HIGH-PRESSURE GD-155 MOSSBAUER EXPERIMENTS ON GD INTERMETALLIC COMPOUNDS COMPARED WITH FIRST-PRINCIPLES BAND-STRUCTURE CALCULATIONS

High-pressure Gd-155 Mossbauer measurements on Gd metal, GdCo5, GdRu2S i2, and Gd2Co17N3 were performed at 4.2 K. The maximum pressures reach ed were about 18 Cpa. The resulting volume reductions of 20-30% were d etermined using high-pressure x-ray diffraction. The pressure dependen ce of the electric-field gradients and hyperfine fields obtained for t he first three systems was compared with predictions from first-princi ples band-structure calculations. Significant changes of the hyperfine parameters are observed, especially for elemental Gd metal. With incr easing pressure, an increase of the electron density at the Gd nuclei is found in all compounds. The values of the hyperfine field initially increase with pressure. For Gd metal the induced structural phase tra nsitions result in large changes in the electric-field gradient at the nucleus (V-zz). The intermetallic compounds show no structural phase transitions and relatively small changes in V-zz. The combination of e xperiment and calculations indicates that the transition-metal magneti c moments decrease at high pressure. Although for zero pressure predic tions of the electric-field gradient and the hyperfine fields, based o n the calculations, are quite accurate, the calculated pressure depend ence of the hyperfine parameters for Gd, GdCo5, and GdRu2Si2 does not in all cases lead to a satisfactory agreement with experiment. The app lication of pressures therefore may give additional stimulus for the i mprovement of the theoretical description of band structures and hyper fine parameters.