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The high-pressure–high-temperature behavior of bassanite

¹ Dipartimento di Scienze della Terra, Università di Perugia, Piazza dell’Università, 06100 Perugia, Italy
² Bayerisches Geoinstitut, Universität Bayreuth, Universität Strasse 30, 95440 Bayreuth, Germany
³ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, BP220, F-38043 Grenoble, France
⁴ Dipartimento di Scienze della Terra, Università di Milano, Via Botticelli 23, 20133 Milano, Italy

Correspondence: ^* E-mail: comodip{at}unipg.it

	ABSTRACT

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 
The pressure evolution of bassanite (CaSO₄·H₂O) was investigated by synchrotron X-ray powder diffraction along three isotherms: at room temperature up to 33 GPa, at 109 °C up to 22 GPa, and at 200 °C up to 12 GPa.

The room-temperature cell-volume data, from 0.001 to 33 GPa, were fitted to a third-order Birch-Murnaghan equation-of-state, and a bulk modulus K₀ = 86(7) GPa with K' = 2.5(3) was obtained. The axial compressibility values are β_a = 3.7(2), β_b = 3.6(1), and β_c = 2.8(1) GPa^–1 (x10^–3) showing a slightly anisotropic behavior, with the least compressible direction along c axis. The strain tensor analysis shows that the main deformation occurs in the (010) plane in a direction 18° from the a axis.

The bulk moduli for isotherms 109 and 200 °C, were obtained by fitting cell-volume data with a second-order Birch-Murnaghan equation-of-state, with K' fixed at 4, and were found to be K₁₀₉ = 79(4) GPa and K₂₀₀ = 63(7) GPa, respectively. The axial compressibility values for isotherm 109 °C are β_a = 2.4(1), β_b = 3.0(1), β_c = 2.5(1) (x10^–3) GPa^–1, and for isotherm 200 °C they are β_a = 3.5(3), β_b = 3.4(3), β_c= 2.6(4) (x10^–3) GPa^–1. These two bulk moduli and the 20 °C bulk modulus, K_0,20 = 69(8) recalculated to a second-order Birch-Murnaghan EoS to be consistent, as well as the axial compressibilities, are similar for the three isotherms indicating that the thermal effect on the bulk moduli is not significant up to 200 °C. The size variation of the pseudo-hexagonal channel with pressure and temperature indicates that the sulfate "host" lattice and the H₂O "guest" molecule in bassanite do not undergo strong change up to 33 GPa and 200 °C.

Key Words: Bassanite • Ca-sulfate • compressibility • high temperature • high pressure • X-ray diffraction

	INTRODUCTION

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 
Several important industrial processes such as the production of phosphoric acid or phosphate fertilizers, gas desulfurization, hydrometallurgic production of zinc and copper, and recovery of natural gas and oil are accompanied by the crystallization of various phases of calcium sulfate, CaSO₄·2H₂O (gypsum), CaSO₄·H₂O (bassanite), and CaSO₄ (anhydrite). In geologic environments, these phases may have important implications in the partitioning of seawater cations during dehydration processes (Freyer and Voigt 2003). Calcium sulfates have also been noted as the main constituting material for ground preparation by ancient southern European painters (Genestar 2002).

A new interest in sulfate minerals has arisen in recent years since the discovery of these phases in extra-terrestrial environments (Rieder et al. 2004). The stability and transformation of gypsum at Mars environmental conditions, for example, was studied by exposure to very low P_H2O (Vaniman et al. 2008; Bish 2007). In fact, the detection of bassanite on Mars may have implications for exobiology, because it occurs in microbial communities and in deep sea medusae as a structural biomineral (Tiemann et al. 2002). Moreover, taking into account that bassanite may be generated from acid sulfate alteration of carbonate, its presence can provide an insight into the environmental evolution of planets.

All the three stable phases of calcium sulfate (gypsum, bassanite, and -anhydrite) contain chains of linked CaO₈ and SO₄ polyhedra. In gypsum, the chains form CaO₈-SO₄ polyhedral layers that are linked along the b axis by water molecules, which at room condition form weak hydrogen bonds with Ca and S polyhedra (Wooster 1936; Pedersen and Semmingsen 1982). Water remains bonded inside the gypsum structure up to 5 GPa by increasing the strength of the hydrogen bonds as shown by three-dimensional structural refinements (Comodi et al. 2008).

