Positron Lifetime Studies of Defect Structures in Ba l-x K x BiO 3
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POSITRON LIFETIME STUDIES OF DEFECT STRUCTURES IN Bal-xKxBi03
J.C. O'BRIENt, R.H. HOWELLt, H.B. RADOUSKYt, P.A. STERNEt, D.G. HINKS§, T.J. FOLKERTS*, and R.N. SHELTON*. (t) Lawrence Livermore National Laboratory, Livermore CA, (§) Argonne National Laboratory, Argonne IL, (t) University of California, Davis CA. ABSTRACT Temperature-dependent positron lifetime experiments have been performed from room temperature to cryogenic temperatures on Bal-xKxBiO3. for x=0.4 and 0.5. From
the temperature dependence
of the positron
lifetime in the normal
state, we
observe a clear signature of competition between separate defect populations to trap the positron. Theoretical calculations of lifetimes of free or trapped positrons have been performed on Bat xK xB i03, to help identify these defects. Lifetime measurements separated
by long times have been performed
and evidence
of aging
effects in the sample defect populations is seen in these materials. INTRODUCTION Positron
annihilation
spectroscopy
(PAS)
has
been
widely
used
to
study
electronic properties and defect structures in solids. 1 We have applied PAS to study the temperature dependence of the positron lifetime in Bal-xKxBiO3, (BKBO). We find that the positron lifetime is dominated by defects in the lattice, and that this effect persists for at least a year, As the material ages at room temperature, the defect populations
change,
Theoretical
LMTO
and
our measured
positron
lifetimes
reflect
this
evolution.
calculations 2 for defect structures in BKBO compare with these
measurements and lead to possible identifications of these defect structures. EXPERIMENTAL The Bal-xKxBiO3
samples, x=0.40 and 0.50, were prepared by a melt-processing
technique which has been previously described 3 . The aged and newly prepared samples had an intense blue color, characteristic of superconducting BKBO. The superconducting transition temperatures magnetization versus temperature data, Magnetometer.
of these samples were determined from measured in a Quantum Designs SQUID
The data for newly prepared material,
presented in Figure 1, indicate
that the transitions are sharp with onset temperatures of 25K and 24K for x= 0.4 and 0.5 respectively, and a width of less than 10 degrees. The superconducting transition was recharacterized
for these samples after one year and no change was observed in
the transition temperature or width from
the original data.
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
878
T=23 c
ZFC (10 Oe)
" 0 .9
-5
cu -10
* Bao. 5 Ko. 5 BiO 3 e Bao. 6 Ko. 4 Bi03 -15 0
10
20
T(K)
30
40
50
Figure 1: Magnetization as a function of temperature for freshly prepared BaO.6K0.4BiO3 and Ba 0 .5 K0 . 5 BiO 3 . Both samples have a sharp transition, < 10K, with an onset temperature of 23K. The positron annihilation rate was measured as a function of temperature in a BaF 2 fast-fast coincidence spectrometer, with a resolution of 260 ps. Two
standard pieces
of each sample
consisting of 20 jiCi of
were mounted 22
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