[Ni(H₂O)₆](NO₃)₂·(15-crown-5)·2H₂O: three phase transitions and an intermediate modulated phase stable over a range of ca 40 K

In the range 308–90 K single crystals of [Ni(H₂O)₆](NO₃)₂·(15-crown-5)·2H₂O continue to diffract well as they undergo three phase transitions. Structures have been determined at 28 different temperatures; data were collected at six more but could not be refined satisfactorily. The transitions identified structurally are in good agreement with those found by thermal analysis. Phase I (above ca 292 K; I2/m, Z = 2, Z′ = ¼) is disordered because the 15-crown-5 molecule is located on a 2/m site. Phase II (ca 292–248 K; P2₁/m, Z = 2, Z′ = ½) is less disordered. Phase III (ca 248–208 K; B2₁, Z = 28, Z′ = 7 in a commensurate approximation) is modulated with some regions resembling phase II and the rest resembling the fully ordered phase IV (P2₁/c, Z = 4, Z′ = 1) that is stable below ca 208 K. The modulated structure could be determined well because crystals flash-cooled to 90 K usually retain the phase III structure. The unusual phase III may be a consequence of the differing thermal contraction of two types of alternating layers, one composed of neutral 15-crown-5 molecules and the other composed of the cations, anions and lattice water molecules, which form a hydrogen-bonded network. The two kinds of layers are linked by O_ether...H₂O_axial—Ni hydrogen bonds.