The structure of bassanite (monoclinic, space group I2, Bezou et al. 1995) contains corner-sharing SO₄ and CaO₈ polyhedra, which form a three-dimensional framework with continuous channels parallel to [001], where the water molecules are located (Fig. 1) forming hydrogen bonds with O atoms of the sulfate tetrahedra (Voigtländer et al. 2003). This framework makes bassanite a nanoporous material with channel diameter around 4 Å, suggesting that the interaction between the sulfate "host" lattice and the H₂O "guest" molecule may be important in exchange processes.

View larger version (25K): [in this window] [in a new window]   FIGURE 1. (001) projection of the bassanite structure at room conditions. Pseudo-hexagonal channels parallel to [001] are shown.

Several papers point to the existence of bassanite with different water content, perhaps up to 0.8 H₂O per mole of CaSO₄, and there have been discussions about whether members of the CaSO₄·nH₂O series have a unique structure, namely that of bassanite with n = 0.5. Chang et al. (1996), reporting the cell parameters and symmetries given for bassanite at different degrees of hydration, suggest that all have a similar structure. Bezou et al. (1995) describes a monoclinic -bassanite with n = 0.5 and a β-bassanite with n = 0.6, suggesting a monoclinic structure for this phase, also. Christensen et al. (2008) revised the β-bassanite structure, determining a trigonal/hexagonal P3₁ symmetry.

Lager et al. (1984) determined the structure of -anhydrite (space group P6₂22, also known as soluble anhydrite) and showed the close relation with the structure of bassanite. Frik and Kuzel (1982) suggested that unit cell and symmetry depend upon water vapor: for n = 0.52, the 6.9 Å parameter is doubled, but Lager et al. (1984) interpreted this as a fine twinning of monoclinic bassanite. Lager et al. (1984) suggested that water mol. in excess than 0.5 would result in a too close proximity of the water molecules, resulting in energetically unfavorable repulsive forces. Theoretical calculations show that the binding energy for H₂O in bassanite is maximized at n = 0.5.

Much attention has been focused on the hydration/dehydration/rehydration behavior of these phases under different conditions, especially at high temperature (see for example Prasad et al. 2001), because of the interest in technical applications of Ca-sulfates as a binder for building materials. Moreover, it is well known that the dehydration process develops excess pore fluid pressure that may cause a decrease in strength, thus favoring brittle failure (Cartwright 1994) by hydrofracturing.

Experiments on the rehydration process in -CaSO₄ to produce bassanite were carried out by Ballirano et al. (2001). The authors resolved and refined the bassanite structure in space group I2 (derived from the P3₁21 structure of -anhydrite), thus allowing the ordering of water molecules over two special and two general positions. They concluded that the lowering of symmetry from the P3₁21 to monoclinic I2 is caused by the ordering of the water molecules inside the channels.

However, some aspects of the Ca-sulfate behavior at high temperature are still not completely understood. In fact, several studies (McConnell et al. 1987; Chang et al. 1996; Putnis et al. 1990) support the hypothesis that the transition from gypsum to bassanite to anhydrite represents steps of a progressive dehydration process. Other studies, in contrast (Prasad et al. 2001, 2005), question the direct formation of bassanite from the dehydration of gypsum, and indicate that bassanite may form by rehydration from -CaSO₄ when the temperature decreases from 117 to 90 °C.

Abriel et al. (1990) studied, by neutron powder diffraction, the thermal decomposition of gypsum in equilibrium with the ambient water vapor pressure showing that gypsum decomposes thermally to a sub-hydrated compound CaSO₄·nH₂O then to -CaSO₄ and finally to insoluble anhydrite, β-CaSO₄.

Christensen et al. (2008) described different dehydration paths in the CaSO₄-H₂O system, by in situ time-resolved synchrotron X-ray powder diffraction. Two different polymorphs of bassanite (-bassanite with the monoclinic structure and β-bassanite with trigonal/hexagonal structure, space group P3₁) form at different conditions of dehydration: (1) -bassanite forms during thermal dehydration in the range 109–140 °C before to transform into soluble anhydrite (in the T range 165–172 °C), and then insoluble anhydrite in the T range 280–462 °C; (2) in a hydrothermal decomposition experiment, gypsum is converted to -bassanite at 99 °C that then transforms to β-bassanite at 118 °C, remaining stable up to 163 °C; when 169 °C is reached, β-bassanite is converted to anhydrite.

