Publications by authors named "Edgar Dachs"

11 Publications

  • Page 1 of 1

Excess enthalpy of mixing of mineral solid solutions derived from density-functional calculations.

Phys Chem Miner 2020 17;47(3):15. Epub 2020 Feb 17.

Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringer-Str. 2a, 5020 Salzburg, Austria.

Calculations using the density-functional theory (DFT) in combination with the single defect method were carried out to determine the heat of mixing behaviour of mineral solid solution phases. The accuracy of this method was tested on the halite-sylvite (NaCl-KCl) binary, pyrope-grossular garnets (MgAlSiO-CaAlSiO), MgO-CaO (halite structure) binary, and on Al/Si ordered alkali feldspars (NaAlSiO-KAlSiO); as members for coupled substitutions, the diopside-jadeite pyroxenes (CaMgSiO-NaAlSiO) and diopside-CaTs pyroxenes (CaMgSiO-CaAlAlSiO) were chosen for testing and, as an application, the heat of mixing of the tremolite-glaucophane amphiboles (CaMgSiO(OH)-NaMgAlSiO(OH)) was computed. Six of these binaries were selected because of their experimentally well-known thermodynamic mixing behaviours. The comparison of the calculated heat of mixing data with calorimetric data showed good agreement for halite-sylvite, pyrope-grossular, and diopside-jadeite binaries and small differences for the Al/Si ordered alkali feldspar solid solution. In the case of the diopside-CaTs binary, the situation is more complex because CaTs is an endmember with disordered cation distributions. Good agreement with the experimental data could be, however, achieved assuming a reasonable disordered state. The calculated data for the Al/Si ordered alkali feldspars were applied to phase equilibrium calculations, i.e. calculating the Al/Si ordered alkali feldspar solvus. This solvus was then compared to the experimentally determined solvus finding good agreement. The solvus of the MgO-CaO binary was also constructed from DFT-based data and compared to the experimentally determined solvus, and the two were also in good agreement. Another application was the determination of the solvus in tremolite-glaucophane amphiboles (CaMgSiO(OH)-NaMgAlSiO(OH)). It was compared to solvi based on coexisting amphiboles found in eclogites and phase equilibrium experiments.
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http://dx.doi.org/10.1007/s00269-020-01085-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7024695PMC
February 2020

Furfuryl Alcohol and Lactic Acid Blends: Homo- or Co-Polymerization?

Polymers (Basel) 2019 Sep 20;11(10). Epub 2019 Sep 20.

Forest Products Technology & Timber Constructions Department, Salzburg University of Applied Sciences, Marktstraße 136a, 5431 Kuchl, Austria.

Furfuryl alcohol (FA) and lactic acid (LA) are two of the most interesting biomolecules, easily obtainable from sugars and hence extremely attractive for green chemistry solutions. These substances undergo homopolymerization and they have been rarely considered for copolymerization. Typically, FA homopolymerizes exothermically in an acid environment producing inhomogeneous porous materials, but recent studies have shown that this reaction can be controlled and therefore we have implemented this process to trigger the copolymerization with LA. The mechanical tests have shown that the blend containing small amount of FA were rigid and the fracture showed patterns more similar to the one of neat polyfurfuryl alcohol (PFA). This LA-rich blend exhibited higher chloroform and water resistances, while thermal analyses (TG and DSC) also indicated a higher furanic character than expected. These observations suggested an intimate interconnection between precursors which was highlighted by the presence of a small band in the ester region of the solid state C-NMR, even if the FT-IR did not evidence any new signal. These studies show that these bioplastics are basically constituted of PLA and PFA homopolymers with some small portion of covalent bonds between the two moieties.
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http://dx.doi.org/10.3390/polym11101533DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6835956PMC
September 2019

A new activity model for Mg-Al biotites determined through an integrated approach.

Contrib Mineral Petrol 2019 23;174(9):76. Epub 2019 Aug 23.

Fachbereich Chemie und Physik der Materialien, Abteilung Mineralogie, Universität Salzburg, Jakob-Haringerstrasse 2a, 5020 Salzburg, Austria.

