Synthesis, crystal structures, substitutional and comparative structural analysis of copper diphosphates LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7 European Journal of Chemistry 9 (3) (2018) 258-268 European Journal of Chemistry View Journal Online View Article Online Synthesis, crystal structures, substitutional and comparative structural analysis of copper diphosphates LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7 Ines Fitouri and Habib Boughzala * Laboratoire de Matériaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Manar II Tunis, Tunisia inesfitouri884@gmail.com (I.F.), habib.boughzala@ipein.rnu.tn (H.B.) * Corresponding author at: Laboratoire de Matériaux et Cristallochimie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 Manar II Tunis, Tunisia. Tel: +216.98.595523 Fax: +216.72.220181 e-mail: habib.boughzala@ipein.rnu.tn (H. Boughzala). 10.5155/eurjchem.9.3.258-268.1762 Received: 11 June 2018 Received in revised form: 17 July 2018 Accepted: 28 July 2018 Published online: 30 September 2018 Printed: 30 September 2018 The title compounds are members of the M2O-CuO-P2O5 system (M = Li, Na, K and Rb), where the lithium, sodium, potassium, rubidium and cesium phases have already been structurally characterized. The studied diphosphates LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7 belong to a large family of materials of general formula, MM’CuP2O7 (M, M’ = Monovalent cation) where the elements M and M’ ionic radii are decisive in the structural type determination. They were synthesized by solid-state reactions. The X-ray structural analysis show that these compounds crystallize in the P21/n monoclinic lattice where the CuO5 pyramidal square are linked to nearly eclipsed P2O7 groups by corner sharing to build up corrugated layers [CuP2O7]2- extending perpendicularly to [010]. The Li+, Na+, K+ and Rb+ cations reside in the interlayer space and in cavities delimited by the anionic network. In this study, the synthesis, the structure, the powder diffraction, the infrared spectroscopy, the thermal analysis (DTA/TGA) and a structural comparison are presented. The structural models were validated by Bond Valence-Sum (BVS) and charge distribution (CHARDI) analysis. Copper Powders Phosphorus X-ray diffraction IR spectroscopy Single crystal X-ray structure Cite this: Eur. J. Chem. 2018, 9(3), 258-268 Journal website: www.eurjchem.com 1. Introduction Many structures of copper diphosphates with general formula MM’CuP2O7 (M, M’ = Monovalent cation), have been widely investigated in recent years because of their storage and energy conduction properties. Owing to the low density and the high reductive power of lithium metal, a great deal of interest has been focused on the development of lithium batteries as power source for portables devices or electric vehicles [1,2]. Indeed, in order to improve their ionic behavior, we attempted to partially replace the sodium and potassium cations in α-Na2CuP2O7 [3] and K2CuP2O7 [4] by smaller lithium cations. This substitution can be of interest for ionic conduction properties such as in rechargeable alkali batteries [5]. Currently, the increasing cost and the high toxicity of heavy metal based batteries encourage the pursuit of the Li- ion alternative. Thanks to environmental friendliness and natural abundance of sodium resources [6], Na-ion batteries are actually under intensive investigation. The Cu2+ ion coordination flexibility, moving between the square pyramidal and the trigonal bipyramid, is able to modulate the phosphate anionic framework and incite to use copper to synthesize new materials. 2. Experimental 2.1. Synthesis and crystallization The synthesis of the studied compounds was carried out starting with the adequate reagent mixed accordingly to the indicated relative proportions in Table 1. The mixtures were finely ground and heated in a porcelain crucible up to 623 K for 12 h to eliminate volatile components. The temperature was then increased to 873 K for LiNaCuP2O7, LiKCuP2O7 and 823 K for Rb0.5Na1.5CuP2O7 reaching the respective melting points and held for 15 days. The samples were slowly cooled (5 K/day) to 500 K and finally allowed to cool radiatively to room temperature. The products were washed with water and rinsed with a 0.5 M aqueous solution of HCl. Only one type of regular light-blue prismatic crystals was observed for each compound. The obtained crystals were ground and checked by powder X-ray diffraction. Rietveld analysis with the program TOPAS 4.2 [7] revealed a single-phase product