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J.L. MacManus-Driscoll et al.: Influence of Mn–O bond length on carrier localization in La1−x(Ca,Sr)xMnO3, where x = 0.28 to 0.375
final sintering at 1400 °C for 60 h in air. Again a buffer
powder of the same composition was used to line the
crucible. From the literature,10,11 sintering at 1400 °C, in
air, was predicted to be a sufficiently reducing condition
to ensure that for the whole of our doping range studied
the samples were on a “stoichiometry versus pO2” pla-
teau, and hence were stoichiometric.
and cooling to check for any hysteresis. There was no
measurable hysteresis detected. Two independent ther-
mocouples were placed above and below the samples to
ensure accuracy in the temperature determination. The
thermocouple readings were found to agree within 1 °C.
The measured resistance values were transformed to re-
sistivity values by using actual sample cross-sectional
areas (i.e., geometrical area × fractional porosity).
X-ray diffraction (XRD) was conducted in the Bragg–
Bretano geometry on finely ground powders from each
of the seven compositions. Room-temperature neutron
diffraction was conducted on two extremes of composi-
tion, 2893 of low doping, and 3693 of high doping. 3693
was studied in preference to 37593 because of the
slightly greater uncertainty of the composition of 37593
(see Table I). The neutron data were collected in the
high-resolution D2B diffractometer at ILL-Grenoble. A
wavelength of 1.594 Å was selected from a Ge mono-
chromator. About 4 g of each sample were contained in
a cylindrical vanadium can. The data collection was com-
pleted in 3 h for each sample. The crystal structures were
refined by the Rietveld method, using the FULLPROF
program. A pseudo-Voigt function was used to generate
the peak shape. Neutron powder diffraction (NPD) pat-
terns for 2893 and 3693 were refined in the space group
R3C, hexagonal description (Z ס
6), with (La,Ca,Sr) at-
oms randomly distributed at 6a (0,0,1/4) positions, Mn at
6b (0,0,0), and O at 18e (x,0,1/4). As NPD data were
collected below TC for both samples, a ferromagnetic
contribution to the scattering was observed on the low-
angle reflections. A magnetic model was included in the
last stages of the refinement, describing the ferromag-
netic structure of the ordered Mn moments. In the final
run the following parameters were refined: scale factors,
positional, thermal isotropic factors, ordered magnetic
moments for Mn cations and unit-cell parameters, as well
as six background coefficients, zero point, half width,
pseudo-Voigt, and asymmetry parameters for the peak
shape. We note that neither the cation nor oxygen pa-
rameters were constrained and hence both cation and
oxygen positions and stoichiometries were refined.
The cation compositions of all the samples were meas-
ured by quantitative energy-dispersive microprobe analy-
sis (EDS). The microstructures were determined by
optical microscopy of thermally etched samples with av-
erage grain sizes determined by the linear intercept
method. Density measurements were undertaken using
the Archimedes method.
III. RESULTS
A. Structural properties
The materials properties of the prepared samples are
shown in Table I. Sample labels are xx(x)93, where xx(x)
represents the doping and 93 represents t. The actual
sample cation compositions measured by EDS were
within 0.01 mol of the starting values, except for 37593,
which is within ±0.015 mol of starting value for the La.
The average grain sizes were between 4 and 25 microns.
The range of grain sizes was larger than expected con-
sidering the identical sintering conditions. It is likely that
the variation in values reflects the variation in starting
powder particle sizes, which is hard to control precisely
by decanting of powders after ultrasonic dispersion,
rather than minor differences in La, Ca, and Sr values.
The smallest grain size measured is above a critical value
for long-range order,12 and therefore there should be no
depression of Tm from grain-size effects in any of the
samples.
The rA values decreased from 1.242 to 1.231 Å for
samples 2893 to 3693. Between sample 3693 and 37593
there was a sharper drop in rA variation with doping,
with a value of 1.217 Å for 37593.
The actual t values were 0.928 ± 0.003, ensuring that
the distortion of the lattice from ion-size effects was
essentially constant from sample to sample. Variance
(2) as listed in Table I is the variance of the A site ionic
radius (ri ס
La, Ca, or Sr, of mole fraction, yi) about the
mean rA, 2 ס
∑yi ri 2 − rA 2 13 It has been shown that
.
the higher the value of 2, the lower is Tm, and this has
been ascribed to inhomogeneous strain effects.13 Our
2
values all lie in the range 0.0022 ± 0.00095 and are low
enough to not cause a depression in Tm.
The density values were in the range of 85 to 94% for
the seven samples. The values are, in general, high for
alkaline earth manganites. It is important to achieve high
densities because of possible peak broadening effects,14
which could be associated with poor grain-boundary cou-
pling. The values we have obtained are not expected to
lead to peak broadening.
Figure 1 shows a typical optical micrograph of a ther-
mally etched sample (3393). Whereas the grain size var-
ies from 5 to 50 microns, the porosity is mainly
Resistivity measurements were made on bar-shaped
samples that were cut from the pellets. The applied cur-
rent was measured to ±0.01 mA and the potential drop to
±0.01 mV. To eliminate any possible electromotive
forces (EMFs) generated by temperature gradients across
the sample, the direction of the current was reversed for
each measurement point and the average voltage re-
corded. Measurements were conducted both on heating
J. Mater. Res., Vol. 15, No. 5, May 2000
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