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.