of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. During the synthesis, no solid solution is observed in spite of the several attempts using different initial proportions. ABSTRACT RESEARCH ARTICLE KEYWORDS European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2018 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. http://dx.doi.org/10.5155/eurjchem.9.3.258-268.1762 http://dx.doi.org/10.5155/eurjchem.9.3.258-268.1762 https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.9.3.258-268.1762&domain=pdf&date_stamp=2018-09-30 http://www.eurjchem.com/ http://dx.doi.org/10.5155/eurjchem.9.3.258-268.1762 mailto:inesfitouri884@gmail.com mailto:habib.boughzala@ipein.rnu.tn mailto:habib.boughzala@ipein.rnu.tn http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.9.3.258-268.1762&domain=pdf&date_stamp=2018-09-30� Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 259 Table 1. Reaction mixtures made for each compound. Compounds Reagents Proportion LiNaCuP2O7 LiH2PO4 / NaH2PO4 / CuO 1:1:2:2 LiKCuP2O7 LiH2PO4 / KH2PO4 / CuO 1:1:2:2 Rb0.5Na1.5CuP2O7 Rb2CO3 / NaH2PO4 /CuO 2:1:1:2 Table 2. Crystallographic data and refinement parameters for compounds LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. Crystal data Formula LiNaCuP2O7 LiKCuP2O7 Rb0.5Na1.5CuP2O7 Crystal system Monoclinic Monoclinic Monoclinic Space group P21/n P21/n P21/n a (Å) 4.988(2) 5.066(1) 7.817(2) b (Å) 13.598(4) 14.180(2) 9.806(3) c (Å) 8.294(2) 8.473(7) 8.644(3) β (°) 97.17(4) 90.189(1) 104.48(2) Unit-cell volume (Å3) 562.1(3) 608.7(5) 641.6(3) Z 4 4 4 Calculated density (g/cm3) 3.160 3.094 3.820 Absorption coefficient (mm–1) 5.262 4.784 11.57 Crystal size (mm) 0.32×0.09×0.13 0.16×0.09×0.13 0.11×0.06×0.13 Data collection Temperature 398 K 398 K 398 K Radiation, wavelength (Å) MoKα, 0.71073 F(000) 516 548 696 θmax (°) 27 27 27 h, k, l ranges -6→5, -2→17, -10→10 -6→5, -1→18, -10→10 -9→3, -1→12, -11→11 Measured reflections 2715 2765 2199 Independent reflections 1230 1323 1393 Reflections with I > 2σ(I) 1156 1201 1181 Refinement Refinement method Full-matrix least-squares on F2 Full-matrix least-squares on F2 Full-matrix least-squares on F2 R1(F), wR2(F2) 1.9%, 5.9% 1.9%, 5.3% 2.8%, 7.4% No. of refined parameters 112 112 116 GooF 1.12 1.00 1.08 (∆/σ)max < 0.001 < 0.001 < 0.001 ∆ϱmin, ∆ϱmax (e.Å-3) 0.86, -1.29 0.44, -0.36 0.69, -1.08 2.2. X‐ray structure determination A light blue single crystal has been selected for X-ray diffraction analysis from each preparation. Data collection was performed on an Enraf-Nonius CAD-4 diffractometer, opera- ting at 298 K with (MoKα radiation, λ = 0.71069 Å). An empi- rical ψ-scan absorption correction was applied in all of the three cases. The structures have been solved by direct methods using SHELXS-2014 software [8] and refined by least- square full-matrix based on F2 using SHELXL-2014 [9] and SHELXLe [10] software. A summary of the crystallographic data and structural determination for LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7 is provided in Table 2. The final reduced atomic coordinates, the equivalent thermal factors and the refined partial occupancies are listed in Table 3. The geometrical characteristics are detailed in Tables 4 and 5. The crystallographic data are deposit in the ICSD database [11] and can be retrieved using the respective Collection Code 433889 for LiKCuP2O7, 433890 for LiNaCuP2O7 and 434654 for Rb0.5Na1.5CuP2O7. The free variable restraint (SUMP) was required to restrain the sum of the monovalent cations occupation factors accordingly to the electro neutrality of the cell content. It’s worthy to note that for LiNaCuP2O7 and LiKCuP2O7 the same cationic site 1 is preferred by the lithium and the site 2 is fully occupied by the sodium and the potassium respectively as it can be seen in Table 3. After minor alignment of the atomic coordinates in NaKCuP2O7 [12] particularly for O6 an O7, selected valance angles are summarized in Table 5. The structural models are validated by the two structural tools, Bond Valence Sum (BVS) [13,14] and Charge Distribution analysis (CD) [15,16]. Both BVS and CD show expected valences (V) and charges (Q) of all the cation sites. The structural model is thus validated, as shown by the dispersion factor measuring the computed charges (Q) deviation in respect to the formal oxidation numbers. The Bond Valence computation and Charge Distribution analysis are summarized in Table 6. 