[Ni(MeCN)(H₂O)₂(_NO3₂] ·(15-crown-5)·MeCN: Detailed study of a four-phase sequence that includes an intermediate modulated phase

A sequence of four phases has been found for an acetonitrile-solvated co-crystal with 15-crown-5 of the nickel complex [acetonitrilediaqua- κ¹ O-nitrato-κ² O-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) (T ≥ ca 240 K; P21/m, Z′ = 1/2), 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; P , 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)·2H ₂O [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.