Thermal “order‐disorder” behaviour in (Na_1‐xK_x)₄B₈O₁₄ solid solutions investigated by X‐ray powder diffraction

Abstract

In the potassium-rich part of the binary system Na 4 B 8 O 14 -K 4 B 8 O 14 solid solutions have been found that can be described with the formula (Na 1-x K x ) 4 B 8 O 14 with 0.45 ≤ x < 1.0. The crystal structures of (Na 0.25 K 0.75 ) 4 B 8 O 14 and (Na 0.45 K 0.55 ) 4 B 8 O 14 were refined at room temperature by the Rietveld method. The solid solutions crystallize like K 4 B 8 O 14 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 (Na 0.25 K 0.75 ) 4 B 8 O 14 was also refined for data collected at 300 and 500 o C. The refinements show that sodium and potassium atoms are less ordered at higher temperatures. Dedicated to Prof. Ladislav Bohatý on the occasion of his 60th birthday