Ab initio structure determination and Rietveld refinement of a high-temperature phase of zirconium hydrogen phosphate and a new polymorph of zirconium pyrophosphate from in situ temperature-resolved powder diffraction data

The collected in situ temperature-resolved synchrotron powder data revealed that the transformation of the recently reported three-dimensional τ-Zr(HPO₄)₂ to cubic ZrP₂O₇ goes through two intermediate phases. The first intermediate phase, ρ-Zr(HPO₄)₂, is formed in a reversible phase transition at 598 K, which involves both rearrangement and disordering of the hydrogen phosphate groups of τ-Zr(HPO₄)₂. At 688 K condensation of the hydrogen phosphate groups leads to the formation of the second intermediate, a new polymorph of zirconium pyrophosphate (β-ZrP₂O₇). Heating above 973 K results in the gradual transformation of β-ZrP₂O₇ to cubic zirconium pyrophosphate (α-ZrP₂O₇). The crystal structures of the two intermediate phases were solved from the in situ powder diffraction data using direct methods and refined using the Rietveld method. Both phases are orthorhombic, space group Pnnm and Z = 2. The lattice parameters for the two phases are: ρ-Zr(HPO₄)₂: a = 8.1935 (2), b = 7.7090 (2), c = 5.4080 (1) Å; β-ZrP₂O₇: a = 8.3127 (5), b = 6.6389 (4), c = 5.3407 (3) Å. The formation mechanism for the new zirconium pyrophosphate polymorph, β-ZrP₂O₇, is discussed in relation to structurally restricted soft chemistry.

Computing details

For both compounds, program(s) used to refine structure: GSAS.

(I) Zirconium Bis(hydrogenortophosphate)

Crystal data

H2O8P2Zr	V = 341.59 (2) Å^3
Mr = 283.18	Z = 2
Orthorhombic, Pnnm	Dx = 2.74 Mg m^−3
Hall symbol: -P 2 2n	Synchrotron radiation, λ = 0.9336 Å
a = 8.1935 (2) Å	µ = 1.04 mm^−1
b = 7.7090 (2) Å	T = 598 K
c = 5.4080 (1) Å	white

Data collection

NSLS beamline X7B, BNL diffractometer	Data collection mode: transmission
Radiation source: X-ray synchrotron	Scan method: Stationary detector
Specimen mounting: quartz capillary

Refinement

Refinement on F^2	0 restraints
Rp = 0.031	Udef
Rwp = 0.042	Weighting scheme based on measured s.u.'s
2142 data points	(Δ/σ)max = 0.01
Profile function: pseudo-Voigt	Background function: Chebychev polynomial
57 parameters

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Zr	0.0	0.5	0.0	0.029 (1)*
P1	0.3491 (10)	0.2442 (11)	0.0	0.040 (2)*	.5
P2	0.2776 (9)	0.1612 (9)	0.0	0.027 (2)*	.5
O1	0.3828 (6)	0.1475 (7)	0.7696 (7)	0.043 (2)*
O2	0.1881 (9)	0.3234 (9)	0.0	0.032 (3)*
O3	−0.0170 (3)	0.0990 (15)	0.5	0.100 (6)*	.5
O4	0.1479 (12)	0.016 (2)	0.0	0.040 (4)*	.5

(II) Zirconium Pyrophosphate

Crystal data

O7P2Zr	V = 294.74 (3) Å^3
Mr = 265.16	Z = 2
Orthorhombic, Pnnm	Dx = 3.01 Mg m^−3
Hall symbol: -P 2 2n	Synchrotron radiation, λ = 0.9336 Å
a = 8.3127 (4) Å	µ = 1.20 mm^−1
b = 6.6389 (4) Å	T = 778 K
c = 5.3407 (3) Å	white

Data collection

NSLS beamline X7B, BNL diffractometer	Data collection mode: transmission
Radiation source: X-ray synchrotron	Scan method: Stationary detector
Specimen mounting: quartz capillary

Refinement

Refinement on F^2	40 parameters
Rp = 0.029	0 restraints
Rwp = 0.037	Weighting scheme based on measured s.u.'s
1843 data points	(Δ/σ)max = 0.01
Profile function: pseudo-Voigt	Background function: Chebychev polynomial

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

	x	y	z	Uiso*/Ueq
Zr	0.0	0.5	0.0	0.0370 (7)*
P	0.3908 (6)	0.3129 (7)	0.0	0.047 (2)*
O1	0.4335 (7)	0.2019 (9)	0.7710 (11)	0.058 (3)*
O2	0.2222 (14)	0.3908 (12)	0.0	0.084 (4)*
O3	0.0	0.0	0.5	0.107 (5)*