occupation factor for Ca0 8r,Pr„ |SMn,.. They all w ere used in the fractal analysis of boundary ch aracteristics b u t not shown here for claritv. T he single crystal size [r]
Trang 1S T U D Y O F S T R U C T U R E A N D E L E C T R I C P R O P E R T Y
O F P E R O V S K I T E S C a ^ P r , , 15M n , JIR u x0 3
P h u n g Q u o c T h a n h , H o a n g N a m N h a t, B a c h T h a n h C o n g
D epartm ent o f Physics, College o f Science, VN U
Abstract We study the perovskite system of composition Ca 0e5Pr0 ,5Mn,„Ru,O3
w h e re X = 0.00, 0.03, 0 0 5 and 0.07 w hich w ere pre pa re d by sta nd ard ce ra m ic
te ch n iq u e T h e d e te rm in a tio n o f stru ctu re s by X-R ay d iffra ctio n a n d R ietve ld
disto rtio n T h e d e p e n d e n c e o f resista n ce o n te m p erature w a s m e a s u re d fo r T
ty p o lo g y T h e e ffe c t o f th e g ra in bo u n d a rie s (b o un da ry length , are a an d sp a ce
ga p) o n c o n d u c tiv ity o f th e sa m p le s w a s an alyze d b y fra c ta l te c h n iq u e a n d th e
sam p le s
1 I n t r o d u c t io n
From th e branch of th e original compound CaMnO;, m any doping m anganates have been prepared including those doped by ruthenium w here th e significantly drop of resistivity in th e low tem p eratu re region and sim ultaneously induced ferrom agnetism of the doped com pounds have been reported For the b e tte r understa n d in g of the conduction
m icrostructure an d th e electric properties would be greatly needed In this paper we are specifically concerned with th e boundary effects on the conductivity of th e slightly doped
ru thenium m an g a n ates of the com positions Ca0 85P r015Mn, ,R ux0 3 (* = 0.00, 0.03, 0.05, 0.07) T h eir s tru c tu re s w ere investigated by m ean of the X-Ray and SEM m ethod; th en the resistance dependence on tem p eratu re an d frequency was m easured The interdependence between th e boundary an d th e electric properties w as studied using th e fractal method
2 E x p e r im e n t
The sam ples w ere prepared using the ceramic technique Powders of CaCOa, M nC 03 (all of 99.9% purity) w ere weighed in th e desired proportions and m illed for lOhrs They com pressed into the re ctan g u lar b ars of (2 X 2 X 15) mm3 an d th en sin te re d a t 1300°c for lOhrs in air
The p h ase of th e sam ples w ere indentified by X-ray powder diffraction with settings: step angle 0.001° The s tru c tu re param eters, including atom ic positions, isotropic therm al
W inM Prof softw are [3] T he single crystal size distribution w as analyzed by fourier stan d ard two electrods technique in the tem perature range from 100 to 500K The typology
of surfaces w as studied by SEM w here im ages with different m agnification T his se t of
im ages w as used in th e analysis of th e boundary effects on conductivity by fractal technique
as discribed in Section 4
3 R e s u lts o f m e a s u r e m e n t s
Figure 1 shows th e diffraction p attern s for all sam ples; th e s e typical perovskite
p attern s show good single phase s ta te and the overlapping of dots an d lines illu stra tes the good profile fitting re su lts from th e R ietveld analysis
Trang 2In Table 1 we list
th e cell co nstants,
lengths and angles For
stru ctu re s ex ist in the
group Pbnm In general,
th e u n it cell co n stants
and volume increased
th is change w as m ere as
the su b stitu tio n w as
altered only slightly
observable increases in
th e C a-0 a n d M n-0
distances; th e change
was seen in th e angles
coordinations w ith in the
th erm a l m otion factors
(B)s0) and th e ir site
occupation factors (s.o.f.)
- corresponding to the
compounds D u ring the
the s.o.f.s for M n an d Ru
for Ca nad P r w ere fixed
a t 0.85 and 0.15 an d the
were refined A mere
deficit was seen in the
Concerning th e therm al
largest for th e oxygens
and sm allest for the
having u n it in A2, are
reasonable
The model typology
shows th e sta tis tic a l
F ig l The X-Ray diffraction diagrams for the samples The dots represent the measured profile points where the lines show the fitted profile function (Pseudo-Voight) Table 1 Cell papameters, some important mean bond lengths and bond angles for Ca0 ajPr„ K,Mii|.xRux0 3 (the standard deviations are given in the parenthesis)
a: =0.05 5.317(3) 5.318(3) 7.541(1) 213.2(3)2.662 1.883 167.0
X =0.07 5.319(1) 5.321(1) 7.545(0) 213.5(2)2.663 1.884 167.5
T able 2 Atomic positions, isotropic thermal motion B,so and site occupation factor for Ca0 8r,Pr„ |SMn, ,R u A
Trang 3orientation of the crystal preferred axis and offer th e chance to estim ate th e averages of the grain isotropies, of th e m easures of grain in h eritanc e of crystal anisotropic properties The m icrostructures of th e sam ples w ere analyzed by SEM Fig.2 show s four im ages for th e sam ples w ith X = 0.00, 0.03, 0.05 and 0.07
consequently; these pictures w ere taken a t the
m agnification of 10|itn scale T here w as an o th e r set
of im ages a t 2^im, 5|im an d 10nm scale, ta k e n at
fractal analysis of boundary ch aracteristics b u t not
to estim ate th e grain average size, th u s th e average
num ber of the single crystal pieces w ithin one grain;
the grain boundary
To study th e dependence of conductivity of
sam ples on te m p e ra tu re we m easured th eir
resistance from 100 to 500K and th e re s u lts are
th e sharp drop of resistivity due to the su b stitu tio n of
Ru A t the room tem p e ra tu re th e resistivity of the
sam ples has practically reduced to several ohm s
4 C o n c lu s io n
The sm all doping of Ru was
enlargem ent in the cell constants
orthorhom bic T he single crystal
stan d ard for perovskites w here
the average sizes moved around
25nm The g ra in s w ere radically larg e r an d contained 40 single crystal pieces by average The resistivity of sam ples w as m easured by sta n d a rd two electrode technique from 100 to 500K T he fast drop of th e resistivity of all sam ples to th e low several ohm s in the room calculation of th e single crystal dom ain average isotropic conductance re su lte d in th e value nea r 1/2 of th e m axim a] anisotropic conductance
R e fe re n c e s
1 Dang Le Minh Hoang Nam Nhat, Phung Quoc Thanh, Bach Thanh Cong, Hoang Van Hai,
"Structure and electric property of the compound C a,,N d,M n03 revisited by fractal length-
‘2004 To appear
2 B.B Mandelbrot, "The Fractal Geometry o f Nature" W.H Freeman and Co., New York, NY 1983.
3 A Jouanneaux, "WinMProf : a visual Rietveld software", CPD newsletter 21 (1999) p.13
4 S Krauss "WinFit 1997", In stitute fur Geologie, Erlangen 1997
5 Phung Quoc Thanh, Bach Thanh Cong and Nguyen Ngoe Dinh, “Influence of Ruthenium