Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270109011457/lg3007sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S0108270109011457/lg3007Isup2.hkl |
Pure Zn2P2O7 and Li4P2O7 were first prepared from analytically pure ZnO, Li2CO3 and NH4H2PO4. Zn2P2O7 was synthesized from ZnO and NH4H2PO4. The starting materials were first mixed in a molar ratio of 1:1, ground in an agate mortar and warmed at 773 K for 10 h. The powders were then pressed into a 1–2 mm thick pellet (diameter around 12 mm), heated to 1173 K and sintered at this temperature for 48 h. Li4P2O7 was obtained similarly by mixing Li2CO3 and NH4H2PO4 in 2:1 molar ratio, but the sintering temperature and time were 1073 K and 96 h, respectively. These two compounds were then weighed and mixed in a molar ratio of 3:1 and ground in an agate mortar. The mixture was heated to 1193 K in a platinum crucible and held at this temperature for 30 min to make the melt homogeneous. The melt was then cooled to 1093 K at a rate of 1 K h−1, followed by cooling to 693 K at a rate of 20 K h−1, and finally it was quenched to room temperature by switching off the furnace. A suitable single-crystal of size 0.30 × 0.28 × 0.25 mm was selected and mounted on a glass fibre for structure determination, and the remaining products were ground into a powder for phase identification by X-ray powder diffraction and for physical property measurements such as second harmonic generation (SHG). No observable SHG signal was detected.
The structure was solved by direct methods implemented in SHELXS97 (Sheldrick, 2008) and refined on F2 using the full-matrix method. All atoms were refined anisotropically. Refinement of the site-occupancy factors of Zn1 and Zn2 shows that the Zn1 site is contaminated with a trace amount (~0.9%) of Li+ and the Zn2 site is statistically disordered by Li+ and Zn2+ in a Zn2+/Li+ ratio of around 1:1. The refined site-occupancy factor of Li1 is also around 0.5. These refined values agree well with the expected composition of the title compound. In the final refinement, the site-occupancy factors of Zn1, Zn2, Li1 and Li2 were fixed, due to the weak scattering power and partial occupancy of the Li+ cations. A small change in the Li+ occupancy factor does not affect the structure features or refinement results significantly.
Data collection: SMART (Bruker, 2000); cell refinement: SAINT (Bruker, 2000); data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).
