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# Michael Lufaso (collaboration with Robert Shull, Virgil Provenzano) <michael.lufaso@nist.gov>
# requesting 3 days, at minimum=2 days
# beamline 33BM, Crystal structure analysis at low temperature of Gd5Ge2Si2 based alloys
# instrument 33BM-C fourc
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# top:/home/www/beamtime-requests/req00519.txt
# UNICAT Member Beam Time Request  #519
# created Sat Dec 06 15:24:10 CST 2003
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beamline:
	33BM

collaboration:
	No

collaborator_Paul:
	ON

contact:
	michael.lufaso@nist.gov

days:
	3

description:
	Since the late 1990’s, when Gschneider and Percharsky and their coworkers at Ames Laboratory first reported observing a large (“giant”) magnetocaloric effect in the Gd5Ge2Si2 compound between 270-300 K, this material has generated a great deal of interest owing to its potential as a suitable refrigerant for near-room temperature magnetic cooling, both in refrigerators and in air-conditioners. The Ames researchers proposed that the large magnetocaloric effect observed in compound in the 270-300 K temperature range is the result of a magnetic field-induced crystallographic structural change from the high-temperature monoclinic paramagnetic phase to the low-temperature orthorhombic ferromagnetic phase. However, together with the pronounced magnetocaloric effect, the Gd5Ge2Si2 compound also exhibits large hysteresis losses. These losses occur at the same temperature range where the compound exhibits its large magnetocaloric effect. Because of the coincidence of hysteretic losses and a large magnetocaloric effect, it is reasonable to conclude that the mechanism is responsible for the large hysteretic losses and for the large magnetocaloric effect. Therefore, before this material can be effectively used as a magnetic refrigerant, its large hysteretic losses need to be significantly reduced.  Recent results obtained at NIST have clearly that shown that the large hysteretic losses can be reduced by more than 90 percent by alloying the Gd5Ge2Si2 compound with a small amount of transition metal, such as iron while still retaining its large magnetocaloric effect. It has been rationalized that one of the major effects of the transition metal addition in the Gd5Ge2Si2 compound is to greatly suppress the formation of the orthorhombic phase, and thus the monoclinic-to- orthorhombic field-induced phase transition in this material is not observed between 260-340 K for H up to 3980 kA/m (5T), resulting in negligible hysteretic losses.  The capability of conducting X-ray diffraction analysis in the transition metal-containing Gd5Ge2Si2 compound below the monoclinic-to-orthorhombic crystallographic phase transition (that is, below 270 K) and be able to show that the monoclinic phase is the stable structure below the phase transition temperature, would greatly support the hypothesis that the greatly reduced hysteresis losses are due to the fact that transition metal addition prevents the formation of the orthorhombic phase even at low temperature and consequently the field-induced phase transition does not occur. The samples are in the form of a polycrystalline powders that are stable when exposed to air. Collection of powder diffraction data at two temperatures (approximately ambient and 250 K) and which is suitable for structure refinement by the Rietveld method is requested. This experiment is in coll

equipment_required:
	Displex (for temperature ~250 K)

experiment:
	Crystal structure analysis at low temperature of Gd5Ge2Si2 based alloys

foreign_nationals:
	None

hazards:
	

instrument:
	33BM-C fourc

instrument_other:
	

minimumdays:
	2

name:
	Michael Lufaso (collaboration with Robert Shull, Virgil Provenzano)

new_request:
	ON

nonmembers:
	None

unacceptable_dates:
	January 8-17, 2004
April 17-22, 2004

z34ID_change_undulator:
	no

z34ID_details:
	

z34ID_on_axis:
	no

z34ID_parasitic:
	no

z34ID_taper:
	no


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