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1.
Cryst Growth Des ; 24(3): 1159-1169, 2024 Feb 07.
Article in English | MEDLINE | ID: mdl-38344675

ABSTRACT

The P-T phase diagram of the hydrated magnesium carbonate nesquehonite (MgCO3·3H2O) has not been reported in the literature. In this paper, we present a joint experimental and computational study of the phase stability and structural behavior of this cementitious material at high-pressure and high-temperature conditions using in situ single-crystal and synchrotron powder X-ray diffraction measurements in resistive-heated diamond anvil cells plus density functional theory calculations. Our results show that nesquehonite undergoes two pressure-induced phase transitions at 2.4 (HP1) and 4.0 GPa (HP2) at ambient temperature. We have found negative axial compressibility and thermal expansivity values, likely related to the directionality of the hydrogen bonds. The equations of state of the different phases have been determined. All the room-temperature compression effects were reversible. Heating experiments at 0.7 GPa show a first temperature-induced decomposition at 115 °C, probably into magnesite and a MgCO3·4H2O phase.

2.
ACS Omega ; 8(11): 10403-10410, 2023 Mar 21.
Article in English | MEDLINE | ID: mdl-36969435

ABSTRACT

We report the structural behavior and compressibility of minrecordite, a naturally occurring Zn-rich dolomite mineral, determined using diamond-anvil cell synchrotron X-ray diffraction. Our data show that this rhombohedral CaZn0.52Mg0.48(CO3)2 carbonate exhibits a highly anisotropic behavior, the c axis being 3.3 times more compressible than the a axis. The axial compressibilities and the equation of state are governed by the compression of the [CaO6] and [ZnO6] octahedra, which are the cations in larger proportion in each layer. We observe the existence of a dense polymorph above 13.4(3) GPa using Ne as a pressure-transmitting medium, but the onset pressure of the phase transition decreases with the appearance of deviatoric stresses in nonhydrostatic conditions. Our results suggest that the phase transition observed in minrecordite is strain-induced and that the high-pressure polymorph is intimately related to the CaCO3-II-type structure. A comparison with other dolomite minerals indicates that the transition pressure decreases when the ratio Zn/Mg in the crystal lattice of pure dolomite is larger than 1. Density functional theory (DFT) calculations predict that a distorted CaCO3-II-type structure is energetically more stable than dolomite-type CaZn(CO3)2 above 10 GPa. However, according to our calculations, the most stable structure above this pressure is a dolomite-V-type phase, a polymorph not observed experimentally.

3.
J Phys Chem C Nanomater Interfaces ; 126(7): 3466-3474, 2022 Feb 24.
Article in English | MEDLINE | ID: mdl-35242269

ABSTRACT

Time-correlated single photon counting has been conducted to gain further insights into the short photoluminescence lifetimes (nanosecond) of lead iodide perovskite (MAPbI3) thin films (∼100 nm). We analyze three different morphologies, compact layer, isolated island, and connected large grain films, from 14 to 300 K using a laser excitation power of 370 nJ/cm2. Lifetime fittings from the Generalized Berberan-Santos decay model range from 0.5 to 6.5 ns, pointing to quasi-direct bandgap emission despite the three different sample strains. The high energy band emission for the isolated-island morphology shows fast recombination rate centers up to 4.8 ns-1, compared to the less than 2 ns-1 for the other two morphologies, similar to that expected in a good quality single crystal of MAPbI3. Low-temperature measurements on samples reflect a huge oscillator strength in this material where the free exciton recombination dominates, explaining the fast lifetimes, the low thermal excitation, and the thermal escape obtained.

4.
ACS Earth Space Chem ; 5(5): 1130-1139, 2021 May 20.
Article in English | MEDLINE | ID: mdl-34901683

ABSTRACT

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO-BaO-CO2 system is more complex than previously thought. We characterized the BaCa(CO3)2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V 0 = 1608(2) Å3, B 0 = 60(3) GPa, B'0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO3)2 polymorphs was investigated.

