• four-phase sequence;
  • modulated phase;
  • phase sequence;
  • disorder

A sequence of four phases has been found for an acetonitrile-solvated co-crystal with 15-crown-5 of the nickel complex [acetonitrilediaqua-κ1O-nitrato-κ2O-nitratonickel(II)]. The structure could be determined at intervals of ca 10 K in the range 90–273 K because crystals remain single through the three transitions. In phase (I) (Tca 240 K; P21/m, Z′ = ½), there is extensive disorder, which is mostly resolved in phase (III) (ca 230–145 K; P21/c, Z′ = 1). Phase (IV) (ca 145–90 K, and probably below; Pinline image, Z′ = 2) is ordered. Phase (II) (ca 238–232 K) is modulated, but the satellite reflections are too weak to allow the structure to be determined within its stability range by standard methods. Most crystals that were flash-cooled from room temperature to 90 K have a metastable P21, Z′ = 5 superstructure that (at least in a commensurate approximation) was identified as similar to the structure of phase (II) by comparison of reconstructed reciprocal-lattice slices and by analogy with the phase behavior of the very similar compound [Ni(H2O)6](NO3)2·(15-crown-5)·2H2O [Siegler et al. (2011). Acta Cryst. B67, 486–498]. In the phase (II) structure slab-like regions that are like the disordered phase (I) structure alternate with regions of similar shape and size that are like the more ordered phase (III) structure.