P-V equation of state of portlandite, Ca(OH) 2 , from powder neutron diffraction data

Abstract

A high pressure neutron powder diffraction study of portlandite [Ca(OH)2] has been performed at ISIS facility (U.K.); nine spectra have been collected increasing the pressure by steps, up to 10.9 GPa, by means of a Paris-Edinburgh cell installed on the POLARIS diffractometer. The tensorial formalism of the lagrangian finite strain theory and the Birch-Murnaghan equation of state have been used to determine, independently, two values of the bulk modulus of portlandite, obtaining K0=38.3(±1.1) GPa [linear incompressibilities: K0a=188.4(±9.9), K0c=64.5(±2.5) GPa] and K0=34.2(±1.4) GPa, respectively. The present results comply with values from previous measurements by X-ray diffraction [K0=37.8(±1.8) GPa] and Brillouin spectroscopy [K0=31.7(±2.5) GPa]. Reasonably, Ca(OH)2 has revealed to be bulkly softer than Mg(OH)2 [K0=41(±2), K0a=313, K0c=57 GPa]. The Ca(OH)2 linear incompressibility values reflect the nature of forces acting to stabilize the (001) layer structure and, further, prove that the replacement Ca/Mg mainly affects the elastic properties in the (001) plane, rather than along the [001] direction. Data from a full refinement of the structure at room pressure are reported.