Mirwald (2008) used in situ differential pressure analysis (DPA), a technique that relies on the high-resolution measurement of pressure and piston displacement as a function of temperature and time. Bassanite appears at lower temperature, around 120 °C at about 2 GPa, and remains stable up to 210 °C. Other papers described the dehydration process of sulfate as strongly influenced by several aspects, such as cooling rate and boundary conditions.

Very few data in the literature exist on the baric behavior of this phase, and no data have been collected under simultaneous high-pressure–high-temperature conditions. Voigtländer et al. (2003), by using high-pressure synchrotron powder diffraction data up to 6.3 GPa, showed an isotropic compression behavior below 3 GPa with a bulk modulus of 67(1) GPa, fitting the volume data to a second-order Birch-Murnaghan EoS. Above 3 GPa, the compression of the structure along a was significantly stronger than along the b and c axes, and the bulk modulus became 101(1) GPa, but the structural evolution at the phase transition was not clarified.

The aim of this study has been to determine the evolution of the lattice parameters of bassanite with pressure and temperature and in turn the evolution of the channels size, by using high-pressure and -temperature synchrotron diffraction data. Accurate lattice parameters were measured, which allowed the equation of state to be determined and to evaluate possible discontinuities and phase transformations with pressure and temperature to be defined.

	EXPERIMENTAL METHODS

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 
Room-temperature—high-pressure experiments
The bassanite sample was obtained by the dehydration of gypsum after heating for 8 h in a furnace at 130 °C. Room-temperature–high-pressure diffraction experiments were performed on the GSECARS-BM13 beamline, at the Advanced Photon Source (Argonne, Chicago), and ID09 beamline, at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), at different pressures up to 33 GPa on compression and decompression. Bassanite powder was loaded into a diamond-anvil cell (300 µm diamond culet and a pre-indented Inconel steel gasket with an 80 µm hole) with a ruby chip as pressure calibrant, an Au filament as second internal calibrant in the APS experiments, and neon as pressure-transmitting medium. Pressure was measured by the ruby-fluorescence method before and after each experiment, and only these latter values are reported here, with an estimated error of ±0.05 GPa. Neon crystallizes at about 4.7 GPa at room temperature, and after its crystallization some effects of non-hydrostaticity may lead to an over-estimation of pressure measured with the ruby-fluorescence method. To check the pressure over-estimation, we used the Au filament as an additional pressure calibrant. We measured the lattice parameter of Au for selected experiments, and we calculated the pressure using the Au equation-of-state reported by Fei et al. (2007). The difference between P_ruby and P_Au is less than 5% at 31 GPa (Table 1).

Experiments on decompression were performed at ESRF by using a similar DAC setup as described above except for the presence of the Au calibrant. An X-ray beam of wavelength 0.4133 Å was used and the diffraction patterns were collected for 25 s with a MarResearch Mar345 image plate detector. The diffraction images were corrected for sample-to-detector distance, tilting, and distortion using a calibration based on the powder diffraction pattern of a Si standard and then integrated by using the FIT2D software package (Hammersley et al. 1996).

High-temperature–high-pressure experiments
The influence of temperature on the baric behavior of bassanite was studied along two isotherms at 109 °C (up to 22 GPa) and 200 °C (up to 12 GPa). HT-HP synchrotron experiments were carried out on beamline ID09, at ESRF (Grenoble, France).

The experimental setup used for isotherm 109 °C consists of bassanite powder loaded on an externally heated DAC (300 µm diamond culet and a pre-indented Inconel steel gasket with a 80 µm hole) with ruby chip as pressure calibrant and Ne as pressure-transmitting medium. We used a high-P,T DAC system developed in BGI (Dubrovinskaia and Dubrovinsky 2003), consisting of a small ceramic heater, mounted directly around the diamond within the DAC. An S-type thermocouple was placed at the diamond-gasket interface, and possible errors in temperature measurements were estimated to be below 5 °C at ~477 °C.

The isotherm 200 °C experimental setup consists of an externally heated membrane-driven DAC (600 µm diamond culet and a pre-indented Inconel steel gasket with a 150 µm hole) loaded with bassanite powder, ruby chip as pressure calibrant, and argon as pressure-transmitting medium. Temperature was measured using a thermocouple placed at the gasket-diamond interface.

