Structural transformation between two cubic phases of (NH4)3SnF7

A non-typical sequence of phase transitions in (NH4)3GeF7: optical and structural characterization

“…The grown single crystals of (NH 4 ) 3 GeF 7 , like (NH 4 ) 3 TiF 7 , 13 have a habitus of elongated rectangular prisms with extinctions along the growth faces. The small single-domain samples were optically uniaxial, which was in agreement with a tetragonal symmetry of the compound.…”

“…At room temperature, the birefringence of the (NH 4 ) 3 GeF 7 crystal (Δn = n o − n e = 0.0034) is higher than that of (NH 4 ) 3 TiF 7 (Δn = 0.0023). 13 The temperature dependence of optical characteristics Δn (T ) of (NH 4 ) 3 GeF 7 is shown in Fig. 3.…”

Sequence of phase transitions in (NH4)3SiF7

“…The temperature stability of the initial tetragonal phase P4/mbm of (NH 4 ) 3 SiF 7 was examined by measurements with a DSM-10M differential scanning microcalorimeter (DSM). Recently it has been shown that among fluorides (NH 4 ) 3 M 4+ F 7 there are at least two crystals undergoing a phase transition into the cubic phase Pm3m above room temperature at ambient -(NH 4 ) 3 SnF 7 13 or high -(NH 4 ) 3 TiF 7 16 pressure.…”

“…11 Moreover, some other fluoride double salts (NH 4 ) 3 M 4+ F 7 (M 4+ = Ti 4+ , Sn 4+ ) undergo phase transitions but of different sequences, and a correlation with the M 4+ ionic radius was found. [11][12][13][14][15][16] Therefore, one might expect the temperature variation would also destabilise the room temperature structure of the silicon double fluoride salt, leading to phase transitions.…”

Serendipitous formation of K15NaSn5F36

“…Interestingly, materials with large cell parameters containing solely isolated octahedral units are only mentioned scarcely. One example is (NH 4 ) 3 SnF 7 , which crystallizes in the cubic crystal system with the cell parameter a = 1208.6(1) pm and a volume of 1.765 nm 3 [20]. Here, we took the chance to characterize K 15 NaSn 5 F 36 , which also exhibits an unusually large unit cell ( hexagonal, a = 1060.3 (2), c = 2011.9(4) pm, V = 1.9588(7) nm 3 ), whilst containing only simple but uniquely arranged building blocks.…”