Structures of two new high-pressure forms of AlPO₄ by X-ray powder diffraction and NMR spectroscopy

The crystal structures of two new high-pressure AlPO₄ phases are reported. One phase synthesized at 6 GPa and 1523 K is triclinic () whilst the other phase synthesized at 7 GPa and 1773 K is monoclinic (). ³¹P MAS (magic-angle spinning) NMR suggests three tetrahedral P sites with equal abundance in both phases. ²⁷Al 3Q MAS NMR spectra provided evidence for two octahedral sites and one five-coordinated Al site in each phase. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing the local structural information from NMR, and were further refined by the Rietveld method. Both phases contain doubly bent chains made of six edge-shared Al polyhedra (including five-coordinated Al), which are joined by PO₄ tetrahedra. The phase is isostructural with FeVO₄ and AlVO₄. The two phases differ in the packing manner of the chains. This study has demonstrated that the combined application of ab initio structure determination via X-ray powder diffraction and solid-state NMR spectroscopy is a powerful approach to the rapid solution of complex inorganic crystal structures.

Computing details

For both structures, program(s) used to solve structure: FOX; program(s) used to refine structure: RIETAN-FP; molecular graphics: VESTA.

AlPO4 (Pbar1)

Crystal data

AlO4P	V = 365.9 (1) Å^3
Mr = 121.95	Z = 6
Triclinic, P1	Synchrotron radiation, λ = 1.00036 Å
a = 6.1328 (1) Å	µ = 3.24 mm^−1
b = 7.5151 (1) Å	T = 300 K
c = 8.5801 (1) Å	Particle morphology: powder
α = 98.2630 (7)°	white
β = 104.6220 (8)°	cylinder, 3.0 × 0.3 mm
γ = 102.1398 (8)°	Specimen preparation: Prepared at 1523 K and 6000000 kPa, cooled at 30000 K min^−1

Data collection

Debye-Scherrer diffractometer	Data collection mode: transmission
Radiation source: synchrotron, SR	Scan method: step
Specimen mounting: capillary	2θmin = 5.910°, 2θmax = 76.990°, 2θstep = 0.010°

Refinement

Rp = 0.033	Profile function: Splitted Pseudo-Voigt
Rwp = 0.045	130 parameters
Rexp = 0.031	0 restraints
RBragg = 0.017	39 constraints
6886 data points	Background function: Legendre polynomial x background data
Excluded region(s): 0-5.9, 8.7-9.3, 12.8-13.7, 15.0-15.7, 77.0-80	Preferred orientation correction: March-Dollase function

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

	x	y	z	Biso*/Beq
Al1	0.7618 (4)	0.6879 (4)	0.4185 (3)	0.89 (6)*
Al2	0.4695 (4)	0.8960 (4)	0.2105 (3)	0.93 (6)*
Al3	0.9820 (5)	0.3015 (4)	0.0226 (3)	0.76 (5)*
P1	0.0083 (4)	0.0074 (3)	0.2594 (2)	0.97 (5)*
P2	0.1993 (3)	0.5970 (3)	0.3444 (3)	1.07 (5)*
P3	0.5223 (4)	0.3020 (3)	0.1316 (3)	1.00 (5)*
O11	0.9463 (7)	0.1423 (6)	0.1537 (5)	0.89 (6)*
O12	0.7696 (8)	0.8770 (6)	0.2642 (3)	0.89 (6)*
O13	0.1342 (7)	0.8740 (6)	0.1690 (5)	0.89 (6)*
O14	0.1768 (7)	0.0967 (6)	0.4309 (5)	0.89 (6)*
O21	0.0558 (7)	0.5307 (6)	0.1543 (4)	0.71 (5)*
O22	0.2652 (7)	0.4339 (6)	0.4180 (5)	0.71 (5)*
O23	0.0650 (7)	0.7026 (6)	0.4317 (5)	0.71 (5)*
O24	0.4380 (8)	0.7370 (5)	0.3525 (5)	0.71 (5)*
O31	0.3516 (7)	0.7298 (5)	0.0127 (5)	0.96 (6)*
O32	0.5181 (7)	0.1299 (5)	0.2119 (5)	0.96 (6)*
O33	0.2772 (7)	0.3054 (6)	0.0374 (5)	0.96 (6)*
O34	0.6533 (6)	0.4772 (6)	0.2497 (5)	0.96 (6)*

AlPO4 (P21c)

Crystal data

AlO4P	Z = 12
Mr = 121.95	Synchrotron radiation, λ = 1.00036 Å
Monoclinic, P21/c	µ = 3.25 mm^−1
a = 6.13219 (6) Å	T = 300 K
b = 14.3465 (1) Å	Particle morphology: powder
c = 8.57620 (9) Å	white
β = 104.7426 (5)°	cylinder, 3.0 × 0.3 mm
V = 729.7 (1) Å^3	Specimen preparation: Prepared at 1773 K and 7000000 kPa, cooled at 30000 K min^−1

Data collection

Debye-Scherrer diffractometer	Data collection mode: transmission
Radiation source: synchrotron, SR	Scan method: step
Specimen mounting: capillary	2θmin = 7.010°, 2θmax = 77.490°, 2θstep = 0.010°

Refinement

Rp = 0.036	Profile function: Pseudo-Voigt
Rwp = 0.049	145 parameters
Rexp = 0.031	0 restraints
RBragg = 0.021	0 constraints
7049 data points	Weighting scheme based on measured s.u.'s
Excluded region(s): 0-7, 77.5-80

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

	x	y	z	Biso*/Beq
Al1	0.7963 (3)	0.5921 (1)	0.6194 (2)	0.79 (4)*
Al2	0.1559 (4)	0.6988 (1)	0.8504 (2)	0.85 (4)*
Al3	0.4535 (3)	0.1023 (1)	0.4374 (2)	0.81 (4)*
P1	0.3214 (3)	0.2484 (1)	0.1412 (2)	0.96 (3)*
P2	0.6681 (3)	0.0474 (1)	0.8262 (2)	1.07 (4)*
P3	0.9140 (3)	0.1027 (1)	0.3284 (2)	0.98 (3)*
O11	0.4271 (6)	0.1809 (2)	0.2734 (4)	0.89 (8)*
O12	0.1566 (6)	0.3211 (2)	0.1907 (5)	0.86 (8)*
O13	0.5212 (6)	0.3057 (3)	0.0988 (5)	1.02 (9)*
O14	0.1792 (6)	0.2042 (2)	0.9896 (4)	0.51 (8)*
O21	0.5466 (6)	0.0144 (2)	0.6494 (4)	0.83 (9)*
O22	0.7705 (6)	0.9662 (2)	0.9264 (5)	0.67 (8)*
O23	0.5052 (6)	0.1054 (3)	0.8955 (4)	0.99 (8)*
O24	0.8652 (6)	0.1143 (3)	0.8068 (4)	0.77 (8)*
O31	0.7815 (5)	0.1243 (2)	0.4700 (4)	0.69 (9)*
O32	0.8616 (6)	0.1881 (2)	0.2109 (4)	1.13 (9)*
O33	0.1633 (6)	0.0946 (3)	0.4195 (4)	0.94 (8)*
O34	0.8310 (6)	0.0121 (2)	0.2476 (4)	0.77 (8)*