Li2Zn3(P2O7)2 | F(000) = 1072 |
Mr = 557.87 | Dx = 3.373 Mg m−3 |
Orthorhombic, Pbcm | Mo Kα radiation, λ = 0.71073 Å |
Hall symbol: -P 2c 2b | Cell parameters from 5442 reflections |
a = 5.1733 (3) Å | θ = 2.5–33.4° |
b = 13.1797 (7) Å | µ = 7.17 mm−1 |
c = 16.1108 (9) Å | T = 298 K |
V = 1098.48 (11) Å3 | Granular, colourless |
Z = 4 | 0.30 × 0.28 × 0.25 mm |
Bruker SMART APEX CCD area-detector diffractometer | 2135 independent reflections |
Radiation source: fine-focus sealed tube | 1994 reflections with I > 2σ(I) |
Graphite monochromator | Rint = 0.022 |
ω scan at different fixed ϕ positions | θmax = 33.5°, θmin = 2.5° |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | h = −8→7 |
Tmin = 0.115, Tmax = 0.165 | k = −19→18 |
9756 measured reflections | l = −24→19 |
Refinement on F2 | 0 restraints |
Least-squares matrix: full | Primary atom site location: structure-invariant direct methods |
R[F2 > 2σ(F2)] = 0.023 | Secondary atom site location: difference Fourier map |
wR(F2) = 0.063 | w = 1/[σ2(Fo2) + (0.0382P)2 + 0.5572P] where P = (Fo2 + 2Fc2)/3 |
S = 1.05 | (Δ/σ)max = 0.003 |
2135 reflections | Δρmax = 0.55 e Å−3 |
126 parameters | Δρmin = −0.46 e Å−3 |
Li2Zn3(P2O7)2 | V = 1098.48 (11) Å3 |
Mr = 557.87 | Z = 4 |
Orthorhombic, Pbcm | Mo Kα radiation |
a = 5.1733 (3) Å | µ = 7.17 mm−1 |
b = 13.1797 (7) Å | T = 298 K |
c = 16.1108 (9) Å | 0.30 × 0.28 × 0.25 mm |
Bruker SMART APEX CCD area-detector diffractometer | 2135 independent reflections |
Absorption correction: multi-scan (SADABS; Sheldrick, 1996) | 1994 reflections with I > 2σ(I) |
Tmin = 0.115, Tmax = 0.165 | Rint = 0.022 |
9756 measured reflections |
R[F2 > 2σ(F2)] = 0.023 | 126 parameters |
wR(F2) = 0.063 | 0 restraints |
S = 1.05 | Δρmax = 0.55 e Å−3 |
2135 reflections | Δρmin = −0.46 e Å−3 |
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. |
Refinement. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. |
x | y | z | Uiso*/Ueq | Occ. (<1) | |
Zn1 | 0.40045 (4) | 0.551978 (13) | 0.583844 (10) | 0.01173 (6) | |
Zn2 | 1.15622 (15) | 0.75848 (5) | 0.83867 (4) | 0.01006 (11) | 0.50 |
Li2 | 1.203 (3) | 0.7504 (11) | 0.8568 (8) | 0.031 (3) | 0.50 |
Li1 | 0.0908 (13) | 0.3797 (7) | 0.6966 (5) | 0.0330 (17) | 0.50 |
P1 | 0.08948 (7) | 0.35767 (3) | 0.50417 (2) | 0.00889 (8) | |
P2 | 0.57046 (10) | 0.44940 (4) | 0.7500 | 0.00920 (10) | |
P3 | 0.65256 (10) | 0.66147 (4) | 0.7500 | 0.00891 (10) | |
O1 | 0.3060 (2) | 0.43500 (8) | 0.49561 (7) | 0.0125 (2) | |