The HT-HP diffraction data were collected using a monochromatic 0.4133 Å wavelength beam for 25 s with a MarResearch Mar345 image plate detector. The diffraction images were calibrated and corrected using the powder diffraction pattern of Si as a standard as described above, and then integrated by using FIT2D software package (Hammersley et al. 1996). Very intense spots possibly due to diamonds and Ar crystals were masked during the integration step.

Lattice refinements
Diffraction reflections collected in the 2 < 2 < 22° range were used for lattice-parameter determinations. The lattice parameters at room temperature and 109 and 200 °C were refined as monoclinic by the Le Bail method (Le Bail et al. 1988) using the GSAS package (Larson and Von Dreele 2000) coupled with the EXPGUI graphical user interface (Toby 2001). Initial structure factors for bassanite were calculated from atomic positions reported by Bezou et al. (1995). Depending on the different experimental setups, pressure media (Ar, Ne) and gold structures were included in the refinements together with bassanite. The background was fitted with a Chebyshev polynomial function with 18 coefficients, and peak shapes were modeled by means of a standard Thompson-Cox-Hasting pseudo-Voigt function (Thompson et al. 1987) modified to incorporate asymmetry (Finger et al. 1994). The peak cut-off was set to 0.5% of the peak maximum. After several cycles of least-squares minimizations, the background and lattice parameters converged. The bassanite lattice parameters obtained are reported in Table 1.

	RESULTS AND DISCUSSION

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 
The lattice parameters of the starting material, obtained from thermal dehydration of gypsum, were measured at room conditions by powder-XRD with CuK radiation at the Department of Earth Sciences (Perugia) before loading the DAC. The obtained data (a = 12.027 Å, b = 6.929 Å, c = 12.671 Å, β = 90.28°) are in good agreement with the monoclinic structure proposed by Bezou et al. (1995) for bassanite with 0.5 H₂O.

The evolution of unit-cell parameters and volumes at different pressures and temperatures are plotted in Figures 2 and 3 and listed in Table 1. The order of the Birch-Murnaghan EoS that best fits the evolution of bassanite volumes for the three isotherms was determined by plotting normalized stress vs. Eulerian finite strain (e.g., Angel 2000) (Fig. 4).

View larger version (10K): [in this window] [in a new window]   FIGURE 2. Evolution of the lattice parameters with pressure and temperature. Squares = room-temperature isotherm; circles = isotherm 109 °C; triangles = isotherm 200 °C. Open symbols = data measured by increasing the pressure; solid symbols = data measured during decompression.

View larger version (7K): [in this window] [in a new window]   FIGURE 3. Volume vs. pressure, for the three isotherms (20, 109, and 200 °C). Symbols as in Figure 2.

View larger version (10K): [in this window] [in a new window]   FIGURE 4. Plot of the "normalized stress," FE vs. finite strain, fE. Error bars are calculated following Angel (2000). Low strain data have large errors because of the relatively high P/P ratio at low pressures and were not included in the calculations, but are reported for a visual estimation of the V0 goodness. Symbols as in Figure 3. Open star = data from Voigtländer et al. (2003); see text for discussion.

The analysis of the unit-strain tensor (Ohashi 1982) has been performed to better clarify the high-pressure deformation mechanism of bassanite (see Thompson and Downs 2008 for a description of the calculation procedures of the unit-strain tensor). The symmetry requires one of the principal axes of the strain ellipsoid to be parallel to the b crystallographic axis, the other two axes being contained in the plane normal to this direction. The unit-strain ellipsoid values for bassanite calculated from room pressure to 33 GPa are ₁ = 2.3(2), ₂ = 2.4(3), ₃ = 4.3(2) GPa^–1 (x10^–3), and the strain axes are oriented with a and c as follows: ₁^a = 72°, ₁^c = 17°, and ₃^a = 18°, ₃^c = 107°, such that the strain ellipsoid of bassanite can be described approximately as an ellipsoid of rotation with one of the two principal axes parallel to b.

Voigtländer et al. (2003) carried out a high-pressure synchrotron powder diffraction experiment at ELETTRA (Trieste, Italy), up to 6.3 GPa using a DAC loaded with methanol:ethanol as pressure-transmitting medium. They showed an isotropic compression behavior below 3 GPa with a bulk modulus of 67(1) GPa, fitting the volume data to a second-order Birch-Murnaghan EoS. Above 3 GPa, the compression of the structure along a was significantly stronger than along b and c axes and the bulk modulus became 101(1) GPa.