2.3. Infrared spectroscopy The infrared (IR) transmission spectra of the compounds LiNaCuP2O7 and LiKCuP2O7 were studied using the Perkin- Elmer spectrophotometer in the range of 1400-400 cm-l wavenumbers containing the most significant solid state absorption bands. The samples were ground with 98% weight KBr and pressed (10 bar) into 10 mm diameter discs. The IR spectra of these two compounds presented in Figure 1 exhibit high similarity. The typical frequency ranges corresponding to symmetric (ʋs) and asymmetric (νas) P-O-P vibrational modes of the P2O7 group are centered at 773/914 cm-1 for the compound LiNaCuP2O7 and 775/908 cm-1 for LiKCuP2O7. The O-P-O bending and P-O stretching vibrational frequencies associated with the PO4 are shown in multiple bands in the 408-604 and 978-1044 cm-1 range, respectively as shown in Table 7. To draw a parallel between the observed spectra of the tow compounds, the noted frequency shift towards the low frequencies for the bands associated to the symmetric deformation mode δ (O-P-O) and the asymmetric elongation mode νas (P-O-P) can be explained by the sodium substitution by a heavier cation, the potassium. 2.4. Powder diffraction Using the structural data obtained by single-crystal diffraction, the calculated powder X-ray pattern can be obtained using the Diamond 3 [17] program. On the other hand, the observed X-ray patterns were registered on a Bruker D8 Advance diffractometer using the Kα1/α2 Copper radiation. The superposition of the observed and calculated raw diffraction of every compound reveals few weak unindexed Bragg pics and confirms the purity of the three synthesized phases. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 260 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 Table 3. Coordinates and isotropic displacement parameters (Å2) of atoms in the crystal structure of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. Atom x y z Ueq = (1/3)ΣiΣjUijai*aj*ai.aj Occupancy LiNaCuP2O7 Li1 0.2398(8) 0.3018(3) 0.1573(5) 0.0154(15) 1.02(2) Na1 0.2398(8) 0.3018(3) 0.1573(5) 0.0154(15) 0.03(2) Li2 0.7691(2) 0.38605(8) 0.37307(2) 0.0175(4) 0.02(5) Na2 0.7691(2) 0.38605(8) 0.37307(2) 0.0175(4) 1.004(5) Cu 0.76586(5) 0.65856(2) 0.25798(3) 0.01163(13) P1 0.27162(2) 0.53003(5) 0.25517(6) 0.00830(15) P2 0.26202(2) 0.68581(4) 0.03920(6) 0.00685(15) O1 0.1804(4) 0.42598(2) 0.2511(2) 0.0153(4) O2 0.5623(3) 0.54255(2) 0.3156(2) 0.0129(3) O3 0.0897(3) 0.59955(2) 0.34668(2) 0.0115(3) O4 0.2614(3) 0.56959(2) 0.06942(2) 0.0103(3) O5 0.3691(3) 0.70280(2) −0.12657(2) 0.0101(3) O6 −0.0249(3) 0.72131(2) 0.05316(2) 0.0105(3) O7 0.4493(3) 0.72874(2) 0.17198(2) 0.0091(3) LiKCuP2O7 Li1 0.2548(8) 0.3064(3) 0.1519(4) 0.0159(14) 1.02(2) K1 0.2548(8) 0.3064(3) 0.1519(4) 0.0159(14) 0.02(2) Li2 0.75511(2) 0.39822(3) 0.38239(6) 0.01542(18) 0.03(3) K2 0.75511(2) 0.39822(3) 0.38239(6) 0.01542(18) 0.997(3) Cu 0.73335(5) 0.65988(2) 0.23902(3) 0.01005(11) P1 0.25017(2) 0.53470(4) 0.23364(6) 0.00910(14) P2 0.24636(2) 0.69267(4) 0.03911(6) 0.00723(14) O1 0.2182(3) 0.43118(2) 0.2169(2) 0.0182(4) O2 0.5034(3) 0.56170(2) 0.31934(2) 0.0144(3) O3 0.0190(3) 0.58294(2) 0.31455(2) 0.0140(3) O4 0.2704(3) 0.58030(2) 0.05834(2) 0.0106(3) O5 0.3576(3) 0.71637(2) −0.12047(2) 0.0114(3) O6 −0.0455(3) 0.71758(2) 0.05421(2) 0.0101(3) O7 0.4138(3) 0.73352(2) 0.17365(2) 0.0101(3) Rb0.5Na1.5CuP2O7 Na1 0.4547(5) 0.5125(4) -0.0634(5) 0.0390(11) 0 .500(1) Na2 0.5178(2) 0.14408(2) 0.16791(2) 0.0202(4) 1.004(9) Rb 0.5000 0.5000 0.5000 0.0683(4) 0.495(2) Cu 0.26021(5) 0.19311(5) 0.42517(5) 0.01255(16) P1 -0.10218(2) 0.33386(2) 0.29638(2) 0.0120(2) P2 0.15248(1) 0.35656(2) 0.11215(2) 0.0097(2) O1 -0.1758(4) 0.4330(3) 0.3923(4) 0.0348(8) O2 -0.2346(4) 0.2795(4) 0.1503(3) 0.0289(8) O3 0.0027(4) 0.2188(3) 0.3929(3) 0.0240(7) O4 0.0412(3) 0.4211(3) 0.2294(3) 0.0125(5) O5 0.2729(4) 0.4684(3) 0.0894(3) 0.0203(6) O6 0.0189(3) 0.3112(3) -0.0387(3) 0.0160(6) O7 0.2505(3) 0.2344(3) 0.2014(3) 0.0177(6) Figure 1. IR spectra of LiNaCuP2O7 diphosphates (in red) and LiKCuP2O7 (in blue). This preliminary step is necessary to control the purity of the product, but remains limited because it is based on visual analysis and do not take care of the relative intensities of the reflections. The use of a more reliable method is crucial in a second step. In this context, a Rietveld study was carried out to identify discrepancies between the results obtained by the two techniques (single crystal and powder) and to ensure the purity of the studied material. Both GSAS [18] and TOPAS 4.2 [7] programs have been used to refine some structural parameters obtained by the single crystal diffraction investigation. The best profile fitting of the raw diffraction are plotted in Figures 2-7, respectively for the powders of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. In order to avoid obtaining incoherent values of the occupancy rates of the cations, restrictions have been applied on their respective sites, as well as a constraint respecting the electrical neutrality of the material. