research papers
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.
Keywords: Rietveld refinement; High-temperature phase; Polymorphism; Powder diffraction; Zirconium hydrogen phosphate; Zirconium pyrophosphate.
Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108768100003645/os0045sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108768100003645/os0045Isup2.hkl | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108768100003645/os0045IIsup3.hkl | |
Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768100003645/os0045Isup4.rtv | |
Rietveld powder data file (CIF format) https://doi.org/10.1107/S0108768100003645/os0045IIsup5.rtv |
(I) Zirconium Bis(hydrogenortophosphate) top
Crystal data top
H2O8P2Zr | V = 341.59 (2) Å3 |
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 top
NSLS beamline X7B, BNL diffractometer | Data collection mode: transmission |
Radiation source: X-ray synchrotron | Scan method: Stationary detector |
Specimen mounting: quartz capillary |
Refinement top
Refinement on F2 | 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 (Å2) top
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 top
Crystal data top
O7P2Zr | V = 294.74 (3) Å3 |
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 top
NSLS beamline X7B, BNL diffractometer | Data collection mode: transmission |
Radiation source: X-ray synchrotron | Scan method: Stationary detector |
Specimen mounting: quartz capillary |
Refinement top
Refinement on F2 | 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 (Å2) top
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)* |