Complex evolution of phase during the thermal investigation of Brushite-type calcium phosphate CaHPO4•2H2O

Abstract

This study focused on a detailed examination of the thermal behavior of Brushite-based calcium phosphate (CaHPO4•2H2O, DCPD) to identify and characterize the intermediate phases which have been the subject of previous several controversies. For that, in situ high-temperature X-ray diffraction (HT-XRD) supported by Fourier transform infrared spectroscopy (FTIR), simultaneous thermal gravimetry and differential thermal analysis (TG/DTA), and scanning electron microscopy (SEM) analysis were used and the combination of results showed that the progressive thermal stress of DCPD in air resulted in a complex heterogeneous formulation consisting of dibasic calcium phosphate anhydrous (CaHPO4, DCPA) and an amorphous calcium phosphate, which appears at low temperatures (~160 °C) and persists up to 375 °C. This amorphous phase was identified as a disordered calcium pyrophosphate (Ca2P2O7, CPP) by the appearance in FTIR spectra of a characteristic band δ(P—O—P) of pyrophosphate groups, located at 740 cm−1. This band is shifted to low frequencies (725 cm−1) as the temperature is increased, indicating the crystallization of the amorphous CPP material into γ-CPP. The high temperature treatment (≥ 375 °C) leads to β−CPP polymorph. The quantification of the amorphous CPP phase by HT-XRD analysis depends on the heating rate of the decomposition of DCPD and presents a maximum at 300 °C for 10 °C min−1. According to the present characterization results, obtaining pure DCPA from the thermal dehydration of DCPD is not effective and leads to biphasic materials including a disordered calcium pyrophosphate phase.