Developments in Ceramic Materials Research

Synthesis, Spectroscopic and Magnetic Studies… 99
ordered phases of some of these compounds, neutron diffraction measurements have been
carried out.
2. SYNTHESIS
The M(PO3)3 metaphoshates were synthesized using the ceramic method. The starting
reagents were the salt or oxide of the metallic cation in its oxidation state of +3. The
phosphorous element was incorporated to the composition of the corresponding
metaphosphate in the form of (NH4)2H(PO4), with a M:P ratio, depending on the kind of
metaphosphate prepared.
The synthesis of the M(PO3)3 metaphosphates with M III = V, Cr, Mo and Fe, was carried
out using the ratio M:P of 1:10. This mixture reaction was heated, under air, in an alumnina
crucible at 800 °C, after of a previously heating in the, approximately, 300-400 °C range, in
order to decompose the (NH4)2H(PO4) reagent. Exceptionally, the Cr2(P6O18) phase was
obtained using the same conditions but with a Cr:P ratio of 1:5. All the resulting phases
exhibit green color [16-18].
Attempts to prepare the Ti(PO3)3 compound by reduction of TiO2 oxide with an excess of
the (NH4)2H(PO4) were unsucceful. This fact can be attributed to the instability of the Ti(III)
cation in air. So, the experimental synthetic method was modified, using as starting reagents
metallic titanium and an excess of (NH4)2H(PO4). A flux of nitrogen was maintained during
the reaction time in order to avoid the oxidation to Ti(IV). The resulting product shows blue
color [19].
3. PHYSICOCHEMICAL CHARACTERIZATION TECHNIQUES
IR spectra (KBr pellets) were obtained with a Nicolet FT-IR 740 spectrophotometer.
Diffuse reflectance spectra were registered at room temperature on a Cary 5000
spectrophotometer, in the range 8000–50000 cm -1 . Mössbauer spectroscopy measurements
were performed in the transmission geometry using a 57 Co-Rh source in the constant
acceleration method. Mössbauer spectra at different temperatures were recordered using a
bath cryostat. The velocity was calibrated using α Fe foil. The spectra were fitted using the
NORMOS program. The isomer shifts are quoted relative to α-Fe at 300 K. A Bruker ESP
300 spectrometer, operating at X and Q-bands, was used to record the ESR polycrystalline
spectra between 4.2 and 300 K. The temperature was stabilized by an Oxford Instrument (ITC
4) regulator. The magnetic field was measured with a Bruker BNM 200 gaussmeter and the
frequency inside the cavity was determined using a Hewlet-Packard 5352B microwave
frequency counter. Magnetic measurements were performed on polycristalline samples
between 1.8 and 300 K, using a Quantum Design SQUID magnetometer (MPMS-7) with a
magnetic field of 0.1 T, at which the magnetization vs. magnetic field is linear even at 1.8 K.
About 5 g of the M(PO3)3 (M III = Cr, Mo and Fe) were employed in the neutron diffraction
experiments, contained in a cylindrical vanadium container and held in a liquid helium
cryostat. The high resolution of D2B was used to obtain extensive and accurate structural data
for both phases at room temperature over the angular range 0≤ 2θ ≤160°. For the iron