Elsevier

Solid State Sciences

Volume 5, Issues 11–12, November–December 2003, Pages 1439-1444
Solid State Sciences

Sodium dithiophosphate(V): Crystal structure, sodium ionic conductivity and dismutation

https://doi.org/10.1016/j.solidstatesciences.2003.07.002Get rights and content

Abstract

Na3PO2S2 was synthesized from the corresponding undecahydrate by freeze–drying. It dismutates at temperatures above 350 °C into Na3PO3S and Na3POS3. The crystal structure of Na3PO2S2 was determined from X-ray powder diffraction data, and refined using the Rietveld technique (Pbca, a=17.5359(2) Å, b=11.3044(1) Å, c=5.8656(1) Å, RF2=8.45%, Rp=4.80%, Rwp=6.79%). The baricenters of the PO2S2-tetrahedra are arranged in the sense of a hexagonal close packing. One of the PO bonds of these tetrahedra is oriented parallel to the crystallographic c-axis with the POS2-groups in an eclipsed mutual orientation. The crystal structure of Na3PO2S2 displays a close relationship to the ones of Ca3CrN3 and Na3POS3. Above 330 °C, Na3PO2S2 can be regarded as a fast ionic conductor (with σ>10−5 S cm−1).

Introduction

Upon heating, most salts consisting of complex anions undergo at least one structural phase transition to rotationally disordered modifications, accompanied by a sharp increase of the cationic conductivity. Empirical studies [1] showed, that this behavior is caused by the “volume effect” as well as by the “paddle-wheel mechanisms.” It is evident, that an increased molar volume of the high temperature phases of the so called rotator phases improves the mobility of the cations (volume effect [2]). The cationic conduction is also enhanced by the rotations of the complex ions that flatten the potential barriers along the pathways of the ions (paddle-wheel mechanisms [3], [4]). Furthermore, recent investigations have revealed, that there is a strong correlation between the rotational and translational motion in the rotator phase Na3PO4 [5], [6], [7], [8], [9], [10], [11], [12].

Beside these effects, other influences on the ionic conductivity need to be considered. One of these is the polarizability of the immobile anionic matrix. The compounds Na3PO4−xSx (x=0–4) offer a convenient access to explore the effect of an increasing substitution for oxygen atoms by the higher polarizable sulfur atoms. So far, we have investigated the crystal structures as well as the temperature-dependent properties and ionic conductivities of Na3PO4 [13], [14], [15], Na3PO3S [16], Na3POS3 [17] and Na3PS4 [18]. In this report, we complete our study with the oxothiophosphate Na3PO2S2.

First indications of the existence of sodium oxothiophosphates were obtained by Berzelius in 1843 [19]. In 1885 Kubierschky reported on the synthesis of Na3PO2S2·11H2O [20]. The anhydride was later studied by Tribodet in 1965 [21]. In the same year, Ladwig published the positions of the reflections in the X-ray powder diffraction pattern without indexing it [22]. Powell and Scott published the Raman data in 1972 [23]. The bond angles and lengths of the anion PO2S23− are known from the single crystal structure determination of NdPO2S2·5.5H2O [24] and of Na3PO2S2·11H2O [25].

Here, we present the crystal structure of Na3PO2S2, which can be considered as the “missing link” among the structures of the series Na3PO4−xSx (x=0–4). Furthermore, we complete our study on the sodium ion conductivities of these substances.

Section snippets

Results and discussion

The title compound was dehydrated by freeze–drying of the corresponding undecahydrate, which is synthesized by controlled alkaline hydrolysis of tetraphosphorusdecasulfide. Traces of water were subsequently removed by heating in vacuum. This smooth method yields a pure product, almost free of by-products. It is colorless, odorless, and air-sensitive.

Na3PO2S2 dismutates at 430 °C into Na3PO3S and Na3POS3 according to DSC measurements [26]. The dismutation can already be observed at 350 °C if the

Synthesis

At first, Na3PO2S2·11H2O was synthesized as described elsewhere [25], based on the prescription of Kubierschky [20], [32]. After twofold recrystallization from water (solution at 20 °C, crystallization at 0 °C) the substance was subjected to a freeze–drying procedure. Subsequently, residual traces of water were removed by heating the sample at 120 °C in vacuum (p=10−3 mbar) for two days. Na3PO2S2 was handled in an atmosphere of dry argon and annealed at 300 °C for 24 days to improve crystallinity.

Thermal investigations

Supplementary material

The supplementary material has been sent to the Fachinformationszentrum Karlsruhe, Abt. PROKA, 76344 Eggenstein-Leopoldshafen, Germany, as supplementary material No. SUP 413043 and can be obtained by contacting the FIZ (quoting the article details and the corresponding SUP number).

Acknowledgments

Financial support by the Deutsche Forschungsgemeinschaft (DFG), the Fonds der Chemischen Industrie (FCI) and the Bundesministerium für Bildung und Forschung (BMBF) is gratefully acknowledged.

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