Crystal structure of the high-P polymorph of Ca2B6O6(OH)102(H2O) (meyerhofferite)

The crystal structure of the high-pressure polymorph of meyerhofferite, ideally Ca2B6O6(OH)102(H2O), has been determined by means of single-crystal synchrotron X-ray diffraction data. Meyerhofferite undergoes a first-order isosymmetric phase transition to meyerhofferite-II, bracketed between 3.15 and 3.75 GPa, with a large volume discontinuity. The phase transition is marked by an increase in the coordination number of the boron B1 site, from III to IV, leading to a more interconnected and less compressible structure. The main structural differences between the two polymorphs and the P-induced deformation mechanisms at the atomic scale are discussed.