1. Chemical context

Double nitrates, which contain two different metal cations and nitrate anions, have applications in various fields. They exhibit unique solubility properties, crystalline structures and special magnetic properties, and act as excellent precursors for the synthesis of mixed oxides. For example, double nitrates of Zn and Cu have been used to produce a mixed Cu and Zn oxide that exhibits high catalytic activity in reactions such as the water gas shift reaction to produce CO₂ and H₂ from CO and H₂O (Smith et al., 2010), as well as selective CO₂ hydrogenation into methanol (Zhong et al., 2020). Rare earth (RE) transition-metal (TM) double nitrates with the general formula RE₂TM₃(NO₃)₁₂·24H₂O attracted much attention in the 1960s, in which the RE is a trivalent cation with an atomic number lower than that of Ho and the TM is a divalent cation, including Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ (Hellwege & Hellwege, 1953; Brochard & Hellwege, 1953; Buckmaster et al., 1968). It should be noted that when RE has a +4 oxidation state, double nitrates are isomorphic with the triclinic MgTh(NO₃)₆(H₂O)₈ salt (Šćavničar & Prodić, 1965). Cerium(III) magnesium and cerium(III) zinc double nitrates have been used extensively in nuclear orientation experiments because very low temperatures can be obtained by adiabatic demagnetization of the salt (Culvahouse, 1961). Their properties make them suitable magnetic thermometers (Thornley, 1963). Like ruby single crystals (Cr³⁺:Al₂O₃), a Ce-doped lanthanum magnesium double nitrate is an ideal medium to study phonon avalanche, a delayed and sudden relaxation of paramagnetic ions by the emission of phonons (Mims & Taylor, 1969). Heat capacity and susceptibility measurements sug­gested that Mn, Ni, Co and Cu lanthanum double nitrates show anti­ferromagnetic transitions below 0.5 K (Mess et al., 1967, 1968). The lack of high-quality crystalline structures of these salts limits the profound understanding of the magnetic properties, as well as their theoretical investigation. We report herein on the growth of centimeter-large crystals of [La(NO₃)₆]₂[Ni(H₂O)₆]₃·6H₂O (Fig. 1) and the crystal structure determined by single-crystal X-ray diffraction.

Figure 1 Left: large green crystals of [La(NO3)6]2[Ni(H2O)6]3·6H2O with a pseudo-hexa­gonal shape placed on a scale paper. The distance between two thick lines is 1 cm. Right: powder X-ray diffraction patterns of [La(NO3)6]2[Ni(H2O)6]3·6H2O. The PDF5 (ID = 00-049-1235) pattern available in the database is shown in green. The PXRD pattern calculated from the structure refined from single-crystal data and the experimental pattern measured on a Panalytical Empyrean diffractometer with Cu Kα1 radiation (λ = 1.540596 Å) are shown in blue and red, respectively. In the inset, the high-intensity diffraction peaks absent in the PDF5 pattern are highlighted with purple rectangles.

2. Structural commentary

Similar to those found in the corresponding magnesium double salt, the title com­pound is made up of two types of ions, [Ni(H₂O)₆]²⁺ and [La(NO₃)₆]³⁻, which are linked together by hydrogen bonds with water mol­ecules in the structure. The La atom on the threefold axis is coordinated by 12 O atoms from six nitrate groups to form a slightly distorted icosa­hedron. The La—O distances range from 2.6339 (8) to 2.7012 (8) Å, which are com­parable to those found in La₂Mg₃(NO₃)₁₂·24H₂O determined by neutron diffraction (Anderson et al., 1977). As depicted in Fig. 2, the structure includes two crystallographically independent positions for Ni²⁺. Three water H7a—O7–H7b mol­ecules and three water H8a—O8—H8b mol­ecules surround Ni1, resulting in a distorted [Ni(H₂O)₆]²⁺ octa­hedron with C3 symmetry. In contrast, the Ni2-containing [Ni(H₂O)₆]²⁺ octa­hedron is highly symmetric, as Ni2 is situated in a site with symmetry. The Ni—O bond lengths in both octa­hedra vary from 2.0471 (8) to 2.0531 (8) Å, similar to those found in [Ni(H₂O)₆](NO₃)₂ (Breternitz et al., 2015).

Figure 2 The mol­ecular structure of [La(NO3)6]2[Ni(H2O)6]3·6H2O, with displace­ment ellipsoids for all non-H atoms drawn at the 50% probability level. H atoms are represented by small spheres of arbitrary radius. H-atom labels have been omitted for clarity. The colour scheme for the different elements can be found in the legend. The viewing direction is slightly tilted from the c axis, in order to prevent overlap between atoms. Hydrogen bonds are indicated with thin dotted lines.

