. This link has largely remained elusive since previous investigations adopted one-technique-at-a-time approach, focusing on limited aspects of any one domain. Reconstruction of structure from experimental data inversion is non-unique for many of these techniques. As a result, complete and precise structural understanding of glass has not emerged yet. In this work, we demonstrate the first experimental pathway for reconstruction of the integrated structure, for Zr 67 Ni 33 and Zr 52 Ti 6 Al 10 Cu 18 Ni 14 glasses. Our strategy engages diverse (× 7) multi-scale techniques [XAFS, 3D-APT, ABED/NBED, FEM, XRD, PAS, FHREM] on the same glass. This strategy complemented mutual limitations of techniques and corroborated common parameters to generate complete, self-consistent and precise parameters. Further, MRO domain size and inter-void separation were correlated to identify the presence of FV at MRO boundaries. This enabled the first experimental reconstruction of hierarchical subset: SRO → MRO → FV → bulk structure. The first ever image of intermediate region between MRO domains emerged from this link. We clarify that determination of all subsets is not our objective; the essence and novelty of this work lies in directing the pathway towards finite solution, in the most logical and unambiguous way.

Polyether phases of formic acid revealed under high pressure.

Evolutionary structure searches reveal that at 20 GPa, formic acid (FA) transforms to an orthorhombic ether-chain polymer phase, ruling out hydrogen bond symmetrization, followed by a novel crowned cyclic-ether tetragonal phase above 60 GPa. Emergence of characteristic polyether and new OH stretching modes in infrared experiments validate the findings. Resemblance of polymer chain with a cosmopolymer polyoxymethylene, shows an engrossing multifaceted evolution of FA.

Design of need-based phosphors and scintillators by compositional modulation in the ZnGa2-xAlxO4:Cr3+ spinel: pure compound versus solid solutions.

Materials that can depict persistent deep red light under both ultraviolet (UV) and X-ray illumination can be a boon to sustainable economy, particularly for optical imaging, solid state lighting, and anticounterfeiting applications. Herein, we have made a series of compounds starting from ZnGa2O4:Cr3+ to ZnAl2O4:Cr3+ (individual spinel) by substituting the varied concentration of Al3+ in place of Ga3+ in ZnGa2-xAlxO4:Cr3+ (solid solution). By virtue of the structural and defect engineering doping strategy, the photo and radioluminescence are expected to be improved. Both Cr and Al doping was found to be energetically favorable in ZnGa2O4, where the same does not hold true for Ga doping in ZnAl2O4, as indicated by the DFT-calculated defect formation energies. There seems to be ordering around the dopant ion in the solid solutions compared to either ZnGa2O4 or ZnAl2O4 and is also reflected to as lower persistent luminescence (PerL) lifetimes. PerL under UV, in general. was found to be lower with the enhancement in the Al3+ content endowed by the formation of Cr-Cr ion pair, lower probability of antisite formation, and widening band gap. On the other hand, X-ray excited emission enhances in the solid solution due to the decrease in cation inversion and associated defects. Confocal Microscopy showed that larger particles depicted much brighter deep red emission but failed to percolate to the human cells to a detectable limit; hence, future work is needed for the functionalization of the ZnGa2-xAlxO4:Cr3+ spinel. This work could be of great implication in designing need-based materials, where UV and X-ray excitation is required, for deep red emission with persistent characteristics from chromium-doped spinels.

Multiphoton light emission in barium stannate perovskites driven by oxygen vacancies, Eu3+ and La3+: accessing the role of defects and local structures.

