Thermal “order-disorder” behaviour in (Na1-xKx)4B8O14 solid solutions investigated by X-ray powder diffraction

In the potassium-rich part of the binary system Na4B8O14-K4B8O14 solid solutions have been found that can be described with the formula (Na1-xKx)4B8O14 with 0.45 ≤ x < 1.0. The crystal structures of (Na0.25K0.75)4B8O14 and (Na0.45K0.55)4B8O14 were refined at room temperature by the Rietveld method. The solid solutions crystallize like K4B8O14 in the triclinic crystal system, space group P-1, with K partially substituted for Na.
An ordered distribution of the alkali atoms over the four cation sites at room temperature has been discovered. The structure of (Na0.25K0.75)4B8O14 was also refined for data collected at 300 and 500°C. The refinements show that sodium and potassium atoms are less ordered at higher temperatures.