Metamorphic temperature investigation of coexisting calcite and dolomite marble––examples from Nikani Ghar marble and Nowshera Formation, Peshawar Basin, Pakistan

Using marble samples from the Nikani Ghar marble and Nowshera Formation from Northern Pakistan the determination of the temperature of metamorphism was undertaken with the help of calcite-dolomite solvus geothermometer. Two types of marbles, that is, calcite-dolomite marble and quartz-bearing calcite-dolomite marble were selected. Petrographic and scanning electron microscope analysis of dolomite samples indicated different grain sizes. X-ray diffraction technique indicated the calcites MgCO₃ content up to 7.93 mol.%. Nikani Ghar marble samples have shown lower contents of MgCO₃ as compared to samples from Nowshera Formation. The calcite-dolomite-quartz marble has also showed relatively lower MgCO₃ content and hence rather low temperature (~500 °C). The temperature reached during peak metamorphism of the investigated marble occurrence, based on calcitedolomite solvus was 628 °C. Metamorphic temperatures derived from the present study were shown as a linear graph and values were in good agreement with the published literature.The authors acknowledge the financial support extended by the Higher Education Commission (HEC), Pakistan and National Academy of Sciences (USA), project ID 131, under the PAK-USA S & T Cooperation Program, Award (No. 0521315). The authors are grateful to the HEC, Pakistan for their support in the form of “International Research Support Initiative Program (IRSIP)” to conduct a part of research at Department of Earth Sciences, University of Cambridge, United 996 Muhammad Fahad, Yaseen Iqbal, Mohammad Riaz, Rick Ubic and Simon A. T. Redfern Kingdom. The financial support extended by the Directorate of S & T, KP regarding minerals upgradation is also acknowledged

Ferroelectric and Incipient Ferroelectric Properties of a Novel Sr_(9-x)PbxCe2Ti2O36 (x=0-9) Ceramic System

Sr_(9-x)PbxCe2Ti12O36 system is derived from the perovskite SrTiO3 and its chemical formula can be written as (Sr_(1-y)Pby)0.75Ce0.167TiO3. We investigated dielectric response of Sr_(9-x)PbxCe2Ti12O36 ceramics (x = 0-9) between 100 Hz and 100 THz at temperatures from 10 to 700 K using low- and high-frequency dielectric, microwave (MW), THz and infrared spectroscopy. We revealed that Sr9Ce2Ti12O36 is an incipient ferroelectric with the R-3c trigonal structure whose relative permittivity e' increases from 167 at 300 K and saturates near 240 below 30 K. The subsequent substitution of Sr by Pb enhances e' to several thousands and induces a ferroelectric phase transition to monoclinic Cc phase for x>=3. Its critical temperature Tc linearly depends on the Pb concentration and reaches 550 K for x=9. The phase transition is of displacive type. The soft mode frequency follows the Barrett formula in samples with x=3. The MW dispersion is lacking and quality factor Q is high in samples with low Pb concentration, although the permittivity is very high in some cases. However, due to the lattice softening, the temperature coefficient of the permittivity is rather high. The best MW quality factor was observed for x=1: Q*f=5800 GHz and e'=250. Concluding, the dielectric properties of Sr_(9- x)PbxCe2Ti12O36 are similar to those of Ba_(1-x)SrxTiO3 so that this system can be presumably used as an alternative for MW devices or capacitors.Comment: subm. to Chem. Mate

An Empirical Model for Perovskite Tetragonality

Tetragonal distortions in perovskites are useful for advanced devices, as they can induce functional properties like capacitance, piezoelectricity, pyroelectricity, and ferroelectricity. These distortions are commonly due to either second-order Jahn-Teller effects or antiferrodistortive instabilities. Accurate empirical composition-structure models can significantly reduce the time and cost involved with developing new functional ceramics; however, until now no such model has existed for tetragonal perovskites, the structures of which are further complicated by extrinsic point defects often used to tune electrical properties. In this work, LeBail refinements of X-ray diffraction patterns obtained from [(PbyBa1−y)(1−3x)La(2x)]TiO3 (P4mm, No. 99) were conducted to measure lattice constants and reveal tetragonality trends. These trends were combined with a model for unit-cell volume in order to derive a general empirical model, based solely on published ionic radii data, for tetragonal lattice constants of perovskites in P4mm and P4/mmm

An Empirical Model for B-Site Cation Ordering in Ba(Mg\u3csub\u3e1/3\u3c/sub\u3eTa\u3csub\u3e2/3\u3c/sub\u3e)O\u3csub\u3e3\u3c/sub\u3e

Processing-structure models are needed in both the lab and industry; however, few exist for cation ordering in perovskites. The perovskite Ba(Mg1/3Ta2/3)O3 in its ordered form is one of the best known high-Q dielectric materials but requires extended high-temperature annealing to achieve high degrees of order; so an empirical model which describes the ordering as a function of an easily obtainable processing parameter would be useful. In this work, powders of Ba(Mg1/3Ta2/3)O3 were synthesized using a conventional solid-state mixed-oxide method. The as-calcined compound had a cubic (lacking long-range B-site cation order) structure but contained short-range-ordered nanodomains. Upon annealing at 1500 °C for up to 40 h an increasingly ordered arrangement of Mg2+ and Ta5+ on the B site was generated, with the ordering causing a trigonal distortion. Empirical modeling as well as first-principles calculations via density functional theory showed that this ordering process was accompanied by a volume decrease despite the fact that ordered planes stack less efficiently. An empirical model was developed to describe the ordering parameter as a function of either annealing time or effective B-site contraction. The implication of this modeling method is that it may be possible to predict the degree of cation ordering in complex perovskite systems from ionic-radii data and experimentally-derived pseudocubic lattice constants alone. Conversely, it may also be possible to predict the degree of volume expansion/contraction upon ordering, which has implications for functional properties like ionic conduction