Defect engineering in perovskites has been found to be the most efficient approach to manipulate their performance in ultraviolet-to-visible photon conversion. Under UV irradiation, BaSnO3 exhibited multicolor photoluminescence (MCPL) in the bluish white region. Its origin has not been well studied in the literature and has been probed in this work using synchrotron radiation, positron annihilation and density functional theory. To achieve desirable performance of doped BaSnO3 in optoelectronics, it is imperative to have correct information on the dopant local site, doping induced defect evolution and efficacy of host to dopant energy transfer (HDET). Extended X-ray absorption fine structure (EXAFS) showed that Eu3+ ions stabilize at both Ba2+ and Sn4+ sites consistent with the highly negative formation energy of around -6.26 eV. Eu3+ doping leads to an intense 5D0→7F1 orange emission and a feeble 5D0→7F2 red emission and an internal quantum yield (IQY) of ∼21% mediated by ET from the defect level of EuBa and EuSn sites to the valence band maximum (VBM). X-ray absorption near edge structure (XANES) ruled out any role of Sn2+ in the PL of BaSnO3 or Eu2+ in the PL of BaSnO3:Eu3+. Interestingly, when co-doped, Eu3+ stabilizes at Sn4+ sites whereas La3+ stabilizes at Ba2+ sites with a formation energy value of -6.44 eV. Based on the asymmetry ratio in emission spectra, it was found that La3+ ions lead to lowering of symmetry around Eu3+ due to increased vacancies and structural distortions, and also suppress the luminescence IQY. We have performed experimental positron annihilation lifetime spectroscopy (PALS) to probe the defects in BaSnO3 in pristine samples and on doping/co-doping. The positron lifetimes for saturation trapping of positrons in various kinds of defects envisaged in BaSnO3 and in the defect free system were calculated using the MIKA Doppler program. Such deep insight into the effect of local structures, dopant sites, defect evolution, ET, etc. on the optical properties of BaSnO3 is expected to provide very deep insight for material scientists into the fabrication of perovskite-based optoelectronic and light-emitting devices.

First principles investigations on the controversy of structural phase transitions in thorium dialuminide under pressure.

We study the high pressure structural aspects of thorium dialuminide, ThAl2, by performing evolutionary crystal structure searches and first principles calculations. We predict a phase transition from the ambient AlB2-type hexagonal structure to a P63/mmc hexagonal structure above 2.4 GPa. We also note that the new structure remains stable up to 50 GPa. This differs from the finding of an earlier study in which three phase transitions were reported close to 5.5, 12 and 25 GPa. An analysis of the electronic structures, Bader charge and electron localization function (ELF) indicates a complex chemical bonding comprising partial metallic, ionic (Th-Al) and covalent (Al-Al) characters in the ambient phase. Comparison of crystal parameters and ELFs for ambient and high pressure phases indicates that a change of Al-Al bonding under pressure is responsible for the structural phase transition. Furthermore, we computed the high P-T phase diagram by including the temperature effects via free energies for the most relevant structures, namely, AlB2-type, MgCu2-type and new hexagonal. We found that a MgCu2-type structure becomes stable only under high P-T conditions. We also provide a plausible explanation for the observed isostructural transition in the ambient AlB2-type structure at 5.5 GPa.

Pressure induced multi-centre bonding and metal-insulator transition in PtAl2.

A study on pressure induced metallization or metal-insulator transition (MIT) of materials is of enormous importance in high pressure research not only in view of various possible device applications but also for basic physical insight into their mechanism. Though the former transition (metallization) is quite common, the latter one (MIT) is comparatively rare, and only a few simple metals (Na, Al, etc.) are predicted to be an insulator at megabar pressure. In this work, we have shown that the binary intermetallic compound (PtAl2) exhibits a pressure induced metal-insulator transition comparatively at low pressure, near ∼28 GPa. By means of first principles calculations, we have established that a unique multi-centre bonding is developed under pressure which causes charge-density-ordering in the system and finally leads to MIT and structural phase transition involving unit cell reduction. In search of some exotic properties, we have found that the system has a good thermoelectric figure-of-merit in its insulating phase.

A unique metallic phase of H3S at high-pressure: sulfur in three different local environments.

We propose a new metallic phase (R3[combining macron]m, Z = 24) for H3S in a large pressure interval (∼108-166 GPa) using an evolutionary crystal structure search algorithm combined with first-principles calculations. This structure consists of SH6, SH3 and S units which are connected through strong SH hydrogen-bonds. It supports four-types of SH hydrogen-bonds that symmetrize at ∼166 GPa. This is the only phase in the H-S family where sulfur exists in three different local environments. Sulfur in SH6 behaves as a cation, like in the SF6 molecule, whereas other sulfurs behave as anions. Hydrogens in SH6 behave like halogens (anions) whereas other hydrogens behave like alkalis (cations). The new structure has a substantially smaller DOS at the Fermi-level in comparison to earlier structures R3m and Im3[combining macron]m. This implies a lower superconducting temperature (TC) for the new phase and thus questions the validity of earlier high-TC explanations. We also find that S-H covalent bonds are different from the B-B bonds of MgB2.

