#
# top.uni.aps.anl.gov:/home/www/beamtime-requests/req00192.txt
# UNICAT Member Beam Time Request  #192

# created Tue Jun 19 15:03:10 CDT 2001
#
collaboration:
	Yes

collaborator+Hawoong:
	on

contact:
	t-chiang@uiuc.edu

days:
	4

description:
	Strontium titanate undergoes a structural phase transition at ~100 K
	from a RT cubic phase to a tetragonal phase at lower temperatures.  This is
	a second order displacive transition driven by the softening of a zone-
	boundary optical phonon mode at the R-point in reciprocal space.  The
	 optical phonon mode when fully softened (at Tc) breaks into two
	new acoustic modes at the new zone center (old R-point).  The hardening
	of these modes under further cooling is responsible for two other phase
	transitions found in transport measurements at 65 K and 37 K (when
	 each of the new modes crosses a previously existing one).
	
	The planned experiment is to use x-ray thermal diffuse scattering to 
	observe phonon frequency change at the R-point over a wide temperature
	range.  This should provide detailed information on the nature of all
	three accessible phase transitions.  Suitable analysis of acquired 
	lineshapes and comparison with available neutron data should also 
	resolve the issue of how the elastic 'central peak' observed in neutron 
	scattering manifests in x-ray diffuse scattering.

equipment+required:
	Diffractometer, helium displex.

experiment:
	Phonon softening in SrTiO3

foreign+nationals:
	

hazards:
	All samples are stable in air.  Usual cryogenic safety procedures
	dealing with closed-cycle helium displex.

minimumdays:
	3

name:
	T.-C. Chiang

new+request:
	on

nonmembers:
	

station:
	33ID-D

unacceptable+dates:
	Aug. 6-11


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