Hexagonal Na1.5Y1.5F6 at high pressures

The high-pressure behavior of hexagonal Na1.5Y1.5F6 is investigated with synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell up to 22 GPa at room temperature. There is no major phase transition in this material in the entire pressure region studied here. At near ambient pressures, the structure is of the Na1.5Nd1.5F6 type (P (6) over bar, Z = 1), found in other low-temperature sodium lanthanide fluorides. Pressure-induced order-disorder phenomena lead to the formation of the gagarinite structure (P6(3)/m, Z = 1) above 20 GPa. Alternatively, the pressure-induced order-disorder phenomena could be related to a polymorphic transition towards the phase of the beta-Na2ThF6 type with P (6) over bar 2m (Z = 1) symmetry. There is no volume change associated with all these reversible structural changes. (C) 2002 Elsevier Science (USA).