Crystal structure of AlPCl₈

The crystal structure of aluminium phospho­rus chloride (systematic name: phospho­rus tetra­chloride tetra­chloridoa­luminate), (PCl₄)[AlCl₄] or AlPCl₈, was determined and refined using single-crystal X-ray diffraction data. The compound crystallizes in the ortho­rhom­bic space group Pbcm. The asymmetric unit comprises one Al atom, one P atom, and five Cl atoms. The structure is characterized by isolated AlCl₄ and PCl₄ tetra­hedra, isostructural with FePCl₈ and GaPCl₈.

1. Chemical context

During our exploratory synthesis in the Mg–Al–P–Cl system, aimed at discovering new magnesium-ion conductors, we initially observed the AlPCl₈ phase. Magnesium-ion conductors, such as MgAl₂Cl₈, exhibit Mg-ion conductivity of approximately 10⁻⁷ S cm⁻¹ at 400 K (Tomita et al., 2021). To enhance this ionic conductivity, we introduced an aliovalent substitution of Al with P to create magnesium-ion vacancies within the structure, following the general formula Mg_1–xAl_2–xP_xCl₈.

Across a wide range of x values (0.1 to 1), we identified a new phase through powder X-ray diffraction (XRD) patterns, which differed significantly from that of MgAl₂Cl₈. Subsequent analysis revealed that this new phase matched the XRD pattern of AlPCl₈ (Fischer & Jübermann, 1938). Since the crystal structure of AlPCl₈ was previously unknown, we proceeded to grow single crystals without Mg to determine its structure. The resulting analysis confirmed that its crystal structure is isostructural with FePCl₈ (Kistenmacher & Stucky, 1968) and GaPCl₈ (Weigand et al., 2009).

2. Structural commentary

Anhydrous aluminium phospho­rus chloride (AlPCl₈) crystallizes in the ortho­rhom­bic space group Pbcm (Fig. 1), with a structure consisting of isolated AlCl₄ and PCl₄ tetra­hedra, and one Al, one P, and five Cl sites in the asymmetric unit. Al³⁺ is tetra­hedrally coordinated by four Cl atoms, with an average Al—Cl bond distance of 2.127 (2) Å, while P⁵⁺ is similarly coordinated, but a shorter average P—Cl bond distance of 1.899 (2) Å. These bond lengths (Table 1) are consistent with the sums of the ionic radii for Al, P, and Cl (Shannon, 1976). The local environment of each tetra­hedron is shown in Fig. 2. The crystal structure was determined to be isostructural with (FeCl₄)(PCl₄) (Kistenmacher & Stucky, 1968).

Table 1
Selected geometric parameters (Å, °)

Al1—Cl2^i	2.1223 (12)	P1—Cl5^ii	1.9018 (12)
Al1—Cl1	2.1343 (16)	P1—Cl4^ii	1.8959 (12)
Al1—Cl2	2.1223 (12)	P1—Cl4	1.8959 (12)
Al1—Cl3	2.128 (2)	P1—Cl5	1.9018 (12)
Cl2^i—Al1—Cl1	108.79 (6)	Cl5^ii—P1—Cl4^ii	109.31 (6)
Cl2^i—Al1—Cl2	112.83 (10)	Cl5^ii—P1—Cl4	110.49 (6)
Cl1—Al1—Cl2	108.79 (6)	Cl4^ii—P1—Cl4	108.26 (9)
Cl2^i—Al1—Cl3	108.77 (6)	Cl5^ii—P1—Cl5	108.97 (8)
Cl1—Al1—Cl3	108.82 (8)	Cl4^ii—P1—Cl5	110.49 (6)
Cl2—Al1—Cl3	108.77 (6)	Cl4—P1—Cl5	109.31 (6)
Symmetry codes: (i) ; (ii) .

Figure 1 The local environments of the AlCl4 (blue) and PCl4 tetra­hedra (purple) are shown. Symmetry codes correspond to those in Table 1.

Figure 2 The displacement of ellipsoids of AlPCl8 drawn at the 50% probability level viewed from two different orientations: (a) approximately along the [111] direction and (b) along the [001] direction. The AlCl4 tetra­hedra are represented in blue, and the PCl4 tetra­hedra in purple.

