Particle size effect on the new phase transition in a tridymite compound, CsCoPO4

A new phase transition (III–IV) was found at 311 K in CsCoPO₄ by DSC measurements. The crystal structure of all the phases, I–IV, in CsCoPO₄ was studied by synchrotron-radiation X-ray powder diffraction. The diffractometry revealed that CsCoPO₄ had the same crystal structure as that of corresponding phases in CsZnPO₄. An extremely large particle size effect was found on III–IV phase transition in CsCoPO₄; the phase transition enthalpy decreases with decreasing the particle size around 0.1 mm. Such large particle size effects had been also observed in CsZnPO₄. However, no III–IV phase transition was observed in the particle smaller than 0.1 mm of CsZnPO₄, while such a critical size was not found in CsCoPO₄.