O2 | −0.0927 (2) | 0.36027 (9) | 0.42966 (7) | 0.0145 (2) | |
O3 | 0.2349 (3) | 0.2500 | 0.5000 | 0.0113 (3) | |
O4 | −0.0492 (3) | 0.36465 (9) | 0.58531 (6) | 0.0160 (2) | |
O5 | 0.4050 (2) | 0.45300 (9) | 0.67297 (8) | 0.0174 (2) | |
O6 | 0.7589 (3) | 0.54661 (11) | 0.7500 | 0.0123 (3) | |
O7 | 0.4941 (2) | 0.67266 (8) | 0.67131 (7) | 0.0157 (2) | |
O8 | 0.8905 (3) | 0.72789 (13) | 0.7500 | 0.0148 (3) | |
O9 | 1.2491 (3) | 0.85975 (11) | 0.7500 | 0.0140 (3) |
U11 | U22 | U33 | U12 | U13 | U23 | |
Zn1 | 0.01255 (10) | 0.01216 (10) | 0.01048 (10) | 0.00154 (6) | 0.00004 (5) | 0.00059 (5) |
Zn2 | 0.0114 (2) | 0.0098 (2) | 0.0090 (3) | 0.00094 (17) | 0.00055 (15) | 0.00057 (17) |
Li2 | 0.044 (8) | 0.025 (4) | 0.023 (6) | 0.023 (4) | 0.018 (5) | 0.015 (4) |
Li1 | 0.018 (3) | 0.055 (5) | 0.026 (3) | −0.014 (3) | −0.001 (2) | 0.008 (3) |
P1 | 0.00906 (16) | 0.00845 (15) | 0.00916 (15) | −0.00097 (11) | 0.00008 (10) | 0.00010 (11) |
P2 | 0.0093 (2) | 0.0083 (2) | 0.0101 (2) | 0.00085 (16) | 0.000 | 0.000 |
P3 | 0.0089 (2) | 0.0089 (2) | 0.0089 (2) | −0.00076 (16) | 0.000 | 0.000 |
O1 | 0.0130 (5) | 0.0115 (4) | 0.0129 (5) | −0.0045 (4) | 0.0019 (4) | −0.0017 (3) |
O2 | 0.0141 (5) | 0.0135 (5) | 0.0159 (5) | 0.0031 (4) | −0.0062 (4) | −0.0019 (4) |
O3 | 0.0093 (6) | 0.0091 (6) | 0.0155 (7) | 0.000 | 0.000 | 0.0006 (5) |
O4 | 0.0197 (5) | 0.0137 (5) | 0.0147 (5) | −0.0004 (4) | 0.0072 (4) | −0.0001 (3) |
O5 | 0.0198 (6) | 0.0157 (5) | 0.0168 (5) | −0.0045 (4) | −0.0077 (4) | 0.0047 (4) |
O6 | 0.0085 (6) | 0.0100 (6) | 0.0182 (7) | −0.0003 (5) | 0.000 | 0.000 |
O7 | 0.0195 (5) | 0.0139 (5) | 0.0138 (5) | 0.0007 (4) | −0.0070 (4) | 0.0000 (4) |
O8 | 0.0136 (7) | 0.0153 (7) | 0.0156 (7) | −0.0058 (5) | 0.000 | 0.000 |
O9 | 0.0137 (7) | 0.0108 (6) | 0.0175 (7) | −0.0033 (5) | 0.000 | 0.000 |
Zn1—O1 | 2.1532 (11) | P2—O9ix | 1.5059 (15) |
Zn1—O1i | 1.9935 (11) | P2—O5vii | 1.5083 (12) |
Zn1—O2ii | 1.9799 (11) | P2—O5 | 1.5083 (12) |
Zn1—O5 | 1.9401 (11) | P2—O6 | 1.6098 (15) |
Zn1—O7 | 2.1796 (11) | P3—O8 | 1.5106 (16) |
Zn1—Zn1i | 3.1995 (4) | P3—O7vii | 1.5168 (11) |
Zn1—Zn2iii | 3.2500 (7) | P3—O7 | 1.5168 (11) |
Zn2—O2iv | 2.1694 (13) | P3—O6 | 1.6107 (15) |
Zn2—O4v | 1.9402 (14) | O1—Zn1i | 1.9935 (11) |
Zn2—O7vi | 2.0883 (14) | O2—Li2x | 1.958 (14) |
Zn2—O8 | 2.0230 (12) | O2—Zn1ii | 1.9799 (11) |
Zn2—O9 | 2.0132 (12) | O2—Zn2x | 2.1694 (13) |
Zn2—Zn2vii | 2.8572 (14) | O3—P1xi | 1.6076 (8) |
Li2—O2iv | 1.958 (14) | O4—Li2viii | 1.942 (15) |