To validate the refined crystal structure, bond-valence sums (BVSs) were calculated using the softBV (Chen et al., 2019) program (V1.3.1). The calculated BVS values closely match the expected ionic charges, further supporting the reliability of the structural model: Al 3.04, P 5.05, Cl1 − 0.77, Cl2 − 0.78, Cl3 − 0.78, Cl4 − 1.25, and Cl5 − 1.24.

3. Synthesis and crystallization

Anhydrous aluminium chloride (AlCl₃, Alfa Aesar, anhydrous, reagent grade) and phospho­rus(V) chloride (PCl₅, Sigma-Aldrich, 95%) were used in the experiment. A stoichiometric mixture of AlCl₃ and PCl₅ was ground using a mortar and pestle and then pressed into a pellet. The pellet was placed in a dry fused-silica ampoule, which was sealed under vacuum and heated in a furnace. The temperature was increased from 303 K to 573 K at a rate of 5 K min⁻¹, then gradually lowered to 373 K at a rate of 0.0694 K min⁻¹. The sample was then allowed to cool naturally to room temperature. Single crystals were collected at 293 K using an optical microscope in a dry room with a dew point of 223 K. A crystal, approximately 0.1 mm in size, was placed into a 0.5 mm diameter glass capillary and sealed with capillary wax (Hampton Research). The same sample was subsequently used for powder analysis.

4. Refinement

Details of the data collection and structure refinement are summarized in Table 2. Single-crystal X-ray diffraction data for AlPCl₈ were collected and processed using APEX2 (Bruker, 2006), with absorption corrections applied through SAINT (Bruker, 2006). The structure was solved using SUPERFLIP (Palatinus & Chapuis, 2007) and refined using CRYSTALS (Betteridge et al., 2003). Three-dimensional Fourier electron-density maps were visualized using MCE (Rohlíček & Hušák, 2007), and structural visualizations were generated using VESTA (Momma & Izumi, 2011).

Table 2
Experimental details

Crystal data
Chemical formula	PCl4^+·AlCl4^−
Mr	341.55
Crystal system, space group	Orthorhombic, Pbcm
Temperature (K)	293
a, b, c (Å)	6.2653 (6), 13.5033 (12), 14.0112 (13)
V (Å^3)	1185.38 (19)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	2.05
Crystal size (mm)	0.2 × 0.2 × 0.2
Data collection
Diffractometer	Bruker D8 Venture
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.664, 0.671
No. of measured, independent and observed [I > 2.0σ(I)] reflections	38491, 1399, 1061
Rint	0.130
(sin θ/λ)max (Å^−1)	0.647
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.093, 0.056, 1.17
No. of reflections	1061
No. of parameters	51
Δρmax, Δρmin (e Å^−3)	0.79, −1.04
Computer programs: APEX2 and SAINT (Bruker, 2006), SUPERFLIP (Palatinus & Chapuis, 2007), CRYSTALS (Betteridge et al., 2003) and VESTA (Momma & Izumi, 2011).

The structure of AlPCl₈ was further confirmed using the powder X-ray Rietveld refinement technique. Data were collected with a Bruker AXS D8 Advance powder X-ray diffractometer, equipped with Cu Kα₁ radiation in Debye-Scherrer geometry, a focusing primary Ge (111) monochromator, and a Vantec position-sensitive detector with a 6° detector slit. The powder sample was homogeneously mixed with carbon (Super C, TIMCAL) at a 1:1 weight ratio to reduce preferred orientation effects, lower effective packing density, and mitigate absorption effects. The sample was placed in a 0.5 mm glass capillary and sealed with wax to prevent air exposure. Measurements were taken over an angular range of 10° ≤ 2θ ≤ 130°, with a step size of 0.016693°, conducted over 13 h at room temperature. Powder profile refinement was performed using GSAS-II software (Toby & Von Dreele, 2013). The final Rietveld plot is shown in Fig. 3.

Figure 3 Powder X-ray Rietveld refinement profile of AlPCl8. Black dots indicate the observed pattern, the red line represents the calculated pattern, the blue line shows the difference between the observed and calculated patterns, and the pink tick marks correspond to the Bragg reflections positions.

