Divergent Structural Chemistry of Uranyl Borates Obtained from Solid State and Hydrothermal Conditions

Abstract

A series of novel uranyl borates, K4Sr4[(UO2)13­(B2O5)2­(BO3)2O12], A6[(UO2)12­(BO3)8O3]­(H2O)6 (A = Rb and Cs), and Rb3[(UO2)3­(BO3)2O­(OH)]­(H2O), were synthesized using conventional conditions. Among them, K4Sr4[(UO2)13­(B2O5)2­(BO3)2O12] and A6[(UO2)12(BO3)8O3]­(H2O)6 were obtained through a high-temperature solid-state reaction method, whereas Rb3[(UO2)3­(BO3)2O­(OH)]­(H2O) was synthesized via a hydrothermal reaction. All compounds adopt novel two-dimensional (2D) layered structures in which basic building units (BBUs) consist of corner- or edge-sharing UO x (x = 6, 7 and 8) polyhedra linked with planar BO3 triangles or B2O5 dimers. K4Sr4[(UO2)13­(B2O5)2­(BO3)2O12] is the first mixed alkali–alkaline earth metal uranyl borate. This compound has the most complex 2D anion topology observed thus far in 2D uranyl borates. The fundamental building block (FBB) in this structure, [(UO2)13(B2O5)2­(BO3)2O12]12–, consists of 3 UO8 hexagonal bipyramids and 10 UO7 pentagonal bipyramids connected with 2 BO3 triangles and 2 B2O5 dimers. The FBB of Rb6[(UO2)12­(BO3)8O3]­(H2O)6 is [(UO2)6­(BO3)4O3]6–, comprised of five edge-sharing UO7 pentagonal bipyramids, one UO6 tetragonal bipyramid, and four BO3 triangles. The simplest FBB, [(UO2)3­(BO3)2O2]4–, occurs in Rb3[(UO2)3(BO3)2O­(OH)]­(H2O), where three UO7 pentagonal bipyramids are linked with two BO3 triangles via edge sharing. The aforementioned FBBs are further polymerized into the corresponding infinite uranyl borates layers. The synthetic methods, novel topologies, thermal stability, and spectroscopic properties of these compounds are reported herein.

Two‐Dimensional Uranyl Borates: From Conventional to Extreme Synthetic Conditions

“…The U/B molar ratios of K 4 [(UO 2 ) 5 (BO 3 ) 2 O 4 ] · H 2 O and M [(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ] · (H 2 O) 5 ( M = Sr, Ba) are 5:2 with the FBB [(UO 2 ) 5 (BO 3 ) 2 O 4 ] 4– . A relatively high U/B ratio (13:6) observed in the first mixed alkali and alkali‐earth metal uranyl borates K 4 Sr 4 [(UO 2 ) 13 (B 2 O 5 ) 2 (BO 3 ) 2 O 12 ] . Its FBB is [(UO 2 ) 13 (B 2 O 5 ) 2 (BO 3 ) 2 O 12 ] 12– , which is built from the connections of three UO 8 , ten UO 7 polyhedra, two B 2 O 5 dimers and two BO 3 triangles.…”

Structural and Spectroscopic Investigation of Novel 2D and 3D Uranium Oxo-Silicates/Germanates and Some Statistical Aspects of Uranyl Coordination in Oxo-Salts

“…A large number of peaks have been found in the range of 200–380 cm –1 , which can be assigned to the ν 2 (UO 2 ) 2+ modes. The Raman spectra of 2D layer structures for uranyl oxo-anions such as arsenates and borates also exhibit a large number of bands in the low frequency region. The ν 3 (GeO 4 ) 4– and ν 4 (O–Ge–O) bands appear at 680 and 429 cm –1 , , respectively.…”

Crystal Growth of Alkali Uranyl Borates from Molten Salt Fluxes: Characterization and Ion Exchange Behavior of A2(UO2)B2O5 (A = Cs, Rb, K)

“…A considerable number of uranium borates have been synthesized by the Albrecht-Schmitt and Alekseev groups; however, the majority of these reported phases were prepared via mild hydrothermal routes − and high-temperature, high-pressure hydrothermal synthesis methods, − while only a handful have been prepared via high-temperature flux synthesis. The latter consist of the simple ternary and quaternary uranium borates UO 2 B 2 O 4 , Mg­(UO 2 )­B 2 O 5 , Ca­(UO 2 ) 2 B 2 O 6 , and A­(UO 2 )­(BO 3 ) (A = Li, Na, K, Rb, Cs), − along with the transition-metal-containing quaternary phase Ni 7 (UO 2 )­(B 4 O 14 ) .…”

“…40 Similarly, a set of highly luminescent uranyl phosphates A 6 [(UO 2 ) 7 O 4 (PO 4 ) 4 ] (A = Cs, Rb) were prepared from a CsCl flux; cation−cation interactions within the layered structure were found to induce intense luminescence in these phases. 41 Additionally, the [Cs 13 single crystal-to-single crystal postsynthetic ion exchange. 42 Given the structurally diverse nature of borates, an investigation of the uranium borate phase space was a natural extension of these systems.…”