Li2—O4v | 1.942 (15) | O4—Zn2viii | 1.9402 (14) |
Li2—O7vi | 1.876 (15) | O7—Li2iii | 1.876 (15) |
Li2—O9 | 2.257 (12) | O7—Zn2iii | 2.0883 (14) |
Li2—O8 | 2.381 (14) | O8—Zn2vii | 2.0230 (12) |
Li1—O4 | 1.944 (8) | O8—Li1v | 2.180 (9) |
Li1—O5 | 1.929 (7) | O8—Li1xii | 2.180 (9) |
Li1—O9viii | 1.975 (7) | O8—Li2vii | 2.381 (14) |
Li1—O8viii | 2.180 (9) | O9—P2xiii | 1.5059 (15) |
P1—O4 | 1.4940 (11) | O9—Li1v | 1.975 (7) |
P1—O1 | 1.5207 (11) | O9—Li1xii | 1.975 (7) |
P1—O2 | 1.5265 (12) | O9—Zn2vii | 2.0132 (12) |
P1—O3 | 1.6076 (8) | O9—Li2vii | 2.257 (12) |
O5—Zn1—O2ii | 118.94 (5) | P1—O2—Zn1ii | 126.58 (7) |
O5—Zn1—O1i | 121.59 (5) | Li2x—O2—Zn1ii | 97.7 (4) |
O2ii—Zn1—O1i | 119.45 (5) | P1—O2—Zn2x | 127.49 (7) |
O5—Zn1—O1 | 90.57 (5) | Zn1ii—O2—Zn2x | 103.02 (5) |
O2ii—Zn1—O1 | 99.40 (5) | P1xi—O3—P1 | 124.19 (10) |
O1i—Zn1—O1 | 79.08 (5) | P1—O4—Li2viii | 124.7 (4) |
O5—Zn1—O7 | 90.54 (5) | P1—O4—Zn2viii | 130.16 (8) |
O2ii—Zn1—O7 | 79.84 (5) | P1—O4—Li1 | 129.4 (2) |
O1i—Zn1—O7 | 100.54 (5) | Li2viii—O4—Li1 | 77.8 (4) |
O1—Zn1—O7 | 178.86 (4) | Zn2viii—O4—Li1 | 66.3 (2) |
O4v—Zn2—O9 | 92.17 (6) | P2—O5—Li1 | 107.5 (2) |
O4v—Zn2—O8 | 113.30 (7) | P2—O5—Zn1 | 129.57 (7) |
O9—Zn2—O8 | 78.06 (6) | Li1—O5—Zn1 | 118.2 (2) |
O4v—Zn2—O7vi | 132.70 (6) | P2—O6—P3 | 122.77 (10) |
O9—Zn2—O7vi | 96.01 (7) | P3—O7—Li2iii | 133.5 (4) |
O8—Zn2—O7vi | 113.98 (7) | P3—O7—Zn2iii | 124.71 (7) |
O4v—Zn2—O2iv | 92.90 (5) | P3—O7—Zn1 | 126.13 (7) |
O9—Zn2—O2iv | 173.68 (7) | Li2iii—O7—Zn1 | 93.7 (4) |
O8—Zn2—O2iv | 103.32 (6) | Zn2iii—O7—Zn1 | 99.17 (5) |
O7vi—Zn2—O2iv | 77.77 (5) | P3—O8—Zn2 | 132.00 (5) |
O7vi—Li2—O4v | 150.3 (10) | P3—O8—Zn2vii | 132.00 (5) |
O7vi—Li2—O2iv | 88.4 (5) | Zn2—O8—Zn2vii | 89.85 (8) |
O4v—Li2—O2iv | 99.8 (7) | P3—O8—Li1v | 124.6 (2) |
O7vi—Li2—O9 | 94.6 (6) | Zn2—O8—Li1v | 60.55 (19) |
O4v—Li2—O9 | 85.1 (5) | Zn2vii—O8—Li1v | 93.8 (2) |
O2iv—Li2—O9 | 164.5 (9) | P3—O8—Li1xii | 124.6 (2) |
O7vi—Li2—O8 | 107.6 (7) | Zn2—O8—Li1xii | 93.8 (2) |
O4v—Li2—O8 | 99.5 (5) | Zn2vii—O8—Li1xii | 60.55 (19) |
O2iv—Li2—O8 | 98.1 (7) | Li1v—O8—Li1xii | 46.5 (4) |
O9—Li2—O8 | 66.4 (3) | P3—O8—Li2 | 128.8 (4) |
O5—Li1—O4 | 100.6 (3) | Zn2vii—O8—Li2 | 91.4 (4) |
O5—Li1—O9viii | 154.6 (5) | Li1v—O8—Li2 | 64.5 (4) |
O4—Li1—O9viii | 93.2 (3) | Li1xii—O8—Li2 | 98.1 (4) |
O5—Li1—O8viii | 120.0 (4) | P3—O8—Li2vii | 128.8 (4) |
O4—Li1—O8viii | 106.7 (4) | Zn2—O8—Li2vii | 91.4 (4) |
O9viii—Li1—O8viii | 75.2 (3) | Li1v—O8—Li2vii | 98.1 (4) |
