High pressure behavior of K-cymrite (KAlSi3O8·H2O) crystal structure

Compressibility and structural evolution of K-cymrite, hexagonal high-pressure KAlSi 3 O 8 ·H 2 O, has been studied up to 18 GPa using synchrotron single crystal X-ray diffraction in Ne pressure medium. K-cymrite retains its original symmetry P 6/ 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 remain 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 P 6/ 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 V 0 = 190.45(12) ų, K 0 = 56.5(7) GPa and K 0 ’ = 3.2(12), with bulk modulus notably deviating from earlier result ( K 0 = 45(2) GPa and K 0 ’ = 1.3(10)) obtained in vaseline media.