Mixtures of lanthanum oxide, europium oxide, calcium carbonate, and phosphoric acid were heated with various ratios of P/(Eu + La + Ca) and La/Ca. Europium ratio was settled at Eu/(Eu + La + Ca) = 0.03. The obtained phosphates were estimated using X-ray diffraction (XRD) patterns, Fourier transform infrared spectroscopy (FT-IR) spectra, and scanning electron micrograph (SEM) images. The fluorescence spectra and resistance against hydrofluoric acid were estimated as functional properties of these phosphate materials. The mixture of lanthanum and calcium phosphates were formed from XRD patterns and IR spectra. Samples prepared in P/(Eu + La + Ca) = 2 and 3 had large particles in SEM images. The condensed phosphates showed a strong peak at 615 nm and high resistance against hydrofluoric acid.
Phosphates are transformed to other forms of phosphates by hydrolysis and dehydration reactions at elevated temperatures [1,2]. Polyphosphate and ultraphosphate are included in a group of condensed phosphates. Polyphosphate has a chain structure in which the PO4 unit shares two oxygen atoms and ultraphosphate has a network structure. Formation of these condensed phosphates was affected by the ratio of phosphorus/cation, heating temperature, time, atmosphere, and so on [3-5]. Therefore, it was difficult to obtain a high yield of the condensed phosphates. Consequently, orthophosphate has been investigated for various uses, but condensed phosphates have been little studied. Orthophosphate materials have been used for ceramic materials, catalysts, fluorescent materials, dielectric substances, metal surface treatment, detergent, food additives, fuel cells, pigments, etc. [6,7]. The condensed phosphates have different properties from those of orthophosphates and can therefore be used as novel functional materials [8,9].
Rare-earth phosphates have a high melting point and large specific surface area in phosphate materials [10,11]. Rare-earth orthophosphates, which are the main component of rare-earth ores, are stable phosphate groups in acidic and basic solutions. Their resistance in acidic and basic solutions was developed into other phosphate materials [
Metals, oxides, and silicates are useful materials, but they are vulnerable to the effects of hydrofluoric acid, which is a reagent used in many industrial applications. However, its wastes are not easily disposed of [
In previous work [
Europium oxide (Eu2O3) was mixed with lanthanum oxide (La2O3) and calcium carbonate (CaCO3) in the ratio of Eu/(Eu + La + Ca) = 0.03 and La/Ca = 10/0, 8/2, 5/5, 2/8, and 0/10. These mixtures were added to phosphoric acid (H3PO4) at mole ratios of P/(La + Ca + Eu) = 1, 2, and 3, and then heated at 700˚C for 20 hr under air conditions.
The respective chemical compositions of these thermal products were analyzed using X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR). X-ray diffraction patterns were recorded on a Rigaku Denki RINT2000 X-Ray diffractometer using monochromated CuKα radiation. The IR spectra were recorded (FT/IR-4200; JASCO Corp.) using a KBr disk method. The particle shapes of phosphate powder were observed from scanning electron micrographs (SEM, JGM-5510LV; JEOL).
The excitation and emission properties were measured using a luminescence spectrometer (LS55; Perkin-Elmer). The emission and excitation wavelengths were 620 and 254 nm, respectively. The resistance of materials against hydrofluoric acid was estimated using the following method. The 0.2 g of thermal products was allowed to stand in 100 ml of 5 wt% of hydrofluoric acid for 1 day. Then, a solid was removed by filtration. The residual ratio was calculated with the dried solid.
Samples prepared in P/(La + Ca + Eu) = 1 and La/Ca = 10/0 indicated the peaks of lanthanum orthophosphate, LaPO4. By the substitution from lanthanum to calcium cation, the peaks of calcium phosphate, Ca3(PO4)2, appeared in XRD patterns. In P/(La + Ca + Eu) = 2, XRD patterns were changed from lanthanum orthophosphate, LaPO4, and polyphosphate, La(PO3)3 to calcium polyphosphate, Ca(PO3)2.
indicated the adsorption due to orthophosphate, on the other hand, sample in La/Ca = 0/10 had the adsorption of condensed phosphates. The most important peak at 770 cm−1 was from P-O-P bonding in condensed phosphates [
From SEM images, the P/(La + Ca + Eu) ratio had more influence on particle shape and size than the La/Ca ratio.