Structure-property correlation over five phases and four transitions in Pb5Al3F19

• Published in: Acta crystallographica. Section B, Structural science Vol. 59; no. 5; pp. 557 - 574
• Main Authors: Abrahams, S. C., Ravez, J., Ritter, H., Ihringer, J.
• Format: Journal Article
• Language: English
• Published: 5 Abbey Square, Chester, Cheshire CH1 2HU, England International Union of Crystallography 01-10-2003; Blackwell
• Subjects: calorimetry; Chemical Sciences; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Crystallographic aspects of phase transformations; pressure effects; dielectric properties; Dielectric, piezoelectric, ferroelectric and antiferroelectric materials; Dielectrics, piezoelectrics, and ferroelectrics and their properties; Elements, oxides, nitrides, borides, carbides, chalcogenides, etc; Exact sciences and technology; Ferroelectricity and antiferroelectricity; Material chemistry; phase transitions; Phase transitions and curie point; Physics; Structure of solids and liquids; crystallography; structure-property correlation; Ferroelastic transformation; Inorganic compounds; Phase transformations; Aluminium fluorides; Entropy; Experimental study; Ion distribution; Lead fluorides; Ferroelectric transitions; Atomic displacements; Substitutional impurities; Impurity effect; Antiferroelectric transitions; Thermal hysteresis; Property structure relationship; Thermodynamic properties; Calorimetry; Structure property correlation; Phase transitions; Dielectric properties
• ISSN: 0108-7681; 1600-5740
• DOI: 10.1107/S0108768103011509

The calorimetric and dielectric properties of Pb5Al3F19 in the five phases stable under ambient pressure are correlated with structure for fuller characterization of each phase. The first‐order transition between ferroelectric phase V and antiferroelectric phase IV at TV,IV = 260 (5) K exhibits a thermal hysteresis of 135 (5) K on heating, with a maximum atomic displacement Δ(xyz)max = 1.21 (6) Å; the transition from phase IV to ferroelastic phase III at 315 (5) K is also first order but with a thermal hysteresis of 10 (5) K and Δ(xyz)max = 0.92 (7)  Å; that from phase III to paraelastic phase II at 360 (5) K is second order without hysteresis and has Δ(xyz)max = 0.69 (4) Å; and the transition from phase II to paraelectric phase I at 670 (5) K is second or higher order, with Δ(xyz)max = 0.7 (4) Å. The measured entropy change ΔS at TV,IV agrees well with ΔS as derived from the increased configurational energy by Stirling's approximation. For all other phase transitions, 0.5 ≥ΔS > 0 J mol−1 K−1 is consistent with an entropy change caused primarily by the changes in the vibrational energy. The structure of phase III is determined both by group theoretical/normal mode analysis and by consideration of the structures of phases II, IV and V reported previously; refinement is by simultaneous Rietveld analysis of the X‐ray and neutron diffraction powder profiles. The structure of prototypic phase I is predicted on the basis of the atomic arrangement in phases II, III, IV and V. The introduction of 3d electrons into the Pb5Al3F19 lattice disturbs the structural equilibrium, the addition of 0.04% Cr3+ causing significant changes in atomic positions and increasing TIV,III by ∼15 K. Substitution of Al3+ by 20% or more Cr3+ eliminates the potential minima that otherwise stabilize phases IV, III and II.