Raman spectroscopy at non-ambient conditions: I - Experimental set-up and a case study (anydrous mineral)

Transcription

1 Raman spectroscopy at non-ambient conditions: I - Experimental set-up and a case study (anydrous mineral) Il contributo della spettroscopia Raman allo studio dei minerali in condizioni non ambientali: I - Metodi e Minerali Anidri G. Diego Gatta Dip. Scienze della Terra Uni Milano Parma, 12 Feb. 2009

2 Raman spectroscopy at non-ambient conditions: Why? - To describe the stability fields (at HP/HT conditions) of materials - To describe the kinetics of solid-state transformations at nonambient conditions - To provide complementary data to the X-ray diffraction Don t forget that: - Raman spectroscopy is a non-destructive analysis - The new micro-raman spectrophotometer allow to operate with sample significantly small in volume (i.e. a few µm 3 )

3 Raman spectroscopy at non-ambient conditions: How? High-Temperature: Heating stages High-Pressure: Diamond Anvil Cells (DAC) High-Pressure/High-Temperature: Heatable DAC - External resistive heating DAC - Laser heating DAC

4 High (and Low) Temperature: Heating stages Temperature range: -196 to 600 C Temperature stability: <0.1 C Sample area 22 mm diameter Gas tight chamber for atmospheric control 100ohm platinum resistor sensor LinKAM TMS94/TMS600 (Hot Stage)

5 Diamond Anvil Cells

6 DAC DAC Source: RJ Angel, VirginiaTech

7 Diamond Anvil Cells (DAC)

8 XRD - DAC Raman - DAC Miletich R., Uni-Heidelberg, Germany

9 Diamond is the premier anvil material as it is the hardest substance known and relatively transparent to electromagnetic radiation over a wide spectral range (from IR to hard X-rays: 5-10 kev). Diamond shows an excellent thermal conductivity and low thermal expansion. The design of the anvils is a critical factor influencing the maximum pressure a DAC can achieve

10 Diamods are classified into two types: Type-I: contain significant amount of nitrogen impurity (i.e. >0.1wt%) and are further classified into subgroups A and B, depending on how the nitrogen impurities aggregate Type-II: do not contain significant nitrogen impurity (i.e. <40 ppm) and are further classified into groups A and B, depending on their electrical conductivity

11 Nitrogen impurities in diamonds: Effects on Raman spectra Banaszak et al. (2006) Cryst. Res. Technol. 41,

12 Despite the relatively high transparency of diamond to light and X-rays, the usual thickness of a pair of 1/4 1/3 carat anvils may create a significant impediment to radiation transmission. For the case of UV and IR radiation, the presence of nitrogen in type-i diamonds (the most common type of natural diamonds) creates absorption bands for UV and IR radiation. For the case of Raman spectroscopy, luminescence in the diamond, produced by the excitation beam, adds significantly to the background in the Raman spectra. For the case of X-rays, the 5 mm diamond thickness of a pair of 0.3 carat anvils will essentially block the transmission of photons for E X-rays <10 kev. Type-II diamonds are better suited to high-pressure spectroscopic studies although they are more expensive Dadashev et al. (2001) Rev. Scient. Instruments, 72,2633

13 The relative transmission through diamond for different photon radiation energies. Note that for a pair of 0.3 carat anvils with ~5 mm thickness, the transmission of the 14.4 kev 57 Fe γ-ray is ~30%, whereas with a pair of anvils with ~1 mm thickness transmission increases to ~75%. Dadashev et al. (2001) Rev. Scient. Instruments, 72,2633

14 Dadashev et al. (2001) Rev. Scient. Instruments, 72,2633

15 HP/HT diamond anvil cells : External resistive heating DAC

16 Schematic view of the high- pressure high- temperature system: (1) diamond, (2) backing plate, (3) guide pin, (4) gasket, (5) cylindrical platens, (6) front-side heater, (7) cylindrical heater, (8) back-side heater, (9) thermocouple, (10) lateral adjusting screw, (11) one of the six M5 screws with hexagonal heads for applying a load during heating, (12) cylindrical plate, (13) one of the six M3 Allen screws used for room-temperature operations.

17 High-pressure/high temperature system mounted for experiments on ID27 beam line at ESRF

18 High-pressure/high temperature system mounted for experiments on Leica DM LM microscope for visual observations and Raman spectroscopy.

19 a) Double-side YLF-laser heating set up for experiments with diamond anvil cells at Bayerisches Geoinstitut: a general view b optical scheme b)

20 Experimental P-transmitting media 16:3:1 Methanol:Ethanol:Water, 4:1 M:E, H 2 O, N 2, Ar, He, H 2, glycerol, silicon-oil P/T-calibration -by the ruby fluorescence method ( ± 0.05 GPa) [low ν/ P, high ν/ T] -by Sm 2+ :BaFCl fluorescence method ( ± 0.05 GPa) [high ν/ P, low ν/ T] -by Sm 2+ :SrB 4 O 7 fluorescence method ( ± 0.05 GPa) [high ν/ P, ν/ T~ 0] - thermocouples - emitted blackbody radiation