5.
Nanoscale ; 11(46): 22378-22386, 2019 Nov 28.
Article in English | MEDLINE | ID: mdl-31730145

ABSTRACT

Photonic applications based on halide perovskites, namely CH3NH3PbI3 (MAPbI3), have recently attracted remarkable attention due to the high efficiencies reported for photovoltaic and light emitting devices. Despite these outstanding results, there are many temperature-, laser excitation power-, and morphology-dependent phenomena that require further research to be completely understood. In this work, we have investigated in detail the nature of exciton optical transitions and recombination dynamics below and above the orthorhombic/tetragonal ('O'-/'T'-) temperature phase transition (∼150 K) depending on the material continuity (continuous-like) or discontinuity (island-like) in MAPbI3 films. At low temperatures, continuous thin films of the perovskite can exhibit strain inhomogeneities associated with the formation of different 'T'-defective domains leading to an energy spread of states over more than 200 meV. On the other hand, a single photoluminescence line peak related to the perovskite 'O'-phase (associated with the distortion of the [PbI3]- anion) is observed in the island-like sample that we attribute to strain relaxation for this morphology. Moreover, the predominantly radiative recombination dynamics of the continuous-like sample mainly originates from nongeminate electron-hole formation of excitons in the 'O'-phase and the internal dynamics with carrier trapping levels. This observation is in strong contrast to the free exciton recombination dominantly found in the island-like sample.

6.
Sci Rep ; 9(1): 7898, 2019 May 27.
Article in English | MEDLINE | ID: mdl-31133679

ABSTRACT

Calcium carbonate is a relevant constituent of the Earth's crust that is transferred into the deep Earth through the subduction process. Its chemical interaction with calcium-rich silicates at high temperatures give rise to the formation of mixed silicate-carbonate minerals, but the structural behavior of these phases under compression is not known. Here we report the existence of a dense polymorph of Ca5(Si2O7)(CO3)2 tilleyite above 8 GPa. We have structurally characterized the two phases at high pressures and temperatures, determined their equations of state and analyzed the evolution of the polyhedral units under compression. This has been possible thanks to the agreement between our powder and single-crystal XRD experiments, Raman spectroscopy measurements and ab-initio simulations. The presence of multiple cation sites, with variable volume and coordination number (6-9) and different polyhedral compressibilities, together with the observation of significant amounts of alumina in compositions of some natural tilleyite assemblages, suggests that post-tilleyite structure has the potential to accommodate cations with different sizes and valencies.

7.
Inorg Chem ; 58(4): 2708-2716, 2019 Feb 18.
Article in English | MEDLINE | ID: mdl-30724075

ABSTRACT

Synchrotron X-ray diffraction measurements on lead sulfate have been performed up to 67 GPa using He as pressure transmitting medium. Experiments reveal the existence of a reversible pressure-induced phase transition from the initial Pnma barite-type to the P212121 post-barite-type structure at pressures above 27 GPa. This phase transition involves a volume collapse of 2.4% and requires a considerable pressure overshoot (large pressure range with coexistence of phases) to overcome the large kinetic barrier of the transition. DFT calculations confirm the experimental observations and support the hypothesis that post-barite-type phase is the thermodynamically stable high-pressure structure for ABO4 ternary oxides with large A and small B atoms. The mechanism of the phase transition is described, and the compressibility and anisotropy of both polymorphs are estimated.

8.
Inorg Chem ; 57(11): 6447-6455, 2018 Jun 04.
Article in English | MEDLINE | ID: mdl-29737842

ABSTRACT

We report the formation of an ultrahigh CO2-loaded pure-SiO2 silicalite-1 structure at high pressure (0.7 GPa) from the interaction of empty zeolite and fluid CO2 medium. The CO2-filled structure was characterized in situ by means of synchrotron powder X-ray diffraction. Rietveld refinements and Fourier recycling allowed the location of 16 guest carbon dioxide molecules per unit cell within the straight and sinusoidal channels of the porous framework to be analyzed. The complete filling of pores by CO2 molecules favors structural stability under compression, avoiding pressure-induced amorphization below 20 GPa, and significantly reduces the compressibility of the system compared to that of the parental empty one. The structure of CO2-loaded silicalite-1 was also monitored at high pressures and temperatures, and its thermal expansivity was estimated.