Supporting information

Computing details

Phosphorus tetrachloride tetrachloridoaluminate

Crystal data

PCl4^+·AlCl4^−	F(000) = 656
Mr = 341.55	Dx = 1.914 Mg m^−3 Dm = 1.914 Mg m^−3 Dm measured by not measured
Orthorhombic, Pbcm	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2c 2b	Cell parameters from 38491 reflections
a = 6.2653 (6) Å	θ = 3.4–27.4°
b = 13.5033 (12) Å	µ = 2.05 mm^−1
c = 14.0112 (13) Å	T = 293 K
V = 1185.38 (19) Å^3	Block, white
Z = 4	0.2 × 0.2 × 0.2 mm

Data collection

Bruker D8 Venture diffractometer	1061 reflections with I > 2.0σ(I)
Graphite monochromator	Rint = 0.130
ω/2θ scans	θmax = 27.4°, θmin = 3.0°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −8→8
Tmin = 0.664, Tmax = 0.671	k = −17→17
38491 measured reflections	l = −18→18
1399 independent reflections

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: other
R[F^2 > 2σ(F^2)] = 0.093	Weighting scheme based on measured s.u.'s Method = SQRT(W) = 1/(Data with the key SIGMA(/FO/) in list 6)
wR(F^2) = 0.056	(Δ/σ)max = 0.0003
S = 1.17	Δρmax = 0.79 e Å^−3
1061 reflections	Δρmin = −1.04 e Å^−3
51 parameters

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Al1	0.2214 (2)	0.47461 (11)	0.2500	0.0427
P1	0.45745 (17)	0.7500	0.5000	0.0479
Cl1	0.55881 (16)	0.49641 (12)	0.2500	0.0638
Cl2	0.09231 (14)	0.53762 (10)	0.37618 (9)	0.0859
Cl3	0.1572 (3)	0.31987 (12)	0.2500	0.0839
Cl4	0.28015 (19)	0.80367 (9)	0.40331 (8)	0.0952
Cl5	0.63379 (17)	0.85187 (8)	0.55069 (10)	0.0984

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Al1	0.0338 (6)	0.0499 (9)	0.0445 (8)	−0.0005 (6)	0.0000	0.0000
P1	0.0443 (6)	0.0493 (7)	0.0501 (7)	0.0000	0.0000	0.0041 (7)
Cl1	0.0288 (6)	0.0861 (10)	0.0764 (8)	−0.0026 (6)	0.0000	0.0000
Cl2	0.0579 (6)	0.1250 (12)	0.0746 (7)	−0.0020 (5)	0.0154 (5)	−0.0428 (8)
Cl3	0.0807 (10)	0.0571 (9)	0.1140 (13)	−0.0146 (8)	0.0000	0.0000
Cl4	0.0936 (9)	0.1129 (12)	0.0791 (9)	0.0081 (7)	−0.0211 (7)	0.0365 (7)
Cl5	0.0842 (8)	0.0780 (9)	0.1329 (12)	−0.0163 (7)	−0.0074 (6)	−0.0404 (7)

Geometric parameters (Å, º)

Al1—Cl2^i	2.1223 (12)	P1—Cl5^ii	1.9018 (12)
Al1—Cl1	2.1343 (16)	P1—Cl4^ii	1.8959 (12)
Al1—Cl2	2.1223 (12)	P1—Cl4	1.8959 (12)
Al1—Cl3	2.128 (2)	P1—Cl5	1.9018 (12)
Cl2^i—Al1—Cl1	108.79 (6)	Cl5^ii—P1—Cl4^ii	109.31 (6)
Cl2^i—Al1—Cl2	112.83 (10)	Cl5^ii—P1—Cl4	110.49 (6)
Cl1—Al1—Cl2	108.79 (6)	Cl4^ii—P1—Cl4	108.26 (9)
Cl2^i—Al1—Cl3	108.77 (6)	Cl5^ii—P1—Cl5	108.97 (8)
Cl1—Al1—Cl3	108.82 (8)	Cl4^ii—P1—Cl5	110.49 (6)
Cl2—Al1—Cl3	108.77 (6)	Cl4—P1—Cl5	109.31 (6)

Symmetry codes: (i) x, y, −z+1/2; (ii) x, −y+3/2, −z+1.

 

Footnotes

‡Contributed equally.