Structural Phase Transitions in PtIn2 at High Pressure: A Theoretical Investigation.

A study of the bonding and electronic properties of intermetallics under pressure is crucial in the design and development of novel materials for useful applications. These properties are largely controlled by the underlying crystal structures. Here we investigated the high-pressure structural behavior of PtIn2 using crystal structure search calculations, which efficiently combine evolutionary algorithms and state-of-the-art density functional theory. Three new crystal structures, namely, Fe2B-type (I4/mcm, Z = 2), cotunnite-type (Pnma, Z = 4), and monoclinic (C2/m, Z = 2), are proposed at about 9.4, 13.5, and 47.5 GPa, respectively. With pressure, the covalent character of the Pt-In pair interaction is found to increase because of enhancement of spd hybridization, and structural transitions are rationalized in terms of the covalency increase.

Prediction of a novel 10-fold gold coordinated structure in AuIn2 above 10 GPa.

A 10-fold gold coordinated tetragonal structure is predicted for AuIn2 above 10 GPa by employing the first-principles crystal structure search method. This structure remains the lowest enthalpy structure up to the highest pressure of this study. Detailed electronic structure analysis is carried out to figure out the underlying factors responsible for the transition. Pressure induced electronic topological transition is found to be one of the main factors behind this transition. Phonon calculations show the softening of TA mode phonons and destruction of the giant Kohn anomaly in close proximity to the transition. Bader charge analysis shows the charge transfer increase from the In to Au atom under pressure. So this study has solved a long-standing structural puzzle of the AuIn2 above 10 GPa. This study is also expected to play an important role in our understanding of the pressure induced metallization of geophysically relevant oxides such as SiO2 and TiO2.

Role of 5f electrons in the structural stability of light actinide (Th-U) mononitrides under pressure.

Pressure induced structural sequences and their mechanism for light actinide (Th-U) mononitrides were studied as a function of 5f-electron number using first-principles total energy and electronic structure calculations. Zero pressure lattice constants, bulk module and C11 elastic module vary systematically with 5f-electron number implying its direct role on crystal binding. There is a critical 5f-electron number below which the system makes B1-B2 and above it B1-R3̄m-B2 structural sequence under pressure. Also, the B1-B2 transition pressure increases with increasing 5f-electron number whereas an opposite trend is obtained for the B1-R3̄m transition pressure. The ascending of N p anti-bonding states through the Fermi level at high pressure is responsible for the structural instability of the system. Above the critical 5f-electron number in the system a narrow 5f-band occurs very close to the Fermi level which allows the system to lower its symmetry via band Jahn-Teller type lattice distortion and the system undergoes a B1-R3̄m phase transition. However, below the critical 5f-electron number this mechanism is not favorable due to a lack of sufficient 5f-state occupancy and thus the system undergoes a B1-B2 phase transition like other ionic solids.

Structural, vibrational, elastic and topological properties of PaN under pressure.

The electronic, structural, vibrational and elastic properties of PaN have been studied at both ambient and high pressures, using first principles methods with several commonly used parameterizations of the exchange-correlation energy. The generalized gradient approximation (GGA) reproduces the ground state properties satisfactorily. The high pressure behavior of the acoustic phonon branch along the [1, 0, 0] and [1, 1, 0] directions and the C44 elastic constant are anomalous, which signals a structural transition. With GGA exchange-correlation, a topological transition in the charge density occurs near the structural transition, which may be regarded as a quantum phase transition, where the order parameter obeys a mean field scaling law. However, here it is found that the topological transition is absent when other exchange-correlation functionals are invoked (local density approximation (LDA) and hybrid functional). This constitutes an example of GGA and LDA leading to qualitatively different predictions, and therefore it is of great interest to examine experimentally whether this topological transition occurs.

Study on the electronic structure and hydrogen adsorption by transition metal decorated single wall carbon nanotubes.

