Equilibrium langbeinite-related phosphates Cs_1 + x Ln _x Zr_2 − x (PO₄)₃ (Ln = Sm–Lu) in the melted systems Cs₂O–P₂O₅–LnF₃–ZrF₄

Abstract

Nine novel phosphates, based upon a combination of caesium, zirconium and lanthanide ions, were obtained from fluoride-containing fluxes using high-temperature crystallization. The structures of Cs(1.80)Eu(0.80)Zr(1.20)(PO(4))(3) (CsEuZrP), Cs(1.79)Gd(0.79)Zr(1.21)(PO(4))(3) (CsGdZrP), Cs(1.87)Tb(0.87)Zr(1.13)(PO(4))(3) (CsTbZrP), Cs(1.67)Dy(0.67)Zr(1.33)(PO(4))(3) (CsDyZrP), Cs(1.75)Ho(0.75)Zr(1.25)(PO(4))(3) (CsHoZrP), Cs(1.78)Er(0.78)Zr(1.22)(PO(4))(3) (CsErZrP), Cs(1.70)Tm(0.70)Zr(1.30)(PO(4))(3) (CsTmZrP), Cs(1.52)Yb(0.52)Zr(1.48)(PO(4))(3) (CsYbZrP) and Cs(1.63)Lu(0.63)Zr(1.37)(PO(4))(3) (CsLuZrP) were solved using single-crystal X-ray diffraction. All compounds are isostructural to the mineral langbeinite (cubic system, space group P2(1)3). Their framework structures originate from the cross-linking of metal octahedra [MO(6)] (M = Zr, Ln) by phosphate tetrahedra. Cs(+) cations are located in the closed cavities of the framework and preferentially occupy one of the two available sites. The principles of crystallization of the equilibrium langbeinite-related phosphates in the fluxes of the system Cs(2)O-P(2)O(5)-LnF(3)-ZrF(4) (Ln = La-Nd, Sm-Lu) are discussed based on their crystal structures.

Partial Substitution of Potassium with Sodium in the K2Ti2(PO4)3 Langbeinite‐Type Framework: Synthesis and Crystalline Structure of K1.75Na0.25Ti2(PO4)3

“…Thus, the crystallographic site of K/Na(1) in K 0.877 Na 0.48 Ti 2 (PO 4 ) 3 is completely filled with equal portions of potassium and sodium atoms (K/Na=0.52:0.48), and the K2 site is filled by potassium for only about a third of the amount (K vac /K=0.643:0.357). Notably, the formation of vacancies in only one of the alkali metal positions was observed earlier, for example, in a number of Cs 1+ x Ln x Zr 2− x (PO 4 ) 3 (Ln=Sm–Lu) solid solutions . The presence of vacancies significantly affects the geometry of the cavities in the anionic [Ti 2 (PO 4 ) 3 ] sublattice.…”

“…This approach was successfully applied for sites concurrently occupied by different elements in langbeinite‐type frameworks in previous investigations . As both K 1.75 Na 0.25 Ti 2 (PO 4 ) 3 and K 2 Ti 2 (PO 4 ) 3 should contain equal quantities of Ti III and Ti IV due to their electroneutrality, three variants of BVS calculations for the positions of Ti1 and Ti2 were performed.…”

“…Notably, the formation of vacanciesi no nly one of the alkali metal positions was observed earlier,f or example, in an umber of Cs 1 + x Ln x Zr 2Àx (PO 4 ) 3 (Ln = Sm-Lu) solid solutions. [22] The presence of vacancies significantly affectsthe geometry of the cavi- [a] Symmetryt ransformationsu sed to generate equivalent atoms:i ii) x, À1 + y, z;i v) 0.5 + y,0 .5Àz, Àx;v i) Àx,0 .5 + y,0 .5Àz;v ii) y, z, x;v iii) z, x, y; xii)0.5 + y, À0.5Àz, Àx. ties in the anionic [Ti 2 (PO 4 ) 3 ]s ublattice.…”

“…This resultsi n the general chemicalf ormula M I x Z 2 (PO 4 ) 3 (M I = K, Rb, Cs;Z = aliovalent metal or metals;1 .0 x 2.0) for stoichiometric compounds or solid solutions, for example, K 2Àx Ti 2 (PO 4 ) 3 , [14,15] K 2 M III Ti(PO 4 ) 3 (M III = Al, [16] Er,Y b, or Y), [17] K 2 M II 0.5 Ti 1.5 (PO 4 ) 3 (M II = Ni, [18] Co, or Mn), [19] Rb 2 Yb x Ti 2Àx (PO 4 ) 3 , [20,21] and Cs 1 + x Ln x Zr 2Àx (PO 4 ) 3 (Ln = Sm-Lu). [22] Due to their high thermal and chemical stability, langbeinitetype phosphates are intensively studied asm atrixes for the storageo fr adioactivew aste from the nucleari ndustry [23] and as basic materials for luminophores. [24] However,i ft he lattice is formed from big alkaline cations (e.g.…”

“…The second packaging, associated with the formation of the langbeinite‐type structure [K 2 Mg 2 (SO 4 ) 3 ], determines the presence of big cations in the framework. This results in the general chemical formula M I x Z 2 (PO 4 ) 3 (M I =K, Rb, Cs; Z=aliovalent metal or metals; 1.0≤ x ≤2.0) for stoichiometric compounds or solid solutions, for example, K 2− x Ti 2 (PO 4 ) 3 , K 2 M III Ti(PO 4 ) 3 (M III =Al, Er, Yb, or Y), K 2 M II 0.5 Ti 1.5 (PO 4 ) 3 (M II =Ni, Co, or Mn), Rb 2 Yb x Ti 2− x (PO 4 ) 3 , and Cs 1+ x Ln x Zr 2− x (PO 4 ) 3 (Ln=Sm–Lu) …”