X-ray Temperature Difference Absorption Spectra (XTDAS): Phase Transitions In NiS1.53Se0.47
T. Norman, T. Tran, T. H. Vu, Q. v. Le, P. Somasundaran, J. M. Honig and J. V. Acrivos
Department of Chemistry San José State University and Purdue University
Abstract
XAS measurements can detect changes in the absorbance as NiS1.53Se0.47 undergoes antiferromagnetic (AF) phase transitions. These effects must be differentiated from for instrumental glitches. We have developed a technique that analyzes the difference between the absorbance at a given temperature T relative to that at say 4 K. The temperature is ramped, at a constant rate of 5 to 10 K per XAS spectrum, by using the remote control Oxford ITC4 program and a PC to produce a continuous temperature cycle from 4 to 100 K - 100 K to 4 K, etc. The changes in the atomic absorbance and in the bond distance that have been detected by conventional XAFS analysis are confirmed in this manner.
Introduction
NiS1.53Se0.47 was prepared at Pudue University (J. M. Honig's Research Group).
Discussion of Results: SSRL 2451M
The AF order increases as the temperature is decreases below TN in NiS1.53Se0.47; this results in an increased atomic absorption above the Se K edge. (figure 1). The explanation for this is that the relaxation time t if for the final states in the post-edge region increases as the AF order increases. The normalized atomic absorption coefficient K versus energy (E= hv) in the neighborhood of an x-ray edge (E0 = hv0) for the absorber A, is given by the sum of Lorentzian curves1 (insert fig. 1a):
K(t , (v-v0)) = S aA/(1+ (2p t AE)2(vAE - v)2) ™ (p /2 + ArcTan(2p t if (v - v0))/p .
The observation is reproducible in the increasing and decreasing parts of the temperature cycle. The magnitude of the slope:
m = (K(E) - K(E0))/(E - E0)/K
versus E - E0 = h(v - v0) was used to calibrate the range of 2p t if /h = 1 to 0. 4 eV-1 for m = 0 to -1E-5 eV-1 in fig. 1a. This shows that the atomic absorption is very sensitive to changes in t if
XAFS analysis at the Ni-K edge data shows a dip in the Ni - Se atomic distance from 2.40 ± 0.01 Å to 2.37 ± 0.01 Å at approximately 25 K. The Ni - S atomic distance showed a similar dip from 2.39 ± 0.01 Å to 2.38 Å ±0.01 Å at approximately 23 K. There was a slight increase in both Ni - Se and Ni - S distance in the 52 K to 80 K temperature range. The structural changes that occurred around 23 ± 4 K may indicate a metal to insulator transition or some other electronic phase change.
Conclusions
The changes measured in the EXAFS analysis are confirmed by XTDAS at the Se edge (fig. 1a); the decrease in the Debye-Waller factor (without change of the Ni-Se bond distance) near 7 K is indicated by the x-ray fine structure observed from 11 to 27 K; the phase transition above 27 K destroys it. Above 80 K, t if decreases by an order of magnitude at 103 eV above the edge, indicating increased disorder.
Acknowledgements
The research, and publications2-6 were supported by NSF DMR9612873 at SJSU.
References
1. F.K. Richtmyer et al., Phys. Rev. 46, 843 (1934)
2. T. Norman, A. T. Nguyen, L. Nguyen and J. Acrivos, Bull. APS 43, 571 (1998)
3. C. Burch, L. Nguyen and J. Acrivos, Bull. APS 43, 584 (1998)
4. Q. v. Le, T. H. Vu, L. Nguyen, A. Alvarado and J.V. Acrivos, ACS Meeting Las Vegas, September 1997
5. J.V Acrivos, ACS Meeting Las Vegas, 1997
6. L. Nguyen, ACS Meeting Las Vegas, 1997
Figure 1: NiS1.53Se.47: (a) Absorbance versus energy at different temperatures showing the slope: m'={A(103 eV) - A(0)}/103 eV, with the calculated atomic absorption coefficient K versus E-E0 and 2p t if /h. (b) XTDAS at the Se edge showing that the transition at 7 K is just due to the hardening of the Ni-Se bond, but with a decrease in the Debye-Waller coefficient; the XAFS due to all the other bonds cancel out completely, leaving only the difference in XAFS amplitudes for the Ni-Se bond. Above 27 K though all the bond distances change cancelling all the XAFS oscillations. (c) S-Se distance showing abrupt change at the first transition temperature near 27 K.