The difference we observed comparing our data with the literature may be due both to the significantly different pressure range investigated, 0.001–6.3 GPa in Voigtländer et al. (2003) and 0.001–33 GPa in the present work, and to a different interpretation of the collected data. Voigtländer et al. (2003) fitted their data with a discontinuity, which implied a strong change in the compression mechanism not yet described, whereas our data were interpolated by a continuous fit, as suggested by plotting normalized stress vs. Eulerian strain (Jeanloz and Hazen 1991; Angel 2000, 2001). We plotted the data of Voigtländer et al. (2003) on the F-f plot (Fig. 4) and observed an agreement within error with our data, and a continuous trend giving a bulk modulus of 85(5) GPa, K' = 4.

If we compare the compression behavior of bassanite with the other phases in the CaSO₄-H₂O system, gypsum has a smaller isothermal bulk modulus of 44(3) GPa with a K' = 3.3(3) in the 0–5 GPa pressure range (Comodi et al. 2008), β-anhydrite in the 0–3 GPa pressure range has a bulk modulus around 47 GPa (Bradbury and Williams 2009), as does -anhydrite [with a K₀ = 66(2) GPa with K' = 4].

The compressibility of -anhydrite calculated by Winkler with quantum mechanical methods (unpublished data, reported in Voigtländer et al. 2003) was higher than the compressibility of bassanite, suggesting a guest-host interaction that is not present in -anhydrite where the channels are completely empty.

Bassanite may be considered as a nanoporous material, thus there is the possibility that the pressure medium might enter within the channels and influence the compressibility, especially when small gas molecules, like Ne or Ar, are used. The pore size in bassanite is certainly large enough to allow the incorporation of gases like Ar and Ne that we used as pressure-transmitting media, although the low reactivity of these molecules suggests that this effect can be neglected. Furthermore, this effect is not measurable even for zeolites that have bigger pore size (unlike He, which could be important).

Isotherms 109 and 200 °C
The compressibilities of the lattice parameters, calculated as described above, were (for isotherm 109 °C) β_a = 2.4(1), β_b = 3.0(1), β_c = 2.5(1) 10^–3 GPa^–1 and (for isotherm 200 °C) β_a = 3.5(3), β_b = 3.4(3), β_c= 2.6(4) 10^–3 GPa^–1.

To bias the uncertainty on V_0,109 and V_0,200 and to compare the bulk moduli for the three isotherms, we followed the procedure suggested by Boffa Ballaran (2009). We fit the volume data (Fig. 3) to a second-order Birch-Murnaghan EoS, in the 4.2–22 GPa pressure range at 109 °C and 4.45–12 GPa at 200 °C and then extrapolated the bulk modulus to zero value, obtaining K₁₀₉ = 79(4) GPa and K₂₀₀ = 63(7) GPa. The comparison of these values with that measured at room conditions, recalculated by using a second-order Birch-Murnaghan equation of state to be consistent [K_0,20 = 69(8) GPa] showed a very limited effect of temperature on the bulk moduli of bassanite. Really, we investigated a restricted temperature range and Ballirano and Melis (2009) found a very low thermal expansion coefficient for this phase. In particular, they collected high-temperature powder diffraction data at room pressure and showed that bassanite has very isotropic thermal behavior with the same thermal expansion coefficient of around 1.1 x 10^–5 °C^–1 for all lattice parameters and around 3.0 x 10^–5 °C^–1 for the cell volume. Moreover, Voigtländer et al. (2003) studied the temperature dependence of the lattice parameters for a deuterated bassanite between –253 and 25 °C by using neutron powder diffraction. They fit a linear thermal expansion for the cell volume to a value of 2.57 x 10^–5 °C^–1, and _a = 7.22 x 10^–6 °C^–1, _b = 5.5 x 10^–6 °C^–1, _c = 1.29 x 10^–5 °C^–1 for the lattice parameters, showing a larger expansion coefficient for c parameter than for a and b parameters.

Many materials survive when compressed at room temperature well beyond their thermodynamic stability. In our experiments, along the three isotherms, only a reversible variation of the diffraction peaks is observed. In particular, only a limited increase of reflection widths was observed, and during decompression these reflection widths recovered to values measured before the compression experiments (Fig. 5). Thus the FWHM enlargement may be related to a partially non-hydrostatic environment within the DAC due to crystallization of the pressure media or to the approaching of the metastability field of bassanite, as observed in other minerals (Comodi et al. 2006), more than due to an amorphization process.