A new activity model for Mg-Al biotites was formulated through an integrated approach combining various experimental results (calorimetry, line-broadening in infrared (IR) spectra, analysis of existing phase-equilibrium data) with density functional theory (DFT) calculations. The resulting model has a sound physical-experimental basis. It considers the three end-members phlogopite (Phl, KMg[(OH)AlSiO]), ordered eastonite (Eas, KMgAl[(OH)AlSiO]), and disordered eastonite (dEas) and, thus, includes Mg-Al order-disorder. The DFT-derived disordering enthalpy, Δ, associated with the disordering of Mg and Al on the M sites of Eas amounts to 34.5 ± 3 kJ/mol. Various biotite compositions along the Phl-Eas join were synthesised hydrothermally at 700 °C and 4 kbar. The most Al-rich biotite synthesized had the composition = 0.77. The samples were characterised by X-ray diffraction (XRD), microprobe analysis and IR spectroscopy. The samples were studied further using relaxation calorimetry to measure their heat capacities () at temperatures from 2 to 300 K and by differential scanning calorimetry between 282 and 760 K. The calorimetric (vibrational) entropy of Phl at 298.15 K, determined from the low- measurements on a pure synthetic sample, is = 319.4 ± 2.2 J/(mol K). The standard entropy, , for Phl is 330.9 ± 2.2 J/(mol K), which is obtained by adding a configurational entropy term, , of 11.53 J/(mol K) due to tetrahedral Al-Si disorder. This value is ~1% larger than those in different data bases, which rely on older calorimetrical data measured on a natural near-Phl mica. Re-analysing phase-equilibrium data on Phl + quartz (Qz) stability with this new , gives a standard enthalpy of formation of Phl, = - 6209.83 ± 1.10 kJ/mol, which is 7-8 kJ/mol less negative than published values. The superambient of Phl is given by the polynomial [J/(mol K)] as follows: . Calorimetric entropies at 298.15 K vary linearly with composition along the Phl-Eas join, indicating ideal vibrational entropies of mixing in this binary. The linear extrapolation of these results to Eas composition gives = 294.5 ± 3.0 J/(mol K) for this end-member. This value is in excellent agreement with its DFT-derived , but ~ 8% smaller than values as appearing in thermodynamic data bases. The DFT-computed superambient of Eas is given by the polynomial [in J/(mol K)] as follows: . A maximum excess enthalpy of mixing, Δ , of ~6 kJ/mol was derived for the Phl-Eas binary using line-broadening from IR spectra (wavenumber region 400-600 cm), which is in accordance with Δ determined from published solution-calorimetry data. The mixing behaviour can be described by a symmetric interaction parameter = 25.4 kJ/mol. Applying this value to published phase-equilibrium data that were undertaken to experimentally determine the Al-saturation level of biotite in the assemblage (Mg-Al)-biotite-sillimanite-sanidine-Qz, gives a = - 6358.5 ± 1.4 kJ/mol in good agreement with the independently DFT-derived value of = - 6360.5 kJ/mol. Application examples demonstrate the effect of the new activity model and thermodynamic standard state data, among others, on the stability of Mg-Al biotite + Qz.
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http://dx.doi.org/10.1007/s00410-019-1606-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6707958PMC
August 2019

The accuracy of standard enthalpies and entropies for phases of petrological interest derived from density-functional calculations.

Contrib Mineral Petrol 2018 16;173(11):90. Epub 2018 Oct 16.

Chemistry and Physics of Materials, University of Salzburg, Jakob-Haringerstr. 2a, 5020 Salzburg, Austria.

The internal energies and entropies of 21 well-known minerals were calculated using the density functional theory (DFT), viz. kyanite, sillimanite, andalusite, albite, microcline, forsterite, fayalite, diopside, jadeite, hedenbergite, pyrope, grossular, talc, pyrophyllite, phlogopite, annite, muscovite, brucite, portlandite, tremolite, and CaTiO-perovskite. These thermodynamic quantities were then transformed into standard enthalpies of formation from the elements and standard entropies enabling a direct comparison with tabulated values. The deviations from reference enthalpy and entropy values are in the order of several kJ/mol and several J/mol/K, respectively, from which the former is more relevant. In the case of phase transitions, the DFT-computed thermodynamic data of involved phases turned out to be accurate and using them in phase diagram calculations yields reasonable results. This is shown for the AlSiO polymorphs. The DFT-based phase boundaries are comparable to those derived from internally consistent thermodynamic data sets. They even suggest an improvement, because they agree with petrological observations concerning the coexistence of kyanite + quartz + corundum in high-grade metamorphic rocks, which are not reproduced correctly using internally consistent data sets. The DFT-derived thermodynamic data are also accurate enough for computing the positions of reactions that are characterized by relatively large reaction enthalpies (> 100 kJ/mol), i.e., dehydration reactions. For reactions with small reaction enthalpies (a few kJ/mol), the DFT errors are too large. They, however, are still far better than enthalpy and entropy values obtained from estimation methods.
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http://dx.doi.org/10.1007/s00410-018-1514-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6208725PMC
October 2018

First-principles investigation of the lattice vibrations in the alkali feldspar solid solution.