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 261 Table 4. Selected bond lengths (Å) in the crystal structures of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. LiNaCuP2O7 LiKCuP2O7 Rb0.5Na1.5CuP2O7 Li-O1 1.888(5) Li-O1 1.863(4) Rb-O1 viii 2.984(3) Li-O5 viii 1.978(4) Li-O5 viii 2.008(4) Rb-O1 ix 2.984(3) Li-O7 ix 1.996(4) Li-O7 ix 1.998(4) Rb-O6 i 3.077(3) Li-O6 vii 2.038(4) Li-O6 vii 2.068(4) Rb-O6 iv 3.077(3) 1.975(4) 1.984(4) Rb-O7 i 3.232(3) Rb-O7 iv 3.232(3) Na-O1 i 2.384(2) K-O2 2.698(2) Rb-O5 3.563(3) Na-O2 2.405(2) K-O1 i 2.776(2) Rb-O5 x 3.563(3) Na-O3 v 2.411(2) K-O5 vi 2.808(2) 3.214(3) Na-O5 viii 2.451(2) K-O3 iv 2.820(3) Na-O7 x 2.593(2) K-O6 ix 2.825(2) Na1-O5 vii 2.205(4) Na-O6 ix 2.633(2) K-O2 iv 2.906(3) Na1-O5 2.211(4) 2.479(2) K-O7 xi 2.915(2) Na1-O3 xi 2.345(5) K-O3 i 2.997(2) Na1-O3 iv 2.476(5) Cu-O3 i 1.924(2) K-O1 3.091(2) Na1-O2 v 2.653(5) Cu-O2 1.945(2) 2.870(2) 2.378(5) Cu-O7 1.956(2) Cu-O5 ii 2.167(2) Cu-O3 i 1.920(2) Na2-O7 2.353(3) Cu-O6 i 2.199(2) Cu-O2 1.940(2) Na2-O2 ix 2.383(3) 2.038(2) Cu-O7 2.003(2) Na2-O4 ii 2.447(3) Cu-O6 i 2.094(2) Na2-O1 ii 2.530(4) P1-O1 1.486(2) Cu-O5 ii 2.211(2) Na2-O6 i 2.571(3) P1-O2 1.525(2) 2.033(2) Na2-O1 xi 2.595(4) P1-O3 1.531(2) Na2-O3 xi 2.708(4) P1-O4 1.630(2) P1-O1 1.483(2) 2.512(3) 1.543(2) P1-O2 1.521(2) P1-O3 1.521(2) Cu-O2 i 1.955(3) P2-O4 1.600(2) P1-O4 1.623(2) Cu-O7 1.959(3) P2-O5 1.511(2) 1.537(2) Cu-O6 i 1.968(3) P2-O6 1.518(2) Cu-O3 1.978(3) P2-O7 1.532(2) P2-O4 1.606(2) Cu-O5 ii 2.218(3) 1.540(2) P2-O5 1.504(2) 2.015(3) P2-O6 1.526(2) P2-O7 1.532(2) P1-O1 1.484(3) 1.542(2) P1-O2 1.516(3) P1-O3 1.517(3) P1-O4 1.627(3) 1.536(3) P2-O5 1.490(3) P2-O6 1.519(3) P2-O7 1.523(3) P2-O4 1.620(3) 1.538(3) Symmetry code: (i) x+1, y, z; (ii) x+1/2, −y+3/2, z+1/2; (iii) −x+3/2, y+1/2, −z+1/2; (iv) −x+1, −y+1, −z+1; (v) x−1, y, z; (vi) −x+1, −y+1, −z; (vii) −x, −y+1, −z; (viii) −x+1/2, y+1/2, −z+1/2; (ix) −x+1/2, y−1/2, −z+1/2; (x) x−1/2, −y+1/2, z−1/2; (xi) −x+3/2, y−1/2, −z+1/2; (xii) x−1/2, −y+3/2, z−1/2. Figure 2. GSAS refinement of the LiNaCuP2O7. If the profile and the background parameters functions are refined, the atomic coordinates and the thermal displacement parameters (obtained by the single crystal study) are fixed. Using the Table 8, a comparison of the main refined crystallo- graphic parameters obtained by single crystal and powder diffraction can be done. Both results are close within the standard errors. Thus the crystal is representative of the powder. 2.5. Thermal analysis (DTA/TGA) To have more information about the thermal properties of the studied compounds and particularly searching any solid state phase transition, coupled Differential Thermal Analysis (DTA) and Thermo Gravimetric Analysis (TGA) was performed on a Perkin Elmer STA6000 thermal analysis system using the polycrystalline samples of LiKCuP2O7 and Rb0.5Na1.5CuP2O7 in the range of 30 to 400 °C. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 262 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 Table 5. Selected angles (°) in the crystal structures of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. Bond LiNaCuP2O7 LiKCuP2O7 NaKCuP2O7 Rb0.5Na1.5CuP2O7 O3i—Cu—O2 90.36(7) 85.88(7) 91.46 (8) O2i—Cu—O7 160.14(1) O3i—Cu—O7 175.44(7) 174.75(6) 160.62 (8) O2i—Cu—O6 i 94.62(1) O2—Cu—O7 93.65(7) 89.17(7) 90.77 (7) O7—Cu—O6 i 86.87(1) O3i—Cu—O5 ii 90.68(7) 93.35(7) 92.15 (8) O2i—Cu—O3 83.69(1) O2—Cu—O5 ii 135.57(7) 123.43(7) 109.22 (7) O7—Cu—O3 92.72(1) O7—Cu—O5 ii 84.94(7) 87.91(6) 89.93 (7) O6i—Cu—O3 173.85(1) O3i—Cu—O6 i 92.36(7) 93.83(7) 91.34 (7) O2i—Cu—O5 ii 99.56(1) O2—Cu—O6 i 140.50(7) 149.97(7) 176.22 (8) O7—Cu—O5 ii 100.03(1) O7—Cu—O6 i 85.93(7) 91.34(6) 87.43 (7) O6i—Cu—O5 ii 95.09(1) O5ii—Cu—O6 i 83.81(6) 86.59(7) 84.52 (7) O3—Cu—O5 ii 91.03(1) O1—P1—O2 113.58(1) 112.72(1) 112.67 (12) O1—P1—O2 114.6(2) O1—P1—O3 114.35(1) 113.83(1) 114.78 (12) O1—P1—O3 114.35(2) O2—P1—O3 109.99(1) 108.72(1) 108.17 (11) O2—P1—O3 111.06(2) O1—P1—O4 107.11(1) 108.30(9) 107.95 (11) O1—P1—O4 104.62(2) O2—P1—O4 105.60(1) 106.31(9) 107.13 (10) O2—P1—O4 106.04(2) O3—P1—O4 105.47(9) 106.51(9) 105.64 (10) O3—P1—O4 105.12(2) O5—P2—O6 112.78(1) 112.93(9) 113.20 (10) O5—P2—O6 115.07(2) O5—P2—O7 111.64(1) 112.08(1) 111.96 (10) O5—P2—O7 113.02(2) O6—P2—O7 