9.
Inorg Chem ; 57(1): 98-105, 2018 Jan 02.
Article in English | MEDLINE | ID: mdl-29227639

ABSTRACT

We report on high-pressure and high-temperature angle-dispersive synchrotron X-ray diffraction and high-pressure Raman data up to 27 GPa and 700 K for natural silicate carbonate Ca5(SiO4)2(CO3) spurrite mineral. No phase transition was found in the studied P-T range. The room-temperature bulk modulus of spurrite using Ne as the pressure-transmitting medium is B0 = 77(1) GPa with a first-pressure derivative of B0' = 5.9(2). The structure compression is highly anisotropic, the b axis being approximately 30% more compressible than the a and c axes. The volumetric thermal expansivity value around 8 GPa was estimated to be 4.1(3) × 10-5 K-1. A comparison with intimately related minerals CaCO3 calcite and aragonite and ß-Ca2SiO4 larnite shows that, as the composition and structural features of spurrite suggest, its compressibility and thermal expansivity lie between those of the silicate and carbonate end members. The crystal chemistry and thermodynamic properties of spurrite are discussed.

10.
Inorg Chem ; 55(20): 10793-10799, 2016 Oct 17.
Article in English | MEDLINE | ID: mdl-27709926

ABSTRACT

The role of carbon dioxide, CO2, as oxidizing agent at high pressures and temperatures is evaluated by studying its chemical reactivity with three transition metals: Au, Pt, and Re. We report systematic X-ray diffraction measurements up to 48 GPa and 2400 K using synchrotron radiation and laser-heating diamond-anvil cells. No evidence of reaction was found in Au and Pt samples in this pressure-temperature range. In the Re + CO2 system, however, a strongly-driven redox reaction occurs at P > 8 GPa and T > 1500 K, and orthorhombic ß-ReO2 is formed. This rhenium oxide phase is stable at least up to 48 GPa and 2400 K and was recovered at ambient conditions. Raman spectroscopy data confirm graphite as a reaction product. Ab-initio total-energy structural and compressibility data of the ß-ReO2 phase shows an excellent agreement with experiments, altogether accurately confirming CO2 reduction P-T conditions in the presence of rhenium metal and the ß-ReO2 equation of state.

11.
Acta Crystallogr B Struct Sci Cryst Eng Mater ; 71(Pt 6): 798-804, 2015 Dec 01.
Article in English | MEDLINE | ID: mdl-26634737

ABSTRACT

We report on high-pressure angle-dispersive X-ray diffraction data up to 15 GPa and ab initio total-energy calculations up to 242 GPa for KBrO3. No phase transition was found below 15 Pa in contrast to previously reported data. Its experimental bulk modulus in the quasi-hydrostatic regime is B0 = 18.8 (9) GPa with a bulk modulus pressure derivative B'0 = 8.2 (4). However, according to our ab initio calculations, KBrO3 significantly reduces its rhombohedral distortion via small cooperative movements of the atoms and the structure progressively approaches the cubic symmetry, where the KBr subarray would adopt a topology similar to that of the corresponding B2-type bromide. This rearrangement of atoms is directly related to the Buerger's mechanism of the B1-B2 phase transition for halides, confirming that cations (second neighbors) do not arrange in an arbitrary way. The O atoms forming the [BrO3] pyramidal units move smoothly with pressure to the center of the [K8] cube faces, where electron localization function calculations present their maxima in other B2-type compounds, eventually adopting the perovskite-type structure at P ≃ 152 GPa. Our data on KBrO3 has been compared with chemically substituted isostructural halates, providing new insights on the compressibility of this family of compounds.

