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Institute
The synthesis of the phosphide telluride Zr2+xPTe2 was accomplished by a solid-state reaction from the elements. Le Bail refinements of the as synthesized crystalline powders as well as the thermal decomposition of Zr2PTe2 along the homogeneity range Zr2+xPTe2 under the release of P4(g) and Te2(g) evidences a maximum zirconium content according to the composition Zr2.5PTe2. The thermal decomposition product of Zr2.5PTe2 undergoes a phase
transformation to “Zr2PTe” adopting the structure motive of the binary phases ZrTe and ZrTe2. The new phase “Zr2PTe” has a wide homogeneity range Zr2–xP1–yTe1+y tolerating a deficit in the cation position and a mixed occupation of the anions. The composition of the crystalline decomposition product was determined to Zr1.95P0.84Te1.16 by Rietveld refinement and by analysing the elemental composition with ICP-OES. Zr1.95P0.84Te1.16 crystallizes in hexagonal space group P63/mmc (no. 194) with lattice constants: a = 3.8726(1) Å and c = 13.008(1) Å.
Phase barograms – phased diagrams of vapour pressure: Eutectoid Phase Formation in Binary Systems
(2012)
Phase barograms are specific representations of conventional (x, T) phase diagrams considering the vapor pressure as additional thermodynamic parameter. Thus, the determination of the vapor pressure is complementary to conventional DTA or DSC measurements. A stringent relationship between the phase diagram and the corresponding phase barogram is derived by thermodynamic considerations of phase relations for complex heterogeneous equilibria. For that purpose, a derivative of the three-dimensional Clausius–Clapeyron equation is utilized. As a result, a validation of DTA/DSC measurements becomes feasible by the transformation of characteristic effects of vapor pressure measurements into thermal effects in conventional phase diagrams.
The system CsBr/SeO2 is presented as a case study of systems with a eutectoid phase formation. CsSeO2Br is formed from the binaries at ϑf = 180(5) °C; the compound decomposes peritectically at ϑp = 470(10) °C.
Inorganic solid-state synthesis with phosphorus and tellurium requires a careful control of the reaction parameters because of the high volatility of the components. This initial disadvantage can be used as a benefit for the investigation of phase formation mechanisms by analyzing the individual vapor pressure behavior. The high-temperature gas-balance is introduced as a device for detection of heterogeneous solid-gas equilibria in closed reaction systems. The experimentally challenging synthesis of the phosphide telluride Zr₂PTe₂ is examined as a model system: optimized synthesis runs at lower temperatures (ϑ = 650 °C) in a faster time, while the quantity as well as the crystalline powder quality is increased. A stepwise solid-solid reaction of zirconium and tellurium according to Ostwald's rule of stages and the shrinking core model is revealed while phosphorus sublimes and subsequently condenses to react to the ternary compound. Additional phenomena such as melting, expansion, and mechanical instabilities can be observed that broaden the possibilities of the gas- balance.