O4—P1—O1 | 113.06 (7) | Li1xii—O8—Li2vii | 64.5 (4) |
O4—P1—O2 | 112.98 (7) | Li2—O8—Li2vii | 92.6 (8) |
O1—P1—O2 | 111.62 (7) | P2xiii—O9—Li1v | 116.6 (3) |
O4—P1—O3 | 108.40 (5) | P2xiii—O9—Li1xii | 116.6 (3) |
O1—P1—O3 | 104.07 (7) | Li1v—O9—Li1xii | 51.7 (4) |
O2—P1—O3 | 106.02 (5) | P2xiii—O9—Zn2vii | 131.92 (5) |
O9ix—P2—O5vii | 112.12 (6) | Li1v—O9—Zn2vii | 100.7 (2) |
O9ix—P2—O5 | 112.12 (6) | Li1xii—O9—Zn2vii | 64.3 (3) |
O5vii—P2—O5 | 110.73 (10) | P2xiii—O9—Zn2 | 131.92 (5) |
O9ix—P2—O6 | 104.42 (9) | Li1v—O9—Zn2 | 64.3 (3) |
O5vii—P2—O6 | 108.57 (5) | Li1xii—O9—Zn2 | 100.7 (2) |
O5—P2—O6 | 108.57 (5) | Zn2vii—O9—Zn2 | 90.41 (7) |
O8—P3—O7vii | 112.58 (6) | P2xiii—O9—Li2vii | 124.5 (4) |
O8—P3—O7 | 112.58 (6) | Li1v—O9—Li2vii | 108.9 (5) |
O7vii—P3—O7 | 113.40 (10) | Li1xii—O9—Li2vii | 70.1 (5) |
O8—P3—O6 | 105.45 (9) | Zn2—O9—Li2vii | 95.3 (4) |
O7vii—P3—O6 | 106.01 (5) | P2xiii—O9—Li2 | 124.5 (4) |
O7—P3—O6 | 106.01 (5) | Li1v—O9—Li2 | 70.1 (5) |
P1—O1—Zn1i | 132.61 (7) | Li1xii—O9—Li2 | 108.9 (5) |
P1—O1—Zn1 | 125.96 (6) | Zn2vii—O9—Li2 | 95.3 (4) |
Zn1i—O1—Zn1 | 100.92 (5) | Li2vii—O9—Li2 | 99.4 (8) |
P1—O2—Li2x | 129.4 (5) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x, −y+1, −z+1; (iii) x−1, y, −z+3/2; (iv) −x+1, −y+1, z+1/2; (v) −x+1, y+1/2, −z+3/2; (vi) x+1, y, −z+3/2; (vii) x, y, −z+3/2; (viii) −x+1, y−1/2, −z+3/2; (ix) −x+2, y−1/2, −z+3/2; (x) −x+1, −y+1, z−1/2; (xi) x, −y+1/2, −z+1; (xii) −x+1, y+1/2, z; (xiii) −x+2, y+1/2, −z+3/2. |
Experimental details
Crystal data | |
Chemical formula | Li2Zn3(P2O7)2 |
Mr | 557.87 |
Crystal system, space group | Orthorhombic, Pbcm |
Temperature (K) | 298 |
a, b, c (Å) | 5.1733 (3), 13.1797 (7), 16.1108 (9) |
V (Å3) | 1098.48 (11) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 7.17 |
Crystal size (mm) | 0.30 × 0.28 × 0.25 |
Data collection | |
Diffractometer | Bruker SMART APEX CCD area-detector diffractometer |
Absorption correction | Multi-scan (SADABS; Sheldrick, 1996) |
Tmin, Tmax | 0.115, 0.165 |
No. of measured, independent and observed [I > 2σ(I)] reflections | 9756, 2135, 1994 |
Rint | 0.022 |
(sin θ/λ)max (Å−1) | 0.777 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.023, 0.063, 1.05 |
No. of reflections | 2135 |
No. of parameters | 126 |
Δρmax, Δρmin (e Å−3) | 0.55, −0.46 |
Computer programs: SMART (Bruker, 2000), SAINT (Bruker, 2000), SHELXS97 (Sheldrick, 2008), SHELXL97 (Sheldrick, 2008), DIAMOND (Brandenburg, 1999).
