We describe the general principle and prototypical applications of a largely unexploited x-ray absorption technique based on single-energy detection during temperature scans. The present developments take advantage of the highly automated experimental set-ups and unique x-ray radiation characteristics available at third-generation synchrotron radiation sources. The sensitivity of the x-ray absorption cross-section to the local structural order makes it possible to investigate the occurrence of phase transitions. We show that by tuning the photon energy to a spectral feature that is highly sensitive to the sample phase it is possible to follow the occurrence and the characteristics of the phase transition. The sensitivity to the atomic fraction of the sample in a certain phase can be in the range. The energy tunability can be used to enhance the sensitivity to structural or electronic transformations. Several applications to prototypical cases are discussed in detail, including examples of solid-solid phase transitions, melting and undercooling, melting of binary alloys, impurity melting, and non-bulk phenomena.

A structural phase transformation in the CuO 2 planes of YBa 2 Cu 3 O x has been observed at the onset of the overdoped regime, x 6.95. We have measured as a function of x the dimpling in the CuO 2 planes by EXAFS, and the O(2,3) in-phase ͑A 1g ͒ mode by Raman scattering. The data show for x $ 6.95 anomalously large static displacements of the Cu (2) atoms off the O(2,3) layer and a gap in the distribution of the O(2,3) in-phase Raman shifts. We conclude the structure of overdoped YBa 2 Cu 3 O x to be a martensitic form of the optimum doped crystal. [S0031-9007(97)04809-6]

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Inelastic neutron scattering (INS) measurements are presented for solid imidazole at 20 K. Both powder and oriented platelets have been measured. Assignments of bands are consistent with earlier infrared and Raman studies except for a band at 961 cm−1 which is definitely assigned as A″. A fit with the proton-weighted vibrational density of states is presented and a full harmonic force field has been established. The mean deviation between measured eigenvalues and those predicted from the force field is ±3 cm−1. The potential energy distribution is compared to quantum chemistry calculations and is found to be in best agreement with an unscaled calculation [J. Sadlej, A. Jaworski, and K. Miaskiewicz, J. Mol. Struct. 274, 247 (1992)]. The INS spectra from a scaled ab initio force field [K. Fan, Y. Xie, and J. Boggs, Theochem, 29, 339 (1986)] are calculated both before and after refinement against only infrared frequencies and compared to experiment. It is demonstrated that comparison with the INS spectrum is a more stringent test of the force field than with just the infrared frequencies alone.