112.01(1) 112.59(9) 111.49 (10) O6—P2—O7 110.79(2) O5—P2—O4 107.23(9) 106.51(9) 107.91 (10) O5—P2—O4 104.53(2) O6—P2—O4 107.13(1) 107.12(8) 105.10 (10) O6—P2—O4 106.88(1) O7—P2—O4 105.56(9) 104.94(9) 106.66 (10) O7—P2—O4 105.71(1) Symmetry code: (i) x+1, y, z; (ii) x+1/2, −y+3/2, z+1/2. Table 6. CHARDI and BVS analysis of ions in LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. Ion q(i).sof(i) Q(i) V(i).sof(i) CN(i) ECoN(i) dar(i) dmed(i) LiNaCuP2O7 Li 1.00 0.98 1.02 4 3.89 1.974 1.975 Na 1.00 0.98 1.08 6 5.80 2.479 2.479 Cu 2.00 1.95 1.98 5 4.42 2.038 2.038 P1 5.00 4.99 4.93 4 3.84 1.543 1.543 P2 5.00 5.10 4.96 4 3.93 1.540 1.540 O1 -2.00 -2.07 -1.88 3 3.00 1.919 1.913 O2 -2.00 -2.05 -1.95 3 3.00 1.959 1.958 O3 -2.00 -2.04 -2.00 3 3.00 1.955 2.207 O4 -2.00 -1.79 -2.15 2 2.99 2.316 1.339 O5 -2.00 -2.05 -1.99 4 4.00 2.702 2.027 O6 -2.00 -1.86 -1.91 3 4.00 2.097 2.097 O7 -2.00 -2.14 -2.08 4 4.00 2.019 2.202 LiKCuP2O7 Li 1.00 0.99 0.97 4 3.79 1.984 1.984 K 1.00 0.98 1.17 9 8.52 2.547 2.870 Cu 2.00 1.95 1.97 5 4.54 2.205 2.033 P1 5.00 4.96 4.97 4 3.85 1.538 1.537 P2 5.00 5.12 4.93 4 3.92 1.542 1.542 O1 -2.00 -2.01 -1.63 4 3.79 2.304 2.040 O2 -2.00 -2.02 -2.02 4 3.93 2.315 2.053 O3 -2.00 -2.02 -2.00 4 3.90 2.266 2.146 O4 -2.00 -1.98 -2.12 2 3.00 2.326 2.081 O5 -2.00 -1.98 -1.73 4 4.00 2.132 2.132 O6 -2.00 -1.99 -1.75 4 4.00 2.128 2.128 O7 -2.00 -1.99 -1.80 4 4.00 2.111 2.286 Rb0.5Na1.5CuP2O7 Rb 0.50 0.47 0.48 8 8.89 3.212 3.214 Na1 0.50 0.48 0.49 5 4.17 2.377 2.378 Na2 1.00 0.92 1.08 7 6.46 2.512 2.512 Cu 2.00 1.78 2.04 5 4.63 2.015 2.015 P1 5.00 4.7 4.98 4 3.83 1.535 1.536 P2 5.00 4.46 5.01 4 3.86 1.538 1.538 O1 -2.00 -2.39 -1.79 3 4.19 2.551 2.398 O2 -2.00 -2.14 -2.02 4 4.79 2.443 2.126 O3 -2.00 -2.03 -2.01 4 4.63 2.204 1.798 O4 -2.00 -1.88 -2.27 3 4.00 2.383 1.898 O5 -2.00 -2.14 -1.94 4 3.00 1.968 2.031 O6 -2.00 -2.47 -1.96 3 4.00 2.284 2.284 O7 -2.00 -2.46 -2.00 3 4.47 2.581 2.266 The obtained results represented in Figures 8 and 9, proof that the compounds are thermally stable and exhibit no phase transition in the explored range of temperature. 3. Results and discussion The compounds LiKCuP2O7 and LiNaCuP2O7 exhibit the same structural arrangement with minor differences. On the other hand, the Rb0.5Na1.5CuP2O7 structure presents some differences that should be explained. 3.1. The structural unit The structural units of the studied compounds are plotted on Figures 10. The PO4 tetrahedra are almost regular, as evidenced by the bonds length and the valence angles grouped in Tables 4 and 5, respectively. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 263 Table 7. Observed vibration frequencies (cm-1) in LiNaCuP2O7 and LiKCuP2O7 and proposed assignments. Observed frequencies Assignments LiNaCuP2O7 LiKCuP2O7 773 775 νs (P-O-P) 914 908 νas (P-O-P) 411-604 408-598 δ (O-P-O) 982-1028 978-1044 ν (P-O) Table 8. Cell parameters and partial cationic occupancy refinements using single crystal and powder X-ray diffraction (GSAS and TOPAS) for compounds LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5CuP2O7. Compound LiNaCuP2O7 LiKCuP2O7 Rb0.5Na1.5CuP2O7 Method Single crystal TOPAS GSAS Single crystal TOPAS GSAS Single crystal TOPAS GSAS Crystal system Monoclinic Space group P21/n a (Å) 4.987(2) 4.992(1) 4.991(9) 5.065(1) 5.072(7) 5.070(6) 7.817(2) 7.826(1) 7.821(3) b (Å) 13.597(4) 13.608(3) 13.606(3) 14.180(2) 14.205(2) 14.199(2) 9.806(3) 9.822(2) 9.816(3) c (Å) 8.293(2) 8.301(2) 8.299(2) 8.473(7) 8.487(1) 8.483(1) 8.644(3) 8.662(2) 8.651(3) β (°) 92.17(4) 92.1664(2) 92.1664(2) 90.189(1) 90.1864(1) 90.189(1) 104.48(2) 104.59(2) 104.48(1) V (Å3) 562.06(6) 563.62(3) 563.28(2) 608.7(5) 611.56(2) 610.811(1) 641.6(3) 644.430(2) 643.144(4) Cationic sites occupancies Li1 1.02(2) 0.939(2) 0.921(2) Li1 1.02(2) 0.994(4) 0.996(4) Na1 0.500(1) 0.498(1) 0.508(3) Na1 0.03(2) 0.061(2) 0.078(2) K1 0.02(2) 0.006(4) 0.004(4) Na2 1.004(9) 0.950(2) 1.015(4) Li2 0.02(5) 0.012(2) 0.015(2) Li2 0.03(3) 0.003(4) 0.003(4) Rb 0.495(2) 0.495(2) 0.502(6) Na2 1.004(5) 0.988(2) 0.984(2) K2 0.997(3) 0.997(4) 0.997(4) Refinement Rint/Rexp (%) 2.70 2.26 2.25 3.10 1.63 1.63 2.40 1.51 2.48 wR/Rwp (%) 5.90 6.90 8.20 5.30 3.60 4.79 7.50 7.34 7.36 R/Rp (%) 1.90 4.20 5.42 1.90 2.60 3.41 2.80 4.63 4.65 GOOF 1.12 3.06 3.64 1.00 2.20 2.94 1.08 2.96 8.79 Figure 3. TOPAS refinement of the LiNaCuP2O7. Figure 4. GSAS refinement of the LiKCuP2O7. The phosphate polyhedra are linked by corner sharing O4 to form diphosphate groups P2O7 assembled with the copper pyramidal square CuO5 by tow vertices O3 and O7. The (CuP2O7)2- units are linked together by vertices forming a corrugated anionic network. 