View larger version (7K): [in this window] [in a new window]   FIGURE 5. Converted intensity vs. 2 plot with the experimental (crosses) and difference (solid line) for two powder patterns of room-temperature isotherm during increasing 12.22 GPa (a) and decreasing 12.06 GPa (b) pressure. Peak positions of bassanite, gold, ruby, and Ne for pattern at 12.22 GPa and of bassanite, gold, and Ne for pattern at 12.06 GPa are reported.

Indirect information on the evolution of the channel size could be extrapolated studying the evolution of selected reflections. As described by Bezou et al. (1995), the bassanite structure has two independent water molecules, OW1 (in a special position) bonded to two H1 and OW2 (in a general position) bonded to H2 and H3. The water molecules are located in the pseudo-hexagonal channel following the sequence OW1-OW2-OW2 with distances of 5.8 Å for OW1-OW2 and 4.0 Å for OW2-OW2. These molecules form hydrogen bonds with O atoms of the sulfate tetrahedral; OW1-water forms a weak hydrogen bond with O32 (H1-O32 = 2.23 Å and OW1-O32 = 3.03 Å), OW2-water forms weak hydrogen bond with O11 (H3-O11 = 2.24 Å and OW2-O11 = 2.85 Å) and with O44 (H2-O44 = 2.17 Å and OW2-O44 = 3.03 Å). The crystallographic (110) plane is almost normal to the OW2-O11 direction. Taking into account that d₁₁₀ shortens by about 9% over the investigated pressure range, OW2-O11 may reach a value of 2.70 Å at 32 GPa. The distance of 2.7 Å is considered the limit below which these O atoms are in contact (Brown 1976) and, when this limit is reached, anomalous structural behavior may occur.

Moreover, the main structural difference of the monoclinic and hexagonal forms of bassanite is based on the symmetry of the continuous channels parallel to [001], where the water molecules are located: in monoclinic bassanite they are pseudo-hexagonal, whereas in the hexagonal bassanite they have a threefold symmetry axis, related to the order/disorder of water molecules. Thus, a further indirect consideration regarding the evolution of the channels can be provided by the peak positions of reflections at d ~ 6 Å and d ~ 3.4 Å, which in the hexagonal symmetry are (100) and (110) reflections. Moreover, d₁₀₀ = 3d₁₁₀ for ideal hexagonal symmetry. Thus the difference between d₁₀₀ and 3d₁₁₀ is zero for hexagonal symmetry and close to zero for a "close to regular" hexagonal symmetry channel, and the greater this difference, the higher the distortion of the channel.

At room conditions, the structure of bassanite exhibits a very small distortion from ideal trigonal symmetry and d₁₀₀ – 3d₁₁₀ () is equal to 0.002 Å. The evolution of the structural distortion with P and T can be followed by the difference in the positions of these peaks up to 33 GPa and 200 °C (Table 1). With an increase in pressure, no significant change was observed at room temperature, as well as for the 109 and 200 °C isotherms, suggesting that the channels should preserve their pseudo-hexagonal shape and continuity along [001] in the investigated P-T range. Thus the channels remain open and bassanite may be considered a nanoporous material also at high-pressure–high-temperature conditions.

	ACKNOWLEDGMENTS

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 
Special thanks to Alexander Kursosov for his kind availability during the experiments at APS. Vitali Prapapenka and the GSECARS-BM13 are acknowledged for the assistance during the experiment at APS. ID-09 beamline is acknowledged for the assistance during the experiment at ESRF. Critical comments by Ross Angel, Martin Kunz, and P.F. Zanazzi helped to improve the quality of the manuscript. Thanks to B. Doherty who revised the English text. The European Synchrotron Facility is acknowledged for allocating beamtime for the experiment. The use of the Advanced Photon Sources of Argonne, Chicago, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. W-31-109-Eng-38. The suggestions of W.A. Crichton, an anonymous referee, and the associate editor, Lars Ehm, were greatly appreciated.

	Footnotes

 
MANUSCRIPT HANDLED BY LARS EHM

MANUSCRIPT RECEIVED March 17, 2009; MANUSCRIPT ACCEPTED July 17, 2009

	REFERENCES CITED

Top Abstract Introduction Experimental methods Results and discussion Acknowledgments References cited

 

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