Zn1—O1 | 2.1532 (11) | Li2—O2iii | 1.958 (14) |
Zn1—O1i | 1.9935 (11) | Li2—O4iv | 1.942 (15) |
Zn1—O2ii | 1.9799 (11) | Li2—O7v | 1.876 (15) |
Zn1—O5 | 1.9401 (11) | Li2—O9 | 2.257 (12) |
Zn1—O7 | 2.1796 (11) | Li2—O8 | 2.381 (14) |
Zn2—O2iii | 2.1694 (13) | Li1—O4 | 1.944 (8) |
Zn2—O4iv | 1.9402 (14) | Li1—O5 | 1.929 (7) |
Zn2—O7v | 2.0883 (14) | Li1—O9vi | 1.975 (7) |
Zn2—O8 | 2.0230 (12) | Li1—O8vi | 2.180 (9) |
Zn2—O9 | 2.0132 (12) |
Symmetry codes: (i) −x+1, −y+1, −z+1; (ii) −x, −y+1, −z+1; (iii) −x+1, −y+1, z+1/2; (iv) −x+1, y+1/2, −z+3/2; (v) x+1, y, −z+3/2; (vi) −x+1, y−1/2, −z+3/2. |
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High-quality II–VI semiconductor zinc oxide (ZnO) crystals have various applications in functional devices (Look, 2001; Tsukazaki et al., 2005). Due to the high melting point (2248 K) and serious volatilization of ZnO at high temperature, a suitable flux is needed for growing high-quality ZnO crystals at a lower temperature. The subsolidus phase relations of the ternary system A2O–ZnO–P2O5 (A = Li, Na or K) were systematically investigated to find such a flux. The title compound, Li2Zn3(P2O7)2, is a possible new phase in this system and Zn2P2O7 and Li4P2O7 were used to synthesize it. The X-ray powder diffraction pattern was measured on the reaction products and all peaks were indexed using TREOR (Werner et al., 1985), with an orthorhombic unit cell a = 5.191 (1), b = 13.226 (4), c = 16.166 (5) Å [M(20) = 17, F(20) = 27], which indicates that the product is a single phase. This unit cell, being in good agreement with that from single-crystal diffraction, is similar but not identical to the cell parameters of γ-Zn2P2O7 (Bataille et al., 1998). Also, the measured powder diffraction pattern did not match PDF entry 49–1240 (ICDD, 2004) for γ-Zn2P2O7 very well. In addition, the warming, heating and cooling scheme guaranteed that the volatilization of Li, Zn and P was avoided, hence the compositon of the product was not significantly different from the starting materials. Therefore, the product was believed to be Li2Zn3(P2O7)2 and single-crystal structure analysis was performed to determine the structure and to verify this new phase. We hereby report the structure of this compound from single-crystal diffraction.
The title compound has two symmetry-independent sites for Zn atoms. One is fully occupied with a trace amount (~0.9%) of Li+ contamination, whereas the other is disordered by Zn2+ and Li+ cations in a Zn2+/Li+ ratio of 1:1. As shown in Fig. 1, Zn atoms are coordinated by five O atoms, three of which are in the equatorial plane while the other two are on the northern and southern pole, respectively. For the fully occupied Zn sites, the average Zn—O bond length in the equatorial plane is 1.97 (2) Å, while the average Zn—O distance between Zn and the polar O atoms is 2.16 (1) Å. The corresponding values for the partially occupied Zn site are 2.01 (6) and 2.09 (7) Å, respectively. Two fully occupied Zn1—O bipyramids connect to each other by sharing one edge to form a [Zn2O8] dimer, and the partially occupied Zn2—O polyhedra build up [Zn2O8] dimers in the same fashion (Fig. 2). Fully and partially occupied [Zn2O8] dimers share edges alternately to form [ZnO5] bipyramid chains running along the c axis. The Zn1···Zn1 interatomic distance between the centres of two adjacent fully occupied [ZnO5] polyhedra is 3.1995 (4) Å, whereas the corresponding Zn2···Zn2 distance for the partially occupied [ZnO5] polyhedra is 2.857 (1) Å. The Zn1···Zn2 distance between the centres of neighbouring fully and partially occupied [ZnO5] pyramids is 3.2500 (7) Å. The difference between the Zn1···Zn1 and Zn2···Zn2 distances leads to Zn2+/Li+ disordering on Zn2 site rather than on both Zn1 and Zn2 sites. The shorter Zn2···Zn2 distance favours lower charges on Zn2 sites, produced by replacing half of the Zn2+ cations with the same number of Li+ cations, whereas the Zn1 site needs a fully occupied Zn2+ ion to stablize the structure. The infinite chains are crosslinked by sharing tetrahedra vertices with [P2O7]4− pyrophosphate groups to build up the three-dimensional framework structure of Li2Zn3(P2O7)2 (Fig. 2). One half of the Li+ cations are disordered with Zn2+ on the partially occupied Zn2 positions and the other half of the Li+cations are situated in the interstitial positions of the framework with an occupancy factor of 1/2, coordinated by four O atoms with an average Li—O distance of 2.0 (1) Å.
