High pressure behavior of K-cymrite (KAlSi3O8 ·H 2 O) crystal structure

Abstract Compressibility and structural evolution of K-cymrite, hexagonal high-pressure KAlSi3O8·H2O, has been studied up to 18 GPa using synchrotron single crystal X-ray diffraction in Ne pressure medium. K-cymrite retains its original symmetry P6/mmm up to a pressure of 7.3 GPa. As the pressure increases from 7.3 to 8.5 GPa the weak satellite reflections appear on diffraction patterns and remains up to maximum applied pressure of 18 GPa indicating incommensurate modulation. However, main reflections can be still indexed in hexagonal cell and structure successfully solved in initial P6/mmm group. After pressure release, K-cymrite reverts to initial non-modulated single-crystal state. The parameters of third-order Birch-Murnaghan equation of state for K-cymrite are V0 = 190.45(12) ų, K0 = 56.5(7) GPa and K’ = 3.2(12), with bulk modulus notably deviating from earlier result (45(2) GPa) obtained in NaCl media.