As illustrated in Fig. 3, each [La(NO₃)₆]³⁻ icosa­hedron is surrounded by three Ni1-containing [Ni(H₂O)₆]²⁺ clusters, and each Ni1-containing octa­hedron is surrounded by three icosa­hedra. These two inter­penetrating honeycomb networks are arranged in a layer parallel to the ab plane. In this layer, the icosa­hedra are linked to the [Ni(H₂O)₆]²⁺ clusters through strong O7—H7A⋯O4, O9—H9A⋯O1 and O10—H10A⋯O9 hydrogen bonds. The six water H9A—O9—H9B mol­ecules per unit cell do not participate in the coordination of either La or Ni. Two successive layers are separated by Ni2-containing [Ni(H₂O)₆]²⁺ clusters, which bridge the layers between them via O10—H10A⋯O9 hydrogen bonds. The com­plex hydrogen-bonding network between the clusters is shown in Fig. 2 and the actual data for the hydrogen bonds are given in Table 1. The network of bonded clusters form sheets that are stacked perpendicular to the c axis (Fig. 4). The sheets are held together by van der Waals forces in the [La(NO₃)₆]₂[Ni(H₂O)₆]₃·6H₂O structure.

Table 1 Hydrogen-bond geometry (Å, °) D—H⋯A D—H H⋯A D⋯A D—H⋯A O8—H8A⋯O9 0.85 2.00 2.8386 (15) 169 O10—H10A⋯O9i 0.85 2.03 2.8149 (13) 153 O10—H10B⋯O5ii 0.86 2.14 2.9626 (13) 161 O7—H7A⋯O4iii 0.86 1.92 2.7656 (11) 168 O7—H7B⋯O6iv 0.86 2.13 2.9393 (11) 158 Symmetry codes: (i) ; (ii) ; (iii) ; (iv) .

Figure 3 The [La(NO3)6]2[Ni(H2O)6]3·6H2O structure represented along the c axis. The colour scheme for the different elements of the structure can be found in the figure inset. [Ni(H2O)6]2+ octa­hedra and [La(NO3)6]3− icosa­hedra are plotted as pink and green front-opening polyhedron, respectively. The unit-cell edges are plotted with blue lines.

Figure 4 The [La(NO3)6]2[Ni(H2O)6]3·6H2O structure represented along the a axis. The colour scheme for the different elements of the structure can be found in the inset. [Ni(H2O)6]2+ octa­hedra and [La(NO3)6]3− icosa­hedra are plotted as pink and green polyhedra, respectively. The unit-cell edges are plotted with blue lines.

3. Database survey

No record of the same com­pound was found in the Crystallography Open Database (COD) or the Inorganic Crystal Structure Database (ICSD). It is listed only once in the Powder Diffraction File (PDF) 2024 version, entry 00-049-1235, without any atomic positions provided. The powder X-ray diffraction (PXRD) pattern available in this database deviates significantly from both the theoretical pattern simulated from the structure refined via single-crystal XRD data and the experimental pattern recorded with powder obtained by crushing a few [La(NO₃)₆]₂[Ni(H₂O)₆]₃·6H₂O single crystals. Notably, peaks below 10°, as well as those in the 22–23° region, are missing in the pattern found in PDF-00-49-1235 (Fig. 1).

4. Synthesis and crystallization

Lanthanum(III) oxide was dissolved in dilute HNO₃ with a concentration of 1 mol l⁻¹ (1 M) and nickel(II) oxide in dilute HNO₃ with a concentration of 0.5 mol l⁻¹ (0.5 M). In order to dissolve the nickel(II) oxide in the dilute HNO₃, the solution was heated at 423 K over a period of 12 h until the nickel(II) oxide com­pletely dissolved and a green transparent solution was obtained. Lanthanum(III) oxide solution (0.2 l) was first mixed with nickel(II) oxide solution (1.2 l) and then 1 mol of citric acid was added to the mixture under vigorous stirring until com­plete dissolution. The solution was transferred to a fume hood for slow evaporation. After 30 d, green hexa­gonal plate-shaped crystals formed with different sizes, the maximum dimension being 2 cm.

5. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms on O atoms were first located in a difference Fourier map and then refined isotropically in riding mode, with U_iso(H) values of 1.5U_eq of the parent O atoms. The O—H distance was refined against the residual peaks, without further constraint.

Table 2 Experimental details Crystal data Chemical formula [La(NO3)6]2[Ni(H2O)6]3·6H2O Mr 1630.45 Crystal system, space group Hexagonal, R Temperature (K) 297 a, c (Å) 11.0230 (1), 34.4826 (4) V (Å3) 3628.53 (8) Z 3 Radiation type Mo Kα μ (mm−1) 3.04 Crystal size (mm) 0.21 × 0.19 × 0.11   Data collection Diffractometer XtaLAB Synergy-i HyPix3000 Absorption correction Gaussian (CrysAlis PRO; Rigaku OD, 2023) Tmin, Tmax 0.573, 1.000 No. of measured, independent and observed [I > 2σ(I)] reflections 27264, 2833, 2663 Rint 0.023 (sin θ/λ)max (Å−1) 0.746   Refinement R[F2 > 2σ(F2)], wR(F2), S 0.013, 0.034, 1.06 No. of reflections 2833 No. of parameters 126 H-atom treatment H-atom parameters constrained Δρmax, Δρmin (e Å−3) 0.30, −0.38 Computer programs: CrysAlis PRO (Rigaku OD, 2023), SHELXT2018 (Sheldrick, 2015a), SHELXL2018 (Sheldrick, 2015b), DIAMOND (Brandenburg, 2006) and publCIF (Westrip, 2010).