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

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 K-0 = 38.3 (+/- 1.1) GPa [linear incompressibilities; K-0a = 188.4 (+/- 9.9), K-0c = 64.5 (+/- 2.5) GPa] and K-0 = 34.2 (+/- 1.4) GPa, respectively. The present results comply with values from previous measurements by Xray diffraction [K-0 = 37.8 (+/- 1.8) GPa] and Brillouin spectroscopy [K-0 = 31.7 (+/- 2.5) GPa]. Reasonably, Ca(OH)(2) has revealed to be bulkly softer than Mg (OH)(2) [K-0 = 41 (+/- 2), K-0a = 313, K-0c = 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.