Evolution towards centrosymmetry of the nonlinear-optical materialRbTiOPO4in the temperature range 293–973 K: Alkaline displacements and titanyl deformations

Abstract

The crystal structure of rubidium titanyl phosphate, RbTiOPO 4 ͑space group Pna2 1 ͒, has been refined at room temperature, and at 473, 673, 873, and 973 K, by using single-crystal x-ray-diffraction techniques. The data show a large anharmonic motion of the rubidium ions increasing with temperature. To describe the importance of this motion in the phase-transition procedure and in the ionic conductivity phenomenon, two models are developed. The study of the deviation to the centrosymmetric structure ͑space group Pnan͒ shows that the change towards centrosymmetry of the titanyl groups correlates directly with the observed variation of the second-harmonic generating intensity I 2 of the RbTiOPO 4 crystal versus temperature.

Direct evidence of an order-disorder nature of ferroelectric phase transitions in K2MgWO2(PO4)2

“…8 The second, much broader CP in KTP ͑more than 150 GHz͒ was attributed to relaxation dynamics of an on site moving nature. 8 The third CP, observed only in a few Kelvin range in the Brillouin study of RbTiOPO 4 , was attributed to coupling of the soft mode to a relaxator. 26 In our case, the observed CPs are related to the phase transitions but the origin of CP in the tetragonal phase cannot be explained by bilinear coupling of the soft optic mode to the diffusive mode since CP relates to a different symmetry ͑E͒ than the diffusive mode ͑A 2 ͒.…”

“…[1][2][3] Many measurements demonstrated that it undergoes a second-order displacive phase transition at 1206 K. [4][5][6][7] However, Raman studies suggested that the phase transition is induced by the diffusive mode. 8 Later studies showed that replacement of Ti IV ͑PO 4 3− ͒ ions by Ge IV , Sn IV , Fe III , or Nb V ͑AsO 4 3− or GeO 4 4− ͒ ions has a significant impact on the phase transition mechanism and properties of KTP isomorphs. [9][10][11] For instance, it was shown that the phase transition in germanate analogs of KTP is both displacive and order-disorder types, 10 whereas the phase transition in KFeFPO 4 is closer to order-disorder type than to displacive.…”

“…K 2 MgWO 2 ͑PO 4 ͒ 2 single crystal, related to famous optical nonlinear material KTiOPO 4 , was investigated by micro-Brillouin scattering with a focus on the central components of the relaxation modes. A critical slowing down has been clearly observed in the vicinity of T 1 = 436 K while suppressed by the coupling with the order parameter.…”

“…© 2007 American Institute of Physics. ͓DOI: 10.1063/1.2713873͔ KTiOPO 4 ͑KTP͒ is a well-known nonlinear-optical material. [1][2][3] Many measurements demonstrated that it undergoes a second-order displacive phase transition at 1206 K. [4][5][6][7] However, Raman studies suggested that the phase transition is induced by the diffusive mode.…”

“…͓DOI: 10.1063/1.2713873͔ KTiOPO 4 ͑KTP͒ is a well-known nonlinear-optical material. [1][2][3] Many measurements demonstrated that it undergoes a second-order displacive phase transition at 1206 K. [4][5][6][7] However, Raman studies suggested that the phase transition is induced by the diffusive mode. 8 Later studies showed that replacement of Ti IV ͑PO 4 3− ͒ ions by Ge IV , Sn IV , Fe III , or Nb V ͑AsO 4 3− or GeO 4 4− ͒ ions has a significant impact on the phase transition mechanism and properties of KTP isomorphs.…”