3.2. The diphosphate group Adopting a nearly eclipsed conformation, the O2–P2–P1– O7 dihedral angle is increasing when the alkaline atomic size decreases at the opposite to the bridging angle P2-O4-P1 as listed in Table 9. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 264 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 Table 9. Values of the geometry index of the four compared diphosphates. Compound LiNaCuP2O7 LiKCuP2O7 KNaCuP2O7 Rb0.5Na1.5CuP2O7 P2-O4-P1 bridging angle (°) 118.269(9) 118.871(7) 119.01(11) 123.02(11) O2–P2–P1–O7 dihedral angle (°) 34.71(1) 23.82(1) 15.90(1) 3.85(1) Geometry index τ for CuO5 polyhedron 0.58 0.41 0.26 0.22 Figure 5. TOPAS refinement of the LiKCuP2O7. Figure 6. GSAS refinement of the Rb0.5Na1.5CuP2O7. Figure 7. TOPAS refinement of the Rb0.5Na1.5CuP2O7. The highest value of 123.033(7)° observed for Rb0.5Na1.5Cu P2O7 is comparable to that for NaCsCuP2O7 [19] 122.5(1)° (the largest value is noted for β-Rb2CuP2O7 [20] 130.3(2)°). Within the diphosphate groups the bonds P1-O4 and P2-O4 are the longest since O4 is the bridging oxygen. Every diphosphate group is linked to three copper polyhedra leaving only one free oxygen (O1) pointing towards the empty space where the alkaline cations are lodged. The oxygen O1 charge (V(O1).sof(O1)) of -1.88, -1.63 and -1.79 in the three structures visible on Table 6 means that this oxygen is not involved in the anionic linkage and it’s the first contributor in the cationic environment presenting the shortest distance Li-O, Na-O, K-O and Rb-O (see Table 4). One of the crucial differences between LiNaCuP2O7 (or LiKCuP2O7) and Rb0.5Na1.5CuP2O7 structures is that in the first one, this corner O1 is located in direction of the base of the CuO5 polyhedron (Figure 10a) and in the second one it is located in the apex side (Figure 10b.). 3.3. The copper polyhedron The crystal structures of the title compounds contain one symmetrically independent copper site surrounded by five oxygens describing a distorted square pyramidal coordination. The four oxygens of the base of the pyramid are shared with two diphosphate groups and its apex is linked to a third P2O7 group. Two CuO5 polyhedra are pointing their pyramidal apex in the same direction as shown in Figure 10a. On the other hand, in Rb0.5Na1.5CuP2O7 the same pair of connected copper polyhedra are pointing in opposite direction (Figure 10b). 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 265 Figure 8. Thermal analysis (DTA/TGA) of LiKCuP2O7. Figure 9. Thermal analysis (DTA/TGA) of Rb0.5Na1.5CuP2O7. (a) (b) Figure 10. Structural units in the crystal structures of LiNaCuP2O7 or LiKCuP2O7 (a) and Rb0.5Na1.5CuP2O7 (b). Symmetry code: (i): x-1, y, z; (ii): 0.5+x, 0.5-y, 0.5+z. The described distorted pyramidal copper coordination is confirmed by the (4+1) bonds length and their angular values in Tables 4 and 5. The τ parameter proposed by Addison [21] through the formula: β α α β− = ≈− +τ 0.01667 0.01667 60 was used to find the appropriate polyhedral description of copper. β and α are the two greatest valence angles of the coordination center (β > α) visible in Table 9. When τ is close to zero, the geometry is similar to square pyramidal and if τ is close to one the geometry is similar to trigonal bipyramidal. The computed values of τ parameter for the studied diphosphates and for KNaCuP2O7 [12] listed in Table 8, seem to be closely related to the monovalent cation size. 3.4. The structural packing The 2-D structural arrangement of α-Na2CuP2O7 is adopted by LiNaCuP2O7, LiKCuP2O7 and KNaCuP2O7. The crystal structure is build up by distorted CuO5 square-pyramids linked to nearly eclipsed P2O7 groups by corners sharing leading to infinite chains running along (100) (Figure 11a). These ribbons are connected by the CuO5 square-pyramids apex forming corrugated layers perpendicular to (010) direction (Figure 12). 