Transmission electron microscopy of Ca oxide nano- and microcrystals in α-tricalcium phosphate prepared by sintering of β-tricalcium phosphate

Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were used to study the porous and non-porous α-tricalcium phosphate (α-Ca3(PO4)2, α-TCP) prepared through a sintering procedure at 1200-1400 °C of β-tricalcium phosphate (β-Ca3(PO4)2, β-TCP). The interpretation of experimental and calculated X-ray and electron diffraction patterns showed that the final product at 1400 °C was primarily α-TCP but roughly 3.0-8.0 wt.% of the starting β-TCP phase and up to 8.0 wt.% of CaO were in the final product. TEM images and electron diffraction patterns showed that the CaO phase - formed by decomposition of TCP - exists as micron-sized areas of various oriented nanocrystals embedded into the bulk α-TCP material and also as self-standing spherulite particles of a few microns in size. Surprisingly, formation of CaO from TCP decomposition occurred at temperatures below those predicted from the phase diagram of the CaO-P2O5 system.