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22 A mineralogical case study: High-pressure X-ray and Raman study of a ferrian magnesian spodumene M2 (Li 0.85 Mg 0.09 Fe )M1 (Fe Mg 0.15 )Si 2 O 6 A ferrian magnesian spodumene with composition: M2 (Li 0.85 Mg 0.09 Fe )M1 (Fe Mg 0.15 )Si 2 O 6 (LMF-cpx) was synthesized with P2 1 /c symmetry at room conditions. Its crystal structure and its HT-behaviour is reported by Cámara et al. (2003). What we already know - A tricritical displacive phase transition from P2 1 /c to HT-C2/c occurs at 389 K. Such a study demonstrates the effect of Mg (and Fe 2+ ) substitution in the octahedral sites on the thermal behaviour of the LiFe 3+ Si 2 O 6 structure: only 0.15 (Mg+Fe 2+ ) a.p.f.u. at the M2 site and 0.15 Mg a.p.f.u. at the M1 site increases the stability field of the P2 1 /c phase at room pressure of about 150 K with respect to the end member. -LiFe 3+ Si 2 O 6 shows a HT-C2/c structure at room conditions. A first-order phase transition from HT-C2/c to P2 1 /c was observed at 0.8 GPa (Pommier 2003).

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24 Evolution of the lattice parameters of LMF-cpx with pressure (Solid lines: M-EoS fit) a (Å) a vs P M-EoS: K TO = 62.8(6) GPa K' = 13.9(3) b (Å) b vs P M-Eos: K T0 = 117(1) GPa K' = -0.3(5) c (Å) c vs P M-EoS: K T0 = 53.6(6) GPa K' = 11.9(3) P (GPa) P (GPa) P (GPa) β vs P V vs P M-EoS: K T0 = 83(1) GPa K' = 9.6(6) β (º) V (Å 3 ) P (GPa) P (GPa)

25 Raman spectra at room conditions of LMF-cpx collected with the crystal in three different orientations. ν 4 ν 11 Intensity (a.u.) ν 1 ν 3 ν 5 ν 6 ν 8 ν 9 ν 10 ν 12,ν 13 Factor group analysis: 60 Raman active modes (30 of Ag symmetry and 30 of Bg symmetry) for the end member LiFeSi 2 O 6 having P2 1 /c symmetry. ν 2 ν Wavenumber (cm -1 ) -Olympus BX40 microscope attached to a Jobin-Yvon LABRAM confocal Raman spectrometer equipped with a charge-coupled detector (CCD-SpectrumOne) nm red light of an Ar + laser. -The un-polarised laser beam was focused a few µm on the sample (objective 50x), the confocal aperture was 1100 µm.

26 Raman spectra of LMF-cpx collected at different pressures. P-precision ±0.05 GPa. P (GPa) 550 ν 11 ν Peak position (cm -1 ) y( ) ν 9 ν 8 ν 7 ν 6 ν 5 ν ν ν ν P (GPa) Wavenumber (cm -1 )

27 Variation as a function of pressure of the peak positions of the doublet corresponding to the Si- O-Si vibrations of the two distinct tetrahedral chains of the P2 1 /c pyroxene structure. A fit with two Lorentzian profiles was used in order to resolve the overlapping peaks ν 12 and ν ν 13 Peak Position (cm -1 ) ν P (GPa) The difference between the Si-O-Si angles for the A- and B-tetrahedral chains (138.4 and respectively) is smaller than those of P2 1 /c clinoenstatite (133.8 and ) and clinoferrosilite (138.2 and ). This may suggest, as in the case of spodumene, that the Si-O-Si modes have a similar vibrational frequency and therefore are not well resolved in the collected Raman spectra.

28 Some concluding remarks A careful monitoring of the intensity of b-type reflections (h+k = 2n+1) confirms that up to the maximum pressure achieved the primitive lattice is maintained. The intensity of the b- reflections was constant, within the esds, on the whole pressure range investigated. The diffraction data collected during decompression showed a reversible/complete restoration of the lattice constants. No new vibrational modes can be observed in the Raman spectra collected for this study, however major changes of the relative intensities can be noticed at the lowest wavenumbers, suggesting that the strength of the local bonds around the M2-site occupied by Li changes as a function of pressure for the LMF-cpx. The peak position of all vibrational modes varies linearly with pressure, moreover the splitting between ν 12 and ν 13 increases with increasing pressure suggesting that the Si-O-Si angles of the two distinct tetrahedral chains become more different.

29 The P2 1 /c phase of LMF-cpx appears also more stable as a function of pressure than P2 1 /c (Mg,Fe) 2 Si 2 O 6 pyroxenes whose critical pressures for the first-order P2 1 /c C2/c transformation are below 7 GPa. A different structural behaviour with respect to the (Mg,Fe) 2 Si 2 O 6 pyroxenes is also suggested by the variation of the Si-O-Si Raman active modes as a function of pressure. Whereas in the case of LMF-cpx the splitting between mode υ 12 and υ 13 increases with pressure, in the case of clinoenstatite the splitting between the vibrations relative to the two distinct tetrahedral chains remains constant as a function of pressure and changes suddenly to a single peak at an intermediate position as the first-order P2 1 /c HP-C2/c phase transition occurs. (Further sources: Shannon 1976, Angel and Hugh-Jones 1994; Ross and Reynard 1999)

30 Generalized phase diagram LiFe 3+ Si 2 O 6 (Li 0.85 Mg 0.09 Fe )(Fe Mg 0.15 )Si 2 O 6 Source: Redhammer et al. (2001), Zhang et al. (2002) and Pommier (2003) for LiFe 3+ Si 2 O 6 ; Camara et al. (2003) and this study for M2 (Li 0.85 Mg 0.09 Fe )M1 (Fe Mg 0.15 )Si 2 O 6

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