Structure cristalline de l'hydrogéno-pyrophosphate tripotassique trihydraté, K3HP2O7.3H2O

Dumas, Y.; Galigné, J. L.; Falgueirettes, J.

doi:10.1107/s0567740873005157

Cited by 9 publications

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“…In contrast to for example CsH 2 PO 4 , which dehydrates via a Cs 2 H 2 P 2 O 7 (=̂ CsPO 3 ·(1/2)H 2 O) intermediate, we could not identify any crystalline intermediate hydrous phase: To our knowledge no structural data of KAsO 3 · n H 2 O phases with 0 < n < 1 have been reported up to now, and attempts to relate the peaks that we did not attribute to γ-KAsO 3 to the diffraction patterns of the hydrous phosphate phases K 2 H 4 P 2 O 7 and K 2 H 4 P 2 O 7 ·H 2 O were unsuccessful. Thermogravimetric analysis confirms the lack of intermediate phases: Even with low heating rates of 1 K/min, we did not observe any steps in the dehydration curve (Figure ).…”

Section: Resultscontrasting

confidence: 65%

Complex Polymorphism and Polytypism of Potassium Metaarsenate, KAsO₃

Stöger

Dušek

2014

Crystal Growth & Design

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The dehydration of KH 2 AsO 4 to KAsO 3 and the complex temperaturedependent polymorphism of KAsO 3 were investigated by high-temperature X-ray powder diffraction. Four of the five KAsO 3 polymorphs were structurally characterized by single crystal or powder X-ray diffraction and are described with attention to polytypism and structurally related phases. On heating, KH 2 AsO 4 subsequently transforms into γ-(150−200 °C), β-(370− 410 °C) and finally α-KAsO 3 (500−510 °C) before melting at 640−650 °C. The dehydration is accompanied by formation of minor amounts of δand β-KAsO 3 . On cooling, α-KAsO 3 subsequently transforms into δ-KAsO 3 (320−300 °C) and γ-KAsO 3 (starting at 270 °C). The α → β transformation was only observed on prolonged heating at 470 °C. The β-KAsO 3 phase reversibly transforms into the metastable β′-KAsO 3 below 250 °C. β-KAsO 3 can be stabilized at lower temperatures by partial substitution of Rb for K. αand γ-KAsO 3 are long-chain polyarsenates with repetition period 2. The crystal structures can be derived from H-MPO 3 (M = K, Rb) and the pyroxene orthoenstatite, respectively. The βand β′-KAsO 3 phases feature cyclotriarsenate units with approximately 3m (β-KAsO 3 ) and 2 (β′-KAsO 3 ) symmetry. Both βand β′-KAsO 3 , are polytypic order−disorder structures composed of nonpolar layers of one kind. The major polytypes are of a maximum degree of order. Alternative stacking sequences are observed by twinning.

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Section: Resultscontrasting

confidence: 65%