The ground state geometry and electronic structure of various 4d transition metal (TM) atom (Y, Zr, Nb and Mo) decorated single wall carbon nanotubes (SWCNTs) are obtained using density functional theory and the projector augmented wave (PAW) method. We found a systematic change in the adsorption site of the transition metal atom with increasing number of d electrons. We also predicted that Y and Zr decorated SWCNTs are metallic whereas Nb and Mo decorated SWCNTs are semiconducting. From detailed electronic structure and Bader charge analysis we found that the systematic variation of the adsorption site with the number of d electrons is related to the decreasing amount of charge transfer from the TM atom to the SWCNT along the 4d series. We have also studied the hydrogen adsorption capabilities of these decorated SWCNTs to understand the role of transition metal d electrons in binding the hydrogen molecules to the system. We found that metallic SWCNT + TM systems are better hydrogen adsorbers. We showed that the hydrogen adsorption by a TM decorated SWCNT will be maximum when all the adsorptions are physisorption and that the retention of magnetism by the system is crucial for physisorption.

Structure of the B2 phase in the Ti-25Al-25Zr alloy: a density functional study.

We report a first-principles density functional theory study of the B2 phase structure in the Ti-25Al-25Zr alloy. Several structural models with different site occupancies have been investigated. In the ideal B2 phase, the Zr atom is found to preferentially occupy the Ti site. However, if the atoms are allowed to relax, Zr occupies both Ti and Al sites, leading to local positional disorder in the structure due to the large size differences of the constituent elements. Further, this alloy is predicted to have better room temperature ductility, enhancing its utility in several technological applications.

Quality of life and alteration in comorbidity following laparoscopic adjustable gastric banding.

OBJECTIVES: Obesity is an increasing problem in the UK and bariatric surgery is likely to increase in volume in the future. While substantial weight loss is the primary outcome following bariatric surgery, the effect on obesity-related morbidity, mortality and quality of life (QOL) is equally important. This study reports on weight loss, QOL, and health outcomes following laparoscopic adjustable gastric banding (LAGB) in a low volume bariatric centre (<20 cases/year) and presents the first assessment of factors relating to the QOL which has been produced from a UK based surgical practice. STUDY DESIGN: Questionnaire based study of patients who had LAGB. Each patients' initial body mass index (BMI), QOL, and comorbidities were recorded. Change in these parameters was measured including excess weight loss, and output from both the Moorehead-Ardelt QOL questionnaire, and the Bariatric Analysis and Reporting Outcome System (BAROS). RESULTS: Eighty-one patients (14 males, 67 females) answered the questionnaire. More than 50% excess weight loss was recorded in 52/81 patients (64%). Sixty-four patients (79%) reported improvement in their QOL including self-esteem, physical activity, social involvement, and ability to work. Seventy-one patients had initial obesity related comorbidity. In 61 of these patients (86%) their comorbidities resolved or improved. Minor port site related complications were recorded in nine patients while two patients had removal of the band because of infection. CONCLUSION: LAGB is a safe method of bariatric surgery. It can achieve satisfactory weight loss with significant improvement in QOL and comorbidity.

Effect of oral clarithromycin on gall-bladder motility in normal subjects and those with gall-stones.

BACKGROUND: Motilin receptor stimulation with erythromycin has been shown to have a prokinetic effect on gall-bladder motility in human beings. AIM: To find out whether oral clarithromycin has similar prokinetic activity to erythromycin on fasting and postprandial gall-bladder emptying in normal humans and those with gall-stone disease. METHODS: In a blinded two-way crossover study clarithromycin 500 mg and a placebo were administered to 10 normal subjects and 10 subjects with gall-stone disease. Gall-bladder volumes were assessed in the fasting and postprandial state. RESULTS: Fasting volumes were significantly less following clarithromycin administration in both normal subjects and subjects with gall-stones compared with placebo (12.1 +/- 1.8 mL vs. 17.8 +/- 2.0 mL, P < 0.05 and 16.7 +/- 2 mL vs. 26.8 +/- 7.2 mL, P < 0.02, mean +/- S.E.M). Postprandial volumes were also significantly less following clarithromycin administration. Ejection fraction significantly increased following clarithromycin in both normal subjects (66 +/- 5.8% vs. 37 +/- 5.9%, P = 0.02) and subjects with gall-stones (45 +/- 3.2 vs. 20 +/- 1.6%, P < 0.02). CONCLUSION: Clarithromycin enhances both fasting and postprandial gall-bladder contraction in normal humans and also in those with gall-stone disease.

Hernias are the most common cause of strangulation in patients presenting with small bowel obstruction.

