The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

Gabriel L Murphy,Eike M Langer,Philip Kegler,Evgeny V Alekseev

doi:10.3390/cryst11080965

Abstract

Herein, the novel uranyl selenate and selenite compounds Rb2[(UO2)2(SeO4)3], Rb2[(UO2)3(SeO3)2O2], Rb2[UO2(SeO4)2(H2O)]·2H2O, and (UO2)2(HSeO3)2(H2SeO3)2Se2O5 have been synthesized using either slow evaporation or hydrothermal methods under acidic conditions and their structures were refined using single crystal X-ray diffraction. Rb2[(UO2)2(SeO4)3] synthesized hydrothermally adopts a layered 2D tetragonal structure in space group P42/ncm with a = 9.8312(4) Å, c = 15.4924(9) Å, and V = 1497.38(15) Å, where it consists of UO7 polyhedra coordinated via SeO4 units to create units UO2(SeO4)58− moieties which interlink to create layers in which Rb+ cations reside in the interspace. Rb2[(UO2)3(SeO3)2O2] synthesized hydrothermally adopts a layered 2D triclinic structure in space group P1¯ with a = 7.0116(6) Å, b = 7.0646(6) Å, c = 8.1793(7) Å, α = 103.318(7)°, β = 105.968(7)°, γ = 100.642(7)° and V = 365.48(6) Å3, where it consists of edge sharing UO7, UO8 and SeO3 polyhedra that form [(UO2)3(SeO3)2O2] layers in which Rb+ cations are found in the interlayer space. Rb2[UO2(SeO4)2(H2O)]·2H2O synthesized hydrothermally adopts a chain 1D orthorhombic structure in space group Pmn21 with a = 13.041(3) Å, b = 8.579(2) Å, c = 11.583(2) Å, and V = 1295.9(5) Å3, consisting of UO7 polyhedra that corner share with one H2O and four SeO42− ligands, creating infinite chains. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 synthesized under slow evaporation conditions adopts a 0D orthorhombic structure in space group Cmc21 with a = 28.4752(12) Å, b = 6.3410(3) Å, c = 10.8575(6) Å, and V = 1960.45(16) Å3, consisting of discrete rings of [(UO2)2(HSeO3)2(H2SeO3)2Se2O5]2. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 is apparently only the second example of a uranyl diselenite compound to be reported. A combination of single crystal X-ray diffraction and bond valance sums calculations are used to characterise all samples obtained in this investigation. The structures uncovered in this investigation are discussed together with the broader family of uranyl selenates and selenites, particularly in the context of the role acidity plays during synthesis in coercing specific structure, functional group, and topology formations.

Highlights

The acquisition of novel non-centrosymmetric (NCS) structural phases has formed a topical focus of inorganic chemical synthesis development due to their prominent application in a variety of non-linear optic (NLO) [1,2], piezoelectric [3], and ferroelectric [4]based materials
Understanding the synthesis conditions and design which pertain to the polymerization of Studies (b) Rb2 [(UO2 )3 (SeO3) 2− units is a pertinent pursuit in the context of generation NLO and second harmonic generation (SHG) material generation in addition to fundamental inorganic synthesis development
There is relatively less known about selenites and selenates in respect of synthesis conditions and control, despite the interest these receive in context of NLO and SHG materials development

Summary

Introduction

The acquisition of novel non-centrosymmetric (NCS) structural phases has formed a topical focus of inorganic chemical synthesis development due to their prominent application in a variety of non-linear optic (NLO) [1,2], piezoelectric [3], and ferroelectric [4]. More contemporary investigations have utilized systematic approaches, which better enable improved synthesis control and increased understanding and prediction of potential reaction outcomes [17,26,27,28,29,30,31,32,33] Many of these more recent studies have highlighted the unique ability and promising application of uranium derived compounds in societally important materials such as radiation detectors, scintillators to proton conductors [34,35,36]. There is relatively less known about selenites and selenates in respect of synthesis conditions and control, despite the interest these receive in context of NLO and SHG materials development It has been more than 30 years since the first and only uranyl polyselenite was reported, UO2 Se2 O5 , possessing the Se2 O5 2− moiety [12]. The results of this investigation are discussed with respect to the broader family of uranyl selenites/selenates regarding the ability for acid to drive novel structure and topology formation

Experimental

Single Crystal X-ray Diffraction

Bond Valence Sums

Results and Discussion

Conclusions