Influence of Synthetic Conditions on Chemistry and Structural Properties of Alkaline Earth Uranyl Borates

Abstract

Four novel alkaline earth metal uranyl borates, namely, A[(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ]•5H 2 O (SrUBO-1, BaUBO-1) and A[(UO 2 ) 2 (B 2 O 5 )O] (SrUBO-2, BaUBO-2) (A = Sr, Ba), have been prepared; SrUBO-1, BaUBO-1, and BaUBO-2 were synthesized using hydrothermal methods, whereas SrUBO-2 was prepared by a traditional hightemperature solid-state method. The compounds SrUBO-1 and BaUBO-1 were found to be iso-structural and crystallized in the centrosymmetric group C2/c. Their structure features t w o -d i m e n s i o n a l a n i o n i c l a y e r s {[(UO 2 ) 5 (BO 3 ) 2 O 2 (OH) 2 ] 2− } n that are composed of [(UO 2 ) 5 O 2 (OH) 2 ] 4+ clusters and [BO 3 ] 3− polyanions. From a topological point of view, this two-dimensional anionic layer can be described as a new 4-nodal net topological type with a point symbol of {3 5 .4 5 } 2 {3 6 .4 6 .5 3 } 3 {3 7 .4 7 .5 7 } 2 . SrUBO-2 and BaUBO-2 are iso-structural and crystallized in space group C2/m. Their structures are based on a three-dimensional framework {[(UO 2 ) 2 (B 2 O 5 )O] 2− } n . Within the three-dimensional framework, two-dimensional U(2)−U(1) = U(3) layers (L 1 ) with cation to cation interactions between [U(3)O 2 ] 2+ groups and one-dimensional edge sharing U(1)−U(2) chains are present. The B 2 O 5 dimers corner sharing with U(1)-U2 chains and U(3)O 6 tetragonal bipyramids forming two-dimensional layers (L 2 ). L 1 further vertically interconnect with L 2 through bridging B 2 O 5 dimers, forming a threedimensional framework [(UO 2 ) 2 (B 2 O 5 )O] 2− . The synthetic conditions, structural characterization, thermal stability, as well as spectroscopic properties of the alkaline earth metal uranyl borates are reported and discussed.

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

“…Whereas, the phases that obtained in this work from hydrothermal method with tetraborates and large amount of water as a reaction medium are based on the isolated oxo‐borate groups. Compound α‐K 4 [(UO 2 ) 5 (BO 3 ) 2 O 4 ] was prepared by a typical high temperature solid state synthesis (980 °C) with excess of tetraborate as the reaction medium similar to that described for Sr[(UO 2 ) 2 (B 2 O 5 )O] synthesis . However, in opposite to the 3D framework structure of Sr[(UO 2 ) 2 (B 2 O 5 )O], α‐K 4 [(UO 2 ) 5 (BO 3 ) 2 O 4 ] possesses a 2D layered structure.…”

“…Compound α‐K 4 [(UO 2 ) 5 (BO 3 ) 2 O 4 ] was prepared by a typical high temperature solid state synthesis (980 °C) with excess of tetraborate as the reaction medium similar to that described for Sr[(UO 2 ) 2 (B 2 O 5 )O] synthesis . However, in opposite to the 3D framework structure of Sr[(UO 2 ) 2 (B 2 O 5 )O], α‐K 4 [(UO 2 ) 5 (BO 3 ) 2 O 4 ] possesses a 2D layered structure. We presumed that a nature of counter cations plays a key role in the formation of these two compounds.…”

“…In the past two decades, uranium oxo‐anion compounds have received considerable attention due to remarkable variety of their structures and interesting physicochemical properties As a result of these studies, the structural chemistry of uranium has been greatly expanded for both synthetic and natural materials . One group of uranium bearing phases, oxo‐anion based compounds such as carbonates, sulfates, silicates, phosphates and borates, etc, has been granted a special attention due to their important role in natural process of uranium migration . Generally, the most common forms of uranium oxides are triuranium octoxide (U 3 O 8 ) and the dioxide UO 2 .…”

“…Both oxides are relatively stable over a wide range of environmental conditions . Oxidation states of uranium ranges from +II to +VI in solid state phases, and depending on the valence state, coordination chemistry of uranium varies dramatically . However, it has to be noted that the oxidation states II, III and V are considerably less stable compared to tetravalent and hexavalent uranium , .…”

Structural Variations in Complex Sodium Thorium Arsenates

“…The structural chemistry of actinides is very rich due to the complex electronic structure of these elements and consequent variety of oxidation states and diversity of the actinide coordination geometries. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] Thorium, whose redox chemistry, unlike most other actinides, is dominated by the +4 oxidation state, shows highly variable coordination numbers (CN) in inorganic materials ranging from 6 to 15. [17][18][19][20][21][22] Coordination chemistry of Th(IV) is similar to those of highly radiotoxic and long-live transuranic element in the oxidation state +4 (e.g.…”