The influence of Na+ and Ca2+ ions on the SiO2–AlPO4 materials structure — IR and Raman studies

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Abstract

The series of samples containing 0–20 mol% of NaCaPO4 and 20–0 mol% of AlPO4, respectively, with the constant amount of SiO2 (80 mol%) have been selected. The materials were prepared using both sol–gel as well as aerosil pseudo-aqua solution method. The AlPO4·SiO2 and NaCaPO4·SiO2 (80 mol% of SiO2) samples have been prepared.

IR and Raman spectra of these samples are presented. The spectra of materials from NaCaPO4–AlPO4–SiO2 system are compared to those of NaCaPO4·SiO2 and AlPO4·SiO2 sample (samples without Al3+ or Na+ and Ca2+ cations, respectively). The studies have enabled us to identify the bands arising from the internal and lattice vibrations. The slight differences between the spectra of sol–gel and aerosil pseudo-aqua solution materials are pointed out and discussed. The influence of Na+ and Ca2+ ions on the AlPO4–SiO2 materials structure is analysed.

Introduction

Berlinit materials are one of the groups of present-day bioceramics [1]. These materials can be obtained from the slow crystallising SiO2–Al2O3–P2O5 glasses modified by the alkaline or alkali earth oxides. The crystallising materials contain aluminium orthophosphate–berlinite (AlPO4), some silicates and phosphates and amorphous phase. Berlinit bioceramics materials exhibit very high chemical and thermal stability and are characterised by very advisable piezoelectric properties.

The SiO2–Al2O3–P2O5 system is rather poorly known. The quasi-binary SiO2–AlPO4 system is of great interest. According our previous studies [2], [3] in the last system, the cristobalite-type solid solutions exist. The substitution limit of SiO2 by AlPO4 is equal to about 20 mol%. The structure of those materials determines their properties. There should be no aluminium ion substitution in the silico-oxygen framework, all Al3+ should exist in the form of stable AlPO4. The addition of alkaline or alkali earth oxides such as Na2O and CaO cause the framework depolymerisation. The changes of the structure are rather small, and they are invisible on the X-ray diffraction patterns because of the similarity of cristobalite and phosphocristobalite X-ray patterns. The aim of our work is to compare the structures of the materials prepared using two different methods and to check if the structure of SiO2–Al2O3–P2O5 modified by Na2O and CaO forms cristobalite-like structure. IR and Raman spectroscopy and X-ray diffraction methods were used.

Section snippets

Experimental

A series of samples containing 80 mol% of SiO2 and various amounts of AlPO4 and NaCaPO4 (0–20 mol%) have been selected. The maximum content of SiO2, taking account of the region of AlPO4 solid solution, has been chosen [3]. The materials have been prepared using sol–gel as well as aerosil pseudo-aqua solution method. Both the methods have allowed one to obtain very good homogeneity of the components in the samples.

In the sol–gel process TEOS (Si(OC2H5)4), aluminium nitrate (Al(NO3)3·9H2O), sodium

Results and discussion

Analysis of X-ray patterns of materials obtained has not shown any other crystalline phase besides cristobalite SiO2 (or cristobalite-type solid solution) in all samples containing AlPO4. On the other hand, there were the reflexes characteristic for at least three phases on the X-ray pattern of 20NaCaPO4·80SiO2 sample (Table 1). We could separate the reflexes characteristic for cristobalite SiO2, sodium calcium orthophosphate NaCaPO4 and sodium calcium orthophosphate Na3Ca6(PO4)5.

The

Summary

All samples examined have cristobalite-like structure. The additions of CaO and Na2O to the AlPO4–SiO2 materials have not caused the polymorphous transition, though a tridymite form could be expected. There are small differences between the spectra of sol–gel and aerosil samples, caused probably by the various crystallinity of samples. The combination of sodium, calcium and aluminium phosphates provide chances to obtain homogenous phospho-silicate materials with steady distributed Ca2+ and Na+

Acknowledgements

This work is supported by the University of Mining and Metallurgy, contribution no. 10.10.160.78.

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