Crystal structures and high-temperature phase-transitions in SrNdMRuO6 (M=Zn,Co,Mg,Ni) new double perovskites studied by symmetry-mode analysis

2013 ◽  
Vol 198 ◽  
pp. 24-38 ◽  
Author(s):  
E. Iturbe-Zabalo ◽  
J.M. Igartua ◽  
A. Faik ◽  
A. Larrañaga ◽  
M. Hoelzel ◽  
...  
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Crystal Structures ◽  
High Temperature Phase ◽  
Mode Analysis ◽  
Temperature Phase ◽  
Double Perovskites ◽  
Symmetry Mode

Mode-crystallography analysis of the crystal structures and the low- and high-temperature phase transitions in Na0.5K0.5NbO3

2015 ◽  
Vol 48 (2) ◽  
pp. 318-333 ◽  
Author(s):  
B. Orayech ◽  
A. Faik ◽  
G. A. López ◽  
O. Fabelo ◽  
J. M. Igartua
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Crystal Structures ◽  
Degrees Of Freedom ◽  
High Temperature Phase ◽  
Structure Evolution ◽  
Internal Space ◽  
Temperature Phase ◽  
Conventional Solid State Reaction ◽  
Space Group

Na0.5K0.5NbO3has been synthesized by the conventional solid-state reaction process. The crystal structures and phase transitions, at low and high temperature, determined from the Rietveld refinements of X-ray and neutron powder diffraction data are reported. The structure evolution of Na0.5K0.5NbO3in the temperature range from 2 to 875 K shows the presence of three phase transitions. The first one, at ∼135 K, is discontinuous from the rhombohedralR3c(No. 161) space group to the room-temperature orthorhombicAmm2 (No. 38) space group; the second is discontinuous from the orthorhombic to the tetragonalP4mmspace group (No. 99) at ∼465 K, and the third is continuous from the tetragonal to the cubic Pm\overline{3}m space group (No. 221) at ∼700 K. The obtained phase-transition sequence isR3c→Amm2 →P4mm→Pm\overline{3}m. No previous studies at low temperature have been carried out on the material with composition Na0.5K0.5NbO3. In the course of the determination of the three experimentally found phases, a novel method of refinement is presented. This is a step forward in the use of the symmetry-adapted modes as degrees of freedom in the refinement process: the parameterization of a direction in the internal space of the, in this case, sole irreducible representation, GM4−, responsible for the symmetry breaking from the parent cubic space group to the polar distorted low-symmetry phases. Eventually, this procedure enables the calculation of the spontaneous polarization.

scholarly journals Orientational order-disorder γ ↔ β ↔ α′ ↔ α phase transitions in Sr2B2O5 pyroborate and crystal structures of β and α phases

2017 ◽  
Vol 73 (6) ◽  
pp. 1056-1067 ◽  
Author(s):  
Sergey Volkov ◽  
Michal Dušek ◽  
Rimma Bubnova ◽  
Maria Krzhizhanovskaya ◽  
Valery Ugolkov ◽  
...  
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Crystal Structures ◽  
Intermediate Phase ◽  
Orientational Order ◽  
High Temperature Phase ◽  
Basic Unit ◽  
Temperature Phase ◽  
Scanning Calorimetry ◽  
X Ray

Crystal structures of γ-, β- and α-Sr2B2O5 polymorphs resulting from the γ ↔ (at 565 K) β ↔ (at 637 K) α′ ↔ (at 651 K) α sequence of reversible first-order phase transitions are studied by high-temperature single-crystal X-ray diffraction, high-temperature X-ray powder diffraction, differential scanning calorimetry and impedance spectroscopy. Out of these phases, the structure of γ-Sr2B2O5 was already known whereas the structures of β- and α-Sr2B2O5 were determined for the first time. The sequence of phase transitions is associated with an unusual change of symmetry, with triclinic intermediate β-Sr2B2O5 phase and monoclinic low-temperature γ-Sr2B2O5 as well as high-temperature α-Sr2B2O5 phase. Taking the α-Sr2B2O5 phase with space group P21/c as a parent structure, the γ-Sr2B2O5 phase was refined as a twofold superstructure with symmetry P21/c, whereas the β-Sr2B2O5 phase was a sixfold superstructure with symmetry P{\overline 1}. To construct a unified structure model for all Sr2B2O5 modifications, phases of γ- and β-Sr2B2O5 were also refined as commensurately modulated structures using the basic unit cell of the parent α-Sr2B2O5. The phase transitions are related to the orientational order–disorder arrangement of B2O5 pyroborate groups, where the degree of disorder grows towards the high-temperature phase. Thermal expansion is strongly anisotropic and dictated by preferable orientations of BO3 triangles in the structure. The intermediate phase α′-Sr2B2O5, stable over a narrow temperature range (637–651 K), features the largest anisotropy of expansion for the known borates: α11 = 205, α22 = 57, α33 = −81 × 10−6 K−1.

High temperature phase transitions and proton conductivity in some kdp-family crystals

1989 ◽  
Vol 100 (1) ◽  
pp. 135-141 ◽  
Author(s):  
A. I. Baranov ◽  
V. P. Khiznichenko ◽  
L. A. Shuvalov
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
Proton Conductivity ◽  
High Temperature Phase ◽  
Temperature Phase

scholarly journals High-temperature phase transitions in dense germanium

2021 ◽  
Vol 154 (17) ◽  
pp. 174702
Author(s):  
Liam C. Kelsall ◽  
Miriam Peña-Alvarez ◽  
Miguel Martinez-Canales ◽  
Jack Binns ◽  
Chris J. Pickard ◽  
...  
Keyword(s):  
Phase Transitions ◽  
High Temperature ◽  
High Temperature Phase ◽  
Temperature Phase
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