Abstract
High temperature X-ray powder diffraction measurements of rubidiumpolyphosphate, Rb2{∞1} [P2O6] were carried out with a position sensitive detector (PSD) and analysed by the Rietveld method. Three new high temperature phases were identified. The monoclinic [P21/n,a = 12.1249(3) Å, b = 4.2279(1) Å, c = 6.4761(2) Å, β = 96.006(2)° room-temperature modification RbPO3-T, transforms at Tc = 661 K to orthorhombic RbPO3-H (Pbnm) with the lattice constants a = 13.1674(4) Å, b = 4.5657(1) å and c = 6.1183(2) å (750 K). A continuous transition at Tc = 914 K leads to RbPO3-HT (Bbmm, 940 K) with a = 13.3517(4) Å, b = 4.5917(1) Å and c = 6.1665(2) Å, for which the empirical power law behaviour of the order parameter Q is Q ∼ (T−Tc)β with β near 0.2 for Tc > T > 0.8Tc. There are saturation effects below 0.8TC.
The T→H transition shows a hysteresis of ca. 110 K and on cooling an intermediate phase, RbPO3-Z, occurs at 591 K, with space group P21/n, a = 14.3476(5) Å, b = 4.5730(1) å, c = 12.0839(4) Å and β = 65.604(2)[unk], which transforms back to the RbPO3-T state at 550 K.
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