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The α↔β phase transitions of Zn 2 P 2 O 7 revisited: existence of an additional intermediate phase with an incommensurately modulated structure
Author(s) -
Stöger Berthold,
Weil Matthias,
Dušek Michal
Publication year - 2014
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.604
H-Index - 33
ISSN - 2052-5206
DOI - 10.1107/s205252061401049x
Subject(s) - phase transition , crystallography , superstructure , differential scanning calorimetry , phase (matter) , chemistry , crystal structure , diffraction , materials science , condensed matter physics , physics , thermodynamics , optics , organic chemistry
Zn 2 P 2 O 7 crystallizes in a thortveitite‐like structure and features temperature‐dependent polymorphism. At high temperatures ( T > 500 K), the aristotype phase β‐Zn 2 P 2 O 7 ( C 2/ m , Z = 2, a ≃ 6.60, b ≃ 8.28, c ≃ 4.53 Å, β≃ 105.4°) is stable. At lower temperatures the lock‐in phase α 1 ‐Zn 2 P 2 O 7 [at 350 K: I 2/ c , Z = 12, a = 20.1131 (13), b = 8.2769 (6), c = 9.109 (3) Å, β = 106.338 (16)°], a sixfold superstructure with commensurate modulation vector q = (1/3, 0, 1/2), is stable. Between the stability ranges of the α 1 ‐ and β‐phases exists the intermediate, incommensurately modulated α 2 ‐Zn 2 P 2 O 7 phase with modulation wavevector q ≃ (0.33, 0, 0.40) and C 2/ m (α, 0, γ)0 s superspace group symmetry. The α 1 →α 2 lock‐in phase transition at T L = 408 K is of first‐order and features virtually no hysteresis. It is immediately followed by the second‐order α 2 →β transition to the non‐modulated phase at T I ≃ 430 K. This transformation is sluggish and even at T = 500 K very weak satellite reflections of the α 2 ‐phase were observed. Both phase transitions were analyzed with differential scanning calorimetry and high‐temperature powder and single‐crystal X‐ray diffraction. The crystal structures of the α 1 ‐ and α 2 ‐phases were refined from single crystal data collected at T = 350, 400, 405, 410, 415, 420, 425, 430, 450 and 500 K. Different models describing the slow transition from the incommensurately modulated α 2 ‐ to the non‐modulated β‐phase were tested. In the model resulting in the best residuals, the bridging O atom of the [P 2 O 7 ] group, which is located on a 2/ m position in the basic structure, is described as an overlap of an atom ordered in internal space and one atom disordered around the mirror plane. The occupancy of the ordered atom decreases with temperature until at T = 500 K virtually only the disordered atom remains. Simultaneously, the amplitude of the modulation functions of the remaining atoms decreases, so that the T = 500 K structure can be considered as the C 2/ m aristotype structure, although the diffraction pattern still features satellite reflections of first order with very low intensities.