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 266 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 (a) (b) Figure 11. Different orientations of copper coordination polyhedra along the chains of the anionic network: (a) in LiNaCuP2O7 structure, (b) in Rb0.5Na1.5CuP2O7. Figure 12. Projection of the LiNaCuP2O7 structural model along [100], showing the corrugated interlayer space and zoom on small (S) and large (L) cations anionic environment. The 2-D anionic stacking leaves interlayer empty space where the largest cations (L) are lodged and ellipsoidal cavities delimited by the vertices of the P2O7 and CuO5 polyhedra housing the smallest cations (S). Figure 12 represents the structure projection along (100) direction where the crystallographic sites L and S are shown. The crystal structure of the Rb0.5Na1.5CuP2O7 is also build up by distorted CuO5 square-pyramids linked to P2O7 groups by corners sharing leading to infinite chains running along (101) (Figure 11b). At the opposite of the LiNaCuP2O7, where the copper square pyramid polyhedra apex are oriented in a common sense in this structure they are pointing in opposite direction. Thus, larger spaces are created able to lodge the Rb+. This double orientation leads to building undulated 2-D anionic framework and leaves empty interlayer space where and Na+ are lodged. A structure projection is plotted on Figure 13. 3.5. The cationic sites occupancies The main important result deduced from this study and that of KNaCuP2O7 is the identification of two kinds of cationic sites in the structure. The first, located in the interlayer space, is booked for large cations (L) and the second for smaller one (S), having an ellipsoidal shape and delimited by the anionic network. Similar results related to the compounds with general formula I III 3 3 4 4A M (XO ) (AI = Alkali metal, MIII = Al, Cr, Fe, … X = As, P) were published by our team [22]. Nevertheless, in the structure of Rb0.5Na1.5CuP2O7 the interlayer space is more regular, thus the large Rb+ cation is lodged on a special position in large intersecting channels of the anionic framework with 50% occupancy. As observed in the studied compounds, the partial cationic site occupancy is a promising structural property offering a large interlayer space where the cationic mobility is possible. Thus, they can be potential candidates for interesting energy storage and conduction behavior. The complex impedance measurements of these compounds are underwork. 3.6. The structural filiation Based on the content of the Table 10, a comparative study can be done between the members of the alkaline copper diphosphate family. The structures of LiNaCuP2O7, LiKCuP2O7, KNaCuP2O7 [12] and Rb0.5Na1.5CuP2O7 are similar to that of α- Na2CuP2O7 [3]. Nevertheless, increasing the alkali cation size seems to introduce further symmetry, indeed, the Rb+ cation adopts a special position in the structure of Rb0.5Na1.5CuP2O7. Accordingly, to the anionic 2D network topology, built up by linked (CuP2O7)2- units, the three studied compounds can be classified into the same group of selected copper diphosphates. 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 267 Table 10. Crystallographic data and topological type of the [CuP2O7]2– anionic network for copper diphosphates MM’CuP2O7 (M and M’ = Monovalent metal). Compound Space group a (Å) b (Å) c (Å) Cell Volume (Å3) Topology of the anionic network References β (°) Band structures Li2CuP2O7 I2/a 14.068(2) 4.8600(8) 8.604(1) 581.07(8) 1D [23] 98.97(1) Li2CuP2O7 C2/c 15.336(1) 4.873(1) 8.626(2) 585.24(16) 1D [24] 114.80(1) β-Na2CuP2O7 C2/c 14.728(1) 5.698(2) 8.067(1) 612.88(2) 1D [25] 115.15(1) Na1.12Ag0.88CuP2O7 C2/c 15.088(2) 5.641(1) 8.171(1) 624.48(2) 1D [26] 116.11(1) Layered structures LiNaCuP2O7 P21/n 4.988(2) 13.598(4) 8.294(2) 562.1(3) 2D This work 92.17(4) α-Na2CuP2O7 P21/n 8.823(3) 13.494(3) 5.108(2) 607.44(3) 2D [3] 92.77(3) LiKCuP2O7 P21/n 5.065(1) 14.180(2) 8.473(7) 608.7(5) 2D This work 90.19(1) KNaCuP2O7 P21/n 5.176(3) 13.972(5) 9.067(3) 655.6 (5) 2D [12] 91.34(2) Rb0.5Na1.5CuP2O7 P21/n 7.811(2) 9.789(3) 8.636(3) 641.6(3) 2D This work 104.60(2) K2CuP2O7 Pbnm 9.509(4) 14.389(6) 5.276(2) 722.0(6) 2D [4] K2CuP2O7 P421m 8.056(2) - 5.460(11) 354.35 (11) 2D [27] NaCsCuP2O7 Pmn21 5.147(2) 15.126(3) 9.717(5) 756.5(5) 2D [19] Framework structures α-Rb2CuP2O7 Pmcn 5.183(8) 10.096(1) 15.146(3) 792.5(2) 3D [28] β-Rb2CuP2O7 Cc 7.002(1) 12.751(3) 9.773(2) 815.0(3) 3D [20] 110.93(3) Cs2CuP2O7 Cc 7.460(6) 12.973(1) 9.980(8) 895.8(12) 3D [29] 111.95(1) Figure 13. 2-D crystal structure of Rb0.5Na1.5CuP2O7 projected along [101] direction with details of Rb+ and Na+ cations environments. The character of binding of these structural units depends on the alkali metal cation and leads to various structural patterns. The increasing of the cationic size seems opening the diphosphate bridging angle and its dihedral angle. It contributes to reduce the copper coordination polyhedron distortion leading to slightly regular tetragonal pyramid as confirmed by τ values visible in Table 10. These effects are visible in Figures 12 and 13. Indeed, an attenuation of the interlayer space and the anionic sheets corrugation is clearly observed. 