Structural, thermal and optical properties of elpasolite-like (NH4)2KZrF7.

New fluoride compound (NH 4) 2 KZrF 7 with the elpasolite structure (sp. gr. Fm -3 m) was synthesized by partial cationic substitution in (NH 4) 3 ZrF 7 cryolite. Due to small difference in ionic radii of K+ and NH 4 +, non-equivalent crystallographic positions 8 c and 4 b are occupied by both cations. Investigations of thermal, optical and structural properties in a wide range of temperature revealed two phase transitions Fm -3 m ↔ P 4 2 / ncm ↔ P 4 2 / nmc of the first order. The results obtained are discussed in comparison with the data for the previously studied related cryolite (NH 4) 3 ZrF 7 Crystal structure of (NH4)2KZrF7 which undergoes two close phase transitions. Image 1 • New fluoride elpasolite (NH 4) 2 KZrF 7 (sp. gr. Fm -3 m) was synthesized. • Non-equivalent crystallographic positions 8 c and 4 b are occupied by both cations. • K+.→NH 4 + substitution in (NH 4) 3 ZrF 7 decreases temperature stability of Fm -3 m phase. • Two phase transitions were found in (NH 4) 2 KZrF 7 instead of six in (NH 4) 3 ZrF 7. • (NH 4) 2 KZrF 7 shows significant decrease in anomalous entropy compared to (NH 4) 3 ZrF 7. [ABSTRACT FROM AUTHOR]