Phys Chem Miner 2015;42(3):243-249. Epub 2014 Oct 11.

Materialforschung und Physik, Universität Salzburg, Hellbrunnerstr. 34, 5020 Salzburg, Austria.

The heat capacities of Al, Si ordered alkali feldspars of different Na, K compositions were calculated using the density functional theory. The effect of the Na, K distribution, if random, ordered or clustered, on the resulting heat capacity was investigated on different cells with AbOr composition. For all compositions and distributions studied, the excess heat capacity of mixing is positive at low temperatures with a maximum at ~60 K. This produces an increasing excess vibrational entropy that reaches a constant value above ~200 K. The amount of the excess heat capacity of AbOr, however, depends on the Na, K distribution. Best agreement with measured excess heat capacities is achieved, if the distribution of Na and K is either ordered or clustered. The positive excess heat capacities can be attributed to a strong softening of the acoustic and the lowest optical modes related to a strong increase of Na-O bond lengths in samples with intermediate compositions. The softening of the lowest optical mode is, however, not reflected by thoroughly measured literature IR data. Comparing calculated and measured IR spectra suggests that the resolution of the measured spectra was insufficient for detecting the lowest IR-active modes.
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http://dx.doi.org/10.1007/s00269-014-0715-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4509561PMC
October 2014

The vibrational and configurational entropy of disordering in CuAu.

J Alloys Compd 2015 May;632:585-590

Materialforschung und Physik, Universität Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria.

The thermodynamics of disordering in CuAu have been investigated by measuring the heat capacity of samples with different degrees of long- and short-range order between  = 5 and 720 K using relaxation and differential scanning calorimetry. The heat capacities of L1-ordered and fcc-disordered samples show similar behaviour at low temperatures (<300 K). They deviate positively from the linear combination of the end-member heat capacities between ∼30 and 160 K. However, small differences between the two samples exist, as the disordered sample has a larger heat capacity producing a vibrational entropy of disordering of ∼0.05 R. At temperatures higher than 300 K, the heat capacity of the ordered sample shows a prominent lambda-type anomaly at 675 K due to the diffusive L1-fcc phase transition. When starting these measurements with disordered samples, ordering effects are observed between 400 and 620 K, and the disordering reaction is observed at 660 K. Evaluation of the data gives an enthalpy and entropy of disordering at 683 K of 2.0 kJ mol and 0.39 R, respectively. However, these values increase with increasing temperature, thereby reducing the short-range order. Because the vibrational and configurational disordering effects become active at different temperature regimes, i.e., the vibrational effects at low temperatures ( ≪ 300 K) and the sum of both effects at higher temperatures ( > 300 K), they have been successfully separated.
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http://dx.doi.org/10.1016/j.jallcom.2014.12.215DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4394139PMC
May 2015

The vibrational and configurational entropy of α-brass.

J Chem Thermodyn 2014 Apr;71(100):126-132

Materialforschung und Physik, Universität Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria.

The heat capacities of two samples of a fcc Cu-Zn alloy with the composition CuZn15 and CuZn34 were measured from  = 5 K to 573 K using relaxation and differential scanning calorimetry. Below ∼90 K, they are characterised by negative excess heat capacities deviating from ideal mixing by up to -0.20 and -0.44 J · mol · K for CuZn15 and CuZn34, respectively. The excess heat capacities produce excess vibrational entropies, which are less negative compared to the excess entropy available from the literature. Since the literature entropy data contain both, the configurational and the vibrational part of the entropy, the difference is attributed to the excess configurational entropy. The thermodynamics of different short-range ordered samples was also investigated. The extent of the short-range order had no influence on the heat capacity below  = 300 K. Above  = 300 K, where the ordering changed during the measurement, the heat capacity depended strongly on the thermal history of the samples. From these data, the heat and entropy of ordering was calculated. The results on the vibrational entropy of this study were also used to test a relationship for estimating the excess vibrational entropy of mixing.
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http://dx.doi.org/10.1016/j.jct.2013.11.012DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4047615PMC
April 2014

Calorimetric study of the entropy relation in the NaCl-KCl system.

J Chem Thermodyn 2013 Jul;62(100):231-235

Materialforschung und Physik, Universität Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria.