4. Conclusion Pure phases of LiNaCuP2O7, LiKCuP2O7 and Rb0.5Na1.5 CuP2O7 were synthesized by solid state reactions. Single crystal X-ray diffraction investigations revealed that these compounds crystallize in the P21/n monoclinic space group and containing nearly eclipsed P2O7 diphosphate groups sharing corners with distorted pyramidal square CuO5 arranged in layered stacking based on two-dimensional anionic network. Small cations prefer crystallographic site located in the interlayer space. Large cations are located in open ellipsoidal cavities situated in the intersection of empty channel delimited by the anionic network. The increasing of the cationic size seems opening the diphosphate bridging angle and its dihedral angle, reducing the copper coordination polyhedron distortion and attenuating the interlayer space and the anionic sheets corrugation. DTA and TGA results revealed the high thermal stability and the absence of any solid state phase transition. The shift observed in the main infrared vibration frequencies of the studied compounds 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 268 Fitouri and Boughzala / European Journal of Chemistry 9 (3) (2018) 258-268 confirms the crystallographic study. Using the structural data of the three compounds, the cell parameters and the cationic sites occupancies refinement using the Rietveld analysis leads to the same results obtained by the single crystal diffraction study. The same study proofs the high purity of the synthe- sized samples. Acknowledgments We acknowledge the assistance of the staff of the Tunisian Laboratory of Materials and Crystallography for the data collection. Supplementary data Further details of the crystal structures investigation are available from the Fachinformationszentrum Karlsruhe, D- 76344 Eggenstein-Leopoldshafen, Germany, on quoting the depository number CSD-433889 for the compound LiKCuP2O7, CSD-433890 for LiNaCuP2O7 and CSD-434654 for Rb0.5Na1.5 CuP2O7 the names of the authors and the citation of the paper. Disclosure statement Conflict of interests: The authors declare that they have no conflict of interest. Author contributions: All authors contributed equally to this work. Ethical approval: All ethical guidelines have been adhered. Sample availability: Samples of the compounds are available from the author. Funding Funding for this research was provided by the Tunisian ministry of high education and scientific research. http://www.mes.tn/ ORCID Ines Fitouri http://orcid.org/0000-0001-6428-3909 Habib Boughzala http://orcid.org/0000-0002-7177-9934 References [1]. Dupre, N.; Gaubicher, J.; Le Mercrier, T.; Wallez, G.; Angenault, J.; Quarton. M. Solid State Ionics 2001, 140, 209-221. [2]. Kurek, P.; Nowinski, J. L.; Jakubowski, W. Solid State Ionics 1989, 36, 243-246. [3]. Etheredge, K. M. S.; Hwu, S. J. Inorg. Chem. 1995, 34, 1495-1499. [4]. ElMaadi, A.; Boukhari, A.; Holt, E. M. J. Alloys Compd. 1995, 223, 13- 17. [5]. Prabaharan, S. R. S.; Michael, M. S.; Radhakrishna, S.; Julien, C. J. Mater. Chem. 1997, 7, 1791-1796. [6]. Kim, S. W.; Seo, D. H.; Ma, X.; Ceder, G.; Kang, K. Adv. Energy Mater. 2012, 2, 710-721. [7]. Coelho, A. A. 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V.; Spiridonova, D. V. Book of Abstracts VI International Conference “Inorganic Materials” Dresden: Elsevier, 2008, 3-143. [29]. Mannasova, A. A.; Chernyatieva, A. P.; Krivovichev, S. V. Z. Kristallogr. 2016, 231, 65-69. Copyright © 2018 by Authors. This work is published and licensed by Atlanta Publishing House LLC, Atlanta, GA, USA. The full terms of this license are available at http://www.eurjchem.com/index.php/eurjchem/pages/view/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). 2018 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.9.3.258-268.1762 http://www.mes.tn/ http://orcid.org/0000-0001-6428-3909 http://orcid.org/0000-0002-7177-9934 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Synthesis and crystallization 2.2. X‐ray structure determination 2.3. Infrared spectroscopy 2.4. Powder diffraction 2.5. Thermal analysis (DTA/TGA) 3. Results and discussion 3.1. The structural unit 3.2. The diphosphate group 3.3. The copper polyhedron 3.4. The structural packing 3.5. The cationic sites occupancies 3.6. The structural filiation 4. Conclusion Acknowledgments Supplementary data Disclosure statement Funding ORCID References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField19: PrintField110: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: PrintField29: PrintField210: