#
# top.uni.aps.anl.gov:/home/www/beamtime-requests/req00225.txt
# UNICAT Member Beam Time Request  #225

# created Sun Dec 02 21:53:19 CST 2001
#
beamline:
	33ID

collaboration:
	No

contact:
	mvholt@uiuc.edu

days:
	2

description:
	Strontium titanate undergoes a structural phase transition at ~100K 
	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 
	mode).
	
	  The planned experiment is to use x-ray thermal diffuse scattering 
	to observe phonon frequency change at the R-point over a temperature 
	range complementing the data of the previous experiment (first 
	experiment 250 deg K - 100 deg K ----> proposed experiment 300 deg K 
	- 250 deg K and 100 deg K - 50 deg K).  We will also collect 
	the intensity of the (1/2 1/2 1/2) superlattice point as a function 
	of temperature.  This should serve to both establish the transition 
	temperature and investigate a possible 'central peak' due to static 
	fluctuations at slightly above the transition.  Suitable analysis of
	acquired lineshapes near Tc may also resolve the issue of how the 
	elastic central peak observed in neutron scattering manifests in 
	x-ray diffuse scattering.

equipment+required:
	He displex, diffractometer.

experiment:
	Phonon softening in SrTiO3

foreign+nationals:
	Dr. Zhong Quan Ma

hazards:
	cryogenics - a closed cycle helium displex will be used.

minimumdays:
	2

name:
	Martin Holt

nonmembers:
	

unacceptable+dates:
	


#REMOTE_HOST: chiangpc9.mrl.uiuc.edu
#REMOTE_ADDR: 130.126.102.93
#CONTENT_LENGTH: 1869
#HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm
#HTTP_USER_AGENT: Mozilla/4.61 [en] (Win95; I)