12.
Inorg Chem ; 51(9): 5289-98, 2012 May 07.
Article in English | MEDLINE | ID: mdl-22530949

ABSTRACT

Angle-dispersive X-ray diffraction measurements have been performed in acanthite, Ag(2)S, up to 18 GPa in order to investigate its high-pressure structural behavior. They have been complemented by ab initio electronic structure calculations. From our experimental data, we have determined that two different high-pressure phase transitions take place at 5 and 10.5 GPa. The first pressure-induced transition is from the initial anti-PbCl(2)-like monoclinic structure (space group P2(1)/n) to an orthorhombic Ag(2)Se-type structure (space group P2(1)2(1)2(1)). The compressibility of the lattice parameters and the equation of state of both phases have been determined. A second phase transition to a P2(1)/n phase has been found, which is a slight modification of the low-pressure structure (Co(2)Si-related structure). The initial monoclinic phase was fully recovered after decompression. Density functional and, in particular, GGA+U calculations present an overall good agreement with the experimental results in terms of the high-pressure sequence, cell parameters, and their evolution with pressure.

13.
Phys Chem Chem Phys ; 13(40): 18200-7, 2011 Oct 28.
Article in English | MEDLINE | ID: mdl-21938287

ABSTRACT

The Layer-by-Layer (LbL) self-assembly of polyelectrolyte multilayers (PEMs) formed by poly(acrylic acid) (PAA) and chitosan (CHI) of two different percentages of acetylation (AC) has been studied by dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results point out that the non-linear growth (exponential growth) of the films is not modified by the percentage of acetylation of the CHI (AC). The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films showing that the multilayers are highly hydrated. This agrees with the values of the complex shear modulus obtained from the analysis of D-QCM data that are in the MPa range, and show a transition from a viscous to mainly elastic behavior depending on the charge density of the CHI chains. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the mechanism of charge compensation depends on the percentage of acetylation of the CHI. It was found that the adsorption kinetics is a bimodal process with characteristic times that depend on the number and nature of each layer. The load capacity of the multilayers for a ß-blocker, propranolol, was found to be higher for the lowest acetylation degree.


Subject(s)
Acrylic Resins/chemistry , Chitosan/chemistry , Acetylation , Quartz Crystal Microbalance Techniques
14.
Langmuir ; 27(11): 6836-45, 2011 Jun 07.
Article in English | MEDLINE | ID: mdl-21561105

ABSTRACT

Multilayers of poly(acrylic acid), PAA, and chitosan, CHI, have been built by the layer-by-layer (LbL) method from aqueous solutions at different pH values and analyzed by the dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The results showed that under all of the assembly conditions considered the growth of the films is nonlinear. The thickness of the PAA layers increases as the pH of the assembling solutions decreases, whereas the adsorption of CHI is almost unaffected by the pH conditions. The comparison of the thickness obtained by D-QCM and by ellipsometry has allowed us to calculate the water content of the films, showing that the multilayers are highly hydrated, with an average water content higher than 20%. The analysis of D-QCM data has provided high-frequency values of the complex shear modulus that are in the megapascal range and shows a transition from mainly viscous to mainly elastic behavior for the added PAA layers, depending on the pH. The monomer surface density in each layer (obtained from the combination of ellipsometry and differential refractive index measurements) indicated that the monomer density depends on the assembly conditions. It was found that the adsorption kinetics is a bimodal process, with characteristic times that depend on the number and nature of the layers. Finally, the possibility of using of these multilayers as a drug storage and delivery system has been evaluated.


Subject(s)
Acrylic Resins/chemistry , Chitosan/chemistry , Drug Carriers/chemistry , Acoustics , Adsorption , Drug Storage , Elasticity , Hydrogen-Ion Concentration , Kinetics , Models, Molecular , Molecular Conformation , Optical Phenomena , Propranolol/chemistry
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