Etude des propriétés cristallographiques, diélectriques et d'optique non linéaire de quelques nouvelles phases de composition BaxLi5−2xNb5(1−y)Ta5yO15 et de structure bronzes oxygénés de tungstène quadratiques

Relaxor-to-Ferroelectric Crossover and Disruption of Polar Order in “Empty” Tetragonal Tungsten Bronzes

“…Such loops, first demonstrated in PbZrO 3 by Shirane et al, , are characteristic of AFE ordering, and consequently, many systems were designated as being AFE based on P – E loops. Few instances of pinched “antiferroelectric” loops have been reported in TTBs. − An AFE-FE-PE phase transition sequence was described by Ravez and colleagues in the TTB-like compounds in the solid solution Ba 2.14 Li 0.71 Nb 5 O 15 –Ba 2.14 Li 0.71 Ta 5 O 15 (i.e., Ba 4.28 Li 1.42 Nb 10 O 30 –Ba 4.28 Li 1.42 Ta 10 O 30 ). , Gagou et al have recently attributed two dielectric anomalies in K 4 Gd 2 Nb 10 O 30 to a FE-AFE-PE sequence of phase transitions based on Raman spectroscopy data; however, no P – E loops were demonstrated, and the low temperature anomaly has previously been associated with a ferroelastic related transition between orthorhombic (pseudotetragonal) and tetragonal structures . Pinched P – E loops have been reported in La and Ti codoped SBN, Sr 0.255 Ba 0.7 La 0.03 Nb 2‑ y Ti y O 6‑δ with y = 0.05 (i.e., Sr 1.275 Ba 3.5 La 0.15 Nb 9.75 Ti 0.25 O 30‑δ ) in which FE and AFE states coexist.…”

Non-centrosymmetric Na3Nb4As3O19

“…For physical properties of this class of compound, see: Masquelier et al (1995); Daidouh et al (1997Daidouh et al ( , 1998Daidouh et al ( , 1999; Sanz et al (1999Sanz et al ( , 2001; Baies et al (2006); Ravez et al (1972Ravez et al ( , 1974; Torardi et al (1985); Krol et al (1980); Blasse et al (1992). For synthetic details, see: Zid et al (1988Zid et al ( , 1989; Ben Amor & Zid (2005; Hizaoui et al (1999a, b); Haddad et al (1988); Harrison et al (1994); Ché rif et al (2011).…”

“…L'occupation partielle des sites par Na + dans la stucture du composé Na 3 Nb 4 As 3 O 19 pourrait conférer à ce matériau des propriétés de conduction ionique (Masquelier et al, 1995;Daidouh et al, 1997Daidouh et al, , 1998Daidouh et al, , 1999Sanz et al, 1999Sanz et al, , 2001Baies et al, 2006). Le composé Na 3 Nb 4 As 3 O 19 , appartenant à une classe non-centrosymétrique (groupe d'espace: C222 1 ) pourrait présenter des propriétés optiques non linéaires (Ravez et al, 1972(Ravez et al, , 1974 de groupements niobyl dans la structure laisse prévoir également des propriétés de luminescence (Torardi et al, 1985;Krol et al, 1980;Blasse et al, 1992).…”

The effect of cation size on structure and properties of Ba-based tetragonal tungsten bronzes Ba4M2Nb10O30 (M = Na, K or Rb) and Ba4M2Nb8Ti2O30 (M = Ca or Sr)

“…The C-sites are, if occupied, typically filled by small cations such as Li + . 5 Furthermore, a certain degree of occupational disorder is expected amid the non-octahedral sites and disorder is increasingly favoured with cation size parity. 6,7 The aristotype TTB structure has a tetragonal centrosymmetric P4/mbm space group, and a whole family of TTB compounds become ferroelectric as they go through one or more phase transitions into non-centrosymmetric space groups.…”

“…The C-sites are, if occupied, typically filled by small cations such as Li + . 5 Furthermore, a certain degree of occupational disorder is expected amid the non-octahedral sites and disorder is increasingly favoured with cation size parity. 6,7…”