Structural Phase Transitions in the Double Salts (NH4)2PO3F<middle dot>NH4NO3 and (NH4)2XO4<middle dot>3NH4NO3 (X = Se, Cr)

In the context of investigating isostructural relationships between sulfates and monofluorophosphates, crystals of the double salts (NH4)(2)PO3F<middle dot>NH4NO3 (AFP<middle dot>AN) and (NH4)(2)XO4<middle dot>3NH(4)NO(3) (AX<middle dot>3AN; X = Se, Cr) were grown from aqueous solutions and structurally characterized using X-ray diffraction and thermal analysis. Whereas the high-temperature forms of the two AX<middle dot>3AN double salts are in fact isostructural with the sulfate analogue, AFP<middle dot>AN crystallizes with a reduced amount of NH4NO3 and thus has a unique crystal structure. Both AFP<middle dot>AN and the two AX<middle dot>3AN compounds exhibit reversible structural phase transitions. Upon cooling, the monofluorophosphate double salt transforms from the monoclinic room-temperature polymorph (I; P2(1)/n, Z = 4) to the intermediate triclinic polymorph (II; P1, Z = 4) that in turn transforms to the monoclinic low-temperature polymorph (III; P2(1)/n, Z = 4). The two phase transitions (I) -> (II) and (II) -> (III) are characterized by a significant increase of the unit cell volumes upon cooling. The two AX<middle dot>3AN double salts transform upon cooling from a disordered monoclinic crystal structure (P2(1), Z = 2) to a monoclinic polymorph with a doubled unit cell (P2(1)/c, Z = 4). Such a phase transition is not observed for the sulfate analogue. All molecular moieties are fully ordered at -93 degrees C for the selenate double salt, whereas one of the nitrate anions remains disordered for the chromate double salt even at -173 degrees C. In all AFP<middle dot>AN and AX<middle dot>3AN crystal structures, the nitrate anions play a crucial role during the phase transitions, and an extensive network of N-H<middle dot><middle dot><middle dot>O hydrogen-bonding interactions is responsible for the cohesion of the crystal.