Structural changes in Sr9In(PO4)7 during antiferroelectric phase transition

Structural changes in Sr9In(PO4)7 during the antiferroelectric (AFE) phase transition are studied by X-ray powder diffraction, electron microscopy, second-harmonic-generation, and dielectric measurements. Sr9In(PO4)7 complements a group of Ca3(VO4)2-type ferroelectric (FE) phosphates and vanadates and is the first example of an AFE material in this family. Antiparallel shifts of Sr atoms from their average positions and ordering of the P1O4 tetrahedra form two contributions in the structural mechanism of the AFE phase transition a displacive contribution and an order-disorder constituent, respectively. The displacive and order-disorder type of structural changes may account for the obtained value of the Curie-Weiss constant (C ~ 104 K) which is in between the value usually observed for pure displacive (C ~ 105 K) and that for orderdisorder phase transitions (C ~ 103 K). The structural mechanism of the AFE phase transition in Sr9In(PO4)7 is very similar to that of the FE phase transition in Ca9R(PO4)7 and Ca9R(VO4)7. Both displacive and orderdisorder contributions are responsible for the physical properties of the Ca3(VO4)2-type materials. © 2016 Pleiades Publishing, Ltd

Structural changes in Sr9In(PO4)7 during antiferroelectric phase transition

International audienceStructural changes in Sr9In(PO4)7 during the antiferroelectric (AFE) phase transition are studied by X-ray powder diffraction, electron microscopy, second-harmonic-generation, and dielectric measurements. Sr9In(PO4)7 complements a group of Ca3(VO4)2-type ferroelectric (FE) phosphates and vanadates and is the first example of an AFE material in this family. Antiparallel shifts of Sr atoms from their average positions and ordering of the P1O4 tetrahedra form two contributions in the structural mechanism of the AFE phase transition a displacive contribution and an order-disorder constituent, respectively. The displacive and order-disorder type of structural changes may account for the obtained value of the Curie-Weiss constant (C ~ 104 K) which is in between the value usually observed for pure displacive (C ~ 105 K) and that for orderdisorder phase transitions (C ~ 103 K). The structural mechanism of the AFE phase transition in Sr9In(PO4)7 is very similar to that of the FE phase transition in Ca9R(PO4)7 and Ca9R(VO4)7. Both displacive and orderdisorder contributions are responsible for the physical properties of the Ca3(VO4)2-type materials. © 2016 Pleiades Publishing, Ltd

Enhanced nonlinear optical activity and Ca2+-conductivity in Са10.5-xPbx(VO4)7 ferroelectrics

Са3(VO4)2-based compositions are considered as promising multifunctional materials combining ferroelectric, optical nonlinear, and Ca2+-ion conductive properties. Their ferroelectric Curie points stretch from minimal for β-Ca3(PO4)2-type compounds temperatures of about 800 K to very high. Investigated in this paper lead substitution for calcium is as a factor controlling ferroelectricity, ionic-conductivity and non-linear optical activity in Ca3(VO4)2-based materials. Polar phase containing powders and ceramics in Ca10.5-xPbx(VO4)7 system are synthesized for 0 ≤ x ≤ 9.5 by the solid state method, and structurally characterized with X-ray powder diffraction and transmission electron microscopy. Dielectric properties, differential thermal analysis and second harmonic generation (SHG) evidence that Ca10.5-xPbx(VO4)7 solid solution (0 ≤ x ≤ 4.5) belongs to whitlockite-type ferroelectrics. SHG activity strongly increases with x up to its maximum at x = 4.5, where it has a record value among all studied before Ca3(XO4)2-related compounds (X = P,V). Ferroelectric Curie temperatures of Ca3(VO4)2 drops from its known value Tc = 1368 K (x = 0) to 770 K (x = 4.5). Crystal symmetry at Tc changes from R3c to R3¯c. After this, one more phase transition to the symmetry R3¯m takes place, its temperature bringing down from 1387 K (x = 0) to 804 K (x = 4.5). Ferroelectric and non-ferroelectric phase transitions in the Са10.5-xPbx(VO4)7 are separated by a broad interval ΔT = 20–50 K and both classified as first-order transformations going in the sequence: R3c↔R3¯c↔R3¯m. Structures of Са10.5-xPbx(VO4)7 compositions with x = 0.5–4 were refined by the Rietveld method and peculiarities of the Pb2+cations distribution in the M1 - M3 and M4 sites of β-Ca3(PO4)2-type structure are discussed regarding the optical nonlinear, ferroelectric and ion-conductive properties. Manifold increased Ca2+- ion conductivity of Са10.5-xPbx(VO4)7 in vicinity of 1000 K at x = 4–4.5 in combination with their ferroelectric and optical nonlinear properties extends applicability of ion-exchange technologies to new promising materials. © 2017 Elsevier B.V