Elsevier

Thermochimica Acta

Volume 543, 10 September 2012, Pages 267-272
Thermochimica Acta

Synthesis and thermal decomposition of scandium hydrogenphosphite Sc2(HPO3)3

https://doi.org/10.1016/j.tca.2012.06.005Get rights and content

Abstract

Simultaneous constant-rate thermogravimetry–difference thermal analysis (TG–DTA) and thermogravimetry–mass spectrometry (TG–MS) of scandium hydrogenphosphite Sc2(HPIIIO3)3 (space group P63/m) were conducted under inert conditions in flowing argon up to 1300 °C. The first significant mass loss step detected between 700 °C and 800 °C is mainly caused by the release of hydrogen resulting in the formation of optical transparent, red nano crystalline agglomerates. Further heating leads to disproportionation into phosphorus vapor and phosphates, namely ScPVO4 (space group I41/amd) and Sc(PVO3)3 (space group Cc).

Highlights

▸ TG–DTA and TG–MS of hydrogenphosphite under inert conditions. ▸ On heating, consecutively release of hydrogen and phosphorus accompanied by the formation of crystalline phosphates, ScPO4 and Sc(PO3)3. ▸ After hydrogen release intermediate formation of red-transparent nano crystalline aggregates.

Introduction

As a continuation of our systematic studies on the crystal chemistry of scandium borophosphates [1], [2], [3], [4], [5], we have recently addressed the question of the possible replacement of PVO4 units by HPIIIO3. These investigations also included the exploration of the constituting systems consisting of phosphate or phosphite, Sc2O3, and MOx (M = alkali or alkaline-earth metal), only.

In the course of our investigations the synthesis and structure determination of the complex scandium phosphite NaSc3[HPIIIO3]2[HPIIIO2(OH)]6 was conducted along with a thermochemical analysis [6]. It turned out that, compared to phosphates little is known about the thermal decomposition of phosphites. One reason is, that for reliable data on the thermal behavior of phosphites the control and knowledge of oxygen and water partial pressures in the thermoanalyser has to be assured. In fact this is true for many reactions although attention to this issue is less developed. Especially at higher temperatures as common for the investigation of inorganic solids this can be a challenging task. Therefore, we decided to install thermoanalysers in argon filled glove boxes allowing simultaneous thermogravimetric, heat flow, and mass spectroscopic investigations under well-defined conditions [7].

To shed more light on the thermal behavior of phosphites Sc2(HPIIIO3)3 [8] was selected because it is chemically less complex than the aforementioned NaSc3[HPIIIO3]2[HPIIIO2(OH)]6 [6]. Furthermore, reports on the thermochemical behavior of Sc2(HPO3)3·xH2O and Sc2(HPO3)3 in air and vacuum were available [9], [10], [11], [12], [13] and first ideas were developed concerning the possible gaseous products and the reaction mechanisms in the respective environments. In order to complete these studies, we decided to conduct experiments under inert conditions to limit the number of additional reactions during decomposition. Furthermore, the analysis of the evolved gases by mass spectrometry should give direct experimental evidence of the reactions involved. Also, the question was addressed, whether Sc2(HPIIIO3)3 can be used as a precursor for the preparation of new compounds.

Herein, we report on the modified hydrothermal synthesis and a comprehensive experimental study on the thermal decomposition of Sc2(HPIIIO3)3 by TG–DTA/TG–MS under inert conditions. Furthermore, first results on the intermediately formed phases are also given.

Section snippets

Synthesis

Sc2(HPO3)3 was prepared under mild hydrothermal conditions [8]. ScCl3·xH2O (0.259 g, Chempur, 99.9%), H3PO3 (0.246 g, Acros Organics, 98%) and deionized water (4 mL) were mixed together under constant stirring followed by transfer to a 10 mL Teflon-lined autoclave. After a reaction time of 4 days at 180 °C under autogenous pressure colorless crystals were obtained, collected by filtration and washed with water, and then dried at 50 °C.

Chemical analyses

The scandium and phosphorous content were determined by

Synthesis and characterization

Sc2(HPO3)3 was prepared under mild hydrothermal conditions as already described [8]. However, educts and temperature program were optimized. The sodium-containing hypophosphite source NaH2PIO2·H2O was modified by free acid H3PO3 treatment and the reported use of sodium borate was not mandatory for the formation of Sc2(HPO3)3. The reaction time was shortened from reported 58 days to 4 days.

The PXRD pattern of the powdered reaction product is shown in Fig. 1. The Rietveld fit [14] indicates the

Summary

Concerning the thermal decomposition of Sc2(HPO3)3, the following conclusions can be drawn from dynamic TG–DTA and TG–MS measurements. The experiments show that oxidation of phosphite to phosphate takes place during heating as indicated by the formation of ScPVO4 and Sc(PVO3)3. The thermal decomposition process up to 1300 °C can be described as 2Sc2(HPO3)3 → 3H2 + 3.6ScPO4 + 0.4Sc(PO3)3 + 1.2P. According to this reaction, a total weight loss of −6.5 wt% is expected, which is in fair agreement

Acknowledgments

We thank Mrs. Susann Scharsach for DTA/TG and TG–MS measurements, Sebastian Schwinger and Dr. Gudrun Auffermann for chemical analyses, Mrs. Petra Scheppan for EDXS analyses and Mr. Steffen Hückmann for PXRD.

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