BACKGROUND: Small bowel obstruction (SBO) is a leading cause of admission to surgical emergency units. Strangulation is associated with a 10-fold increase in mortality. The aim of the present study was to identify the most frequent causes of strangulation in patients presenting with small bowel obstruction. METHODS: A prospective study was conducted of all patients presenting with SBO in one teaching hospital between 2003 and 2004. RESULTS: One hundred and sixty-one patients with symptoms and signs of small bowel obstruction were admitted. Eighty-three were confirmed with contrast studies. The male:female ratio was 1:1.6. The aetiology of obstruction was adhesions in 97 patients (60.2%), hernia in 29 (18%), malignancy in 17 (10.6%) and miscellaneous causes in 18 (11.2%). Operative procedures were performed on 74 patients (46%), 31 of them (42%) with adhesions, 25 (34%) with hernias and 18 (24%) due to other causes. Strangulated bowel occurred in 15 patients (9.3%); 12 had hernias whilst three had adhesions (P < 0.0001). Of the strangulated hernias, ten were femoral, one was inguinal and one was paraumbilical. There were seven deaths; three occurred in patients declared unfit for surgery, while four occurred post-operatively - two had strangulated bowel, the other two had advanced cancer. CONCLUSION: Whilst adhesions are the most common cause of small bowel obstruction, hernias remain the most frequent cause of strangulation in patients presenting with this condition.

Prospective study of laparoscopic adjustable gastric banding in the west of Scotland.

UNLABELLED: Obesity is an increasing problem in Scotland and Laparoscopic Adjustable Gastric Bands (LAGB) are an effective method of weight reduction. Most outcome data are reported from high volume units with extensive experience or dedicated bariatric practice. We aimed to assess an experienced laparoscopic surgeon's outcome working outwith a dedicated bariatric practice in the west of Scotland. METHODS: All LAGB procedures performed by a single surgeon were prospectively assessed from 1997 to 2004. LAGB were inserted using pars flaccida approach. Patient selection was based on BMI >35 or significant obesity related co-morbidities. Outcomes included percentage excess weight loss (%EWL) and excess BMI loss (EBL). We assessed total operating time to assess the learning curve for LAGB placement. RESULTS: 125 patients were assessed (107 F:18 M). 123 patients were in regular follow-up (98%). Median age was 44 years (range 25-63). Mean follow-up was 34 months (range 11-91). Median initial BMI was 49 (range 37-73). 31% were BMI 35-45, 36% were BMI 45-50 and 33% were BMI>50. %EWL at 1,3 and 5 years was 45, 58 and 74, respectively. EBL at 1, 3 and 5 years was 11.7, 16.1, and 21.7, respectively. Complications included 4 converted procedures, 1 failed band insertion after conversion and re-operation for removal in five. Eight patients had tubing access port problems requiring intervention. The median overall total operation time was 80 minutes (range 50 - 160). CONCLUSIONS: In this cohort LAGB insertion by an experienced laparoscopic surgeon is safe with few re-operations. Satisfactory weight loss is obtained and patient compliance with follow-up is high.

Some characteristics of rural acceptors of family planning measures.

PIP: In order to identify socioeconomic and demographic factors involved in the acceptance of family planning among rural people in Bangladesh, researcher conducted a survey of village households in the Mymensingh district. With an annual growth rate of 2.32, the population of Bangladesh, currently at 89.9 million, will double over the next 30 years. Although the government has shown great commitment to family planning, a large segment of the population does not practice family planning. Since 85% of the population lives in rural areas, this study of the differences among users and nonusers focused on rural people. It investigated the following characteristics: age, income, education, farm size, family size, wife's age, wife's age at marriage, wife's education, newspaper reading, and radio listening. The study examined data from 166 randomly selected households in 5 villages. 93% of the respondents indicated some knowledge concerning family planning methods, 58% expressed support for the family planning program, and 23% reported practicing some family planning method at the time of the study. Out of a total of 129 nonusers, 46 chose to explain their reasons for nonacceptance. 505 cited religious reasons, 11% mentioned health concerns, and 39% cited old age. Of the 10 characteristics examined, 5 were found to differ significantly between users and nonusers, including age, farm size, education, newspaper reading, and radio listening. Young, educated people with smaller farms were found to more receptive to family planning. The report recommends promoting education in rural areas to improve the success of family planning. Also, it stresses the need to popularize contraceptive use among newly married couples and to motivate them towards continued use.^ieng