Аннотация:The lanthanide hafnates Ln2Hf2O7 (Ln = Nd, Dy) belong to the large class of compounds,
which show different structural, chemical, physical properties. Rare-earth hafnates are typically
prepared via coprecipitation, sol–gel processing or mechanical activation (m/a) of oxides,
followed by high-temperature heat treatment at 1600-1650°С [1, 2]. In some cases, on
the XRD pictures of hafnates synthesized at 1600–1650 ° С, it is noted the presence of the
line 26.6 ° in 2θ (CuKα), which refers to graphite [2]. The likely reason for the presence of
carbon in materials prepared via m/a is that, during m/a process, the Ln2O3 (Ln = Nd, Dy)
absorb CO2 and H2O from air and partially convert into X-ray amorphous LnOHCO3• nH2O
basic carbonates [3].
The purpose of this work was to determinate the phase formation features of Nd2Hf2O7 pyrochlore
and Dy2Hf2O7 (DyHfO4-δ) fluorite in a wide temperature range, from 550 to 1600°C.
We studied precursors (1) obtained using mechanical activation and samples heat-treated in
air for a long time (2–5) under the following conditions: (2) 550°C, 22.5 days; (3) 1200°C,
54 hrs; (4) 1600°C up to 10 hrs. The powders thus prepared were characterized by XRD and
DTA/TG in combination with mass spectrometric analysis of the gas released during heating
at 10°C/min to 1200°C in oxygen or helium containing 0.1% oxygen. According to XRD
data, the as-prepared precursor (1) consisted of a mixture of start oxides, sample (2) consisted
of start oxides with wider diffraction lines, sample (3) consisted of well-crystallized
Nd2Hf2O7 pyrochlore with HfO2 admixture and phase-pure DyHfO4-δ fluorite, and samples
(4) consisted of a phase-pure Nd2Hf2O7 pyrochlore and DyHfO4-δ fluorite phase, respectively.
Heating the samples in a DSC cell was accompanied by a multistep gas release, usually in
parallel with a weight loss and thermal events. The most interesting data were obtained for
samples (3) and (4). Even though no XRD evidence for the presence of carbon in the hafnates
was obtained, heating these samples was accompanied by CO2 release, a weight loss, and
exothermic peaks between 300 and 450°C. It is shown that the CO2 formation is due to the reaction
of carbon with O2 (g). At higher temperatures, we observed an increase in the amount
of released CO2, with broad maxima in the range 600–1000°C. For samples (1) and (2), the
CO2 release also accompanies the start of the synthesis of final products. It should be noted
that main synthesis from mechanically activated oxides occurs in the region of 800–900°C
for Nd2Hf2O7 and 1000–1100°C for DyHfO4-δ , respectively, and is accompanied by weight
loss, which is more significant in the first case than in the second. In general, all effects associated
with the CO2 release are less pronounced for the synthesized DyHfO4-δ fluorite than
for the Nd2Hf2O7 pyrochlore.
