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Physica B 208&209 (1995) 263-264

In situ high-temperature (

2100 K) XAFS and anharmonicity

F. Farges a'*, G. Fiquet b, D. Andrault ~, J.-P. Iti6 d aLab. de Physique des Gkomatbriaux, Universitb Marne-la-Vallbe, IPGP and URA CNRS 734, 93160 Noisy le Grand, France bDept, de Gbologie, ENS, 46 alike d'Italie, F-69007 Lyon. France °Dept. des Gbomatkriaux. IPGP, 2 place Jussieu, F-75005 Paris dLPMC, Univ. Paris 6 and LURE, 2 place Jussieu, F-75005 Paris

Abstract High-temperature (up to 2100 K) XAFS data in the energy-dispersive mode has been collected at Ga and Ge K-edges on a new type of micro-furnace (heating wire) that can run up to 3500 K. Another advantage is the fast data collection time (few seconds per spectra). Data were collected in a variety of Ga- and Ge-oxide compounds. At Ge-K edge, [,]Ge 4 + polyhedra shows a weak anharmonicity, related to the high G e - O bond strength in those compounds. In contrast, [6]Ge4+ polyhedra has a slightly anharmonic behaviour, but much smaller compared to more weaker bonds, like in [4, 61Ga23+O3 or olivine-t6]Niz2 + SiO,. The high-temperature, time-resolved analysis of a t6lGe--* [4]Ge coordination change can also be followed in situ, as well as the melting of these high temperature phases.

1. Experimental We have used a new device to collect XAFS spectra for a variety of systems and temperatures. Our version of the Mysen and Frantz heating wire technique [1] has been implemented on the X A S l l energy-dispersive XAFS spectrometer of DCI (1.85 GeV, 350 mA) at LURE (Orsay, France). Because of the presence of a bent Si-1 1 1 crystal, this particular spectrometer focuses X-ray on a 500 I~m o hole that has been drilled within a Pt9oRhlo wire. The hole contains the sample (100-300 llm thickness available depending on the wire used). The wire is heated by Joule effect. A power-temperature calibration, based on 10-model compound melting points (T are + 20°C on the average) is used together with an optical pyrometer to measure the actual temperature of the wire (gradients are + 10 K at 2000 K). The maximum temperature possible is that of the wire * Corresponding author. 0921-4526./95/$9.50 © 1995 Elsevier Science B.V. All rights reserved SSD! 0 9 2 1 - 4 5 2 6 ( 9 4 ) 0 0 6 7 5 - X

melting point used (here, 2100 K). However, much higher temperatures can be achieved by the use of lr, W, Ta... wires. Data collection is done in the transmission mode, every 20-50 °C (3 s spectrum, 3-13 A-1 k-domain used). Glass-based mirrors were used to reject harmonics from the incident beam. Energy was calibrated using AsGa and Ge slides. Powders from high purity, X-ray crystalline model compounds were set into the holes in order to get absorbance steps between 1 and 3 units at Ga and Ge K-edges ( < 100 ~tm thick samples). Samples studied include GeO 2 quartz (['HGe4 + ) and rutile ([6]Ge '*+ ), Mg2-, Ca2-, and C a M g G e O , olivines (['HGe4+); Ca-, and SrGeO3 pyroxenes (also [4]Ge4+), as well as Sr- and CaGeO3 perovskites ([61Ge4+). Ga-samples includes Ga2 03 ([*'6]Ga3 + ) and G a P O , ([alGa 3+ ). XAFS spectra were normalized using Victoreen and spline functions and recalculated into k space, assuming an Eo energy value arbitrary chosen as the "half-way" point of the absorption step. The k3-XAFS signal was

264

F. Farges et al./Physica B 208&209 (1995) 263- 264

Ge~Ge Ge-OI/l~

~.=''~

=

,'~21 ~ / 0 - 6 K -I ~:~ ++~a,~...~.~¥~...-+ + ~...-~--

ILv/

4

8 12 k(~-] )

,.

,

.." ,,.'•..

