Growth and characterization of a new inorganic metal–halide crystal structure, InPb2Cl5

A new solid-state inorganic compound, InPb2Cl5, was synthesized by melting InCl and PbCl2 in a vacuum-sealed quartz ampoule. Bulk InPb2Cl5 was separated from PbCl2 and characterized by single-crystal X-ray diffraction.


Chemical context
Indium lead chloride, InPb 2 Cl 5 is a metal halide that has been studied as a new material to be used in optoelectronic semiconducting applications.Other isostructural metal halides that have the structure APb 2 Cl 5 (where A = K, Rb, Tl) have gained interest in fields such as optoelectronics (Vu et al., 2020), and photovoltaics as a tunable laser (Isaenko et al., 2013;Khyzhun et al., 2014;Brown et al., 2013).There has been success in growing metal-halide semiconducting crystals such as RbPb 2 Cl 5 and KPb 2 Cl 5 (Isaenko et al., 2013;Rowe et al., 2014;Isaenko et al., 2009), however single-crystal InPb 2 Cl 5 has not been reported and has only been computationally studied as the InPbCl 3 phase (Khan et al., 2022).Similar to RbPb 2 Cl 5 , KPb 2 Cl 5 , and TlPb 2 Cl 5 , InPb 2 Cl 5 crystallizes in a monoclinic structure and has a space group of type P2 1 /c.Bulk InPb 2 Cl 5 samples were prepared, which contained a mixture of black/ grey metallic polycrystalline InPb 2 Cl 5 at the bottom of the ampoule and white/yellow PbCl 2 crystals above.When the black/grey crystals were broken up, the crystals appeared to have a much clearer and lighter green hue.The broken-up crystal was examined under an optical microscope and clear colourless crystal pieces were seen.The clear colourless singlecrystal pieces were handpicked and characterized by singlecrystal X-ray diffraction (XRD).The bulk material was ground using a mortar and pestle and the powder was characterized by powder-XRD.The powder-XRD pattern had low intensity peaks of In 7 Cl 9 and PbCl 2 impurities, but matched closely with the InPb 2 Cl 5 phase.When InPb 2 Cl 5 was left in ambient conditions over four months, the bulk absorbed moisture over time and left a light-grey film around the bulk with moisture build up on the side of the material.

Structural commentary
The single-crystal structure of InPb 2 Cl 5 was found to adopt a monoclinic P2 1 /c space group.The single-crystal structure refinement confirmed the composition as InPb 2 Cl 5 .The bond lengths in the asymmetric unit (Fig. 1) of InPb 2 Cl 5 are listed in Table 1.The unit cell of InPb 2 Cl 5 (Fig. 2) has four symmetryrelated formula units.The Pb atoms in the unit cell (Fig. 2) coordinate multiple chlorine atoms that give a range of bond lengths from 2.868 (5)-3.3145(15) A ˚.The Pb1 atoms coordinate with seven chlorine atoms in the structure, with bond lengths ranging from 2.868 (5)-3.1371(14) A ˚.The Pb1 atoms form a nine-face polyhedron with a volume of 37.374 A ˚3 (Fig. 3).The Pb2 atoms have a coordination number of 8 with bond lengths from 2.916 (7)-3.3145(15) A ˚.The Pb2 atoms form a 12-face dodecahedron with a volume of 49.796A ˚3 (Fig. 3).The shortest bond length is between Cl1 and Pb1, which is 2.868 (5) A ˚.The largest bond lengths are between the Pb2 atom and a Cl3 atom at 3.3145 (15) A ˚.The typical bond length between Pb and Cl atoms is 2.44 A ˚in the binary structure.There is an increase in bond lengths from the binary PbCl 2 to the InPb 2 Cl 5 structure.The indium atom interstitially coordinates eight chlorine atoms in a distorted octahedral geometry.The range of indium-chlorine bonds range from 3.1447 (18)-3.588(8) A ˚.The indium atom forms a 12-face dodecahedron with a volume of 62.568 A ˚3 (Fig. 3).The typical In-Cl bond length is around 2.56 A ˚, indicating that the indium-chlorine bonds have a much weaker interaction in the InPb 2 Cl 5 structure.The largest bond angles seen in the unit cell (Fig. 2) are between the Cl1 iii -Pb1-Cl2 atoms at 156.02 (3) � (symmetry codes as per Fig. 2).The shortest bond angle in the structure is between the Cl4 ii -In1-Cl5 i atoms at 63.2587 ( 3

Database survey
The InPb 2 Cl 5 structure is isostructural with other compounds such as RbPb 2 Cl 5 (Isaenko et al., 2013;Isaenko et al., 2009), Figure 1 A view of the asymmetric unit.Ellipsoids are drawn at the 50% probability level.
A partial packing plot viewed down the b-axis.Ellipsoids are drawn at the 50% probability level.Symmetry codes: (i) x, y, z; KPb 2 Cl 5 (Rowe et al., 2014;Isaenko et al., 2009) and TlPb 2 Cl 5 (Khyzhun et al., 2014).The cell dimensions of the monoclinic InPb 2 Cl 5 cell are compared with the isostructural compounds in Table 2.The cell dimensions for InPb 2 Cl 5 match very closely with TlPb 2 Cl 5 .There is no significant difference between the InPb 2 Cl 5 structure and the isostructural structures in Table 2, the small difference is due to the atomic size difference for indium.The indium atom has the smallest atom size, so it is expected to fit tighter into the unit cell compared to the other structures, so we see that InPb 2 Cl 5 has the smallest unit cell volume.The thallium atom is most comparable to the indium atom size, which is why its cell dimensions are most similar.

Synthesis and crystallization
Stoichiometric amounts of the binary compounds In I Cl (4g, ThermoFisher Scientific 99.995%, metals basis) and Pb II Cl 2 [3.7278 (5)g, Acros Organics 99%] were mixed in a glove box under an argon environment (<0.1ppm O 2 and H 2 O).The binary compounds were ground together with a mortar and pestle and then loaded into a quartz ampoule.The quartz ampoule was flame sealed under high vacuum (5.5 � 10 À 5 mbar).The loaded quartz ampoule was heated at 3K min À 1 in a vertical furnace to 793K.The ampoule was cooled at 0.5 K min À 1 to room temperature.A 1 mm 3 piece of the metallic black and grey crystal was separated from the excess yellow Pb II Cl 2 crystals and sent for characterization by powder-XRD and single-crystal XRD.

Refinement
The crystallographic data, data collection and structure refinement are summarized in Table 3.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å
) � .The Pb atoms have stronger interactions with the chlorine atoms resulting in shorter bond lengths and a wide range of bond lengths from 69.7087 (3)-156.02(3) � .The indium atoms have weaker interactions and are interstitially located throughout the structure.The weaker interactions of the indium atoms is evident because of the shorter bond lengths and smaller range of bond angles from 63.2587 (3)-144.1653(3) � .A complete list of bond lengths and bond angles is given in the supporting information.

Figure 3 A
Figure 3A polyhedron view of the crystal structure packing, viewed down the baxis.

Table 3
Experimental details.