Structural, vibrational study and conductivity investigation of a new cesium–bismuth polyphosphate CsBi(PO3)4

Cyclophosphate MBi(P4O12) (M = Cs, Rb): Structure Change Giving Rise to Property Enhancement

“…These linear [PO 3 ] ∞ chains in CsBi(PO 3 ) 4 are also connected by [BiO 8 ] polyhedra resulting in a 3D framework with Cs + reside in the voids. 14 The Bi−O bond varies from 2.342 to 2.498 Å and the Cs−O bond from 2.974 to 3.496 Å, similar to those in the title compound.…”

“…As discussed above, the crystal structure of the title cubic CsBi­(P 4 O 12 ) are closely related with the monoclinic CsBi­(PO 3 ) 4 , and the monoclinic cyclo KGd­(P 4 O 12 ), and the latter two are both centrosymmetric structures which are SHG-inert in nature. Therefore, to exhibit their distinct properties governed by the crystal structure symmetries, the SHG intensity was measured on the polycrystalline CsBi­(P 4 O 12 ) sample by the standard Kurtz–Perry method with a Q-switched Nd:YAG laser of λ = 1064 nm.…”

“…Besides, the coordination number of the monovalent cations varies from 9 for Cs + to 6 More interestingly, the previously reported monoclinic CsBi(PO 3 ) 4 (S.G.: P2 1 /n) 14 shares the same stoichiometry with the title CsBi(P 4 O 12 ); however, their structures are totally different. As presented in Figure 1f, CsBi(PO 3 ) 4 14 features an infinite linear [PO 3 ] ∞ chain formed by the [PO 4 ] tetrahedra run along the [101] direction, rather than the isolated tetramer cyclo [P 4 O 12 ] ring in the title CsBi(P 4 O 12 ) (Figure 1b). These linear [PO 3 ] ∞ chains in CsBi(PO 3 ) 4 are also connected by [BiO 8 ] polyhedra resulting in a 3D framework with Cs + reside in the voids.…”

“…Although the existence of the cyclic [P 4 O 12 ] structural motif was first confirmed in 1937 by Pauling, only about 20 cyclotetraphosphates in this system have been structurally identified (Table S1). − …”

“…Unlike the RE ions exhibiting the f–f transitions, Bi 3+ has no absorption in the visible region, leading to a wider band gap ( E g ) of 4.52 eV for the title MBi­(P 4 O 12 ). Interestingly, compared to the monoclinic infinite [PO 3 ] ∞ chains, and the centrosymmetric uniformly oriented [P 4 O 12 ] rings in KGd­(P 4 O 12 ), the rearrangement of the PO 4 tetrahedra into such cyclic [P 4 O 12 ] rings not only changes the symmetry from a centrosymmetric P 2 1 / n , or C 2/ c , to the non-centrosymmetric I 3 d , leading to a large non-phase-matchable second harmonic generation (SHG) response, but also gives rise to an excellent bulk ionic conductivity with the lowest activation energy of 0.18 eV among phosphates known to date. These property–structure relationships may provide some useful insights to the future structure design and property enhancement of the functional materials.…”