295 K 550 K ~NI K II1111K 12511K 1315 K 1321) K 1325 K 1400 K 14511K 151~1K

+ r- GeO2 (anharmonic) - r-GeO2 (harmonic) o q - G e O 2 (harmonic)

+ t

-~=5.71S)× 10 "6 K ~

1

3 R(A)

5 21X)

400

600

800

10~)

1200

14(X)

10(X)

T (K)

Fig. 1. Typical Ge-K edge normalized XAFS spectra for GeO2 and their Fourier transform for a variety of temperatures. Fourier transformed (FT) over the range 3-13 A 1 using a Kaiser window. The inverse Fourier transform ( F T - 1) for the O-contribution around Ga or Ge was calculated in the 0.9 1.7 ,~ range of the F T (max. 5 variable parameters). The FT-filtered XAFS oscillations were modelled in k3-space with the curved-wave formalism [2] and the cumulant expansion theory [3]. The spectra collected at room temperature were used to derive empirical amplitude and phase-shifts in order to fit the high temperature spectra (Fig. 1). This technique allows excellent reproducibilities in the XAFS-derived distances and number of neighbor, about + 0.003 A and 0.2 atoms.

2. Results and discussion At Ge-K edge, ['*lGe4 + (GeO2 quartz, Ge-olivines and pyroxenes) shows a weak anharmonicitiy. The third cumulant tends to zero values and the harmonic model is still approximable up to relatively high temperatures 11500 K). The XAFS-derived G e - O thermal expansion values are consistent and XRD-measurements (~ ~ 4 - 8 × 10 6 K - 1). In contrast, t6]Ge4+-structures (GeO/-rutile and CaGeO3) show a slightly anharmonic behaviour (Fig. 2), but smaller compared to [,*,6iGa 23 + 0 3 . The linear thermal expansion coefficient ~ is correlated to the Pauling bond strength v according to the empirical relation [4]: ~SZaV

= 4 × 10 -6,

(1)

where s is an ionicity factor (set to 0.5 in oxide compounds), Z, is the absolute charge for the anion involved in the bond considered. Therefore, quasi-zero values for the relative third cumulant (Ao"~31) are predicted when

Fig. 2. Evolution of the average Ge O distance as a function of temperature for the two GeO2 polymorphis. Rutile: (e) uncorrect harmonic model; ( + ) correct anharmonic model. Quartz: (o) correct harmonic model. Near 1300 K, rutile modifies into the quartz polymorph, prior melting. Quartz and liquid phases show 141Ge.

v tends to 1 valence units. This trend has been observed here for all t4]G e 4 + - O polyhedra studied, despite located in a variety of different structures. Anharmonicity appears to increase with deceasing bond strength, in the order 141Ga3+-O (v ~ ) , [61Ge4+-O (23), [4'6]Ga3+-O (0.71, ['*]Ni2+-O (½), t6]Ni2+-O (13)... The analysis of phase relations for GeO 2 polymorphs in temperature rutile, quartz, liquid and glass; Fig. 2 shows a [6]Ge --~ t4]Ge coordination change near 1320 K, related to the rutile ~ quartz modification known at this temperature. A drop in Ao-131 from 3 x 10 -4/dk 3 t o negligible values ( ~ 10 5 ~3) is observed during this 6 --, 4 transition around Ge, in agreement with our previous bond strength considerations. Near 1400 K, GeO2 melts, resulting in regular GeO4 tetrahedra and quasi-zero values for Atr 131when GeO2 is molten, The quench of the liquid gives a glass, again with GeO4 units.

References [I] P. Richet, P. Gillet, A. Pierre, A. Bouhfid, I. Daniel and G. Fiquet, J. Appl. Phys. 74 (1993) 5451. [2] J.J. Rehr, R.C. Albers, C.R. Natoli and E.A. Stern, Phys. Rev. B 34 11986) 4350. [33 E.A. Stern, P. Livins and Z. Zhang, Phys. Rev. B 43 119911 8850. [4] R.M. Hazen and L.W. Finger, Comparative Crystal Chemistry (Wiley, New York, 1982)•