The basic structural features of the title compound are very similar to those of γ-Zn2(P2O7) (Bataille et al., 1998). The latter compound also consists of infinite corrugated Zn—O bipyramid chains running along c, which are crosslinked together by pyrophosphate groups to form the crystal structure. The title compound is essentially an Li-doped variant of γ-Zn2(P2O7). The Li2Zn3(P2O7)2 phase can be derived from γ-Zn2(P2O7) by replacing half of the Zn2+ cations on one of the two symmetry-independent Zn sites with Li+ cations, and by doping the same number of Li+ cations in the interstatial positions of the structure to balance the net charge due to Zn2+–Li+ substitution. Similar to the title compound, the Zn···Zn distances in the γ-Zn2(P2O7) structure also fall into two groups of 3.084 and 3.242/3.252 Å, respectively. The former distance (3.084 Å) is significantly longer than the corresponding value [2.857 (1) Å] in the title compound, whereas the values in the second group are comparable with their counterparts in the Li2Zn3(P2O7)2 structure. In the γ-Zn2(P2O7) structure, it is the Zn2+ cations on the Zn sites involving shorter Zn···Zn distances (3.084 Å) that are partially substituted by Li+ cations to form the Li2Zn3(P2O7)2 phase.
The structures of several Li–Zn–P–O quaternary compounds have been reported, examples being α-LiZnPO4 (Elammari & Elouadi, 1989), δ1-LiZnPO4 (Jensen et al., 1995), ε-LiZnPO4 (Bu et al., 1996), LiZnPO4-CR1 (Bu et al., 1998), and α-Li4Zn(PO4)2 and β-Li4Zn(PO4)2 (Jensen et al., 2002). All these compounds are orthophosphates and all the Zn atoms in these structures are tetrahedrally coordinated by O atoms, with an average Zn—O distance of 1.95 (1) Å. This fact indicates that the Zn2+ cation has a strong preference for tetrahedral coordination in Li–Zn–P–O system. However, under the conditions reported in this paper, the Zn2+ cations are coordinated by five O atoms to form bipyramids, and these polyhedra build up infinite chains by sharing edges. Pyrophosphate groups, [P2O7]4−, rather than orthophosphate groups, [PO4]3−, are observed in the structure and crosslink the [ZnO5] bipyramid chains.
The Li+ cations in all the reported structures are tetrahedrally coordinated by four O atoms, whether they are situated in the walls of channels (ε-LiZnPO4), in negatively charged cages (LiZnPO4-CR1), or disordered with Zn2+ cations to a minor degree [δ1-LiZnPO4, α-Li4Zn(PO4)2 and β-Li4Zn(PO4)2]. In contrast, the coordination of the Li+ cations in the title compound is rather complicated. The interstitial Li+ cations are tetrahedrally coordinated, whereas the Li+ cations disordered on the Zn sites are five-coordinated. Due to the similarity in the coordination radii of Li+ and Zn2+ cations, statistical disorder of Li+ and Zn2+ is possible in principle. In fact, the mean Li—O and Zn—O distances calculated from the reported Li–Zn–P–O compounds are 1.97 (5) and 1.95 (1) Å, respectively. A minor degree of disorder was proposed in β-Li4Zn(PO4)2 (prepared from supercritical water at 875 K) and δ1-LiZnPO4 (obtained from solution at 365 K), but in the compounds synthesized by high-temperature solid-state reactions at 973–1275 K, a much higher degree of Li+/Zn2+ statistical disorder (Li+:Zn2+ = 1:1) was reported in compounds such as NaLiZnP2O7 (Shepelev et al., 2005). Similarly, the title compound was prepared at 1123 K and a consequent 1:1 Li+/Zn2+ disorder is found. All Li+ cations in NaLiZnP2O7 are disordered with Zn2+ cations, but only half of the Li+ cations in the title compound are disordered on the Zn2+ sites.