The heat capacity of one Na-rich and two K-rich samples of the NaCl-KCl (halite-sylvite) crystalline solution was investigated between 5 and 300 K. It deviated positively from ideal behaviour with a maximum at 40 K. The thereby produced excess entropy at 298.15 K was described by a symmetric Margules mixing model yielding [Formula: see text] = 8.73 J/mol/K. Using enthalpy of mixing data from the literature and our data on the entropy, the solvus was calculated for a pressure of 10 Pa and compared with the directly determined solvus. The difference between them can be attributed to the effect of Na-K short range ordering (clustering).
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http://dx.doi.org/10.1016/j.jct.2013.03.015DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4047613PMC
July 2013

A relationship to estimate the excess entropy of mixing: Application in silicate solid solutions and binary alloys.

J Alloys Compd 2012 Jun;527(2):127-131

Materialforschung und Physik, Universität Salzburg, Hellbrunnerstr. 34, A-5020 Salzburg, Austria.

The paper presents new calorimetric data on the excess heat capacity and vibrational entropy of mixing of Pt-Rh and Ag-Pd alloys. The results of the latter alloy are compared to those obtained by calculations using the density functional theory. The extent of the excess vibrational entropy of mixing of these binaries and of some already investigated binary mixtures is related to the differences of the end-member volumes and the end-member bulk moduli. These quantities are used to roughly represent the changes of the bond length and stiffness in the substituted and substituent polyhedra due to compositional changes, which are assumed to be the important factors for the non-ideal vibrational behaviour in solid solutions.
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http://dx.doi.org/10.1016/j.jallcom.2012.03.007DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3587341PMC
June 2012

Experimentally determined standard thermodynamic properties of synthetic MgSO(4)·4H(2)O (Starkeyite) and MgSO(4)·3H(2)O: a revised internally consistent thermodynamic data set for magnesium sulfate hydrates.

Astrobiology 2012 Nov 24;12(11):1042-54. Epub 2012 Oct 24.

Institute of Geosciences, Mineralogy, Friedrich-Schiller University, D-07745 Jena, Germany.

The enthalpies of formation of synthetic MgSO(4)·4H(2)O (starkeyite) and MgSO(4)·3H(2)O were obtained by solution calorimetry at T=298.15 K. The resulting enthalpies of formation from the elements are [Formula: see text] (starkeyite)=-2498.7±1.1 kJ·mol(-1) and [Formula: see text] (MgSO(4)·3H(2)O)=-2210.3±1.3 kJ·mol(-1). The standard entropy of starkeyite was derived from low-temperature heat capacity measurements acquired with a physical property measurement system (PPMS) in the temperature range 5 K
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http://dx.doi.org/10.1089/ast.2012.0823DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3491617PMC
November 2012

A sample-saving method for heat capacity measurements on powders using relaxation calorimetry.

Cryogenics (Guildf) 2011 Aug;51(8):460-464

Department of Material Research and Physics, Division Mineralogy, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria.

An experimental method is described for determining the low-temperature heat capacity (C(p)) of mg-sized powder samples using the Quantum Design "Physical Properties Measurement System" (PPMS). The powder is contained in an Al pan as an ∼1 mm thick compressed layer. The sample is not mixed with Apiezon N grease, as compared to other methods. Thus, it is not contaminated and can be used for further study. This is necessary for samples that are only available in tiny amounts. To demonstrate the method various samples, all insulating in nature, were studied including benzoic acid, sapphire and different silicate minerals. The measurements show that the method has an accuracy in C(p) to better than 1% at T above 30-50 K and ±3-5% up to ±10% below. The experimental procedure is based on three independent PPMS and three independent differential scanning calorimetry (DSC) measurements. The DSC C(p) data are used to slightly adjust the PPMS C(p) data by a factor CpDSC/CpPPMSat298K. This is done because heat capacities measured with a DSC device are more accurate around ambient T (⩽0.6%) than PPMS values and is possible because the deviation of PPMS heat capacities from reference values is nearly constant between about 50 K and 300 K. The resulting standard entropies agree with published reference values within 0.21% for the silicates, by 0.34% for corundum, and by 0.9% for powdered benzoic acid. The method thus allows entropy determinations on powders with an accuracy of better than 1%. The advantage of our method compared to other experimental techniques is that the sample powder is not contaminated with grease and that heat capacity values show less scatter at high temperatures.
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http://dx.doi.org/10.1016/j.cryogenics.2011.04.011DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3145170PMC
August 2011